const API_URL = '/api';
let allDrugs = [];
let currentDrug = null;
let showLowStockOnly = false;
let selectedLocationFilter = '';
let searchTerm = '';
let expandedDrugs = new Set();
let currentUser = null;
let accessToken = null;
let deliveryDrugId = null;
let deliveryLineCounter = 0;
let deliveryLocations = [];
let activeVariantPacksVariantId = null;

// Toast notification system
function showToast(message, type = 'info', duration = 3000) {
    const container = document.getElementById('toastContainer');
    if (!container) return;
    
    const toast = document.createElement('div');
    toast.className = `toast ${type}`;
    
    const icons = {
        success: '✓',
        error: '✕',
        warning: '⚠',
        info: 'ℹ'
    };
    
    toast.innerHTML = `
        <span class="toast-icon">${icons[type] || icons.info}</span>
        <span class="toast-message">${message}</span>
    `;
    
    container.appendChild(toast);
    
    // Auto remove after duration
    setTimeout(() => {
        toast.classList.add('fade-out');
        setTimeout(() => {
            container.removeChild(toast);
        }, 300);
    }, duration);
}

// Initialize on page load
document.addEventListener('DOMContentLoaded', () => {
    checkAuth();
});

// Check if user is already logged in
function checkAuth() {
    const token = localStorage.getItem('accessToken');
    const user = localStorage.getItem('currentUser');
    
    if (token && user) {
        accessToken = token;
        currentUser = JSON.parse(user);
        showMainApp();
    } else {
        showLoginPage();
    }
}

// Show login page
function showLoginPage() {
    document.getElementById('loginPage').style.display = 'flex';
    document.getElementById('mainApp').style.display = 'none';
    
    const loginForm = document.getElementById('loginForm');
    if (loginForm) loginForm.addEventListener('submit', handleLogin);
}

// Show main app
function showMainApp() {
    document.getElementById('loginPage').style.display = 'none';
    document.getElementById('mainApp').style.display = 'block';
    
    // Handle backward compatibility: convert old is_admin format to role
    if (!currentUser.role && currentUser.is_admin !== undefined) {
        currentUser.role = currentUser.is_admin ? 'admin' : 'user';
    }
    // Default to 'user' if role is still undefined
    if (!currentUser.role) {
        currentUser.role = 'user';
    }
    
    const userDisplay = document.getElementById('currentUser');
    if (userDisplay) {
        const roleLabel = currentUser.role.charAt(0).toUpperCase() + currentUser.role.slice(1);
        userDisplay.textContent = `👤 ${currentUser.username} [${roleLabel}]`;
    }
    
    const adminBtn = document.getElementById('adminBtn');
    if (adminBtn) {
        adminBtn.style.display = currentUser.role === 'admin' ? 'block' : 'none';
    }
    
    const locationsBtn = document.getElementById('locationsBtn');
    if (locationsBtn) {
        locationsBtn.style.display = currentUser.role === 'admin' ? 'block' : 'none';
    }

    const reportsBtn = document.getElementById('reportsBtn');
    if (reportsBtn) {
        reportsBtn.style.display = currentUser.role === 'admin' ? 'block' : 'none';
    }
    
    // Hide action buttons for read-only users
    const isReadOnly = currentUser.role === 'readonly';
    const addDrugBtn = document.getElementById('addDrugBtn');
    const dispenseBtn = document.getElementById('dispenseBtn');
    const printNotesBtn = document.getElementById('printNotesBtn');
    
    if (addDrugBtn) addDrugBtn.style.display = isReadOnly ? 'none' : 'block';
    if (dispenseBtn) dispenseBtn.style.display = isReadOnly ? 'none' : 'block';
    if (printNotesBtn) printNotesBtn.style.display = isReadOnly ? 'none' : 'block';
    
    setupEventListeners();
    loadDrugs();
}

// Handle login
async function handleLogin(e) {
    e.preventDefault();
    const username = document.getElementById('loginUsername').value;
    const password = document.getElementById('loginPassword').value;
    
    try {
        const response = await fetch(`${API_URL}/auth/login`, {
            method: 'POST',
            headers: { 'Content-Type': 'application/json' },
            body: JSON.stringify({ username, password })
        });
        
        if (!response.ok) {
            throw new Error('Invalid credentials');
        }
        
        const data = await response.json();
        accessToken = data.access_token;
        currentUser = data.user;
        
        localStorage.setItem('accessToken', accessToken);
        localStorage.setItem('currentUser', JSON.stringify(currentUser));
        
        document.getElementById('loginForm').reset();
        const errorDiv = document.getElementById('loginError');
        if (errorDiv) errorDiv.style.display = 'none';
        showMainApp();
    } catch (error) {
        const errorDiv = document.getElementById('loginError');
        if (errorDiv) {
            errorDiv.textContent = error.message;
            errorDiv.style.display = 'block';
        }
    }
}

// Handle register
// Logout
function handleLogout() {
    localStorage.removeItem('accessToken');
    localStorage.removeItem('currentUser');
    accessToken = null;
    currentUser = null;
    const loginForm = document.getElementById('loginForm');
    if (loginForm) loginForm.reset();
    const registerForm = document.getElementById('registerForm');
    if (registerForm) {
        registerForm.style.display = 'none';
    }
    const form = document.getElementById('loginForm');
    if (form) form.style.display = 'block';
    showLoginPage();
}

// API helper with authentication
async function apiCall(endpoint, options = {}) {
    const headers = {
        'Content-Type': 'application/json',
        ...options.headers
    };
    
    if (accessToken) {
        headers['Authorization'] = `Bearer ${accessToken}`;
    }
    
    const response = await fetch(`${API_URL}${endpoint}`, {
        ...options,
        headers
    });
    
    if (response.status === 401) {
        handleLogout();
        throw new Error('Authentication expired');
    }
    
    return response;
}

// Setup event listeners
function setupEventListeners() {
    const drugForm = document.getElementById('drugForm');
    const variantForm = document.getElementById('variantForm');
    const editVariantForm = document.getElementById('editVariantForm');
    const dispenseForm = document.getElementById('dispenseForm');
    const prescribeForm = document.getElementById('prescribeForm');
    const editForm = document.getElementById('editForm');
    const printNotesForm = document.getElementById('printNotesForm');
    const addModal = document.getElementById('addModal');
    const addVariantModal = document.getElementById('addVariantModal');
    const editVariantModal = document.getElementById('editVariantModal');
    const dispenseModal = document.getElementById('dispenseModal');
    const prescribeModal = document.getElementById('prescribeModal');
    const editModal = document.getElementById('editModal');
    const printNotesModal = document.getElementById('printNotesModal');
    const batchReceiveModal = document.getElementById('batchReceiveModal');
    const receiveDeliveryModal = document.getElementById('receiveDeliveryModal');
    const addDrugBtn = document.getElementById('addDrugBtn');
    const dispenseBtn = document.getElementById('dispenseBtn');
    const printNotesBtn = document.getElementById('printNotesBtn');
    const cancelAddBtn = document.getElementById('cancelAddBtn');
    const cancelVariantBtn = document.getElementById('cancelVariantBtn');
    const cancelEditVariantBtn = document.getElementById('cancelEditVariantBtn');
    const cancelDispenseBtn = document.getElementById('cancelDispenseBtn');
    const cancelPrescribeBtn = document.getElementById('cancelPrescribeBtn');
    const cancelEditBtn = document.getElementById('cancelEditBtn');
    const cancelBatchReceiveBtn = document.getElementById('cancelBatchReceiveBtn');
    const cancelReceiveDeliveryBtn = document.getElementById('cancelReceiveDeliveryBtn');
    const addDeliveryLineBtn = document.getElementById('addDeliveryLineBtn');
    const addVariantFromDeliveryBtn = document.getElementById('addVariantFromDeliveryBtn');
    const addVariantPackRowBtn = document.getElementById('addVariantPackRowBtn');
    const variantUnitSelect = document.getElementById('variantUnit');
    const variantStrengthInput = document.getElementById('variantStrength');
    const dispenseModeSelect = document.getElementById('dispenseMode');
    const variantPacksForm = document.getElementById('variantPacksForm');
    const closeVariantPacksBtn = document.getElementById('closeVariantPacksBtn');
    const showAllBtn = document.getElementById('showAllBtn');
    const showLowStockBtn = document.getElementById('showLowStockBtn');
    const locationFilterSelect = document.getElementById('locationFilterSelect');
    const userMenuBtn = document.getElementById('userMenuBtn');
    const adminBtn = document.getElementById('adminBtn');
    const locationsBtn = document.getElementById('locationsBtn');
    const reportsBtn = document.getElementById('reportsBtn');
    const logoutBtn = document.getElementById('logoutBtn');
    const changePasswordBtn = document.getElementById('changePasswordBtn');

    // Modal close buttons
    const closeButtons = document.querySelectorAll('.close');

    if (drugForm) drugForm.addEventListener('submit', handleAddDrug);
    if (variantForm) variantForm.addEventListener('submit', handleAddVariant);
    if (editVariantForm) editVariantForm.addEventListener('submit', handleEditVariant);
    if (dispenseForm) dispenseForm.addEventListener('submit', handleDispenseDrug);
    if (prescribeForm) prescribeForm.addEventListener('submit', handlePrescribeDrug);
    if (editForm) editForm.addEventListener('submit', handleEditDrug);
    if (printNotesForm) printNotesForm.addEventListener('submit', handlePrintNotes);
    
    const batchReceiveForm = document.getElementById('batchReceiveForm');
    if (batchReceiveForm) batchReceiveForm.addEventListener('submit', handleBatchReceive);
    if (cancelBatchReceiveBtn) cancelBatchReceiveBtn.addEventListener('click', () => closeModal(batchReceiveModal));

    const receiveDeliveryForm = document.getElementById('receiveDeliveryForm');
    if (receiveDeliveryForm) receiveDeliveryForm.addEventListener('submit', handleReceiveDelivery);
    if (cancelReceiveDeliveryBtn) cancelReceiveDeliveryBtn.addEventListener('click', () => closeModal(receiveDeliveryModal));
    if (addDeliveryLineBtn) addDeliveryLineBtn.addEventListener('click', () => appendDeliveryLine());
    if (addVariantFromDeliveryBtn) addVariantFromDeliveryBtn.addEventListener('click', handleAddVariantFromDelivery);
    if (addVariantPackRowBtn) addVariantPackRowBtn.addEventListener('click', () => appendVariantPackRow());
    if (variantUnitSelect) {
        variantUnitSelect.addEventListener('change', () => {
            refreshVariantPackRowLabels();
        });
    }
    if (variantStrengthInput && variantUnitSelect) {
        variantStrengthInput.addEventListener('blur', () => {
            variantUnitSelect.value = inferBaseUnitFromStrength(variantStrengthInput.value);
            refreshVariantPackRowLabels();
        });
    }
    if (dispenseModeSelect) dispenseModeSelect.addEventListener('change', updateDispenseModeUi);
    
    if (addDrugBtn) addDrugBtn.addEventListener('click', () => openModal(addModal));
    if (printNotesBtn) printNotesBtn.addEventListener('click', () => openModal(printNotesModal));
    if (dispenseBtn) dispenseBtn.addEventListener('click', () => {
        updateDispenseDrugSelect();
        updateDispenseModeUi();
        openModal(dispenseModal);
    });
    
    if (cancelAddBtn) cancelAddBtn.addEventListener('click', () => closeModal(addModal));
    if (cancelVariantBtn) cancelVariantBtn.addEventListener('click', () => closeModal(addVariantModal));
    if (cancelEditVariantBtn) cancelEditVariantBtn.addEventListener('click', () => closeModal(editVariantModal));
    if (cancelDispenseBtn) cancelDispenseBtn.addEventListener('click', () => closeModal(dispenseModal));
    if (cancelPrescribeBtn) cancelPrescribeBtn.addEventListener('click', () => closeModal(prescribeModal));
    if (cancelEditBtn) cancelEditBtn.addEventListener('click', closeEditModal);
    
    const cancelPrintNotesBtn = document.getElementById('cancelPrintNotesBtn');
    if (cancelPrintNotesBtn) cancelPrintNotesBtn.addEventListener('click', () => closeModal(printNotesModal));
    
    const closeHistoryBtn = document.getElementById('closeHistoryBtn');
    if (closeHistoryBtn) closeHistoryBtn.addEventListener('click', () => closeModal(document.getElementById('historyModal')));

    const closeUserManagementBtn = document.getElementById('closeUserManagementBtn');
    if (closeUserManagementBtn) closeUserManagementBtn.addEventListener('click', () => closeModal(document.getElementById('userManagementModal')));
    
    const closeLocationManagementBtn = document.getElementById('closeLocationManagementBtn');
    if (closeLocationManagementBtn) closeLocationManagementBtn.addEventListener('click', () => closeModal(document.getElementById('locationManagementModal')));

    if (variantPacksForm) variantPacksForm.addEventListener('submit', handleCreateVariantPack);
    if (closeVariantPacksBtn) closeVariantPacksBtn.addEventListener('click', () => closeModal(document.getElementById('variantPacksModal')));
    
    const createLocationForm = document.getElementById('createLocationForm');
    if (createLocationForm) createLocationForm.addEventListener('submit', createLocation);
    
    const changePasswordForm = document.getElementById('changePasswordForm');
    if (changePasswordForm) changePasswordForm.addEventListener('submit', handleChangePassword);
    
    const cancelChangePasswordBtn = document.getElementById('cancelChangePasswordBtn');
    if (cancelChangePasswordBtn) cancelChangePasswordBtn.addEventListener('click', () => closeModal(document.getElementById('changePasswordModal')));
    
    const adminChangePasswordForm = document.getElementById('adminChangePasswordForm');
    if (adminChangePasswordForm) adminChangePasswordForm.addEventListener('submit', handleAdminChangePassword);
    
    const cancelAdminChangePasswordBtn = document.getElementById('cancelAdminChangePasswordBtn');
    if (cancelAdminChangePasswordBtn) cancelAdminChangePasswordBtn.addEventListener('click', () => closeModal(document.getElementById('adminChangePasswordModal')));
    
    closeButtons.forEach(btn => btn.addEventListener('click', (e) => {
        const modal = e.target.closest('.modal');
        closeModal(modal);
    }));
    
    if (showAllBtn) showAllBtn.addEventListener('click', () => {
        showLowStockOnly = false;
        updateFilterButtons();
        renderDrugs();
    });
    if (showLowStockBtn) showLowStockBtn.addEventListener('click', () => {
        showLowStockOnly = true;
        updateFilterButtons();
        renderDrugs();
    });
    if (locationFilterSelect) locationFilterSelect.addEventListener('change', (e) => {
        selectedLocationFilter = e.target.value;
        renderDrugs();
    });
    
    // User menu
    if (userMenuBtn) userMenuBtn.addEventListener('click', () => {
        const dropdown = document.getElementById('userDropdown');
        if (dropdown) dropdown.style.display = dropdown.style.display === 'none' ? 'block' : 'none';
    });
    
    if (changePasswordBtn) changePasswordBtn.addEventListener('click', openChangePasswordModal);
    if (adminBtn) adminBtn.addEventListener('click', openUserManagement);
    if (locationsBtn) locationsBtn.addEventListener('click', openLocationManagement);
    if (reportsBtn) reportsBtn.addEventListener('click', openReportsPage);
    if (logoutBtn) logoutBtn.addEventListener('click', handleLogout);

    // Search functionality
    const drugSearch = document.getElementById('drugSearch');
    if (drugSearch) {
        let searchTimeout;
        drugSearch.addEventListener('input', (e) => {
            clearTimeout(searchTimeout);
            searchTimeout = setTimeout(() => {
                searchTerm = e.target.value.toLowerCase().trim();
                renderDrugs();
            }, 150);
        });
    }

    const dispenseQuantityInput = document.getElementById('dispenseQuantity');
    if (dispenseQuantityInput) {
        dispenseQuantityInput.addEventListener('input', () => {
            const mode = document.getElementById('dispenseMode')?.value || 'subunit';
            if (mode !== 'subunit') {
                return;
            }

            const packSelect = document.getElementById('dispensePackSelect');
            const packCount = document.getElementById('dispensePackCount');
            const packPreview = document.getElementById('dispensePackPreview');
            const variantId = parseInt(document.getElementById('dispenseDrugSelect')?.value || '', 10);
            const variant = getVariantById(variantId);

            if (packSelect) packSelect.value = '';
            if (packCount) packCount.value = '';
            if (packPreview && variant) {
                packPreview.textContent = `Enter direct quantity in ${variant.unit}.`;
            }
        });
    }

    // Close modal when clicking outside
    window.addEventListener('click', (e) => {
        if (e.target.classList.contains('modal')) {
            closeModal(e.target);
        }
    });
}

// Load drugs from API
async function loadDrugs() {
    try {
        const response = await apiCall('/drugs');
        if (!response.ok) throw new Error('Failed to load drugs');
        allDrugs = await response.json();

        updateLocationFilterOptions();
        renderDrugs();
        updateDispenseDrugSelect();
    } catch (error) {
        console.error('Error loading drugs:', error);
        document.getElementById('drugsList').innerHTML = 
            '<p class="empty">Error loading drugs. Make sure the backend is running.</p>';
    }
}

// Modal utility functions
function openModal(modal) {
    // Find the highest z-index among currently visible modals
    const visibleModals = document.querySelectorAll('.modal.show');
    let maxZIndex = 1000;
    
    visibleModals.forEach(m => {
        const zIndex = parseInt(window.getComputedStyle(m).zIndex, 10) || 1000;
        if (zIndex > maxZIndex) {
            maxZIndex = zIndex;
        }
    });
    
    // Set the new modal's z-index higher than any existing modal
    modal.style.zIndex = (maxZIndex + 100).toString();
    modal.classList.add('show');
    document.body.style.overflow = 'hidden';
}

function closeModal(modal) {
    modal.classList.remove('show');
    modal.style.zIndex = '1000';
    document.body.style.overflow = 'auto';
}

function closeEditModal() {
    closeModal(document.getElementById('editModal'));
}
function updateDispenseDrugSelect() {
    const select = document.getElementById('dispenseDrugSelect');
    select.innerHTML = '<option value="">-- Select a drug variant --</option>';
    
    allDrugs.forEach(drug => {
        drug.variants.forEach(variant => {
            const option = document.createElement('option');
            option.value = variant.id;
            option.textContent = `${drug.name} ${variant.strength} (${variant.quantity} ${variant.unit})`;
            select.appendChild(option);
        });
    });

    const packSelect = document.getElementById('dispensePackSelect');
    const packCount = document.getElementById('dispensePackCount');
    const packPreview = document.getElementById('dispensePackPreview');
    const modeSelect = document.getElementById('dispenseMode');
    if (packSelect) {
        packSelect.innerHTML = '<option value="">-- Select pack --</option>';
    }
    if (packCount) {
        packCount.value = '';
    }
    if (modeSelect) {
        modeSelect.value = 'subunit';
    }
    if (packPreview) {
        packPreview.textContent = 'Select a pack and whole-number count.';
    }

    updateDispenseModeUi();
}

function populateDispensePackSelect(variant) {
    const packSelect = document.getElementById('dispensePackSelect');
    const packCount = document.getElementById('dispensePackCount');
    const packPreview = document.getElementById('dispensePackPreview');
    if (!packSelect) return;

    const activePacks = getActivePacksForVariant(variant);
    packSelect.innerHTML = '<option value="">-- Select pack --</option>';

    activePacks.forEach(pack => {
        const option = document.createElement('option');
        option.value = String(pack.id);
        option.textContent = `${pack.label} (${pack.pack_size_in_base_units} ${variant.unit})`;
        packSelect.appendChild(option);
    });

    if (packCount) packCount.value = '';
    if (packPreview) {
        packPreview.textContent = activePacks.length > 0
            ? `Select a pack and whole-number count (${variant.unit} base unit).`
            : `No active packs for this variant.`;
    }
}

function updateDispenseModeUi() {
    const mode = document.getElementById('dispenseMode')?.value || 'subunit';
    const quantityGroup = document.getElementById('dispenseQuantityGroup');
    const packRow = document.getElementById('dispensePackRow');
    const quantityInput = document.getElementById('dispenseQuantity');
    const packSelect = document.getElementById('dispensePackSelect');
    const packCount = document.getElementById('dispensePackCount');

    if (quantityGroup) {
        quantityGroup.style.display = mode === 'subunit' ? '' : 'none';
    }
    if (packRow) {
        packRow.style.display = mode === 'pack' ? '' : 'none';
    }

    if (quantityInput) {
        quantityInput.required = mode === 'subunit';
    }
    if (packSelect) {
        packSelect.required = mode === 'pack';
    }
    if (packCount) {
        packCount.required = mode === 'pack';
    }

    updateAllocationPreview();
}

function updateDispenseQuantityFromPack() {
    const mode = document.getElementById('dispenseMode')?.value || 'subunit';
    if (mode !== 'pack') return;

    const variantId = parseInt(document.getElementById('dispenseDrugSelect')?.value || '', 10);
    const packId = parseInt(document.getElementById('dispensePackSelect')?.value || '', 10);
    const packCount = parseFloat(document.getElementById('dispensePackCount')?.value || '');
    const quantityInput = document.getElementById('dispenseQuantity');
    const preview = document.getElementById('dispensePackPreview');

    const variant = getVariantById(variantId);
    if (!quantityInput || !preview || !variant) return;

    const selectedPack = getActivePacksForVariant(variant).find(pack => pack.id === packId);
    if (selectedPack && !Number.isNaN(packCount) && packCount > 0) {
        if (Math.abs(packCount - Math.round(packCount)) > 1e-6) {
            preview.textContent = 'Whole-pack mode requires a whole-number pack count.';
            return;
        }
        const quantity = packCount * selectedPack.pack_size_in_base_units;
        quantityInput.value = String(quantity);
        preview.textContent = `${packCount} × ${selectedPack.pack_size_in_base_units} = ${quantity} ${variant.unit}`;
        updateAllocationPreview();
        return;
    }

    preview.textContent = selectedPack
        ? `1 ${selectedPack.pack_unit_name} = ${selectedPack.pack_size_in_base_units} ${variant.unit}`
        : `Select a pack to calculate quantity.`;
}

function formatDisplayDate(value) {
    if (!value) {
        return 'Unknown';
    }

    const parsed = new Date(value);
    if (Number.isNaN(parsed.getTime())) {
        return value;
    }

    return parsed.toLocaleDateString();
}

function getBatchLocationLabel(batch) {
    return batch.location_name || batch.location?.name || `Location #${batch.location_id}`;
}

function updateLocationFilterOptions() {
    const locationFilterSelect = document.getElementById('locationFilterSelect');
    if (!locationFilterSelect) return;

    const previousValue = selectedLocationFilter;
    const locations = new Set();

    allDrugs.forEach(drug => {
        drug.variants.forEach(variant => {
            (variant.batches || []).forEach(batch => {
                if (batch.quantity > 0) {
                    locations.add(getBatchLocationLabel(batch));
                }
            });
        });
    });

    locationFilterSelect.innerHTML = '<option value="">All Locations</option>';
    Array.from(locations)
        .sort((a, b) => a.localeCompare(b))
        .forEach(location => {
            const option = document.createElement('option');
            option.value = location;
            option.textContent = location;
            locationFilterSelect.appendChild(option);
        });

    if (previousValue && locations.has(previousValue)) {
        selectedLocationFilter = previousValue;
        locationFilterSelect.value = previousValue;
    } else {
        selectedLocationFilter = '';
        locationFilterSelect.value = '';
    }
}

function populateDispenseBatchSelect(activeBatches) {
    const batchSelect = document.getElementById('dispenseBatchSelect');
    const selectedVariantId = parseInt(document.getElementById('dispenseDrugSelect').value || '', 10);
    const unitLabel = getVariantById(selectedVariantId)?.unit || 'units';
    const previousValue = batchSelect.value;

    batchSelect.innerHTML = '<option value="">Automatic FEFO Selection</option>';

    activeBatches.forEach((batch, index) => {
        const option = document.createElement('option');
        const expiryLabel = formatDisplayDate(batch.expiry_date);
        const locationLabel = getBatchLocationLabel(batch);
        const fefoLabel = index === 0 ? ' [FEFO default]' : '';
        option.value = batch.id;
        option.textContent = `${batch.batch_number} | ${batch.quantity} ${unitLabel} | ${locationLabel} | Expires ${expiryLabel}${fefoLabel}`;
        batchSelect.appendChild(option);
    });

    if (previousValue && activeBatches.some(batch => String(batch.id) === previousValue)) {
        batchSelect.value = previousValue;
    }
}

// Update batch info display when variant is selected
async function updateBatchInfo() {
    const variantId = parseInt(document.getElementById('dispenseDrugSelect').value);
    const batchInfoSection = document.getElementById('batchInfoSection');
    const batchInfoContent = document.getElementById('batchInfoContent');
    const batchSelect = document.getElementById('dispenseBatchSelect');
    
    if (!variantId) {
        batchInfoSection.style.display = 'none';
        batchSelect.innerHTML = '<option value="">Automatic FEFO Selection</option>';
        const packSelect = document.getElementById('dispensePackSelect');
        if (packSelect) packSelect.innerHTML = '<option value="">-- Select pack --</option>';
        return;
    }

    const variant = getVariantById(variantId);
    if (variant) {
        populateDispensePackSelect(variant);
    }
    updateDispenseModeUi();
    
    batchInfoSection.style.display = 'block';
    batchInfoContent.innerHTML = '<p class="loading">Loading batches...</p>';
    
    try {
        const response = await apiCall(`/variants/${variantId}/batches`);
        if (!response.ok) throw new Error('Failed to load batches');
        
        const batches = await response.json();
        
        // Filter out empty batches
        const activeBatches = batches.filter(b => b.quantity > 0);
        
        if (activeBatches.length === 0) {
            populateDispenseBatchSelect([]);
            batchInfoContent.innerHTML = '<p style="color: #d32f2f; margin: 0;">⚠️ No active batches available for this variant</p>';
            return;
        }
        
        // Sort by expiry date (FEFO order)
        activeBatches.sort((a, b) => new Date(a.expiry_date) - new Date(b.expiry_date));
        populateDispenseBatchSelect(activeBatches);
        
        const batchHtml = activeBatches.map((batch, index) => {
            const expiryDate = new Date(batch.expiry_date);
            const locationLabel = getBatchLocationLabel(batch);
            const expiryLabel = formatDisplayDate(batch.expiry_date);
            const today = new Date();
            const isExpired = expiryDate < today;
            const daysToExpiry = Math.ceil((expiryDate - today) / (1000 * 60 * 60 * 24));
            
            let expiryStatus = '✓ OK';
            let statusColor = '#4caf50';
            if (isExpired) {
                expiryStatus = '✕ EXPIRED';
                statusColor = '#d32f2f';
            } else if (daysToExpiry <= 7) {
                expiryStatus = `⚠️ ${daysToExpiry}d left`;
                statusColor = '#ff9800';
            }
            
            const isFEFO = index === 0;
            
            return `
                <div style="padding: 8px; margin: 5px 0; background: white; border: 1px solid #e0e0e0; border-radius: 3px; ${isFEFO ? 'border-left: 3px solid #2196F3; background: #f0f8ff;' : ''}">
                    <div style="display: flex; justify-content: space-between; align-items: center;">
                        <div>
                            <strong>${batch.batch_number}</strong> ${isFEFO ? '<span style="background: #2196F3; color: white; padding: 2px 6px; border-radius: 2px; font-size: 0.8em; margin-left: 5px;">FIRST</span>' : ''}
                            <div style="font-size: 0.9em; color: #666; margin-top: 3px;">
                                Qty: <strong>${batch.quantity}</strong> | 
                                Location: <strong>${escapeHtml(locationLabel)}</strong> | 
                                Expiry: <strong>${expiryLabel}</strong> <span style="color: ${statusColor};">(${expiryStatus})</span>
                            </div>
                        </div>
                    </div>
                </div>
            `;
        }).join('');
        
        batchInfoContent.innerHTML = batchHtml;
    } catch (error) {
        console.error('Error loading batches:', error);
        batchInfoContent.innerHTML = '<p style="color: #d32f2f; margin: 0;">Error loading batches</p>';
    }
    
    // Update allocation preview when batches load
    updateAllocationPreview();
}

// Update allocation preview based on quantity and allow_split flag
async function updateAllocationPreview() {
    const variantId = parseInt(document.getElementById('dispenseDrugSelect').value);
    const unitLabel = getVariantById(variantId)?.unit || 'units';
    const quantity = parseFloat(document.getElementById('dispenseQuantity').value);
    const allowSplit = document.getElementById('dispenseAllowSplit').checked;
    const preferredBatchId = parseInt(document.getElementById('dispenseBatchSelect').value);
    const allocationPreviewSection = document.getElementById('allocationPreviewSection');
    const allocationPreviewContent = document.getElementById('allocationPreviewContent');
    
    if (!variantId || isNaN(quantity) || quantity <= 0) {
        allocationPreviewSection.style.display = 'none';
        return;
    }
    
    allocationPreviewSection.style.display = 'block';
    allocationPreviewContent.innerHTML = '<p class="loading">Calculating allocation...</p>';
    
    try {
        const response = await apiCall(`/variants/${variantId}/batches`);
        if (!response.ok) throw new Error('Failed to load batches');
        
        const batches = await response.json();
        let activeBatches = batches.filter(b => b.quantity > 0)
            .sort((a, b) => new Date(a.expiry_date) - new Date(b.expiry_date));
        
        if (activeBatches.length === 0) {
            allocationPreviewContent.innerHTML = '<p style="color: #d32f2f; margin: 0;">⚠️ No active batches available</p>';
            return;
        }

        if (!Number.isNaN(preferredBatchId)) {
            const preferredBatch = activeBatches.find(batch => batch.id === preferredBatchId);
            if (!preferredBatch) {
                allocationPreviewContent.innerHTML = '<p style="color: #d32f2f; margin: 0;">✕ Selected preferred batch is no longer available.</p>';
                return;
            }

            activeBatches = [preferredBatch, ...activeBatches.filter(batch => batch.id !== preferredBatchId)];
        }
        
        // Simulate FEFO allocation
        const allocations = [];
        let remainingQty = quantity;
        
        for (const batch of activeBatches) {
            if (remainingQty <= 0) break;
            
            const allocQty = Math.min(remainingQty, batch.quantity);
            allocations.push({
                batchNumber: batch.batch_number,
                batchId: batch.id,
                quantity: allocQty,
                location: getBatchLocationLabel(batch),
                expiryDate: batch.expiry_date,
                preferred: !Number.isNaN(preferredBatchId) && batch.id === preferredBatchId
            });
            remainingQty -= allocQty;
            
            if (!allowSplit) break;
        }
        
        if (remainingQty > 0 && !allowSplit) {
            const failureContext = !Number.isNaN(preferredBatchId)
                ? 'Preferred batch cannot fully satisfy this request. Enable split to fall through to FEFO batches.'
                : 'Insufficient stock in first batch. Check "Allow Split" to use multiple batches.';
            allocationPreviewContent.innerHTML = `<p style="color: #d32f2f; margin: 0;">✕ ${failureContext}</p>`;
            return;
        }
        
        if (remainingQty > 0 && allowSplit) {
            allocationPreviewContent.innerHTML = `
                <p style="color: #d32f2f; margin: 0 0 10px 0;">✕ Warning: Only ${quantity - remainingQty} ${escapeHtml(unitLabel)} available across all batches (${remainingQty} short)</p>
                <div>${allocations.map(a => `
                    <div style="padding: 5px; margin: 3px 0; background: white; border-radius: 2px; font-size: 0.9em;">
                        <strong>${a.batchNumber}</strong>${a.preferred ? ' <span style="color: #1565c0;">(preferred)</span>' : ''}: ${a.quantity} ${escapeHtml(unitLabel)} (${escapeHtml(a.location)}) - Expires ${formatDisplayDate(a.expiryDate)}
                    </div>
                `).join('')}</div>
            `;
            return;
        }
        
        const allocationHtml = allocations.map(a => `
            <div style="padding: 5px; margin: 3px 0; background: white; border-radius: 2px; font-size: 0.9em;">
                <strong>${a.batchNumber}</strong>${a.preferred ? ' <span style="color: #1565c0;">(preferred)</span>' : ''}: ${a.quantity} ${escapeHtml(unitLabel)} (${escapeHtml(a.location)}) - Expires ${formatDisplayDate(a.expiryDate)}
            </div>
        `).join('');
        
        const pluralText = allocations.length === 1 ? 'batch' : 'batches';
        const introText = !Number.isNaN(preferredBatchId)
            ? `✓ Will start from your preferred batch, then use FEFO for any remainder across <strong>${allocations.length} ${pluralText}</strong>:`
            : `✓ Will dispense from <strong>${allocations.length} ${pluralText}</strong>:`;
        allocationPreviewContent.innerHTML = `
            <p style="margin: 0 0 8px 0; color: #333;">${introText}</p>
            <div>${allocationHtml}</div>
        `;
    } catch (error) {
        console.error('Error calculating allocation:', error);
        allocationPreviewContent.innerHTML = '<p style="color: #d32f2f; margin: 0;">Error calculating allocation</p>';
    }
}

// Render drugs list
function renderDrugs() {
    const drugsList = document.getElementById('drugsList');
    let drugsToShow = allDrugs;

    // Apply search filter
    if (searchTerm) {
        drugsToShow = drugsToShow.filter(drug => 
            drug.name.toLowerCase().includes(searchTerm) ||
            (drug.description && drug.description.toLowerCase().includes(searchTerm)) ||
            drug.variants.some(variant => variant.strength.toLowerCase().includes(searchTerm))
        );
    }

    // Apply stock filter
    if (showLowStockOnly) {
        drugsToShow = drugsToShow.filter(drug => 
            drug.variants.some(variant => variant.quantity <= variant.low_stock_threshold)
        );
    }

    // Apply location filter
    if (selectedLocationFilter) {
        drugsToShow = drugsToShow.filter(drug =>
            drug.variants.some(variant =>
                (variant.batches || []).some(batch =>
                    batch.quantity > 0 && getBatchLocationLabel(batch) === selectedLocationFilter
                )
            )
        );
    }

    // Sort alphabetically by drug name
    drugsToShow = drugsToShow.sort((a, b) => 
        a.name.toLowerCase().localeCompare(b.name.toLowerCase())
    );

    if (drugsToShow.length === 0) {
        drugsList.innerHTML = '<p class="empty">No drugs found matching your criteria</p>';
        return;
    }

    drugsList.innerHTML = drugsToShow.map(drug => {
        const totalVariants = drug.variants.length;
        const lowStockVariants = drug.variants.filter(v => v.quantity <= v.low_stock_threshold).length;
        const totalQuantity = drug.variants.reduce((sum, v) => sum + v.quantity, 0);
        const isLowStock = lowStockVariants > 0;
        const isExpanded = expandedDrugs.has(drug.id);
        const isReadOnly = currentUser.role === 'readonly';
        const isControlled = drug.is_controlled;
        
        const variantsHtml = isExpanded ? `
            ${drug.variants.map(variant => {
                const variantIsLowStock = variant.quantity <= variant.low_stock_threshold;
                return `
                    <div class="variant-item ${variantIsLowStock ? 'low-stock' : ''}">
                        <div class="variant-info">
                            <div class="variant-details">
                                <div class="variant-name">${escapeHtml(drug.name)} ${escapeHtml(variant.strength)}</div>
                                <div class="variant-quantity">${variant.quantity} ${escapeHtml(variant.unit)}</div>
                            </div>
                            <div class="variant-status">
                                <span class="variant-badge ${variantIsLowStock ? 'badge-low' : 'badge-normal'}">
                                    ${variantIsLowStock ? 'Low Stock' : 'OK'}
                                </span>
                            </div>
                        </div>
                        <div class="variant-actions">
                            ${!isReadOnly ? `
                                <button class="btn btn-info btn-small" onclick="event.stopPropagation(); openVariantPacksModal(${variant.id})">📦 Packs</button>
                                <button class="btn btn-primary btn-small" onclick="prescribeVariant(${variant.id}, '${drug.name.replace(/'/g, "\\'")}', '${variant.strength.replace(/'/g, "\\'")}', '${variant.unit.replace(/'/g, "\\'")}')">🏷️ Prescribe & Print</button>
                                <button class="btn btn-success btn-small" onclick="dispenseVariant(${variant.id})">💊 Dispense</button>
                                <button class="btn btn-warning btn-small" onclick="openEditVariantModal(${variant.id})">Edit</button>
                                <button class="btn btn-danger btn-small" onclick="deleteVariant(${variant.id})">Delete</button>
                            ` : ''}
                        </div>
                    </div>
                `;
            }).join('')}` : '';
        
        return `
            <div class="drug-item ${isLowStock ? 'low-stock' : ''} ${isExpanded ? 'expanded' : ''}" onclick="toggleDrugExpansion(${drug.id})">
                <div class="drug-info">
                    <div class="drug-name">
                        ${escapeHtml(drug.name)}
                        ${isControlled ? '<span style="background: #d32f2f; color: white; padding: 2px 6px; border-radius: 2px; font-size: 0.75em; margin-left: 8px; display: inline-block;">⚠️ CONTROLLED</span>' : ''}
                    </div>
                    <div class="drug-description">${drug.description ? escapeHtml(drug.description) : 'No description'}</div>
                    <div class="drug-quantity">${totalVariants} variant${totalVariants !== 1 ? 's' : ''} (${totalQuantity} total units)</div>
                    <div class="drug-status">
                        <span class="drug-badge ${isLowStock ? 'badge-low' : 'badge-normal'}">
                            ${isLowStock ? `${lowStockVariants} low` : 'All OK'}
                        </span>
                    </div>
                </div>
                <div class="drug-actions">
                    ${!isReadOnly ? `
                        <button class="btn btn-success btn-small" onclick="event.stopPropagation(); openReceiveDeliveryModal(${drug.id})">📦 Receive Delivery</button>
                        <button class="btn btn-primary btn-small" onclick="event.stopPropagation(); openAddVariantModal(${drug.id})">➕ Add</button>
                    ` : ''}
                    <button class="btn btn-info btn-small" onclick="event.stopPropagation(); showDrugHistory(${drug.id})">📋 History</button>
                    ${!isReadOnly ? `
                        <button class="btn btn-warning btn-small" onclick="event.stopPropagation(); openEditModal(${drug.id})">Edit Drug</button>
                        <button class="btn btn-danger btn-small" onclick="event.stopPropagation(); deleteDrug(${drug.id})">Delete</button>
                    ` : ''}
                    <span class="expand-icon">${isExpanded ? '▼' : '▶'}</span>
                </div>
            </div>
            <div class="drug-variants ${isExpanded ? 'expanded' : ''}" id="variants-${drug.id}">
                ${variantsHtml}
            </div>
        `;
    }).join('');
}

// Handle add drug form
async function handleAddDrug(e) {
    e.preventDefault();

    const drugData = {
        name: document.getElementById('drugName').value,
        description: document.getElementById('drugDescription').value,
        is_controlled: document.getElementById('drugIsControlled').checked
    };

    try {
        // Create the drug first
        const drugResponse = await apiCall('/drugs', {
            method: 'POST',
            body: JSON.stringify(drugData)
        });

        if (!drugResponse.ok) throw new Error('Failed to add drug');
        const createdDrug = await drugResponse.json();

        // Check if initial variant data was provided
        const variantStrength = document.getElementById('initialVariantStrength').value.trim();
        if (variantStrength) {
            const variantData = {
                strength: variantStrength,
                quantity: parseFloat(document.getElementById('initialVariantQuantity').value) || 0,
                unit: document.getElementById('initialVariantUnit').value || 'units',
                low_stock_threshold: parseFloat(document.getElementById('initialVariantThreshold').value) || 10
            };

            const variantResponse = await apiCall(`/drugs/${createdDrug.id}/variants`, {
                method: 'POST',
                body: JSON.stringify(variantData)
            });

            if (!variantResponse.ok) throw new Error('Failed to add variant');
        }

        document.getElementById('drugForm').reset();
        document.getElementById('initialVariantUnit').value = 'units';
        document.getElementById('initialVariantThreshold').value = '10';
        closeModal(document.getElementById('addModal'));
        await loadDrugs();
        showToast('Drug added successfully!', 'success');
    } catch (error) {
        console.error('Error adding drug:', error);
        showToast('Failed to add drug. Check the console for details.', 'error');
    }
}

// Handle dispense drug form
async function handleDispenseDrug(e) {
    e.preventDefault();

    const variantId = parseInt(document.getElementById('dispenseDrugSelect').value);
    let quantity = parseFloat(document.getElementById('dispenseQuantity').value);
    const dispenseMode = (document.getElementById('dispenseMode').value || 'subunit').toLowerCase();
    const preferredBatchIdValue = document.getElementById('dispenseBatchSelect').value;
    const requestedPackIdValue = document.getElementById('dispensePackSelect').value;
    const requestedPackCountValue = document.getElementById('dispensePackCount').value;
    const animalName = document.getElementById('dispenseAnimal').value;
    const userName = document.getElementById('dispenseUser').value;
    const notes = document.getElementById('dispenseNotes').value;
    const allowSplit = document.getElementById('dispenseAllowSplit').checked;

    const selectedPackId = requestedPackIdValue ? parseInt(requestedPackIdValue, 10) : null;
    const selectedPackCount = requestedPackCountValue ? parseFloat(requestedPackCountValue) : null;
    const variant = getVariantById(variantId);
    const selectedPack = variant && selectedPackId
        ? getActivePacksForVariant(variant).find(pack => pack.id === selectedPackId)
        : null;

    if (!['subunit', 'pack'].includes(dispenseMode)) {
        showToast('Please select a valid dispense mode.', 'warning');
        return;
    }

    if (dispenseMode === 'pack') {
        if (!selectedPack) {
            showToast('Please select a pack type for whole-pack dispensing.', 'warning');
            return;
        }
        if (selectedPackCount == null || Number.isNaN(selectedPackCount) || selectedPackCount <= 0) {
            showToast('Please enter a valid pack count greater than zero.', 'warning');
            return;
        }
        if (Math.abs(selectedPackCount - Math.round(selectedPackCount)) > 1e-6) {
            showToast('Whole-pack dispensing requires a whole-number pack count.', 'warning');
            return;
        }

        quantity = selectedPackCount * selectedPack.pack_size_in_base_units;
    }

    if (!variantId || isNaN(quantity) || quantity <= 0 || !userName) {
        showToast('Please fill in all required fields (Drug Variant, Quantity > 0, Dispensed by)', 'warning');
        return;
    }

    const dispensingData = {
        drug_variant_id: variantId,
        quantity: quantity,
        dispense_mode: dispenseMode,
        batch_id: preferredBatchIdValue ? parseInt(preferredBatchIdValue) : null,
        requested_pack_id: dispenseMode === 'pack' ? selectedPackId : null,
        requested_pack_count: dispenseMode === 'pack' ? selectedPackCount : null,
        animal_name: animalName || null,
        user_name: userName,
        notes: notes || null,
        allow_split: allowSplit
    };

    try {
        const response = await apiCall('/dispense', {
            method: 'POST',
            body: JSON.stringify(dispensingData)
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to dispense drug');
        }

        document.getElementById('dispenseForm').reset();
        closeModal(document.getElementById('dispenseModal'));
        await loadDrugs();
        showToast('Drug dispensed successfully!', 'success');
    } catch (error) {
        console.error('Error dispensing drug:', error);
        showToast('Failed to dispense drug: ' + error.message, 'error');
    }
}

// Open edit modal
function openEditModal(drugId) {
    const drug = allDrugs.find(d => d.id === drugId);
    if (!drug) return;

    document.getElementById('editDrugId').value = drug.id;
    document.getElementById('editDrugName').value = drug.name;
    document.getElementById('editDrugDescription').value = drug.description || '';
    document.getElementById('editDrugIsControlled').checked = drug.is_controlled || false;

    document.getElementById('editModal').classList.add('show');
}

// Close edit modal
function closeEditModal() {
    document.getElementById('editModal').classList.remove('show');
    document.getElementById('editForm').reset();
}

// Show variants for a drug
function toggleDrugExpansion(drugId) {
    if (expandedDrugs.has(drugId)) {
        expandedDrugs.delete(drugId);
    } else {
        expandedDrugs.add(drugId);
    }
    renderDrugs();
}

// Open add variant modal
function openAddVariantModal(drugId) {
    const drug = allDrugs.find(d => d.id === drugId);
    if (!drug) return;
    
    currentDrug = drug;
    const form = document.getElementById('variantForm');
    if (form) form.reset();
    document.getElementById('variantDrugId').value = drug.id;
    initializeVariantPackRows();
    document.getElementById('addVariantModal').classList.add('show');
}

function inferBaseUnitFromStrength(strength) {
    const value = String(strength || '').toLowerCase();
    if (value.includes('/ml') || value.includes('ml')) return 'ml';
    if (value.includes('tablet')) return 'tablets';
    if (value.includes('capsule')) return 'capsules';
    return 'units';
}

function getVariantPackRowsContainer() {
    return document.getElementById('variantPackRows');
}

function refreshVariantPackRowLabels() {
    const container = getVariantPackRowsContainer();
    const baseUnit = document.getElementById('variantUnit')?.value || 'units';
    if (!container) return;

    container.querySelectorAll('.variant-pack-row').forEach(row => {
        const packUnit = row.querySelector('.variant-pack-unit')?.value || 'pack';
        const label = row.querySelector('.variant-pack-size-label');
        if (!label) return;
        const titleCasePack = packUnit.charAt(0).toUpperCase() + packUnit.slice(1);
        label.textContent = `${titleCasePack} Size (${baseUnit}) *`;
    });
}

function appendVariantPackRow(prefill = {}) {
    const container = getVariantPackRowsContainer();
    if (!container) return;

    const row = document.createElement('div');
    row.className = 'delivery-line variant-pack-row';

    const selectedPackUnit = prefill.packUnit || 'bottle';
    const selectedSize = prefill.packSize || '';
    const baseUnit = document.getElementById('variantUnit')?.value || 'units';

    row.innerHTML = `
        <div class="delivery-line-grid" style="grid-template-columns: 1.2fr 1.2fr auto;">
            <div class="form-group">
                <label>Pack Type *</label>
                <select class="variant-pack-unit" required>
                    <option value="bottle" ${selectedPackUnit === 'bottle' ? 'selected' : ''}>Bottle</option>
                    <option value="box" ${selectedPackUnit === 'box' ? 'selected' : ''}>Box</option>
                    <option value="vial" ${selectedPackUnit === 'vial' ? 'selected' : ''}>Vial</option>
                    <option value="packet" ${selectedPackUnit === 'packet' ? 'selected' : ''}>Packet</option>
                </select>
            </div>
            <div class="form-group">
                <label class="variant-pack-size-label">Bottle Size (${baseUnit}) *</label>
                <input type="number" class="variant-pack-size" min="0.0001" step="0.0001" value="${selectedSize}" required>
            </div>
            <button type="button" class="btn btn-danger btn-small variant-pack-remove-btn">Remove</button>
        </div>
    `;

    const removeBtn = row.querySelector('.variant-pack-remove-btn');
    const unitSelect = row.querySelector('.variant-pack-unit');
    if (removeBtn) {
        removeBtn.addEventListener('click', () => {
            if (container.querySelectorAll('.variant-pack-row').length <= 1) {
                showToast('At least one pack size is required', 'warning');
                return;
            }
            row.remove();
        });
    }

    if (unitSelect) {
        unitSelect.addEventListener('change', refreshVariantPackRowLabels);
    }

    container.appendChild(row);
    refreshVariantPackRowLabels();
}

function initializeVariantPackRows() {
    const container = getVariantPackRowsContainer();
    if (!container) return;
    container.innerHTML = '';

    const strengthValue = document.getElementById('variantStrength')?.value || '';
    const inferredBaseUnit = inferBaseUnitFromStrength(strengthValue);
    const variantUnitSelect = document.getElementById('variantUnit');
    if (variantUnitSelect) {
        variantUnitSelect.value = inferredBaseUnit;
    }

    appendVariantPackRow({ packUnit: 'bottle' });
}

// Handle add variant form
async function handleAddVariant(e) {
    e.preventDefault();

    const drugId = parseInt(document.getElementById('variantDrugId').value);
    const baseUnit = document.getElementById('variantUnit').value;
    const rows = Array.from(document.querySelectorAll('#variantPackRows .variant-pack-row'));

    if (rows.length === 0) {
        showToast('Please add at least one pack size', 'warning');
        return;
    }

    const packPayloads = [];
    for (let i = 0; i < rows.length; i += 1) {
        const row = rows[i];
        const packUnit = row.querySelector('.variant-pack-unit')?.value;
        const packSize = parseFloat(row.querySelector('.variant-pack-size')?.value || '');

        if (!packUnit || Number.isNaN(packSize) || packSize <= 0) {
            showToast(`Pack row ${i + 1} is incomplete`, 'warning');
            return;
        }

        const normalizedPackUnit = packUnit.trim().toLowerCase();
        const titleCasePack = normalizedPackUnit.charAt(0).toUpperCase() + normalizedPackUnit.slice(1);
        packPayloads.push({
            label: `${titleCasePack} ${packSize} ${baseUnit}`,
            pack_unit_name: normalizedPackUnit,
            pack_size_in_base_units: packSize,
            is_active: true
        });
    }

    const variantData = {
        strength: document.getElementById('variantStrength').value,
        quantity: 0,
        unit: baseUnit,
        base_unit: baseUnit,
        low_stock_threshold: parseFloat(document.getElementById('variantThreshold').value)
    };

    try {
        const response = await apiCall(`/drugs/${drugId}/variants`, {
            method: 'POST',
            body: JSON.stringify(variantData)
        });

        if (!response.ok) throw new Error('Failed to add variant');

        const createdVariant = await response.json();

        for (const packPayload of packPayloads) {
            const packResponse = await apiCall(`/variants/${createdVariant.id}/packs`, {
                method: 'POST',
                body: JSON.stringify(packPayload)
            });

            if (!packResponse.ok) {
                const packError = await packResponse.json();
                throw new Error(packError.detail || 'Variant created but pack size creation failed');
            }
        }

        // Archive the auto-created default 1:1 pack when custom pack sizes are configured.
        const packsResponse = await apiCall(`/variants/${createdVariant.id}/packs`);
        if (packsResponse.ok) {
            const packs = await packsResponse.json();
            const defaultPack = packs.find(
                p => p.is_active && Number(p.pack_size_in_base_units) === 1 && (p.pack_unit_name || '').toLowerCase() === baseUnit.toLowerCase()
            );
            if (defaultPack && packs.filter(p => p.is_active).length > 1) {
                await apiCall(`/variant-packs/${defaultPack.id}`, {
                    method: 'PUT',
                    body: JSON.stringify({ is_active: false })
                });
            }
        }

        document.getElementById('variantForm').reset();
        closeModal(document.getElementById('addVariantModal'));
        await loadDrugs();
        if (document.getElementById('receiveDeliveryModal')?.classList.contains('show')) {
            refreshDeliveryVariantSelects();
        }
        renderDrugs();
        showToast('Variant added successfully!', 'success');
    } catch (error) {
        console.error('Error adding variant:', error);
        showToast('Failed to add variant. Check the console for details.', 'error');
    }
}

// Open edit variant modal
function openEditVariantModal(variantId) {
    // Find the variant from all drugs
    let variant = null;
    for (const drug of allDrugs) {
        variant = drug.variants.find(v => v.id === variantId);
        if (variant) break;
    }
    
    if (!variant) return;

    document.getElementById('editVariantId').value = variant.id;
    document.getElementById('editVariantStrength').value = variant.strength;
    document.getElementById('editVariantQuantity').value = variant.quantity;
    document.getElementById('editVariantUnit').value = variant.unit;
    document.getElementById('editVariantThreshold').value = variant.low_stock_threshold;

    document.getElementById('editVariantModal').classList.add('show');
}

function inferPackUnitName(baseUnit) {
    const value = String(baseUnit || 'pack').trim().toLowerCase();
    if (!value) return 'pack';
    if (value.endsWith('s') && value.length > 1) return value.slice(0, -1);
    return value;
}

// Handle edit variant form
async function handleEditVariant(e) {
    e.preventDefault();

    const variantId = parseInt(document.getElementById('editVariantId').value);
    const variantData = {
        strength: document.getElementById('editVariantStrength').value,
        quantity: parseFloat(document.getElementById('editVariantQuantity').value),
        unit: document.getElementById('editVariantUnit').value,
        low_stock_threshold: parseFloat(document.getElementById('editVariantThreshold').value)
    };

    try {
        const response = await apiCall(`/variants/${variantId}`, {
            method: 'PUT',
            body: JSON.stringify(variantData)
        });

        if (!response.ok) throw new Error('Failed to update variant');

        closeModal(document.getElementById('editVariantModal'));
        await loadDrugs();
        renderDrugs();
        showToast('Variant updated successfully!', 'success');
    } catch (error) {
        console.error('Error updating variant:', error);
        showToast('Failed to update variant. Check the console for details.', 'error');
    }
}

async function openVariantPacksModal(variantId) {
    const variant = getVariantById(variantId);
    if (!variant) {
        showToast('Variant not found', 'error');
        return;
    }

    const parentDrug = allDrugs.find(d => (d.variants || []).some(v => v.id === variantId));
    activeVariantPacksVariantId = variantId;

    const label = document.getElementById('variantPacksLabel');
    if (label) {
        const drugName = parentDrug ? parentDrug.name : 'Drug';
        label.textContent = `${drugName} ${variant.strength} | Base Unit: ${variant.unit}`;
    }

    const form = document.getElementById('variantPacksForm');
    if (form) form.reset();
    document.getElementById('variantPacksVariantId').value = String(variantId);
    document.getElementById('variantPacksNewUnit').value = inferPackUnitName(variant.unit);

    await refreshVariantPacksList();
    openModal(document.getElementById('variantPacksModal'));
}

async function refreshVariantPacksList() {
    const variantId = parseInt(document.getElementById('variantPacksVariantId')?.value || '', 10);
    const list = document.getElementById('variantPacksList');
    if (!variantId || !list) return;

    list.innerHTML = '<p class="loading">Loading packs...</p>';

    try {
        const response = await apiCall(`/variants/${variantId}/packs`);
        if (!response.ok) throw new Error('Failed to load pack presentations');
        const packs = await response.json();

        if (!Array.isArray(packs) || packs.length === 0) {
            list.innerHTML = '<p class="empty">No pack presentations defined.</p>';
            return;
        }

        list.innerHTML = `
            <div class="locations-table">
                ${packs.map(pack => `
                    <div class="location-item" style="${pack.is_active ? '' : 'opacity: 0.6;'}">
                        <div style="flex: 1;">
                            <strong>${escapeHtml(pack.label)}</strong>
                            <div style="font-size: 0.88em; color: #666;">
                                ${escapeHtml(pack.pack_unit_name)} | ${pack.pack_size_in_base_units} base units
                                ${pack.is_active ? '' : ' | archived'}
                            </div>
                        </div>
                        <button class="btn ${pack.is_active ? 'btn-danger' : 'btn-secondary'} btn-small" onclick="toggleVariantPackActive(${pack.id}, ${pack.is_active ? 'false' : 'true'})">
                            ${pack.is_active ? 'Archive' : 'Restore'}
                        </button>
                    </div>
                `).join('')}
            </div>
        `;
    } catch (error) {
        console.error('Error loading variant packs:', error);
        list.innerHTML = '<p class="empty">Failed to load pack presentations</p>';
    }
}

async function handleCreateVariantPack(e) {
    e.preventDefault();

    const variantId = parseInt(document.getElementById('variantPacksVariantId').value, 10);
    const label = document.getElementById('variantPacksNewLabel').value.trim();
    const packUnitName = document.getElementById('variantPacksNewUnit').value.trim();
    const size = parseFloat(document.getElementById('variantPacksNewSize').value);

    if (!variantId || !label || !packUnitName || Number.isNaN(size) || size <= 0) {
        showToast('Please complete all pack fields', 'warning');
        return;
    }

    try {
        const response = await apiCall(`/variants/${variantId}/packs`, {
            method: 'POST',
            body: JSON.stringify({
                label,
                pack_unit_name: packUnitName,
                pack_size_in_base_units: size,
                is_active: true
            })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to add pack presentation');
        }

        document.getElementById('variantPacksForm').reset();
        const variant = getVariantById(variantId);
        document.getElementById('variantPacksNewUnit').value = inferPackUnitName(variant?.unit || 'pack');
        await loadDrugs();
        await refreshVariantPacksList();
        if (document.getElementById('receiveDeliveryModal')?.classList.contains('show')) {
            refreshDeliveryVariantSelects();
        }
        showToast('Pack presentation added', 'success');
    } catch (error) {
        console.error('Error creating pack presentation:', error);
        showToast('Failed to add pack presentation: ' + error.message, 'error');
    }
}

async function toggleVariantPackActive(packId, nextActiveState) {
    try {
        const response = await apiCall(`/variant-packs/${packId}`, {
            method: 'PUT',
            body: JSON.stringify({ is_active: Boolean(nextActiveState) })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to update pack state');
        }

        await loadDrugs();
        await refreshVariantPacksList();
        if (document.getElementById('receiveDeliveryModal')?.classList.contains('show')) {
            refreshDeliveryVariantSelects();
        }
        showToast('Pack updated', 'success');
    } catch (error) {
        console.error('Error updating pack state:', error);
        showToast('Failed to update pack: ' + error.message, 'error');
    }
}

// Dispense from variant
function dispenseVariant(variantId) {
    // Update the dropdown display with all variants
    updateDispenseDrugSelect();
    
    // Pre-select the variant in the dispense modal
    const drugSelect = document.getElementById('dispenseDrugSelect');
    drugSelect.value = variantId;
    
    // Update batch info for selected variant
    updateBatchInfo();
    
    // Open dispense modal
    openModal(document.getElementById('dispenseModal'));
}

// Prescribe variant and print label
function prescribeVariant(variantId, drugName, variantStrength, unit) {
    // Set hidden fields
    document.getElementById('prescribeVariantId').value = variantId;
    document.getElementById('prescribeDrugName').value = drugName;
    document.getElementById('prescribeVariantStrength').value = variantStrength;
    document.getElementById('prescribeUnit').value = unit || 'units';
    
    // Pre-fill user name if available
    if (currentUser) {
        document.getElementById('prescribeUser').value = currentUser.username;
    }
    
    // Set default expiry date to 1 month from now
    const defaultExpiry = new Date();
    defaultExpiry.setMonth(defaultExpiry.getMonth() + 1);
    document.getElementById('prescribeExpiry').value = defaultExpiry.toISOString().split('T')[0];
    
    // Open prescribe modal
    openModal(document.getElementById('prescribeModal'));
}

// Handle prescribe drug form submission
async function handlePrescribeDrug(e) {
    e.preventDefault();

    const variantId = parseInt(document.getElementById('prescribeVariantId').value);
    const drugName = document.getElementById('prescribeDrugName').value;
    const variantStrength = document.getElementById('prescribeVariantStrength').value;
    const unit = document.getElementById('prescribeUnit').value;
    const quantity = parseFloat(document.getElementById('prescribeQuantity').value);
    const animalName = document.getElementById('prescribeAnimal').value;
    const dosage = document.getElementById('prescribeDosage').value;
    const expiryDate = document.getElementById('prescribeExpiry').value;
    const userName = document.getElementById('prescribeUser').value;
    const notes = document.getElementById('prescribeNotes').value;

    if (!variantId || isNaN(quantity) || quantity <= 0 || !animalName || !dosage || !expiryDate || !userName) {
        showToast('Please fill in all required fields', 'warning');
        return;
    }

    // Convert expiry date to DD/MM/YYYY format
    const expiryParts = expiryDate.split('-');
    const formattedExpiry = `${expiryParts[2]}/${expiryParts[1]}/${expiryParts[0]}`;

    try {
        // First, print the label
        const labelData = {
            variables: {
                practice_name: "Many Tears Animal Rescue",
                animal_name: animalName,
                drug_name: `${drugName} ${variantStrength}`,
                dosage: dosage,
                quantity: `${quantity} ${unit}`,
                expiry_date: formattedExpiry
            }
        };

        const labelResponse = await apiCall('/labels/print', {
            method: 'POST',
            body: JSON.stringify(labelData)
        });

        if (!labelResponse.ok) {
            const error = await labelResponse.json();
            throw new Error(error.detail || 'Label printing request failed');
        }

        const labelResult = await labelResponse.json();
        console.log('Label print result:', labelResult);
        
        if (!labelResult.success) {
            // Label printing failed - don't dispense the drug
            const isError = labelResult.message && (
                labelResult.message.includes('not found') || 
                labelResult.message.includes('error') ||
                labelResult.message.includes('failed')
            );
            const toastType = isError ? 'error' : 'warning';
            showToast('Cannot dispense: ' + labelResult.message, toastType, 5000);
            return;
        }

        // Label printed successfully, now dispense the drug
        const dispensingData = {
            drug_variant_id: variantId,
            quantity: quantity,
            animal_name: animalName,
            user_name: userName,
            notes: notes || null
        };

        const dispenseResponse = await apiCall('/dispense', {
            method: 'POST',
            body: JSON.stringify(dispensingData)
        });

        if (!dispenseResponse.ok) {
            const error = await dispenseResponse.json();
            throw new Error(error.detail || 'Failed to dispense drug');
        }

        // Both operations succeeded
        showToast('Drug prescribed and label printed successfully!', 'success');

        document.getElementById('prescribeForm').reset();
        closeModal(document.getElementById('prescribeModal'));
        await loadDrugs();
    } catch (error) {
        console.error('Error prescribing drug:', error);
        showToast('Failed to prescribe drug: ' + error.message, 'error');
    }
}

// Handle print notes form submission
async function handlePrintNotes(e) {
    e.preventDefault();

    const animalName = document.getElementById('notesAnimalName').value.trim();
    const notes = document.getElementById('notesContent').value.trim();

    if (!animalName || !notes) {
        showToast('Please fill in all required fields', 'warning');
        return;
    }

    try {
        // Send notes to print endpoint
        const notesData = {
            variables: {
                animal_name: animalName,
                notes: notes
            }
        };

        const response = await apiCall('/notes/print', {
            method: 'POST',
            body: JSON.stringify(notesData)
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Notes printing request failed');
        }

        const result = await response.json();
        console.log('Notes print result:', result);
        
        if (!result.success) {
            // Printing failed
            const isError = result.message && (
                result.message.includes('not found') || 
                result.message.includes('error') ||
                result.message.includes('failed')
            );
            const toastType = isError ? 'error' : 'warning';
            showToast(result.message, toastType, 5000);
            return;
        }

        // Printing succeeded
        showToast('Notes printed successfully!', 'success');

        document.getElementById('printNotesForm').reset();
        closeModal(document.getElementById('printNotesModal'));
    } catch (error) {
        console.error('Error printing notes:', error);
        showToast('Failed to print notes: ' + error.message, 'error');
    }
}

// Delete variant
async function deleteVariant(variantId) {
    if (!confirm('Are you sure you want to delete this variant?')) return;

    try {
        const response = await apiCall(`/variants/${variantId}`, {
            method: 'DELETE'
        });

        if (!response.ok) throw new Error('Failed to delete variant');

        await loadDrugs();
        renderDrugs();
        showToast('Variant deleted successfully!', 'success');
    } catch (error) {
        console.error('Error deleting variant:', error);
        showToast('Failed to delete variant. Check the console for details.', 'error');
    }
}

// Show dispensing history for a drug
async function showDrugHistory(drugId) {
    const drug = allDrugs.find(d => d.id === drugId);
    if (!drug) return;

    const historyModal = document.getElementById('historyModal');
    const historyContent = document.getElementById('historyContent');
    document.getElementById('historyDrugName').textContent = drug.name;
    
    historyContent.innerHTML = '<p class="loading">Loading history...</p>';
    openModal(historyModal);
    
    try {
        const response = await apiCall(`/dispense/history`);
        if (!response.ok) throw new Error('Failed to fetch history');
        
        const allHistory = await response.json();
        
        // Filter history for this drug's variants
        const variantIds = drug.variants.map(v => v.id);
        const drugHistory = allHistory.filter(item => variantIds.includes(item.drug_variant_id));
        
        if (drugHistory.length === 0) {
            historyContent.innerHTML = '<p class="empty">No dispensing history for this drug.</p>';
            return;
        }
        
        // Sort by dispensed_at descending (most recent first)
        drugHistory.sort((a, b) => new Date(b.dispensed_at) - new Date(a.dispensed_at));
        
        const historyHtml = drugHistory.map(item => {
            const variant = drug.variants.find(v => v.id === item.drug_variant_id);
            const date = new Date(item.dispensed_at).toLocaleDateString();
            const time = new Date(item.dispensed_at).toLocaleTimeString();
            
            return `
                <div class="history-item">
                    <div class="history-header">
                        <div class="history-variant">${drug.name} ${variant.strength}</div>
                        <div class="history-datetime">${date} ${time}</div>
                    </div>
                    <div class="history-details">
                        <div class="history-row">
                            <span class="history-label">Quantity:</span>
                            <span class="history-value">${item.quantity} ${variant.unit}</span>
                        </div>
                        <div class="history-row">
                            <span class="history-label">Animal:</span>
                            <span class="history-value">${escapeHtml(item.animal_name)}</span>
                        </div>
                        <div class="history-row">
                            <span class="history-label">User:</span>
                            <span class="history-value">${escapeHtml(item.user_name)}</span>
                        </div>
                        ${item.notes ? `
                        <div class="history-row">
                            <span class="history-label">Notes:</span>
                            <span class="history-value">${escapeHtml(item.notes)}</span>
                        </div>
                        ` : ''}
                    </div>
                </div>
            `;
        }).join('');
        
        historyContent.innerHTML = historyHtml;
    } catch (error) {
        console.error('Error fetching history:', error);
        historyContent.innerHTML = '<p class="error">Failed to load history. Check the console for details.</p>';
    }
}

// Handle edit drug form
async function handleEditDrug(e) {
    e.preventDefault();

    const drugId = parseInt(document.getElementById('editDrugId').value);
    const drugData = {
        name: document.getElementById('editDrugName').value,
        description: document.getElementById('editDrugDescription').value,
        is_controlled: document.getElementById('editDrugIsControlled').checked
    };

    try {
        const response = await apiCall(`/drugs/${drugId}`, {
            method: 'PUT',
            body: JSON.stringify(drugData)
        });

        if (!response.ok) throw new Error('Failed to update drug');

        closeEditModal();
        await loadDrugs();
        showToast('Drug updated successfully!', 'success');
    } catch (error) {
        console.error('Error updating drug:', error);
        showToast('Failed to update drug. Check the console for details.', 'error');
    }
}

// Delete drug
async function deleteDrug(drugId) {
    if (!confirm('Are you sure you want to delete this drug?')) return;

    try {
        const response = await apiCall(`/drugs/${drugId}`, {
            method: 'DELETE'
        });

        if (!response.ok) throw new Error('Failed to delete drug');

        await loadDrugs();
        showToast('Drug deleted successfully!', 'success');
    } catch (error) {
        console.error('Error deleting drug:', error);
        showToast('Failed to delete drug. Check the console for details.', 'error');
    }
}

// Password Management
function openChangePasswordModal() {
    const modal = document.getElementById('changePasswordModal');
    document.getElementById('changePasswordForm').reset();
    
    // Close dropdown
    const dropdown = document.getElementById('userDropdown');
    if (dropdown) dropdown.style.display = 'none';
    
    openModal(modal);
}

async function handleChangePassword(e) {
    e.preventDefault();
    
    const currentPassword = document.getElementById('currentPassword').value;
    const newPassword = document.getElementById('newPassword').value;
    const confirmPassword = document.getElementById('confirmNewPassword').value;
    
    if (newPassword !== confirmPassword) {
        showToast('New passwords do not match!', 'warning');
        return;
    }
    
    if (newPassword.length < 1) {
        showToast('New password cannot be empty!', 'warning');
        return;
    }
    
    try {
        const response = await apiCall('/auth/change-password', {
            method: 'POST',
            body: JSON.stringify({
                current_password: currentPassword,
                new_password: newPassword
            })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to change password');
        }

        showToast('Password changed successfully!', 'success');
        closeModal(document.getElementById('changePasswordModal'));
    } catch (error) {
        console.error('Error changing password:', error);
        showToast('Failed to change password: ' + error.message, 'error');
    }
}

async function openAdminChangePasswordModal(userId, username) {
    const modal = document.getElementById('adminChangePasswordModal');
    document.getElementById('adminChangePasswordForm').reset();
    document.getElementById('adminChangePasswordUserId').value = userId;
    document.getElementById('adminChangePasswordUsername').value = username;
    openModal(modal);
}

async function handleAdminChangePassword(e) {
    e.preventDefault();
    
    const userId = document.getElementById('adminChangePasswordUserId').value;
    const newPassword = document.getElementById('adminChangePasswordNewPassword').value;
    const confirmPassword = document.getElementById('adminChangePasswordConfirm').value;
    
    if (newPassword !== confirmPassword) {
        showToast('Passwords do not match!', 'warning');
        return;
    }
    
    if (newPassword.length < 1) {
        showToast('Password cannot be empty!', 'warning');
        return;
    }
    
    try {
        const response = await apiCall(`/users/${userId}/change-password`, {
            method: 'POST',
            body: JSON.stringify({
                new_password: newPassword
            })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to change password');
        }

        showToast('Password changed successfully!', 'success');
        closeModal(document.getElementById('adminChangePasswordModal'));
        openUserManagement();
    } catch (error) {
        console.error('Error changing password:', error);
        showToast('Failed to change password: ' + error.message, 'error');
    }
}

// Update filter button states
function updateFilterButtons() {
    document.getElementById('showAllBtn').classList.toggle('active', !showLowStockOnly);
    document.getElementById('showLowStockBtn').classList.toggle('active', showLowStockOnly);
}

// Escape HTML to prevent XSS
function escapeHtml(text) {
    const div = document.createElement('div');
    div.textContent = text;
    return div.innerHTML;
}

async function openReportsPage() {
    const dropdown = document.getElementById('userDropdown');
    if (dropdown) dropdown.style.display = 'none';

    window.location.href = 'reports.html';
}

// User Management
async function openUserManagement() {
    const modal = document.getElementById('userManagementModal');
    document.getElementById('newUsername').value = '';
    document.getElementById('newUserPassword').value = '';
    document.getElementById('newUserRole').value = '';
    
    const usersList = document.getElementById('usersList');
    usersList.innerHTML = '<h3>Users</h3><p class="loading">Loading users...</p>';
    
    try {
        const response = await apiCall('/users');
        if (!response.ok) throw new Error('Failed to load users');

        const users = await response.json();
        
        const usersHtml = `
            <h3>Users</h3>
            <div class="users-table">
                ${users.map(user => {
                    const roleLabel = user.role.charAt(0).toUpperCase() + user.role.slice(1);
                    const roleBadge = user.role === 'admin' ? '👑 Admin' : 
                                     user.role === 'readonly' ? '👁️ Read-Only' : '👤 Regular';
                    return `
                        <div class="user-item">
                            <span>${user.username}</span>
                            <span class="admin-badge">${roleBadge}</span>
                            <button class="btn btn-secondary btn-small" onclick="openAdminChangePasswordModal(${user.id}, '${escapeHtml(user.username)}')">🔑 Password</button>
                            ${user.id !== currentUser.id ? `
                                <button class="btn btn-danger btn-small" onclick="deleteUser(${user.id})">Delete</button>
                            ` : ''}
                        </div>
                    `;
                }).join('')}
            </div>
        `;
        
        usersList.innerHTML = usersHtml;
    } catch (error) {
        console.error('Error loading users:', error);
        usersList.innerHTML = '<h3>Users</h3><p class="empty">Error loading users</p>';
    }
    
    const createUserForm = document.getElementById('createUserForm');
    if (createUserForm) {
        createUserForm.onsubmit = createUser;
    }
    
    openModal(modal);
}

// Create user
async function createUser(e) {
    e.preventDefault();
    
    const username = document.getElementById('newUsername').value;
    const password = document.getElementById('newUserPassword').value;
    const role = document.getElementById('newUserRole').value;
    
    if (!role) {
        showToast('Please select a role', 'warning');
        return;
    }
    
    try {
        const response = await apiCall('/users', {
            method: 'POST',
            body: JSON.stringify({ username, password, role })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to create user');
        }

        document.getElementById('newUsername').value = '';
        document.getElementById('newUserPassword').value = '';
        document.getElementById('newUserRole').value = '';
        showToast('User created successfully!', 'success');
        openUserManagement();
    } catch (error) {
        console.error('Error creating user:', error);
        showToast('Failed to create user: ' + error.message, 'error');
    }
}

// Delete user
async function deleteUser(userId) {
    if (!confirm('Are you sure you want to delete this user?')) return;

    try {
        const response = await apiCall(`/users/${userId}`, { method: 'DELETE' });

        if (!response.ok) throw new Error('Failed to delete user');

        showToast('User deleted successfully!', 'success');
        openUserManagement();
    } catch (error) {
        console.error('Error deleting user:', error);
        showToast('Failed to delete user: ' + error.message, 'error');
    }
}

// Location Management
async function openLocationManagement() {
    const modal = document.getElementById('locationManagementModal');
    document.getElementById('newLocationName').value = '';
    
    const locationsList = document.getElementById('locationsList');
    locationsList.innerHTML = '<h3>Active Locations</h3><p class="loading">Loading locations...</p>';
    
    try {
        const response = await apiCall('/locations');
        if (!response.ok) throw new Error('Failed to load locations');

        const locations = await response.json();
        
        const activeLocations = locations.filter(loc => loc.is_active);
        const inactiveLocations = locations.filter(loc => !loc.is_active);
        
        let locationsHtml = '<h3>Active Locations</h3>';
        if (activeLocations.length === 0) {
            locationsHtml += '<p class="empty">No active locations</p>';
        } else {
            locationsHtml += `<div class="locations-table">
                ${activeLocations.map(location => `
                    <div class="location-item">
                        <div style="flex: 1;">
                            <strong>${location.name}</strong>
                            <div style="font-size: 0.85em; color: #666;">Created: ${new Date(location.created_at).toLocaleDateString()}</div>
                        </div>
                        <button class="btn btn-danger btn-small" onclick="archiveLocation(${location.id}, '${location.name.replace(/'/g, "\\'")}')">Archive</button>
                    </div>
                `).join('')}
            </div>`;
        }
        
        if (inactiveLocations.length > 0) {
            locationsHtml += `
                <h3 style="margin-top: 20px;">Archived Locations</h3>
                <div class="locations-table">
                    ${inactiveLocations.map(location => `
                        <div class="location-item" style="opacity: 0.6;">
                            <div style="flex: 1;">
                                <strong>${location.name}</strong> <span style="color: #999;">(archived)</span>
                            </div>
                            <button class="btn btn-secondary btn-small" onclick="restoreLocation(${location.id}, '${location.name.replace(/'/g, "\\'")}')">Restore</button>
                        </div>
                    `).join('')}
                </div>
            `;
        }
        
        locationsList.innerHTML = locationsHtml;
    } catch (error) {
        console.error('Error loading locations:', error);
        locationsList.innerHTML = '<h3>Active Locations</h3><p class="empty">Error loading locations</p>';
    }
    
    openModal(modal);
}

// Create location
async function createLocation(e) {
    e.preventDefault();
    
    const name = document.getElementById('newLocationName').value.trim();
    
    if (!name) {
        showToast('Please enter a location name', 'warning');
        return;
    }
    
    try {
        const response = await apiCall('/locations', {
            method: 'POST',
            body: JSON.stringify({ name })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to create location');
        }

        document.getElementById('newLocationName').value = '';
        showToast('Location created successfully!', 'success');
        openLocationManagement();
    } catch (error) {
        console.error('Error creating location:', error);
        showToast('Failed to create location: ' + error.message, 'error');
    }
}

// Archive location
async function archiveLocation(locationId, locationName) {
    if (!confirm(`Archive location "${locationName}"?\n\nYou can restore it later if needed.`)) return;

    try {
        const response = await apiCall(`/locations/${locationId}`, {
            method: 'PUT',
            body: JSON.stringify({ is_active: false })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to archive location');
        }

        showToast('Location archived successfully!', 'success');
        openLocationManagement();
    } catch (error) {
        console.error('Error archiving location:', error);
        showToast('Failed to archive location: ' + error.message, 'error');
    }
}

// Restore location
async function restoreLocation(locationId, locationName) {
    if (!confirm(`Restore location "${locationName}"?`)) return;

    try {
        const response = await apiCall(`/locations/${locationId}`, {
            method: 'PUT',
            body: JSON.stringify({ is_active: true })
        });

        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to restore location');
        }

        showToast('Location restored successfully!', 'success');
        openLocationManagement();
    } catch (error) {
        console.error('Error restoring location:', error);
        showToast('Failed to restore location: ' + error.message, 'error');
    }
}

// Batch Management
async function openBatchReceiveModal(variantId) {
    const batchReceiveModal = document.getElementById('batchReceiveModal');
    document.getElementById('batchReceiveForm').reset();
    document.getElementById('batchVariantId').value = variantId;
    
    // Initialize locations
    await initializeBatchLocations();
    
    openModal(batchReceiveModal);
}

async function initializeBatchLocations() {
    const locationSelect = document.getElementById('batchLocation');
    
    try {
        const response = await apiCall('/locations');
        if (!response.ok) throw new Error('Failed to load locations');
        
        const locations = await response.json();
        
        locationSelect.innerHTML = '<option value="">-- Select location --</option>';
        locations.forEach(location => {
            if (location.is_active) {
                const option = document.createElement('option');
                option.value = location.id;
                option.textContent = location.name;
                locationSelect.appendChild(option);
            }
        });
    } catch (error) {
        console.error('Error loading locations:', error);
        showToast('Failed to load storage locations', 'error');
    }
}

async function handleBatchReceive(e) {
    e.preventDefault();
    
    const variantId = parseInt(document.getElementById('batchVariantId').value);
    const batchNumber = document.getElementById('batchNumber').value.trim();
    const quantity = parseFloat(document.getElementById('batchQuantity').value);
    const expiryDate = document.getElementById('batchExpiryDate').value;
    const locationId = parseInt(document.getElementById('batchLocation').value);
    const notes = document.getElementById('batchNotes').value.trim();
    
    if (!batchNumber || isNaN(quantity) || quantity <= 0 || !expiryDate || !locationId) {
        showToast('Please fill in all required fields', 'warning');
        return;
    }
    
    const batchData = {
        batch_number: batchNumber,
        quantity: quantity,
        expiry_date: expiryDate,
        location_id: locationId,
        notes: notes || null
    };
    
    try {
        const response = await apiCall(`/variants/${variantId}/batches`, {
            method: 'POST',
            body: JSON.stringify(batchData)
        });
        
        if (!response.ok) {
            const error = await response.json();
            throw new Error(error.detail || 'Failed to receive batch');
        }
        
        document.getElementById('batchReceiveForm').reset();
        closeModal(document.getElementById('batchReceiveModal'));
        await loadDrugs();
        showToast('Batch received successfully!', 'success');
    } catch (error) {
        console.error('Error receiving batch:', error);
        showToast('Failed to receive batch: ' + error.message, 'error');
    }
}

function getActiveDeliveryDrug() {
    return allDrugs.find(d => d.id === deliveryDrugId);
}

function getVariantById(variantId) {
    for (const drug of allDrugs) {
        const found = (drug.variants || []).find(v => v.id === variantId);
        if (found) return found;
    }
    return null;
}

function buildDeliveryVariantOptions(drug, selectedVariantId = '') {
    if (!drug || !drug.variants || drug.variants.length === 0) {
        return '<option value="">-- No variants available --</option>';
    }

    return [`<option value="">-- Select variant --</option>`, ...drug.variants.map(v => {
        const selected = String(v.id) === String(selectedVariantId) ? ' selected' : '';
        return `<option value="${v.id}"${selected}>${escapeHtml(v.strength)} (${escapeHtml(v.unit)})</option>`;
    })].join('');
}

function getActivePacksForVariant(variant) {
    if (!variant || !Array.isArray(variant.packs)) return [];
    return variant.packs.filter(pack => pack.is_active);
}

function buildDeliveryPackOptions(variant, selectedPackId = '') {
    const packs = getActivePacksForVariant(variant);
    if (packs.length === 0) {
        return '<option value="">-- No active packs --</option>';
    }

    return [`<option value="">-- Select pack --</option>`, ...packs.map(pack => {
        const selected = String(pack.id) === String(selectedPackId) ? ' selected' : '';
        const label = `${pack.label} (${pack.pack_size_in_base_units} ${variant.unit})`;
        return `<option value="${pack.id}"${selected}>${escapeHtml(label)}</option>`;
    })].join('');
}

function buildDeliveryLocationOptions(selectedLocationId = '') {
    return [`<option value="">-- Select location --</option>`, ...deliveryLocations.map(location => {
        const selected = String(location.id) === String(selectedLocationId) ? ' selected' : '';
        return `<option value="${location.id}"${selected}>${escapeHtml(location.name)}</option>`;
    })].join('');
}

function updateDeliveryLineQuantityDisplay(line) {
    const variantId = parseInt(line.querySelector('.delivery-variant-select')?.value || '', 10);
    const packSelect = line.querySelector('.delivery-pack-select');

    const variant = getVariantById(variantId);
    if (!variant || !packSelect) {
        return;
    }

    const currentPackId = packSelect.value;
    packSelect.innerHTML = buildDeliveryPackOptions(variant, currentPackId);
}

function wireDeliveryLineEvents(line) {
    const variantSelect = line.querySelector('.delivery-variant-select');
    const packSelect = line.querySelector('.delivery-pack-select');
    const packCountInput = line.querySelector('.delivery-pack-count');

    if (variantSelect && packSelect) {
        variantSelect.addEventListener('change', () => {
            const variantId = parseInt(variantSelect.value || '', 10);
            const variant = getVariantById(variantId);
            packSelect.innerHTML = buildDeliveryPackOptions(variant, '');
            if (packCountInput) packCountInput.value = '';
            updateDeliveryLineQuantityDisplay(line);
        });
    }

    if (packSelect) {
        packSelect.addEventListener('change', () => {
            updateDeliveryLineQuantityDisplay(line);
        });
    }

    if (packCountInput) {
        packCountInput.addEventListener('input', () => {
            updateDeliveryLineQuantityDisplay(line);
        });
    }
}

function appendDeliveryLine(prefill = {}) {
    const container = document.getElementById('deliveryLinesContainer');
    const drug = getActiveDeliveryDrug();
    if (!container || !drug) return;

    deliveryLineCounter += 1;
    const lineId = `delivery-line-${deliveryLineCounter}`;

    const line = document.createElement('div');
    line.className = 'delivery-line';
    line.dataset.lineId = lineId;

    const initialVariant = drug.variants.find(v => String(v.id) === String(prefill.variantId)) || drug.variants[0] || null;
    const initialVariantId = prefill.variantId || (initialVariant ? initialVariant.id : '');
    const initialPackId = prefill.packId || '';
    const initialPackCount = prefill.packCount || '';

    line.innerHTML = `
        <div class="delivery-line-grid">
            <div class="form-group">
                <label>Variant</label>
                <select class="delivery-variant-select" required>
                    ${buildDeliveryVariantOptions(drug, initialVariantId)}
                </select>
            </div>
            <div class="form-group">
                <label>Pack Type</label>
                <select class="delivery-pack-select" required>
                    ${buildDeliveryPackOptions(initialVariant, initialPackId)}
                </select>
            </div>
            <div class="form-group">
                <label>Pack Count</label>
                <input type="number" class="delivery-pack-count" min="0.0001" step="0.0001" value="${initialPackCount}" required>
            </div>
            <div class="form-group">
                <label>Batch Number</label>
                <input type="text" class="delivery-batch-number" value="${prefill.batchNumber || ''}" placeholder="e.g. ABC123" required>
            </div>
            <div class="form-group">
                <label>Expiry</label>
                <input type="date" class="delivery-expiry-date" value="${prefill.expiryDate || ''}" required>
            </div>
            <div class="form-group">
                <label>Location</label>
                <select class="delivery-location-select" required>
                    ${buildDeliveryLocationOptions(prefill.locationId || '')}
                </select>
            </div>
            <button type="button" class="btn btn-danger btn-small delivery-remove-btn">Remove</button>
        </div>
    `;

    const removeBtn = line.querySelector('.delivery-remove-btn');
    if (removeBtn) {
        removeBtn.addEventListener('click', () => {
            if (container.children.length <= 1) {
                showToast('At least one delivery line is required', 'warning');
                return;
            }
            line.remove();
        });
    }

    wireDeliveryLineEvents(line);
    updateDeliveryLineQuantityDisplay(line);

    container.appendChild(line);
}

function refreshDeliveryVariantSelects() {
    const drug = getActiveDeliveryDrug();
    const container = document.getElementById('deliveryLinesContainer');
    if (!drug || !container) return;

    container.querySelectorAll('.delivery-line').forEach(line => {
        const select = line.querySelector('.delivery-variant-select');
        const packSelect = line.querySelector('.delivery-pack-select');
        if (!select) return;

        const currentVariantId = select.value;
        select.innerHTML = buildDeliveryVariantOptions(drug, currentVariantId);

        const variant = getVariantById(parseInt(select.value || '', 10));
        if (packSelect) {
            const currentPackId = packSelect.value;
            packSelect.innerHTML = buildDeliveryPackOptions(variant, currentPackId);
        }

        updateDeliveryLineQuantityDisplay(line);
    });
}

async function initializeDeliveryLocations() {
    try {
        const response = await apiCall('/locations');
        if (!response.ok) throw new Error('Failed to load locations');
        const locations = await response.json();
        deliveryLocations = locations.filter(location => location.is_active);
    } catch (error) {
        console.error('Error loading delivery locations:', error);
        showToast('Failed to load storage locations', 'error');
        deliveryLocations = [];
    }
}

async function openReceiveDeliveryModal(drugId) {
    deliveryDrugId = drugId;
    const drug = getActiveDeliveryDrug();
    if (!drug) {
        showToast('Drug not found', 'error');
        return;
    }

    const form = document.getElementById('receiveDeliveryForm');
    const container = document.getElementById('deliveryLinesContainer');
    const label = document.getElementById('receiveDeliveryDrugLabel');

    if (form) form.reset();
    if (container) container.innerHTML = '';
    if (label) label.textContent = `Drug: ${drug.name}`;

    await initializeDeliveryLocations();
    appendDeliveryLine();

    openModal(document.getElementById('receiveDeliveryModal'));
}

function handleAddVariantFromDelivery() {
    if (!deliveryDrugId) {
        showToast('Select a drug first', 'warning');
        return;
    }
    openAddVariantModal(deliveryDrugId);
}

async function handleReceiveDelivery(e) {
    e.preventDefault();

    const drug = getActiveDeliveryDrug();
    const container = document.getElementById('deliveryLinesContainer');
    if (!drug || !container) {
        showToast('Delivery context unavailable', 'error');
        return;
    }

    const lines = Array.from(container.querySelectorAll('.delivery-line'));
    if (lines.length === 0) {
        showToast('Add at least one delivery line', 'warning');
        return;
    }

    const payloads = [];

    for (let i = 0; i < lines.length; i += 1) {
        const line = lines[i];
        const variantId = parseInt(line.querySelector('.delivery-variant-select')?.value || '', 10);
        const packIdRaw = line.querySelector('.delivery-pack-select')?.value || '';
        const packId = packIdRaw ? parseInt(packIdRaw, 10) : null;
        const packCountRaw = line.querySelector('.delivery-pack-count')?.value || '';
        const packCount = packCountRaw ? parseFloat(packCountRaw) : null;
        const batchNumber = (line.querySelector('.delivery-batch-number')?.value || '').trim();
        const expiryDate = line.querySelector('.delivery-expiry-date')?.value || '';
        const locationId = parseInt(line.querySelector('.delivery-location-select')?.value || '', 10);

        if (!variantId || !packId || packCount === null || Number.isNaN(packCount) || packCount <= 0 || !batchNumber || !expiryDate || !locationId) {
            showToast(`Delivery line ${i + 1} is incomplete`, 'warning');
            return;
        }

        const variant = drug.variants.find(v => v.id === variantId);
        const selectedPack = variant ? getActivePacksForVariant(variant).find(pack => pack.id === packId) : null;
        if (!selectedPack) {
            showToast(`Delivery line ${i + 1} has an invalid pack selection`, 'warning');
            return;
        }
        const computedQuantity = packCount * selectedPack.pack_size_in_base_units;

        payloads.push({
            variantId,
            payload: {
                batch_number: batchNumber,
                received_pack_id: packId,
                received_pack_count: packCount,
                expiry_date: expiryDate,
                location_id: locationId,
                notes: `Received ${packCount} ${selectedPack.pack_unit_name}(s), total ${computedQuantity} ${variant ? variant.unit : 'units'}`
            }
        });
    }

    try {
        for (let i = 0; i < payloads.length; i += 1) {
            const entry = payloads[i];
            const response = await apiCall(`/variants/${entry.variantId}/batches`, {
                method: 'POST',
                body: JSON.stringify(entry.payload)
            });

            if (!response.ok) {
                const error = await response.json();
                throw new Error(`Line ${i + 1}: ${error.detail || 'Failed to receive delivery line'}`);
            }
        }

        closeModal(document.getElementById('receiveDeliveryModal'));
        await loadDrugs();
        showToast(`Delivery received successfully (${payloads.length} line${payloads.length === 1 ? '' : 's'})`, 'success');
    } catch (error) {
        console.error('Error receiving delivery:', error);
        showToast('Failed to receive delivery: ' + error.message, 'error');
    }
}