Optimize React rendering and data processing performance

PERFORMANCE_IMPROVEMENTS.md

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+# Performance Improvements
+
+## Overview
+This document outlines the performance optimizations made to the BentaKo POS system to improve rendering performance, reduce unnecessary re-renders, and optimize data processing.
+
+## Key Improvements
+
+### 1. React Component Memoization
+
+#### ProductCard Component
+- **Issue**: ProductCard was re-rendering on every cart update, even when its props didn't change
+- **Solution**: Wrapped component with `React.memo()`
+- **Impact**: Prevents unnecessary re-renders for products not affected by cart changes
+- **File**: `src/components/dashboard/ProductCard.tsx`
+
+#### ProductGrid Component  
+- **Issue**: ProductGrid was calling `cart.some()` for every product on each render (O(n*m) complexity)
+- **Solution**: 
+  - Wrapped component with `React.memo()`
+  - Memoized cart item IDs into a Set for O(1) lookups instead of O(n) array searches
+  - Changed from `cart.some(item => item.id === product.id)` to `cartItemIds.has(product.id)`
+- **Impact**: Significant performance improvement when displaying many products with items in cart
+- **File**: `src/components/dashboard/ProductGrid.tsx`
+
+### 2. Custom Hook Optimizations
+
+#### useCart Hook
+- **Issue**: `totalPrice` and `totalItems` were recalculated on every render
+- **Solution**: Wrapped calculations with `useMemo()` to only recalculate when cart changes
+- **Impact**: Reduces CPU usage when cart data hasn't changed
+- **File**: `src/hooks/useCart.ts`
+
+#### useProductSearch Hook
+- **Issue**: Product filtering was happening on every render
+- **Solution**: Wrapped `filteredProducts` calculation with `useMemo()`
+- **Impact**: Prevents redundant filtering operations when dependencies haven't changed
+- **File**: `src/hooks/useProductSearch.ts`
+
+### 3. Mobile Page Optimizations
+
+- **Issue**: Mobile.tsx had duplicate formatPrice function and inefficient cart lookups
+- **Solution**:
+  - Imported shared `formatPrice` utility
+  - Added `useMemo` for cart item IDs Set
+  - Changed from `cart.some()` to `cartItemIds.has()`
+- **Impact**: Consistent formatting logic and faster cart lookups
+- **File**: `src/pages/Mobile.tsx`
+
+### 4. Reports Page Major Optimization
+
+This was the most significant performance improvement in the codebase.
+
+#### Problems Identified:
+1. **Multiple filter passes**: Sales data was being filtered 7+ times separately for different metrics
+2. **No memoization**: All calculations ran on every render
+3. **Date object recreation**: Date helpers were created on every render
+4. **Repeated operations**: Similar operations were done multiple times
+
+#### Solutions Implemented:
+
+**Single-Pass Data Processing**:
+```javascript
+// Before: 7+ separate filter operations
+const todaysSales = sales?.filter(sale => isToday(new Date(sale.date)))...
+const yesterdaySales = sales?.filter(sale => ...)...
+const weeklySales = sales?.filter(sale => ...)...
+// ... and more
+
+// After: Single pass through data
+sales.forEach(sale => {
+  const saleDate = new Date(sale.date);
+  const revenue = sale.price * sale.quantity;
+
+  totalRevenue += revenue;
+  if (dateHelpers.isToday(saleDate)) todaysSales += revenue;
+  if (dateHelpers.isThisWeek(saleDate)) weeklySales += revenue;
+  // ... all calculations in one pass
+});
+```
+
+**Memoization Strategy**:
+- `dateHelpers`: Memoized with empty dependency array (only created on mount)
+- `salesMetrics`: Memoized with `[sales, dateHelpers]` dependencies
+- `debtMetrics`: Memoized with `[debts]` dependency
+- `inventoryValue`: Memoized with `[inventoryItems]` dependency
+- `reportsData`: Memoized with all metric dependencies
+
+**Performance Impact**:
+- Reduced from O(7n) to O(n) time complexity for sales data processing
+- Eliminated redundant Date object creation
+- Prevented recalculation when data hasn't changed
+- Reduced CPU usage on every render cycle
+
+**File**: `src/pages/Reports.tsx`
+
+### 5. Code Duplication Elimination
+
+#### formatPrice Utility
+- **Issue**: Same `formatPrice` function was duplicated in 3 places:
+  - `src/utils/formatPrice.ts` 
+  - `src/components/dashboard/ProductCard.tsx`
+  - `src/pages/Mobile.tsx`
+- **Solution**: Consolidated to single utility and imported everywhere
+- **Impact**: DRY principle, easier maintenance, consistent behavior
+- **Files Modified**: 
+  - `src/components/dashboard/ProductCard.tsx`
+  - `src/pages/Mobile.tsx`
+
+### 6. Service Layer Optimization
+
+#### Inventory Service
+- **Issue**: `localStorage.getItem()` and `JSON.parse()` were called every time, even when data was already loaded
+- **Solution**: 
+  - Added `isInitialized` flag to cache the parsed data
+  - Only parse localStorage once on first access
+  - Subsequent calls use the in-memory cache
+- **Impact**: Eliminates redundant parsing operations
+- **File**: `src/services/inventoryService.ts`
+
+## Performance Metrics
+
+### Before Optimizations:
+- ProductCard: Re-rendered on every cart change (unnecessary)
+- ProductGrid: O(n*m) cart lookups on every render
+- Reports: 7+ filter passes through sales data on every render
+- localStorage: Parsed on every inventory access
+
+### After Optimizations:
+- ProductCard: Only re-renders when its own props change
+- ProductGrid: O(n) cart lookups with Set-based memoized IDs
+- Reports: Single O(n) pass through sales data, memoized results
+- localStorage: Parsed once, cached in memory
+
+## Best Practices Applied
+
+1. **React.memo**: Used for pure functional components that render the same output for same props
+2. **useMemo**: Applied for expensive calculations and derived data
+3. **Set for lookups**: Using Set instead of Array for O(1) lookups instead of O(n)
+4. **Single-pass processing**: Process data once instead of multiple filter operations
+5. **Lazy initialization**: Only parse/process data when needed, then cache
+6. **DRY principle**: Consolidated duplicate code into shared utilities
+
+## Recommendations for Future
+
+1. **Code splitting**: The build shows a 562 kB bundle. Consider:
+   - Dynamic imports for routes
+   - Lazy loading for heavy components
+   - Splitting vendor chunks
+
+2. **Virtual scrolling**: If product lists grow large, consider react-window or react-virtual
+
+3. **Debouncing**: Already implemented for search (300ms), good practice
+
+4. **Service Worker**: For offline support and faster subsequent loads
+
+5. **React Query optimizations**: 
+   - Already using React Query for caching
+   - Consider adjusting staleTime and cacheTime based on data freshness needs
+
+## Testing Recommendations
+
+1. Test with large datasets (100+ products, 1000+ sales records)
+2. Monitor React DevTools Profiler to verify reduced renders
+3. Use Performance tab in Chrome DevTools to measure improvements
+4. Test on low-end mobile devices to ensure smooth experience
+
+## Conclusion
+
+These optimizations significantly improve the application's performance, especially on the Reports page which had the most complex calculations. The changes follow React best practices and maintain code readability while improving efficiency.