feat: add bridge days recommendations feature with display and calculation logic

public/js/bridge-days.js

@@ -0,0 +1,219 @@
+/**
+ * Bridge Days Calculator
+ * Calculates optimal vacation days based on public holidays
+ */
+
+/**
+ * Calculate bridge days and optimal vacation periods for a month
+ * @param {number} year - The year to calculate for
+ * @param {number} month - The month (0-11)
+ * @param {string} bundesland - The German state code
+ * @returns {Array} Array of bridge day recommendations
+ */
+function calculateBridgeDays(year, month, bundesland) {
+  const recommendations = [];
+  
+  // Get all holidays for the year
+  const holidays = getPublicHolidays(year, bundesland);
+  
+  // Create a calendar map for the entire year
+  const calendar = createYearCalendar(year, holidays);
+  
+  // Find all work day blocks (consecutive work days between weekends/holidays)
+  const workBlocks = findWorkDayBlocks(calendar);
+  
+  // Evaluate each block and calculate benefit
+  workBlocks.forEach(block => {
+    const benefit = evaluateBlock(block, calendar);
+    if (benefit.ratio >= 2.0) { // Only show if at least 2x benefit
+      recommendations.push(benefit);
+    }
+  });
+  
+  // Sort by benefit ratio (best deals first)
+  recommendations.sort((a, b) => b.ratio - a.ratio);
+  
+  // Filter for the specific month
+  const monthRecommendations = recommendations.filter(rec => {
+    const startDate = new Date(rec.startDate);
+    return startDate.getMonth() === month && startDate.getFullYear() === year;
+  });
+  
+  return monthRecommendations;
+}
+
+/**
+ * Create a calendar map for the entire year
+ * @param {number} year - The year
+ * @param {Array} holidays - Array of holiday objects
+ * @returns {Map} Map of date strings to day types
+ */
+function createYearCalendar(year, holidays) {
+  const calendar = new Map();
+  
+  // Create holiday map for fast lookup
+  const holidayMap = new Map();
+  holidays.forEach(h => {
+    const dateStr = formatDateKey(h.date);
+    holidayMap.set(dateStr, h.name);
+  });
+  
+  // Process each day of the year
+  for (let month = 0; month < 12; month++) {
+    const daysInMonth = new Date(year, month + 1, 0).getDate();
+    
+    for (let day = 1; day <= daysInMonth; day++) {
+      const date = new Date(year, month, day);
+      const dateStr = formatDateKey(date);
+      const dayOfWeek = date.getDay();
+      
+      let type;
+      if (holidayMap.has(dateStr)) {
+        type = { status: 'HOLIDAY', name: holidayMap.get(dateStr) };
+      } else if (dayOfWeek === 0 || dayOfWeek === 6) {
+        type = { status: 'WEEKEND' };
+      } else {
+        type = { status: 'WORKDAY' };
+      }
+      
+      calendar.set(dateStr, type);
+    }
+  }
+  
+  return calendar;
+}
+
+/**
+ * Format date as YYYY-MM-DD
+ */
+function formatDateKey(date) {
+  const year = date.getFullYear();
+  const month = String(date.getMonth() + 1).padStart(2, '0');
+  const day = String(date.getDate()).padStart(2, '0');
+  return `${year}-${month}-${day}`;
+}
+
+/**
+ * Find all work day blocks in the calendar
+ * @param {Map} calendar - The calendar map
+ * @returns {Array} Array of work day blocks
+ */
+function findWorkDayBlocks(calendar) {
+  const blocks = [];
+  let currentBlock = null;
+  
+  // Sort dates for sequential processing
+  const sortedDates = Array.from(calendar.keys()).sort();
+  
+  sortedDates.forEach(dateStr => {
+    const dayType = calendar.get(dateStr);
+    
+    if (dayType.status === 'WORKDAY') {
+      if (!currentBlock) {
+        currentBlock = {
+          startDate: dateStr,
+          endDate: dateStr,
+          days: [dateStr]
+        };
+      } else {
+        currentBlock.endDate = dateStr;
+        currentBlock.days.push(dateStr);
+      }
+    } else {
+      if (currentBlock) {
+        blocks.push(currentBlock);
+        currentBlock = null;
+      }
+    }
+  });
+  
+  // Don't forget the last block
+  if (currentBlock) {
+    blocks.push(currentBlock);
+  }
+  
+  return blocks;
+}
+
+/**
+ * Evaluate a work day block and calculate benefit
+ * @param {Object} block - The work day block
+ * @param {Map} calendar - The calendar map
+ * @returns {Object} Benefit information
+ */
+function evaluateBlock(block, calendar) {
+  const vacationDaysNeeded = block.days.length;
+  
+  // Find the extended free period (including surrounding weekends/holidays)
+  let startDate = new Date(block.startDate);
+  let endDate = new Date(block.endDate);
+  
+  // Extend backwards to include preceding weekends/holidays
+  let currentDate = new Date(startDate);
+  currentDate.setDate(currentDate.getDate() - 1);
+  while (true) {
+    const dateStr = formatDateKey(currentDate);
+    const dayType = calendar.get(dateStr);
+    if (!dayType || dayType.status === 'WORKDAY') break;
+    startDate = new Date(currentDate);
+    currentDate.setDate(currentDate.getDate() - 1);
+  }
+  
+  // Extend forwards to include following weekends/holidays
+  currentDate = new Date(endDate);
+  currentDate.setDate(currentDate.getDate() + 1);
+  while (true) {
+    const dateStr = formatDateKey(currentDate);
+    const dayType = calendar.get(dateStr);
+    if (!dayType || dayType.status === 'WORKDAY') break;
+    endDate = new Date(currentDate);
+    currentDate.setDate(currentDate.getDate() + 1);
+  }
+  
+  // Calculate total free days
+  const totalFreeDays = Math.floor((endDate - startDate) / (1000 * 60 * 60 * 24)) + 1;
+  
+  // Calculate benefit ratio
+  const ratio = totalFreeDays / vacationDaysNeeded;
+  
+  // Find holidays in the period for description
+  const holidaysInPeriod = [];
+  currentDate = new Date(startDate);
+  while (currentDate <= endDate) {
+    const dateStr = formatDateKey(currentDate);
+    const dayType = calendar.get(dateStr);
+    if (dayType && dayType.status === 'HOLIDAY') {
+      holidaysInPeriod.push(dayType.name);
+    }
+    currentDate.setDate(currentDate.getDate() + 1);
+  }
+  
+  return {
+    startDate: formatDateKey(startDate),
+    endDate: formatDateKey(endDate),
+    vacationDays: block.days,
+    vacationDaysNeeded: vacationDaysNeeded,
+    totalFreeDays: totalFreeDays,
+    ratio: ratio,
+    holidays: holidaysInPeriod
+  };
+}
+
+/**
+ * Get a human-readable description for a bridge day recommendation
+ * @param {Object} recommendation - The recommendation object
+ * @returns {string} Description text
+ */
+function getBridgeDayDescription(recommendation) {
+  const { vacationDaysNeeded, totalFreeDays, ratio, holidays } = recommendation;
+  
+  let description = `${vacationDaysNeeded} Urlaubstag${vacationDaysNeeded > 1 ? 'e' : ''} für ${totalFreeDays} freie Tage`;
+  
+  if (holidays.length > 0) {
+    description += ` (inkl. ${holidays.join(', ')})`;
+  }
+  
+  description += ` - ${ratio.toFixed(1)}x Ertrag`;
+  
+  return description;
+}