// Date Utilities with Timezone Support
// Fixes inconsistent date handling across the app

// ============================================
// TYPES
// ============================================

export type DateFormat = 'display' | 'input' | 'iso' | 'short' | 'long';

interface DateFormatOptions {
    format?: DateFormat;
    includeTime?: boolean;
    timezone?: 'local' | 'utc';
}

// ============================================
// CONSTANTS
// ============================================

// Indian Standard Time offset (UTC+5:30)
const IST_OFFSET_HOURS = 5;
const IST_OFFSET_MINUTES = 30;

// ============================================
// TIMEZONE UTILITIES
// ============================================

/**
 * Get current date in IST timezone
 */
export const getCurrentDateIST = (): Date => {
    const now = new Date();
    const utc = now.getTime() + (now.getTimezoneOffset() * 60000);
    return new Date(utc + (IST_OFFSET_HOURS * 3600000) + (IST_OFFSET_MINUTES * 60000));
};

/**
 * Convert date to IST timezone
 */
export const toIST = (date: Date): Date => {
    const utc = date.getTime() + (date.getTimezoneOffset() * 60000);
    return new Date(utc + (IST_OFFSET_HOURS * 3600000) + (IST_OFFSET_MINUTES * 60000));
};

/**
 * Get start of day in IST
 */
export const getStartOfDayIST = (date: Date = new Date()): Date => {
    const ist = toIST(date);
    return new Date(ist.getFullYear(), ist.getMonth(), ist.getDate(), 0, 0, 0, 0);
};

/**
 * Get end of day in IST
 */
export const getEndOfDayIST = (date: Date = new Date()): Date => {
    const ist = toIST(date);
    return new Date(ist.getFullYear(), ist.getMonth(), ist.getDate(), 23, 59, 59, 999);
};

// ============================================
// FORMATTING
// ============================================

/**
 * Format date to DD/MM/YYYY (Indian format)
 */
export const formatDisplayDate = (dateStr: string | Date): string => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '-';
    
    return date.toLocaleDateString('en-IN', {
        day: '2-digit',
        month: '2-digit',
        year: 'numeric',
        timeZone: 'Asia/Kolkata'
    });
};

/**
 * Format date to YYYY-MM-DD (input field format)
 */
export const formatInputDate = (dateStr: string | Date): string => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '';
    
    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}`;
};

/**
 * Format date to DD/MM/YY (short format)
 */
export const formatShortDate = (dateStr: string | Date): string => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '';
    
    const dd = date.getDate().toString().padStart(2, '0');
    const mm = (date.getMonth() + 1).toString().padStart(2, '0');
    const yy = date.getFullYear().toString().slice(-2);
    
    return `${dd}/${mm}/${yy}`;
};

/**
 * Format date with time (DD/MM/YYYY HH:mm)
 */
export const formatDateTime = (dateStr: string | Date): string => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '-';
    
    const datePart = formatDisplayDate(date);
    const timePart = date.toLocaleTimeString('en-IN', {
        hour: '2-digit',
        minute: '2-digit',
        hour12: true,
        timeZone: 'Asia/Kolkata'
    });
    
    return `${datePart} ${timePart}`;
};

/**
 * Format date to long format (e.g., "15 March 2024")
 */
export const formatLongDate = (dateStr: string | Date): string => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '-';
    
    return date.toLocaleDateString('en-IN', {
        day: 'numeric',
        month: 'long',
        year: 'numeric',
        timeZone: 'Asia/Kolkata'
    });
};

/**
 * Format date in Marathi (e.g., "१५ मार्च २०२४")
 */
export const formatMarathiDate = (dateStr: string | Date): string => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '-';
    
    return date.toLocaleDateString('mr-IN', {
        day: 'numeric',
        month: 'long',
        year: 'numeric',
        timeZone: 'Asia/Kolkata'
    });
};

// ============================================
// CONVENIENCE FUNCTIONS
// ============================================

/**
 * Get today's date in YYYY-MM-DD format (for input fields)
 */
export const getTodayDate = (): string => {
    return formatInputDate(new Date());
};

/**
 * Get today's date in DD/MM/YYYY format (for display)
 */
export const getTodayDisplayDate = (): string => {
    return formatDisplayDate(new Date());
};

/**
 * Parse date string to Date object (handles multiple formats)
 */
export const parseDate = (dateStr: string): Date | null => {
    if (!dateStr) return null;
    
    // Try ISO format (YYYY-MM-DD)
    if (/^\d{4}-\d{2}-\d{2}$/.test(dateStr)) {
        return new Date(dateStr + 'T00:00:00');
    }
    
    // Try DD/MM/YYYY format
    if (/^\d{2}\/\d{2}\/\d{4}$/.test(dateStr)) {
        const [day, month, year] = dateStr.split('/');
        return new Date(`${year}-${month}-${day}T00:00:00`);
    }
    
    // Try DD/MM/YY format
    if (/^\d{2}\/\d{2}\/\d{2}$/.test(dateStr)) {
        const [day, month, year] = dateStr.split('/');
        return new Date(`20${year}-${month}-${day}T00:00:00`);
    }
    
    // Fallback to native parsing
    const parsed = new Date(dateStr);
    return isNaN(parsed.getTime()) ? null : parsed;
};

// ============================================
// DATE CALCULATIONS
// ============================================

/**
 * Add days to a date
 */
export const addDays = (date: Date, days: number): Date => {
    const result = new Date(date);
    result.setDate(result.getDate() + days);
    return result;
};

/**
 * Get difference in days between two dates
 */
export const getDaysDifference = (date1: Date, date2: Date): number => {
    const oneDay = 24 * 60 * 60 * 1000;
    const d1 = new Date(date1.getFullYear(), date1.getMonth(), date1.getDate());
    const d2 = new Date(date2.getFullYear(), date2.getMonth(), date2.getDate());
    return Math.round(Math.abs((d2.getTime() - d1.getTime()) / oneDay));
};

/**
 * Check if date is today
 */
export const isToday = (dateStr: string | Date): boolean => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    const today = new Date();
    return (
        date.getFullYear() === today.getFullYear() &&
        date.getMonth() === today.getMonth() &&
        date.getDate() === today.getDate()
    );
};

/**
 * Check if date is in the past
 */
export const isPastDate = (dateStr: string | Date): boolean => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    const today = getStartOfDayIST();
    return date < today;
};

/**
 * Check if date is in the future
 */
export const isFutureDate = (dateStr: string | Date): boolean => {
    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    const today = getEndOfDayIST();
    return date > today;
};

// ============================================
// FINANCIAL YEAR UTILITIES (India: April-March)
// ============================================

/**
 * Get current financial year
 */
export const getCurrentFinancialYear = (): { start: Date; end: Date; label: string } => {
    const now = new Date();
    const year = now.getFullYear();
    const month = now.getMonth(); // 0-indexed
    
    // If before April (month < 3), FY is previous year to current year
    // If April or later (month >= 3), FY is current year to next year
    const fyStartYear = month < 3 ? year - 1 : year;
    
    const start = new Date(fyStartYear, 3, 1); // April 1
    const end = new Date(fyStartYear + 1, 2, 31); // March 31
    
    const label = `FY ${fyStartYear}-${(fyStartYear + 1).toString().slice(-2)}`;
    
    return { start, end, label };
};

/**
 * Get all financial years between two dates
 */
export const getFinancialYears = (startDate: Date, endDate: Date): string[] => {
    const years: string[] = [];
    let current = new Date(startDate);
    
    while (current <= endDate) {
        const fy = getCurrentFinancialYear();
        const year = current.getMonth() < 3 ? current.getFullYear() - 1 : current.getFullYear();
        const label = `FY ${year}-${(year + 1).toString().slice(-2)}`;
        
        if (!years.includes(label)) {
            years.push(label);
        }
        
        current.setMonth(current.getMonth() + 1);
    }
    
    return years;
};

/**
 * Check if date falls in a specific financial year
 */
export const isInFinancialYear = (date: Date, fyLabel: string): boolean => {
    const match = fyLabel.match(/FY (\d{4})-(\d{2})/);
    if (!match) return false;
    
    const startYear = parseInt(match[1]);
    const fyStart = new Date(startYear, 3, 1);
    const fyEnd = new Date(startYear + 1, 2, 31, 23, 59, 59);
    
    return date >= fyStart && date <= fyEnd;
};

// ============================================
// UNIVERSAL FORMAT FUNCTION
// ============================================

/**
 * Format date with options
 */
export const formatDate = (
    dateStr: string | Date,
    options: DateFormatOptions = {}
): string => {
    const { format = 'display', includeTime = false, timezone = 'local' } = options;
    
    let date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
    if (isNaN(date.getTime())) return '-';
    
    if (timezone === 'utc') {
        date = new Date(date.toUTCString());
    }
    
    switch (format) {
        case 'input':
            return formatInputDate(date);
        case 'short':
            return formatShortDate(date);
        case 'long':
            return formatLongDate(date);
        case 'iso':
            return date.toISOString();
        default:
            if (includeTime) {
                return formatDateTime(date);
            }
            return formatDisplayDate(date);
    }
};

export default {
    getCurrentDateIST,
    toIST,
    getStartOfDayIST,
    getEndOfDayIST,
    formatDisplayDate,
    formatInputDate,
    formatShortDate,
    formatDateTime,
    formatLongDate,
    formatMarathiDate,
    getTodayDate,
    getTodayDisplayDate,
    parseDate,
    addDays,
    getDaysDifference,
    isToday,
    isPastDate,
    isFutureDate,
    getCurrentFinancialYear,
    getFinancialYears,
    isInFinancialYear,
    formatDate
};