import {
  format,
  formatDistanceToNow,
  isThisMonth,
  isThisWeek,
  isToday,
  isValid,
  isYesterday,
  parse,
  startOfMonth,
  startOfWeek,
  startOfYear,
  subMonths,
} from 'date-fns';
import { MAX_RELATIVE_DAYS } from './constants';

function isDateOnlyInput(input: string): boolean {
  return /^\d{4}-\d{2}-\d{2}$/.test(input.trim());
}

/**
 * Parse date string from various formats
 */
export function parseDateInput(input: string): Date | null {
  // Try ISO format
  const iso = parse(input, 'yyyy-MM-dd', new Date());
  if (isValid(iso)) return iso;

  // Try with time
  const isoWithTime = parse(input, "yyyy-MM-dd'T'HH:mm:ss", new Date());
  if (isValid(isoWithTime)) return isoWithTime;

  // Try relative dates
  const lower = input.toLowerCase().trim();

  if (lower === 'today') {
    const today = new Date();
    today.setHours(0, 0, 0, 0);
    return today;
  }

  if (lower === 'yesterday') {
    const yesterday = new Date();
    yesterday.setDate(yesterday.getDate() - 1);
    yesterday.setHours(0, 0, 0, 0);
    return yesterday;
  }

  if (lower === 'week' || lower === 'this week') {
    return startOfWeek(new Date());
  }

  if (lower === 'month' || lower === 'this month') {
    return startOfMonth(new Date());
  }

  if (lower === 'year' || lower === 'this year') {
    return startOfYear(new Date());
  }

  // Try "N days ago"
  const daysAgoMatch = lower.match(/^(\d+)\s*days?\s*ago$/);
  if (daysAgoMatch) {
    const [, daysText] = daysAgoMatch;
    if (!daysText) return null;
    const days = parseInt(daysText, 10);
    if (days > MAX_RELATIVE_DAYS) return null;
    const date = new Date();
    date.setDate(date.getDate() - days);
    date.setHours(0, 0, 0, 0);
    return date;
  }

  // Try "N weeks ago"
  const weeksAgoMatch = lower.match(/^(\d+)\s*weeks?\s*ago$/);
  if (weeksAgoMatch) {
    const [, weeksText] = weeksAgoMatch;
    if (!weeksText) return null;
    const weeks = parseInt(weeksText, 10);
    if (weeks * 7 > MAX_RELATIVE_DAYS) return null;
    const date = new Date();
    date.setDate(date.getDate() - weeks * 7);
    date.setHours(0, 0, 0, 0);
    return date;
  }

  // Try "N months ago"
  const monthsAgoMatch = lower.match(/^(\d+)\s*months?\s*ago$/);
  if (monthsAgoMatch) {
    const [, monthsText] = monthsAgoMatch;
    if (!monthsText) return null;
    const months = parseInt(monthsText, 10);
    if (months * 30 > MAX_RELATIVE_DAYS) return null;
    // Use subMonths (not setMonth) so month-ends clamp instead of overflow
    // (e.g. Mar 31 minus 1 month -> Feb 28, not Mar 3).
    const date = subMonths(new Date(), months);
    date.setHours(0, 0, 0, 0);
    return date;
  }

  return null;
}

/**
 * Format date for display
 */
export function formatDateDisplay(date: Date | number | null | undefined): string {
  if (date == null) return 'Unknown';

  const d = typeof date === 'number' ? new Date(date) : date;
  if (!isValid(d)) return 'Invalid date';

  if (isToday(d)) {
    return `Today at ${format(d, 'HH:mm')}`;
  }

  if (isYesterday(d)) {
    return `Yesterday at ${format(d, 'HH:mm')}`;
  }

  if (isThisWeek(d)) {
    return format(d, "EEEE 'at' HH:mm");
  }

  if (isThisMonth(d)) {
    return format(d, "MMM d 'at' HH:mm");
  }

  return format(d, 'MMM d, yyyy');
}

/**
 * Format relative time
 */
export function formatRelativeTime(date: Date | number | null | undefined): string {
  if (date == null) return 'Unknown';

  const d = typeof date === 'number' ? new Date(date) : date;
  if (!isValid(d)) return 'Invalid date';

  return formatDistanceToNow(d, { addSuffix: true });
}

/**
 * Format date for export
 */
export function formatDateExport(date: Date | number | null | undefined): string {
  if (date == null) return '';

  const d = typeof date === 'number' ? new Date(date) : date;
  if (!isValid(d)) return '';

  return format(d, "yyyy-MM-dd'T'HH:mm:ssxxx");
}

/**
 * Parse date range string
 */
export function parseDateRange(range: string): { start?: Date; end?: Date } | null {
  // Single date
  const single = parseDateInput(range);
  if (single) {
    const end = new Date();
    end.setHours(23, 59, 59, 999);
    return { start: single, end };
  }

  // Date range with ".." or " to "
  const rangeMatch = range.match(/^(.+?)\s*(?:\.\.|\s+to\s+)(.+)$/i);
  if (rangeMatch) {
    const [, startText, endText] = rangeMatch;
    if (!startText || !endText) return null;
    const start = parseDateInput(startText.trim());
    const end = parseDateInput(endText.trim());

    if (start && end) {
      // A calendar date means its whole day; an explicit timestamp at midnight
      // remains a precise endpoint.
      if (isDateOnlyInput(endText)) {
        end.setHours(23, 59, 59, 999);
      }
      return { start, end };
    }
  }

  return null;
}