export type SearchDateParameter = "since" | "before";
type RelativeDateUnit = "day" | "week" | "month" | "year";

function startOfLocalDay(date: Date): Date {
  const result = new Date(date);
  result.setHours(0, 0, 0, 0);
  return result;
}

function shiftCalendarMonths(date: Date, months: number): Date {
  const result = startOfLocalDay(date);
  const originalDay = result.getDate();

  result.setDate(1);
  result.setMonth(result.getMonth() + months);

  const lastDay = new Date(result.getFullYear(), result.getMonth() + 1, 0).getDate();
  result.setDate(Math.min(originalDay, lastDay));
  return result;
}

/**
 * Parse exact and relative dates accepted by email_search.
 *
 * Supported examples:
 * - 2026-08-05
 * - today / yesterday / tomorrow
 * - 2 days ago / a month ago
 * - last week / last 3 months
 */
export function parseEmailSearchDate(
  input: string,
  parameter: SearchDateParameter,
  now: Date = new Date(),
): Date {
  const raw = input.trim();
  const normalized = raw.toLowerCase().replace(/\s+/g, " ");
  if (!normalized) throw new Error(`${parameter} date cannot be empty.`);

  const today = startOfLocalDay(now);

  if (normalized === "today") return today;
  if (normalized === "yesterday") {
    const result = new Date(today);
    result.setDate(result.getDate() - 1);
    return result;
  }
  if (normalized === "tomorrow") {
    const result = new Date(today);
    result.setDate(result.getDate() + 1);
    return result;
  }

  // Bare YYYY-MM-DD is parsed explicitly as local time instead of UTC.
  const isoMatch = /^(\d{4})-(\d{2})-(\d{2})$/.exec(normalized);
  if (isoMatch) {
    const year = Number(isoMatch[1]);
    const month = Number(isoMatch[2]);
    const day = Number(isoMatch[3]);
    const result = new Date(year, month - 1, day);
    result.setHours(0, 0, 0, 0);

    if (
      result.getFullYear() !== year ||
      result.getMonth() !== month - 1 ||
      result.getDate() !== day
    ) {
      throw new Error(
        `Invalid ${parameter} date "${raw}". Use a real date in YYYY-MM-DD format.`,
      );
    }
    return result;
  }

  let amount: number | undefined;
  let unit: RelativeDateUnit | undefined;

  const agoMatch = /^(?:(\d+)|a|an|one)\s+(day|week|month|year)s?\s+ago$/.exec(
    normalized,
  );
  if (agoMatch) {
    amount = agoMatch[1] ? Number(agoMatch[1]) : 1;
    unit = agoMatch[2] as RelativeDateUnit;
  } else {
    const lastMatch = /^last\s+(?:(\d+)\s+)?(day|week|month|year)s?$/.exec(normalized);
    if (lastMatch) {
      amount = lastMatch[1] ? Number(lastMatch[1]) : 1;
      unit = lastMatch[2] as RelativeDateUnit;
    }
  }

  if (amount === undefined || unit === undefined) {
    throw new Error(
      `Unsupported ${parameter} date "${raw}". Use YYYY-MM-DD, today, yesterday, ` +
        `"2 weeks ago", or "last 2 weeks".`,
    );
  }
  if (!Number.isSafeInteger(amount) || amount < 0) {
    throw new Error(`Invalid ${parameter} date quantity in "${raw}".`);
  }

  let result: Date;
  switch (unit) {
    case "day":
      result = new Date(today);
      result.setDate(result.getDate() - amount);
      break;
    case "week":
      result = new Date(today);
      result.setDate(result.getDate() - amount * 7);
      break;
    case "month":
      result = shiftCalendarMonths(today, -amount);
      break;
    case "year":
      result = shiftCalendarMonths(today, -amount * 12);
      break;
  }

  if (Number.isNaN(result.getTime())) {
    throw new Error(`The ${parameter} date "${raw}" is outside the supported range.`);
  }
  return result;
}

export function validateDateRange(since?: Date, before?: Date): void {
  if (since && before && since.getTime() >= before.getTime()) {
    throw new Error(
      `Invalid date range: since (${since.toISOString()}) must be earlier than before (${before.toISOString()}).`,
    );
  }
}

export type SearchDateParameter = "since" | "before";
type RelativeDateUnit = "day" | "week" | "month" | "year";

function startOfLocalDay(date: Date): Date {
  const result = new Date(date);
  result.setHours(0, 0, 0, 0);
  return result;
}

function shiftCalendarMonths(date: Date, months: number): Date {
  const result = startOfLocalDay(date);
  const originalDay = result.getDate();

  result.setDate(1);
  result.setMonth(result.getMonth() + months);

  const lastDay = new Date(result.getFullYear(), result.getMonth() + 1, 0).getDate();
  result.setDate(Math.min(originalDay, lastDay));
  return result;
}

/**
 * Parse exact and relative dates accepted by email_search.
 *
 * Supported examples:
 * - 2026-08-05
 * - today / yesterday / tomorrow
 * - 2 days ago / a month ago
 * - last week / last 3 months
 */
export function parseEmailSearchDate(
  input: string,
  parameter: SearchDateParameter,
  now: Date = new Date(),
): Date {
  const raw = input.trim();
  const normalized = raw.toLowerCase().replace(/\s+/g, " ");
  if (!normalized) throw new Error(`${parameter} date cannot be empty.`);

  const today = startOfLocalDay(now);

  if (normalized === "today") return today;
  if (normalized === "yesterday") {
    const result = new Date(today);
    result.setDate(result.getDate() - 1);
    return result;
  }
  if (normalized === "tomorrow") {
    const result = new Date(today);
    result.setDate(result.getDate() + 1);
    return result;
  }

  // Bare YYYY-MM-DD is parsed explicitly as local time instead of UTC.
  const isoMatch = /^(\d{4})-(\d{2})-(\d{2})$/.exec(normalized);
  if (isoMatch) {
    const year = Number(isoMatch[1]);
    const month = Number(isoMatch[2]);
    const day = Number(isoMatch[3]);
    const result = new Date(year, month - 1, day);
    result.setHours(0, 0, 0, 0);

    if (
      result.getFullYear() !== year ||
      result.getMonth() !== month - 1 ||
      result.getDate() !== day
    ) {
      throw new Error(
        `Invalid ${parameter} date "${raw}". Use a real date in YYYY-MM-DD format.`,
      );
    }
    return result;
  }

  let amount: number | undefined;
  let unit: RelativeDateUnit | undefined;

  const agoMatch = /^(?:(\d+)|a|an|one)\s+(day|week|month|year)s?\s+ago$/.exec(
    normalized,
  );
  if (agoMatch) {
    amount = agoMatch[1] ? Number(agoMatch[1]) : 1;
    unit = agoMatch[2] as RelativeDateUnit;
  } else {
    const lastMatch = /^last\s+(?:(\d+)\s+)?(day|week|month|year)s?$/.exec(normalized);
    if (lastMatch) {
      amount = lastMatch[1] ? Number(lastMatch[1]) : 1;
      unit = lastMatch[2] as RelativeDateUnit;
    }
  }

  if (amount === undefined || unit === undefined) {
    throw new Error(
      `Unsupported ${parameter} date "${raw}". Use YYYY-MM-DD, today, yesterday, ` +
        `"2 weeks ago", or "last 2 weeks".`,
    );
  }
  if (!Number.isSafeInteger(amount) || amount < 0) {
    throw new Error(`Invalid ${parameter} date quantity in "${raw}".`);
  }

  let result: Date;
  switch (unit) {
    case "day":
      result = new Date(today);
      result.setDate(result.getDate() - amount);
      break;
    case "week":
      result = new Date(today);
      result.setDate(result.getDate() - amount * 7);
      break;
    case "month":
      result = shiftCalendarMonths(today, -amount);
      break;
    case "year":
      result = shiftCalendarMonths(today, -amount * 12);
      break;
  }

  if (Number.isNaN(result.getTime())) {
    throw new Error(`The ${parameter} date "${raw}" is outside the supported range.`);
  }
  return result;
}

export function validateDateRange(since?: Date, before?: Date): void {
  if (since && before && since.getTime() >= before.getTime()) {
    throw new Error(
      `Invalid date range: since (${since.toISOString()}) must be earlier than before (${before.toISOString()}).`,
    );
  }
}