wordle/server/src/daily-word.ts

/**
 * Compute the daily word ID deterministically from the current date.
 * Timezone: America/New_York (EST/EDT).
 *
 * Formula: daysSinceEpoch mod targetCount (0-indexed)
 * Epoch: 2026-01-01 (arbitrary fixed reference)
 *
 * Uses Intl.DateTimeFormat to extract the date in the target timezone,
 * avoiding toLocaleString+re-parse pitfalls that break when the server
 * timezone differs from America/New_York.
 */

const EPOCH_DATE = new Date('2026-01-01T00:00:00-05:00'); // EST

function getDateInTz(date: Date, tz: string): string {
  // en-CA locale gives YYYY-MM-DD format
  return new Intl.DateTimeFormat('en-CA', {
    timeZone: tz,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
  }).format(date);
}

function daysBetween(a: string, b: string): number {
  // Parse YYYY-MM-DD as UTC midnight for consistent arithmetic
  const da = new Date(a + 'T00:00:00Z').getTime();
  const db = new Date(b + 'T00:00:00Z').getTime();
  return Math.floor((da - db) / 86_400_000);
}

function getDaysSinceEpoch(now: Date): number {
  const nyDate = getDateInTz(now, 'America/New_York');
  const epochDate = getDateInTz(EPOCH_DATE, 'America/New_York');
  return daysBetween(nyDate, epochDate);
}

export function getDailyWordId(targetCount: number, now: Date = new Date()): number {
  if (targetCount <= 0) {
    throw new Error('targetCount must be positive');
  }
  const days = getDaysSinceEpoch(now);
  return days % targetCount;
}