// The business day for every tenant is the Kenyan calendar day
// (Africa/Nairobi, EAT). EAT is a fixed UTC+3 with no daylight saving, so a
// constant offset is exact — no timezone database needed.
//
// Never use `new Date().setHours(0)` or `toISOString().slice(0, 10)` for
// "today" on the server: on Vercel the server clock is UTC, which would
// start the business day at 03:00 EAT and push early-morning sales onto
// the previous day.

const EAT_OFFSET_MS = 3 * 60 * 60 * 1000;
const DAY_MS = 24 * 60 * 60 * 1000;
const DATE_RE = /^\d{4}-\d{2}-\d{2}$/;

/** Today's business day as YYYY-MM-DD. */
export function todayNairobi(now: Date = new Date()): string {
  return new Date(now.getTime() + EAT_OFFSET_MS).toISOString().slice(0, 10);
}

export function isBusinessDate(value: string): boolean {
  if (!DATE_RE.test(value)) return false;
  const d = new Date(value + "T00:00:00.000Z");
  return !Number.isNaN(d.getTime()) && d.toISOString().slice(0, 10) === value;
}

/** The UTC instants [start, end) covering one business day. */
export function dayRangeNairobi(date: string): { start: Date; end: Date } {
  if (!isBusinessDate(date)) throw new Error(`Invalid business date: ${date}`);
  const start = new Date(new Date(date + "T00:00:00.000Z").getTime() - EAT_OFFSET_MS);
  return { start, end: new Date(start.getTime() + DAY_MS) };
}

/** Start of the business day containing `now`. */
export function startOfTodayNairobi(now: Date = new Date()): Date {
  return dayRangeNairobi(todayNairobi(now)).start;
}

/** Start of the current business month (1st, 00:00 EAT). */
export function startOfMonthNairobi(now: Date = new Date()): Date {
  return dayRangeNairobi(todayNairobi(now).slice(0, 8) + "01").start;
}