// Pure streak logic over local calendar days — no React, no storage. Days are
// "YYYY-MM-DD" keys in the device's local time zone, so a session at 23:50
// counts for the day it was actually done, not the UTC one.

export type DayKey = string;

export function dayKey(date: Date): DayKey {
  const y = date.getFullYear();
  const m = String(date.getMonth() + 1).padStart(2, "0");
  const d = String(date.getDate()).padStart(2, "0");
  return `${y}-${m}-${d}`;
}

/** Local-midnight Date for a day key. */
export function fromKey(key: DayKey): Date {
  const [y, m, d] = key.split("-").map(Number);
  return new Date(y, m - 1, d);
}

function shiftKey(key: DayKey, days: number): DayKey {
  const date = fromKey(key);
  date.setDate(date.getDate() + days);
  return dayKey(date);
}

/** Unique, ascending day keys from a list of ISO session timestamps. */
export function sessionDays(sessionsIso: string[]): DayKey[] {
  return Array.from(new Set(sessionsIso.map((iso) => dayKey(new Date(iso))))).sort();
}

/** The last `n` day keys ending on `today`, oldest first. */
export function lastNDays(n: number, today: DayKey): DayKey[] {
  const out: DayKey[] = [];
  for (let i = n - 1; i >= 0; i--) out.push(shiftKey(today, -i));
  return out;
}

/**
 * Current streak = consecutive trained days ending today or yesterday (a
 * streak isn't broken until a full day is missed). Best = longest run ever.
 */
export function computeStreaks(days: DayKey[], today: DayKey): { current: number; best: number } {
  const sorted = Array.from(new Set(days)).sort();
  if (sorted.length === 0) return { current: 0, best: 0 };

  let best = 1;
  let run = 1;
  for (let i = 1; i < sorted.length; i++) {
    run = shiftKey(sorted[i - 1], 1) === sorted[i] ? run + 1 : 1;
    if (run > best) best = run;
  }

  const last = sorted[sorted.length - 1];
  const alive = last === today || last === shiftKey(today, -1);
  return { current: alive ? run : 0, best };
}