/* Pure helpers for streaks, totals, and the goal-window countdown. */

const DAY_MS = 86_400_000;

export function todayISO(d = new Date()) {
  const y = d.getFullYear();
  const m = String(d.getMonth() + 1).padStart(2, '0');
  const day = String(d.getDate()).padStart(2, '0');
  return `${y}-${m}-${day}`;
}

export function isoToDate(iso) {
  const [y, m, d] = iso.split('-').map(Number);
  return new Date(y, m - 1, d);
}

export function addDays(iso, n) {
  const d = isoToDate(iso);
  d.setDate(d.getDate() + n);
  return todayISO(d);
}

export function eachDay(startISO, endISO) {
  const out = [];
  let cur = startISO;
  while (cur <= endISO) {
    out.push(cur);
    cur = addDays(cur, 1);
  }
  return out;
}

export function clampWindowEnd(goal, refISO = todayISO()) {
  return goal.end_date < refISO ? goal.end_date : refISO;
}

export function daysRemaining(endISO, refISO = todayISO()) {
  if (endISO < refISO) return 0;
  return Math.round((isoToDate(endISO) - isoToDate(refISO)) / DAY_MS);
}

/**
 * Build a Set of completed ISO dates for (user, goal) from a flat completions array.
 */
export function completedSet(completions, user, goalId) {
  const set = new Set();
  for (const c of completions) {
    if (c.user === user && c.goal_id === goalId && c.completed) set.add(c.date);
  }
  return set;
}

/**
 * Current streak ending at (or just before) `refISO`, walking back day-by-day
 * while completions are present, never crossing the goal start.
 */
export function currentStreak(completedSetForUser, goal, refISO = todayISO()) {
  const start = goal.start_date;
  let end = clampWindowEnd(goal, refISO);
  if (refISO < start) return 0;

  // If today isn't completed yet and the goal is still active,
  // start from yesterday — the day isn't over so don't break the streak.
  if (end === refISO && !completedSetForUser.has(end)) {
    end = addDays(end, -1);
  }

  let cur = end;
  let n = 0;
  while (cur >= start && completedSetForUser.has(cur)) {
    n += 1;
    cur = addDays(cur, -1);
  }
  return n;
}

export function longestStreak(completedSetForUser, goal, refISO = todayISO()) {
  const start = goal.start_date;
  const end = clampWindowEnd(goal, refISO);
  if (end < start) return 0;
  let best = 0;
  let run = 0;
  for (const d of eachDay(start, end)) {
    if (completedSetForUser.has(d)) {
      run += 1;
      if (run > best) best = run;
    } else {
      run = 0;
    }
  }
  return best;
}

export function totalCompleted(completedSetForUser, goal, refISO = todayISO()) {
  const start = goal.start_date;
  const end = clampWindowEnd(goal, refISO);
  if (end < start) return 0;
  let n = 0;
  for (const d of eachDay(start, end)) {
    if (completedSetForUser.has(d)) n += 1;
  }
  return n;
}

export function windowLength(goal) {
  return eachDay(goal.start_date, goal.end_date).length;
}