life-helper/app/lib/domain/streak/compute_streak.dart

import '../models/tracker_entry.dart';

enum RewardTier { t0, t1, t2, t3, t4 }

extension RewardTierX on RewardTier {
  String get dbValue {
    switch (this) {
      case RewardTier.t0:
        return 'T0';
      case RewardTier.t1:
        return 'T1';
      case RewardTier.t2:
        return 'T2';
      case RewardTier.t3:
        return 'T3';
      case RewardTier.t4:
        return 'T4';
    }
  }

  int get rank => RewardTier.values.indexOf(this);
}

class StreakState {
  final int currentStreak;
  final int longestStreak;
  final int doneCountInPhase42;
  final int doneCountInWindow30;
  final RewardTier currentTier;
  final bool neverMissTwiceBroken;

  const StreakState({
    required this.currentStreak,
    required this.longestStreak,
    required this.doneCountInPhase42,
    required this.doneCountInWindow30,
    required this.currentTier,
    required this.neverMissTwiceBroken,
  });

  static const StreakState empty = StreakState(
    currentStreak: 0,
    longestStreak: 0,
    doneCountInPhase42: 0,
    doneCountInWindow30: 0,
    currentTier: RewardTier.t0,
    neverMissTwiceBroken: false,
  );
}

/// fn-compute-streak: computes streak + 5-tier milestone with Never miss twice.
///
/// OQ-5 decision (2026-06-11):
/// - 2+ consecutive blank → tier demoted (T3→T2, T2→T1, T1→T0), streak = 0,
///   neverMissTwiceBroken = true.
/// - 1 blank → streak = 0, tier stays. Next done starts at 1.
///
/// Pure function. [habitStartedAt] is the habit's started_at (YYYY-MM-DD).
StreakState computeStreak({
  required Iterable<TrackerEntryModel> entries,
  required DateTime asOf,
  required String habitStartedAt,
}) {
  // 1. Index by date string (YYYY-MM-DD).
  final byDate = <String, TrackerEntryModel>{};
  for (final e in entries) {
    byDate[e.date] = e;
  }

  if (byDate.isEmpty) return StreakState.empty;

  final startDate = DateTime.parse(habitStartedAt);
  final asOfDate = DateTime(asOf.year, asOf.month, asOf.day);

  // 2. currentStreak (Never miss twice).
  //
  // Semantics:
  // - Walk back from asOf, stopping at habit start.
  // - "No entry record" for a date means tracking hasn't reached that day (or
  //   user hasn't synced) — treat as the end of streak history, do not penalize.
  // - Explicit TrackerValue.blank is the penalty signal. 1 blank zeroes the
  //   streak (tier stays); 2 consecutive blanks set neverMissTwiceBroken.
  var streak = 0;
  var consecutiveBlank = 0;
  var neverMissTwiceBroken = false;
  var cursor = asOfDate;

  while (!cursor.isBefore(startDate)) {
    final e = byDate[_ymd(cursor)];
    if (e == null) {
      // End of recorded history.
      break;
    }
    if (e.value == TrackerValue.blank) {
      consecutiveBlank += 1;
      if (consecutiveBlank >= 2) {
        neverMissTwiceBroken = true;
        streak = 0;
        break;
      }
      // 1 blank so far: streak is broken, but keep walking one more day to
      // detect a possible double-blank.
      streak = 0;
      cursor = cursor.subtract(const Duration(days: 1));
      continue;
    }
    // Done.
    if (consecutiveBlank > 0) {
      // Previous step saw a single blank, now done → single-blank confirmed.
      break;
    }
    streak += 1;
    cursor = cursor.subtract(const Duration(days: 1));
  }

  // 3. longestStreak over all entries.
  final sortedDates = byDate.keys.toList()..sort();
  var longest = 0;
  var run = 0;
  for (final d in sortedDates) {
    if (byDate[d]!.value == TrackerValue.done) {
      run += 1;
      if (run > longest) longest = run;
    } else {
      run = 0;
    }
  }

  // 4. Window counts.
  final win30Start = asOfDate.subtract(const Duration(days: 29));
  final win42Start = asOfDate.subtract(const Duration(days: 41));
  var window30 = 0;
  var window42 = 0;
  for (final e in byDate.values) {
    if (e.value != TrackerValue.done) continue;
    final d = DateTime.parse(e.date);
    if (!d.isBefore(win30Start) && !d.isAfter(asOfDate)) window30 += 1;
    if (!d.isBefore(win42Start) && !d.isAfter(asOfDate)) window42 += 1;
  }

  // 5. Tier judgment.
  var tier = RewardTier.t0;
  if (streak >= 3) tier = RewardTier.t1;
  if (streak >= 7) tier = RewardTier.t2;
  if (window30 >= 24 && tier.rank < RewardTier.t3.rank) tier = RewardTier.t3;
  final phaseDay = asOfDate.difference(startDate).inDays + 1;
  if (phaseDay >= 42 && window42 >= 30) tier = RewardTier.t4;

  return StreakState(
    currentStreak: streak,
    longestStreak: longest,
    doneCountInPhase42: window42,
    doneCountInWindow30: window30,
    currentTier: tier,
    neverMissTwiceBroken: neverMissTwiceBroken,
  );
}

String _ymd(DateTime d) =>
    '${d.year.toString().padLeft(4, '0')}-'
    '${d.month.toString().padLeft(2, '0')}-'
    '${d.day.toString().padLeft(2, '0')}';