// Backlog query helpers for the ADHD scheduling core.
//
// The backlog is not a separate database table in this starter model. It is a
// view over tasks whose `Task.status` is `TaskStatus.backlog`. This file keeps
// display-oriented backlog filtering, sorting, and age markers out of the main
// scheduler so the engine can focus on moving tasks through time.

import 'models.dart';

/// Derived backlog filters for a unified backlog list.
///
/// These filters do not store separate task collections. They are projections
/// over the same master task list. That is important because a task can move
/// between today's timeline and backlog by changing [Task.status], without
/// needing to copy it between separate stores.
enum BacklogFilter {
  /// Uncategorized captured tasks in the default inbox project.
  inbox,

  /// Tasks that have been manually or automatically pushed at least once.
  pushed,

  /// Critical tasks that have missed at least once and need recovery attention.
  criticalMissed,

  /// Someday/maybe tasks that are intentionally kept out of normal pressure.
  wishlist,

  /// Tasks whose [Task.updatedAt] age exceeds the configured stale threshold.
  stale,

  /// Tasks still missing a reward estimate. Useful during cleanup/review.
  noRewardSet,
}

/// Sort options for a unified backlog list.
///
/// Sort keys are intentionally product-facing rather than database-facing. For
/// example, `rewardVsEffort` maps to a simple derived score instead of a stored
/// field. Persistence can later index the underlying fields if needed.
enum BacklogSortKey {
  /// Highest priority first.
  priority,

  /// Best simple reward-minus-difficulty score first.
  rewardVsEffort,

  /// Oldest created task first.
  age,

  /// Lexicographic project id grouping. Future UI can replace this with project
  /// display order while keeping the same public key.
  project,

  /// Most frequently pushed tasks first.
  timesPushed,
}

/// Visual age bucket for backlog display.
///
/// This supports the design rule that old backlog items should visually age
/// from green to blue to purple. The enum names describe semantic buckets; UI
/// code should translate them into actual theme colors.
enum BacklogStalenessMarker {
  /// Fresh backlog item. Default: created within seven days.
  green,

  /// Aging backlog item. Default: created within thirty days.
  blue,

  /// Old/stale backlog item. Default: created more than thirty days ago.
  purple,
}

/// Configurable thresholds for backlog age markers.
///
/// The defaults match the current design spec: less than a week is fresh, less
/// than a month is aging, and anything older is stale. Keeping the thresholds in
/// a value object makes future settings/preferences easy to inject in tests or
/// user configuration.
class BacklogStalenessSettings {
  const BacklogStalenessSettings({
    this.greenMaxAge = const Duration(days: 7),
    this.blueMaxAge = const Duration(days: 30),
  });

  /// Maximum age that still counts as fresh/green.
  final Duration greenMaxAge;

  /// Maximum age that still counts as aging/blue. Anything older is purple.
  final Duration blueMaxAge;

  /// Return the visual age marker for [task] relative to [now].
  ///
  /// This uses [Task.createdAt], not [Task.updatedAt], because the marker is
  /// meant to show how long the idea has existed in the system. A task edited
  /// yesterday but created two months ago should still feel old in the backlog.
  BacklogStalenessMarker markerFor({
    required Task task,
    required DateTime now,
  }) {
    final age = now.difference(task.createdAt);

    if (age <= greenMaxAge) {
      return BacklogStalenessMarker.green;
    }

    if (age <= blueMaxAge) {
      return BacklogStalenessMarker.blue;
    }

    return BacklogStalenessMarker.purple;
  }
}

/// Read-only backlog projection over the unified task list.
///
/// [BacklogView] is a query/helper object. It does not mutate tasks or own data;
/// it receives the current task list and exposes common backlog slices for UI.
/// That keeps backlog display logic out of widgets and avoids duplicating the
/// same filtering rules in multiple screens.
class BacklogView {
  BacklogView({
    required List<Task> tasks,
    required this.now,
    this.staleAfter = const Duration(days: 31),
    this.stalenessSettings = const BacklogStalenessSettings(),
  }) : tasks = List<Task>.unmodifiable(tasks);

  /// Master task list supplied by the caller. Only `status == backlog` items are
  /// shown by this view.
  final List<Task> tasks;

  /// Clock value supplied by the caller so age/staleness behavior is testable.
  final DateTime now;

  /// Age since [Task.createdAt] that qualifies for the `stale` filter.
  ///
  /// V1 does not yet store a separate "entered backlog at" timestamp. Until
  /// persistence adds that field, both stale filtering and visual staleness use
  /// task creation age so they do not disagree after a task edit.
  final Duration staleAfter;

  /// Color-bucket threshold configuration for backlog aging indicators.
  final BacklogStalenessSettings stalenessSettings;

  /// All tasks currently in backlog status.
  ///
  /// The returned list is a snapshot. It is not intended to be modified and then
  /// written back; state changes should go through scheduling/action services.
  List<Task> get backlogTasks {
    return tasks.where((task) => task.isBacklog).toList(growable: false);
  }

  /// Return backlog tasks matching a single user-facing filter.
  ///
  /// Filtering always starts from [backlogTasks], so a completed or planned task
  /// will never appear here even if it has matching statistics.
  List<Task> filter(BacklogFilter filter) {
    return backlogTasks.where((task) => _matchesFilter(task, filter)).toList(
          growable: false,
        );
  }

  /// Return all backlog tasks sorted by a user-facing ordering.
  ///
  /// A new list is created before sorting so the original [tasks] list is never
  /// reordered by a read operation. The final list is unmodifiable to make that
  /// intent explicit to callers.
  List<Task> sorted(BacklogSortKey sortKey) {
    final sortedTasks = backlogTasks
        .asMap()
        .entries
        .map(
          (entry) => _IndexedTask(
            task: entry.value,
            originalIndex: entry.key,
          ),
        )
        .toList(growable: false);
    sortedTasks.sort((a, b) {
      final comparison = _compareTasks(a.task, b.task, sortKey);
      if (comparison != 0) {
        return comparison;
      }

      return a.originalIndex.compareTo(b.originalIndex);
    });

    return List<Task>.unmodifiable(
      sortedTasks.map((indexedTask) => indexedTask.task),
    );
  }

  /// Return the green/blue/purple marker for one task.
  BacklogStalenessMarker stalenessMarkerFor(Task task) {
    return stalenessSettings.markerFor(task: task, now: now);
  }

  /// Private predicate implementing every [BacklogFilter] option.
  ///
  /// Keeping this as a switch expression makes new filters obvious: add the enum
  /// value and the compiler forces this method to handle it.
  bool _matchesFilter(Task task, BacklogFilter filter) {
    return switch (filter) {
      BacklogFilter.inbox => task.projectId == 'inbox',
      BacklogFilter.pushed =>
        task.stats.manuallyPushedCount > 0 || task.stats.autoPushedCount > 0,
      BacklogFilter.criticalMissed =>
        task.type == TaskType.critical && task.stats.missedCount > 0,
      BacklogFilter.wishlist => task.backlogTags.contains(BacklogTag.wishlist),
      BacklogFilter.stale => now.difference(task.createdAt) >= staleAfter,
      BacklogFilter.noRewardSet => task.reward == RewardLevel.notSet,
    };
  }

  /// Comparison callback used by [sorted].
  ///
  /// Sort directions are encoded here. Higher priority/reward/push counts should
  /// appear earlier, while older age uses the earliest [Task.createdAt] first.
  int _compareTasks(Task a, Task b, BacklogSortKey sortKey) {
    return switch (sortKey) {
      BacklogSortKey.priority =>
        _priorityRank(b.priority).compareTo(_priorityRank(a.priority)),
      BacklogSortKey.rewardVsEffort =>
        _rewardVsEffortScore(b).compareTo(_rewardVsEffortScore(a)),
      BacklogSortKey.age => a.createdAt.compareTo(b.createdAt),
      BacklogSortKey.project => a.projectId.compareTo(b.projectId),
      BacklogSortKey.timesPushed => _timesPushed(b).compareTo(_timesPushed(a)),
    };
  }
}

/// Convert nullable priority into a stable numeric rank for sorting.
///
/// Null priority is treated like medium so partially imported data behaves like
/// normal starter tasks instead of sinking to the bottom.
int _priorityRank(PriorityLevel? priority) {
  return switch (priority) {
    PriorityLevel.veryLow => 0,
    PriorityLevel.low => 1,
    PriorityLevel.medium || null => 2,
    PriorityLevel.high => 3,
    PriorityLevel.veryHigh => 4,
  };
}

/// Convert reward enum values to numeric ranks for derived scoring.
int _rewardRank(RewardLevel reward) {
  return switch (reward) {
    RewardLevel.notSet => 0,
    RewardLevel.veryLow => 1,
    RewardLevel.low => 2,
    RewardLevel.medium => 3,
    RewardLevel.high => 4,
    RewardLevel.veryHigh => 5,
  };
}

/// Convert difficulty enum values to numeric ranks for derived scoring.
int _difficultyRank(DifficultyLevel difficulty) {
  return switch (difficulty) {
    DifficultyLevel.notSet => 0,
    DifficultyLevel.veryEasy => 1,
    DifficultyLevel.easy => 2,
    DifficultyLevel.medium => 3,
    DifficultyLevel.hard => 4,
    DifficultyLevel.veryHard => 5,
  };
}

/// Simple motivation score: reward minus difficulty.
///
/// Positive scores suggest high payoff for lower activation cost. Negative scores
/// suggest high effort for lower payoff. This is deliberately simple for V1 and
/// can be replaced by richer heuristics later without changing the public sort
/// key.
int _rewardVsEffortScore(Task task) {
  return _rewardRank(task.reward) - _difficultyRank(task.difficulty);
}

/// Total manual and automatic pushes recorded on the task.
int _timesPushed(Task task) {
  return task.stats.manuallyPushedCount + task.stats.autoPushedCount;
}

/// Backlog task paired with its source-list position for stable sorting.
class _IndexedTask {
  const _IndexedTask({
    required this.task,
    required this.originalIndex,
  });

  final Task task;
  final int originalIndex;
}