namespace Mr.BrickAdventures.Progression;

/// <summary>
/// One game from New Game until game over. Several exist at once and the player picks between them;
/// "save slot" is the word the UI uses for the same thing.
/// </summary>
/// <remarks>
/// Deliberately exposes no bare mutators — no LoseLife, no AddCoins. An ended Attempt is settled,
/// and settling is atomic and idempotent because it defers to the Attempt's own terminal outcome.
/// See docs/adr/0005-playthrough-settles-attempts.md.
/// </remarks>
public sealed class Playthrough
{
    private readonly EventLog _events = new();

    private Playthrough(PlaythroughId id, LevelId currentLevel, Lives lives)
    {
        Id = id;
        CurrentLevel = currentLevel;
        Lives = lives;
    }

    public static Playthrough StartNew(PlaythroughId id, Campaign campaign, Lives startingLives) =>
        new(id, campaign.First, startingLives);

    public PlaythroughId Id { get; }

    public LevelId CurrentLevel { get; private set; }

    public Lives Lives { get; private set; }

    /// <summary>Coins this Playthrough holds. Only a Commit changes it — never a purchase.</summary>
    public Coins Bank { get; private set; } = Coins.Zero;

    private readonly Dictionary<SkillId, SkillLevel> _ownedSkills = new();

    public IReadOnlyDictionary<SkillId, SkillLevel> OwnedSkills => _ownedSkills;

    private readonly HashSet<LevelId> _clearedLevels = new();

    /// <summary>
    /// Levels this Playthrough has completed at least once. Reachability is derived from this and
    /// never stored beside it.
    /// </summary>
    public IReadOnlySet<LevelId> ClearedLevels => _clearedLevels;

    public bool HasCleared(LevelId level) => _clearedLevels.Contains(level);

    /// <summary>
    /// Levels this Playthrough can enter: the Campaign's first, everything cleared, and whatever
    /// follows each cleared Level. Computed on demand — storing it beside <see cref="ClearedLevels"/>
    /// would be a second source of the same truth.
    /// </summary>
    public IReadOnlySet<LevelId> ReachableLevels(Campaign campaign)
    {
        var reachable = new HashSet<LevelId> { campaign.First };

        foreach (var cleared in _clearedLevels)
        {
            reachable.Add(cleared);
            if (campaign.Next(cleared) is { } next)
                reachable.Add(next);
        }

        return reachable;
    }

    public bool IsRunOver => Lives.IsExhausted;

    /// <summary>
    /// Whether <paramref name="cost"/> can be met from the Bank plus what this Attempt has found,
    /// less what it has already spent. Spans both scopes, which is why it lives here rather than on
    /// the Attempt — the Attempt cannot see the Bank.
    /// </summary>
    public bool CanAfford(Attempt attempt, Coins cost) =>
        Bank.Add(attempt.CoinsFound).TrySubtract(attempt.CoinsSpent, out var available)
        && available.IsAtLeast(cost);

    /// <summary>
    /// Ends the given Attempt in completion, then Commits it if this is a First Clear or Discards it
    /// if it is a Replay. Decides which from the Attempt's own Level, so a Replay cannot advance
    /// campaign position (ADR-0003). Does nothing if the Attempt has already ended.
    /// </summary>
    public void SettleCompletion(Attempt attempt, Campaign campaign)
    {
        var isFirstClear = !HasCleared(attempt.Level);

        if (!attempt.TryEndInCompletion()) return;
        if (!isFirstClear) return;

        if (!Bank.Add(attempt.CoinsFound).TrySubtract(attempt.CoinsSpent, out var settled))
            throw new InvalidOperationException(
                "Settling this attempt would drive the bank negative, which means affordability was never checked.");

        Bank = settled;

        foreach (var (skill, level) in attempt.SkillsBought)
            _ownedSkills[skill] = level;

        _clearedLevels.Add(attempt.Level);

        if (campaign.Next(attempt.Level) is { } next)
            CurrentLevel = next;
    }

    /// <summary>
    /// Ends the given Attempt in death and charges one life for it. Does nothing if the Attempt has
    /// already ended, so a repeated death event costs no further lives.
    /// </summary>
    public void SettleDeath(Attempt attempt)
    {
        if (!attempt.TryEndInDeath()) return;

        Lives = Lives.Decrement();

        if (Lives.IsExhausted)
            _events.Raise(new RunEnded());
    }

    /// <summary>
    /// Takes the events raised since the last drain, clearing them. The composition root drains
    /// once after each use case and republishes.
    /// </summary>
    public IReadOnlyList<ProgressionEvent> DrainEvents() => _events.Drain();
}