using System;

namespace AibisDream
{
    public enum GameSessionPhase
    {
        MainMenu,
        Starting,
        Transitioning,
        Restoring,
        Playing,
        ReturningToMenu
    }

    /// <summary>
    /// 单次游戏会话的纯状态模型。它不执行流程，也不发送全局事件。
    /// </summary>
    public sealed class GameSession
    {
        public GameSessionPhase Phase { get; private set; } = GameSessionPhase.MainMenu;
        public TalkSceneSO CurrentTalkScene { get; private set; }
        public bool IsPaused { get; private set; }
        public bool IsDialogueActive { get; private set; }

        public bool IsActive => Phase != GameSessionPhase.MainMenu;

        public bool IsBusy => Phase is GameSessionPhase.Starting
            or GameSessionPhase.Transitioning
            or GameSessionPhase.Restoring
            or GameSessionPhase.ReturningToMenu;

        public bool CanPause => Phase == GameSessionPhase.Playing && !IsPaused;
        public bool CanResume => Phase == GameSessionPhase.Playing && IsPaused;
        public bool CanAdvanceDialogue => Phase == GameSessionPhase.Playing && IsDialogueActive && !IsPaused;

        internal bool TryTransitionTo(GameSessionPhase next, out string error)
        {
            if (Phase == next)
            {
                error = null;
                return true;
            }

            if (!IsTransitionAllowed(Phase, next))
            {
                error = $"Illegal game session transition: {Phase} -> {next}";
                return false;
            }

            Phase = next;
            error = null;
            return true;
        }

        internal void SetCurrentTalkScene(TalkSceneSO scene)
        {
            CurrentTalkScene = scene;
        }

        internal void SetPaused(bool paused)
        {
            IsPaused = paused;
        }

        internal void SetDialogueActive(bool active)
        {
            IsDialogueActive = active;
        }

        internal void ResetRuntimeState()
        {
            CurrentTalkScene = null;
            IsPaused = false;
            IsDialogueActive = false;
        }

        private static bool IsTransitionAllowed(GameSessionPhase current, GameSessionPhase next)
        {
            return current switch
            {
                GameSessionPhase.MainMenu => next is GameSessionPhase.Starting or GameSessionPhase.Restoring,
                GameSessionPhase.Starting => next is GameSessionPhase.Playing or GameSessionPhase.ReturningToMenu,
                GameSessionPhase.Transitioning => next is GameSessionPhase.Playing or GameSessionPhase.ReturningToMenu,
                GameSessionPhase.Restoring => next is GameSessionPhase.Playing
                    or GameSessionPhase.MainMenu
                    or GameSessionPhase.ReturningToMenu,
                GameSessionPhase.Playing => next is GameSessionPhase.Transitioning
                    or GameSessionPhase.Restoring
                    or GameSessionPhase.ReturningToMenu,
                GameSessionPhase.ReturningToMenu => next == GameSessionPhase.MainMenu,
                _ => throw new ArgumentOutOfRangeException(nameof(current), current, null)
            };
        }
    }
}