using System.Collections.Generic;
using System;
using UnityEngine;

namespace Ichni.Menu
{
    public class DifficultySelectionContainer : MonoBehaviour
    {
        public List<DifficultySelectionButton> buttons;
        public DifficultySelectionButton selectedButton;

        /// <summary>
        /// 在当前歌曲中解析一个可实际游玩的难度。
        /// <para>preferredListIndex 是 <see cref="SongSelectionRecord.difficultyListIndex"/>，即难度在当前歌曲
        /// List 中的位置；它不能替代 <see cref="DifficultyData.saveDifficultyId"/>，后者是游戏记录的稳定 ID。</para>
        /// <para>规则：只考虑已配置 Button 且 <c>isAvailable</c> 为 true 的难度，优先选择与目标位置距离最小者；
        /// 距离相同则选择位置较低者。因此目标难度不存在时，玩家会落到最接近、且偏低的可用难度。</para>
        /// </summary>
        private bool TryResolveAvailableDifficultyIndex(
            IReadOnlyList<DifficultyData> difficulties,
            int preferredListIndex,
            out int resolvedListIndex)
        {
            resolvedListIndex = -1;
            if (difficulties == null || buttons == null)
            {
                return false;
            }

            int candidateCount = Mathf.Min(difficulties.Count, buttons.Count);
            long bestDistance = long.MaxValue;

            for (int index = 0; index < candidateCount; index++)
            {
                DifficultyData difficulty = difficulties[index];
                if (difficulty == null || !difficulty.isAvailable || buttons[index] == null)
                {
                    continue;
                }

                long distance = Math.Abs((long)index - preferredListIndex);
                if (distance < bestDistance || (distance == bestDistance && index < resolvedListIndex))
                {
                    bestDistance = distance;
                    resolvedListIndex = index;
                }
            }

            return resolvedListIndex >= 0;
        }

        /// <summary>
        /// 清除当前的难度选择，并同时关闭标准的进入游戏按钮。
        /// SongSelectionTab 的快速进入入口也会单独检查 <c>selectedDifficulty</c>，避免绕过此 UI 状态。
        /// </summary>
        private void ClearDifficultySelection()
        {
            selectedButton?.Deselect();
            selectedButton = null;

            SongSelectionUIPage songSelectionUIPage = MenuManager.instance.songSelectionUIPage;
            songSelectionUIPage.selectedDifficulty = null;
            songSelectionUIPage.playSongUI.SetEnterGameAvailable(false);
        }

        /// <summary>
        /// 根据当前歌曲刷新难度按钮，并将章节内存缓存解析为一个实际可玩的难度。
        /// 该方法用于首次进入、从游戏返回以及拖动切歌，所以所有自动难度回退都必须经过此处。
        /// </summary>
        public void SetUp(List<DifficultyData> difficulties)
        {
            int difficultyCount = difficulties?.Count ?? 0;
            int buttonCount = buttons?.Count ?? 0;

            ClearDifficultySelection();

            if (difficultyCount > buttonCount)
            {
                Debug.LogError($"Song '{MenuManager.instance.songSelectionUIPage.selectedSong?.songName}' has {difficultyCount} difficulties but only {buttonCount} difficulty buttons. " +
                               "Only difficulties with configured buttons can be selected.");
            }

            for (int i = 0; i < buttonCount; i++)
            {
                DifficultySelectionButton button = buttons[i];
                if (button == null)
                {
                    Debug.LogError($"Difficulty button at index {i} is missing.");
                    continue;
                }

                bool hasDifficulty = i < difficultyCount && difficulties[i] != null;
                button.gameObject.SetActive(hasDifficulty);
                if (hasDifficulty)
                {
                    // SetUp 会进一步按 isAvailable 隐藏不可用难度。
                    button.SetUp(difficulties[i]);
                }
            }

            if (difficultyCount == 0)
            {
                Debug.LogWarning($"Song '{MenuManager.instance.songSelectionUIPage.selectedSong?.songName}' has no configured difficulties.");
                return;
            }

            if (!MenuInformationRecorder.instance.TryGetRecordOfThisChapter(out SongSelectionRecord songSelectionRecord))
            {
                Debug.LogWarning("Cannot select a difficulty because the current chapter has no valid song selection record.");
                return;
            }

            if (!TryResolveAvailableDifficultyIndex(difficulties, songSelectionRecord.difficultyListIndex, out int resolvedListIndex))
            {
                Debug.LogWarning($"Song '{MenuManager.instance.songSelectionUIPage.selectedSong?.songName}' has no available difficulty to select.");
                return;
            }

            // Select 会把实际选中的难度写回章节缓存；之后继续切歌时，以这次真实选择作为新的偏好。
            buttons[resolvedListIndex].Select();

            // 难度可用不等于内容已解锁。保持按钮状态与统一授权服务一致，
            // 而 PlaySongUI / 快速点击仍会在真正进入前再次检查，形成双层防御。
            UnlockSaveModule unlockSaveModule = GameSaveManager.instance?.UnlockSaveModule;
            bool canEnterSong = unlockSaveModule != null && unlockSaveModule.CanEnterSong(
                ChapterSelectionManager.instance?.currentChapter,
                MenuManager.instance.songSelectionUIPage.selectedSong);
            MenuManager.instance.songSelectionUIPage.playSongUI.SetEnterGameAvailable(canEnterSong);
        }
    }
}