存档重构，游戏内容解锁机制；教程完善（未完成）

Assets/Scripts/UI/SongSelection/DifficultySelectionContainer.cs

@@ -1,5 +1,5 @@
-using System.Collections;
 using System.Collections.Generic;
+using System;
 using UnityEngine;
 
 namespace Ichni.Menu
@@ -9,28 +9,123 @@ namespace Ichni.Menu
         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;
+            int difficultyCount = difficulties?.Count ?? 0;
+            int buttonCount = buttons?.Count ?? 0;
 
-            for (var i = 0; i < buttons.Count; i++)
+            ClearDifficultySelection();
+
+            if (difficultyCount > buttonCount)
             {
-                buttons[i].gameObject.SetActive(i < difficultyCount);
+                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 < difficultyCount; i++)
+            for (int i = 0; i < buttonCount; i++)
             {
-                buttons[i].SetUp(difficulties[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]);
+                }
             }
 
-            SongSelectionRecord songSelectionRecord = MenuInformationRecorder.instance.GetRecordOfThisChapter();
-
-            if (songSelectionRecord.difficultyIndex >= difficultyCount)
+            if (difficultyCount == 0)
             {
-                songSelectionRecord.difficultyIndex = difficultyCount - 1;
+                Debug.LogWarning($"Song '{MenuManager.instance.songSelectionUIPage.selectedSong?.songName}' has no configured difficulties.");
+                return;
             }
 
-            buttons[songSelectionRecord.difficultyIndex % buttons.Count].Select();
+            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);
         }
     }
-}
+}