Этап 5

Minint.Core/Models/MinintDocument.cs

@@ -21,6 +21,13 @@ public sealed class MinintDocument
 
     public List<MinintLayer> Layers { get; }
 
+    /// <summary>
+    /// Reverse lookup cache: RgbaColor → palette index. Built lazily, invalidated
+    /// on structural palette changes (compact, clear). Call <see cref="InvalidatePaletteCache"/>
+    /// after bulk palette modifications.
+    /// </summary>
+    private Dictionary<RgbaColor, int>? _paletteCache;
+
     public MinintDocument(string name)
     {
         Name = name;
@@ -50,4 +57,48 @@ public sealed class MinintDocument
         <= 16_777_215 => 3,
         _ => 4
     };
+
+    /// <summary>
+    /// O(1) lookup of a color in the palette. Returns the index, or -1 if not found.
+    /// Lazily builds an internal dictionary on first call.
+    /// </summary>
+    public int FindColorCached(RgbaColor color)
+    {
+        var cache = EnsurePaletteCache();
+        return cache.TryGetValue(color, out int idx) ? idx : -1;
+    }
+
+    /// <summary>
+    /// Returns the index of <paramref name="color"/>. If absent, appends it to the palette
+    /// and updates the cache. O(1) amortized.
+    /// </summary>
+    public int EnsureColorCached(RgbaColor color)
+    {
+        var cache = EnsurePaletteCache();
+        if (cache.TryGetValue(color, out int idx))
+            return idx;
+
+        idx = Palette.Count;
+        Palette.Add(color);
+        cache[color] = idx;
+        return idx;
+    }
+
+    /// <summary>
+    /// Drops the reverse lookup cache. Must be called after any operation that
+    /// reorders, removes, or bulk-replaces palette entries (e.g. compact, grayscale).
+    /// </summary>
+    public void InvalidatePaletteCache() => _paletteCache = null;
+
+    private Dictionary<RgbaColor, int> EnsurePaletteCache()
+    {
+        if (_paletteCache is not null)
+            return _paletteCache;
+
+        var cache = new Dictionary<RgbaColor, int>(Palette.Count);
+        for (int i = 0; i < Palette.Count; i++)
+            cache.TryAdd(Palette[i], i); // first occurrence wins (for dupes)
+        _paletteCache = cache;
+        return cache;
+    }
 }