Support setting custom alignment heads for dtw

Whisper.net/Ggml/GgmlType.cs

@@ -35,4 +35,16 @@ public enum WhisperAlignmentHeadsPreset
     LargeV2,
     LargeV3,
     LargeV3Turbo
-}
+}
+
+public class WhisperAlignmentHead
+{
+    public int TextLayer;
+    public int Head;
+
+    public WhisperAlignmentHead(int textLayer, int head)
+    {
+        TextLayer = textLayer;
+        Head = head;
+    }
+}

Whisper.net/Internals/ModelLoader/WhisperProcessorModelBufferLoader.cs

@@ -10,15 +10,23 @@ namespace Whisper.net.Internals.ModelLoader;
 internal class WhisperProcessorModelBufferLoader(byte[] buffer) : IWhisperProcessorModelLoader
 {
     private readonly GCHandle pinnedBuffer = GCHandle.Alloc(buffer, GCHandleType.Pinned);
+    private GCHandle aheadsHandle;
 
     public void Dispose()
     {
         pinnedBuffer.Free();
+        if (aheadsHandle.IsAllocated)
+        {
+            aheadsHandle.Free();
+        }
     }
 
     public IntPtr LoadNativeContext(INativeWhisper nativeWhisper)
     {
         var bufferLength = new UIntPtr((uint)buffer.Length);
+
+        var aHeads = WhisperProcessorModelFileLoader.GetWhisperAlignmentHeads(RuntimeOptions.Instance.CustomAlignmentHeads, ref aheadsHandle);
+
         return nativeWhisper.Whisper_Init_From_Buffer_With_Params_No_State(pinnedBuffer.AddrOfPinnedObject(), bufferLength,
             new WhisperContextParams()
             {
@@ -28,11 +36,7 @@ public IntPtr LoadNativeContext(INativeWhisper nativeWhisper)
                 DtwTokenLevelTimestamp = RuntimeOptions.Instance.UseDtwTimeStamps ? (byte)1 : (byte)0,
                 HeadsPreset = (WhisperAlignmentHeadsPreset)RuntimeOptions.Instance.HeadsPreset,
                 DtwNTop = -1,
-                WhisperAheads = new WhisperAheads()
-                {
-                    NHeads = 0,
-                    Heads = IntPtr.Zero
-                },
+                WhisperAheads = aHeads,
                 Dtw_mem_size = 1024 * 1024 * 128,
             });
     }

Whisper.net/Internals/ModelLoader/WhisperProcessorModelFileLoader.cs

@@ -1,5 +1,6 @@
 // Licensed under the MIT license: https://opensource.org/licenses/MIT
 
+using System.Runtime.InteropServices;
 using Whisper.net.Internals.Native;
 using Whisper.net.LibraryLoader;
 using Whisper.net.Native;
@@ -8,13 +9,49 @@ namespace Whisper.net.Internals.ModelLoader;
 
 internal sealed class WhisperProcessorModelFileLoader(string pathModel) : IWhisperProcessorModelLoader
 {
+    private GCHandle aheadsHandle;
+
     public void Dispose()
     {
+        if (aheadsHandle.IsAllocated)
+        {
+            aheadsHandle.Free();
+        }
+    }
+
+    public static WhisperAheads GetWhisperAlignmentHeads(Ggml.WhisperAlignmentHead[]? alignmentHeads, ref GCHandle aHeadsHandle)
+    {
+        var aHeadsPtr = IntPtr.Zero;
+        var nHeads = alignmentHeads?.Length ?? 0;
 
+        if (nHeads > 0)
+        {
+            var aHeads = new int[nHeads * 2];
+            if (aHeadsHandle.IsAllocated)
+            {
+                aHeadsHandle.Free();
+            }
+            aHeadsHandle = GCHandle.Alloc(aHeads, GCHandleType.Pinned);
+            aHeadsPtr = aHeadsHandle.AddrOfPinnedObject();
+
+            for (var i = 0; i < nHeads; i++)
+            {
+                aHeads[i * 2] = alignmentHeads![i].TextLayer;
+                aHeads[i * 2 + 1] = alignmentHeads[i].Head;
+            }
+        }
+
+        return new WhisperAheads()
+        { 
+            NHeads = (nuint)nHeads,
+            Heads = aHeadsPtr
+        };
     }
 
     public IntPtr LoadNativeContext(INativeWhisper nativeWhisper)
     {
+        var aHeads = GetWhisperAlignmentHeads(RuntimeOptions.Instance.CustomAlignmentHeads, ref aheadsHandle);
+
         return nativeWhisper.Whisper_Init_From_File_With_Params_No_State(pathModel,
            new WhisperContextParams()
            {
@@ -24,11 +61,7 @@ public IntPtr LoadNativeContext(INativeWhisper nativeWhisper)
                DtwTokenLevelTimestamp = RuntimeOptions.Instance.UseDtwTimeStamps ? (byte)1 : (byte)0,
                HeadsPreset = (WhisperAlignmentHeadsPreset)RuntimeOptions.Instance.HeadsPreset,
                DtwNTop = -1,
-               WhisperAheads = new WhisperAheads()
-               {
-                   NHeads = 0,
-                   Heads = IntPtr.Zero
-               },
+               WhisperAheads = aHeads,
                Dtw_mem_size = 1024 * 1024 * 128,
            });
     }

Whisper.net/LibraryLoader/RuntimeOptions.cs

@@ -13,6 +13,7 @@ public class RuntimeOptions
     internal bool UseFlashAttention { get; private set; }
     internal bool UseDtwTimeStamps { get; private set; }
     internal WhisperAlignmentHeadsPreset HeadsPreset { get; private set; }
+    internal WhisperAlignmentHead[]? CustomAlignmentHeads { get; private set; }
     internal int GpuDevice { get; private set; }
     internal List<RuntimeLibrary> RuntimeLibraryOrder { get; private set; }
     internal RuntimeLibrary? LoadedLibrary { get; private set; }
@@ -27,6 +28,7 @@ private RuntimeOptions()
         UseFlashAttention = false;
         UseDtwTimeStamps = false;
         HeadsPreset = WhisperAlignmentHeadsPreset.None;
+        CustomAlignmentHeads = null;
         RuntimeLibraryOrder = defaultRuntimeOrder;
         GpuDevice = 0;
     }
@@ -127,6 +129,17 @@ public void SetHeadsPreset(WhisperAlignmentHeadsPreset headsPreset)
         HeadsPreset = headsPreset;
     }
 
+    /// <summary>
+    /// Sets custom attention heads array for DTW. 
+    /// </summary>
+    /// <remarks>
+    /// By default, it is null. Required when using DTW with models which don't have a matching WhisperAlignmentHeadsPreset.
+    /// </remarks>
+    public void SetAlignmentHeads(WhisperAlignmentHead[]? alignmentHeads)
+    {
+        CustomAlignmentHeads = alignmentHeads;
+    }
+
     /// <summary>
     /// Resets the runtime options to their default values.
     /// </summary>
@@ -138,6 +151,7 @@ public void Reset()
         UseFlashAttention = false;
         UseDtwTimeStamps = false;
         HeadsPreset = WhisperAlignmentHeadsPreset.None;
+        CustomAlignmentHeads = null;
         RuntimeLibraryOrder = defaultRuntimeOrder;
         GpuDevice = 0;
     }