add riif helper

src/ImageSharp/Common/Helpers/RiffHelper.cs

@@ -0,0 +1,117 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Buffers.Binary;
+using System.Text;
+
+namespace SixLabors.ImageSharp.Common.Helpers;
+
+internal class RiffHelper
+{
+    /// <summary>
+    /// The header bytes identifying RIFF file.
+    /// </summary>
+    public static readonly uint RiffFourCc = 0x52_49_46_46;
+
+    public static void WriteRiffFile(Stream stream, string formType, Action<Stream> func) =>
+        WriteChunk(stream, RiffFourCc, s =>
+        {
+            s.Write(Encoding.ASCII.GetBytes(formType));
+            func(s);
+        });
+
+    public static void WriteChunk(Stream stream, uint fourCc, Action<Stream> func)
+    {
+        Span<byte> buffer = stackalloc byte[4];
+
+        // write the fourCC
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer, fourCc);
+        stream.Write(buffer);
+
+        long sizePosition = stream.Position;
+        stream.Position += 4;
+
+        func(stream);
+
+        long position = stream.Position;
+        stream.Position = sizePosition;
+
+        uint dataSize = (uint)(position - sizePosition - 4);
+
+        // padding
+        if (dataSize % 2 == 1)
+        {
+            stream.WriteByte(0);
+            position++;
+        }
+
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer, dataSize);
+        stream.Write(buffer);
+
+        stream.Position = position;
+    }
+
+    public static void WriteChunk(Stream stream, uint fourCc, ReadOnlySpan<byte> data)
+    {
+        Span<byte> buffer = stackalloc byte[4];
+
+        // write the fourCC
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer, fourCc);
+        stream.Write(buffer);
+        uint size = (uint)data.Length;
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer, size);
+        stream.Write(buffer);
+        stream.Write(data);
+
+        // padding
+        if (size % 2 == 1)
+        {
+            stream.WriteByte(0);
+        }
+    }
+
+    public static unsafe void WriteChunk<TStruct>(Stream stream, uint fourCc, in TStruct chunk)
+        where TStruct : unmanaged
+    {
+        fixed (TStruct* ptr = &chunk)
+        {
+            WriteChunk(stream, fourCc, new Span<byte>(ptr, sizeof(TStruct)));
+        }
+    }
+
+    public static long BeginWriteChunk(Stream stream, uint fourCc)
+    {
+        Span<byte> buffer = stackalloc byte[4];
+
+        // write the fourCC
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer, fourCc);
+        stream.Write(buffer);
+
+        long sizePosition = stream.Position;
+        stream.Position += 4;
+
+        return sizePosition;
+    }
+
+    public static void EndWriteChunk(Stream stream, long sizePosition)
+    {
+        Span<byte> buffer = stackalloc byte[4];
+
+        long position = stream.Position;
+        stream.Position = sizePosition;
+
+        uint dataSize = (uint)(position - sizePosition - 4);
+
+        // padding
+        if (dataSize % 2 == 1)
+        {
+            stream.WriteByte(0);
+            position++;
+        }
+
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer, dataSize);
+        stream.Write(buffer);
+
+        stream.Position = position;
+    }
+}