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Brotli-Builder/BrotliLib/Collections/MoveToFront.cs

57 lines
1.7 KiB
C#

using System;
using System.Linq;
namespace BrotliLib.Collections{
public static class MoveToFront{
/// <summary>
/// Performs an in-place move-to-front transformation on the input byte array.
/// </summary>
public static readonly Action<byte[]> Encode = contextMap => {
byte[] mtf = CreateAlphabet(contextMap.Max() + 1);
for(int index = 0; index < contextMap.Length; index++){
byte mtfIndex = (byte)Array.IndexOf(mtf, contextMap[index]);
byte value = mtf[mtfIndex];
contextMap[index] = mtfIndex;
MoveValueToFront(mtf, mtfIndex, value);
}
};
/// <summary>
/// Performs an in-place inverse-move-to-front transformation on the input byte array.
/// </summary>
public static readonly Action<byte[]> Decode = contextMap => {
byte[] mtf = CreateAlphabet(256);
for(int index = 0; index < contextMap.Length; index++){
byte mapIndex = contextMap[index];
byte value = mtf[mapIndex];
contextMap[index] = value;
MoveValueToFront(mtf, mapIndex, value);
}
};
// Helpers
private static byte[] CreateAlphabet(int size){
byte[] mtf = new byte[size];
for(int index = 0; index < mtf.Length; index++){
mtf[index] = (byte)index;
}
return mtf;
}
private static void MoveValueToFront(byte[] mtf, byte index, byte value){
while(index > 0){
mtf[index] = mtf[--index];
}
mtf[0] = value;
}
}
}