Brotli-Builder/BrotliLib/Brotli/Components/Data/DistanceCode.Type.Last.cs

using System.Collections.Generic;
using System.Linq;
using BrotliLib.Serialization.Reader;
using BrotliLib.Serialization.Writer;

namespace BrotliLib.Brotli.Components.Data{
    public abstract partial class DistanceCode{
        /// <inheritdoc />
        /// <summary>
        /// Represents a distance code which uses the global ring buffer of last distances.
        /// </summary>
        public sealed class Last : DistanceCode{
            private static readonly byte[] BufferIndexes = {
                3, 2, 1, 0,
                3, 3, 3, 3, 3, 3,
                2, 2, 2, 2, 2, 2
            };

            private static readonly sbyte[] BufferValueOffsets = {
                0, 0, 0, 0,
                -1, 1, -2, 2, -3, 3,
                -1, 1, -2, 2, -3, 3
            };

            public const int CodeCount = 16;

            public static readonly IReadOnlyList<DistanceCode> Codes = Enumerable.Range(0, CodeCount)
                                                                                 .Select(code => new Last(code))
                                                                                 .ToArray<DistanceCode>();

            private readonly byte index;
            private readonly sbyte offset;

            private Last(int code) : base(code){
                this.index = BufferIndexes[code];
                this.offset = BufferValueOffsets[code];
            }

            public override int ExtraBits => 0;

            public override bool CanEncodeValue(BrotliGlobalState state, int value){
                return value == state.DistanceBuffer[index] + offset;
            }

            protected override int ReadValue(BrotliGlobalState state, IBitReader reader){
                return state.DistanceBuffer[index] + offset;
            }

            protected override void WriteValue(BrotliGlobalState state, int value, IBitWriter writer){
                // no extra bits
            }

            public override string ToString(){
                return base.ToString() + " | Value = buffer[" + index + "] " + (offset < 0 ? "- " + (-offset) : "+ " + offset);
            }
        }
    }
}