SjkScripts/Collections/BitBuffer.cs

using System;

namespace SJK.Collections;

/// <summary>
/// Stores an Array of bits that can be retrieved by index. Can be used to return multiple bits as an int or long.
/// </summary>
public sealed class BitBuffer
{
    // Number of bits in a byte
    private const int BitsInByte = 8;
    private const float BitsInByteFloat = 8f;
    private byte[] data;

    /// <summary>
    /// Length of the bit buffer in bits.
    /// </summary>
    public int Length
    {
        get { return length; }
        set
        {
            AppendNewArray(value);
            length = value;
        }
    }

    /// <summary>
    /// Capacity of the bit buffer.
    /// </summary>
    public int Capacity { get { return data.Length; } }

    private int length;

    /// <summary>
    /// Initializes a new instance of the BitBuffer class with the specified length in bits.
    /// </summary>
    /// <param name="length">The length of the bit buffer in bits.</param>
    public BitBuffer(int length)
    {
        data = new byte[length / BitsInByte + 5];
    }

    /// <summary>
    /// Initializes a new instance of the BitBuffer class with the specified byte array.
    /// </summary>
    /// <param name="_data">The byte array to initialize the bit buffer.</param>
    public BitBuffer(byte[] _data)
    {
        data = _data;
        length = data.Length * BitsInByte;
    }

    private void AppendNewArray(int length)
    {
        int capacity = length / BitsInByte + 1;
        byte[] newData = new byte[capacity];
        Array.Copy(data, newData, System.Math.Min(data.Length, newData.Length));
        data = newData;
    }

    /// <summary>
    /// Gets the bit at the specified index.
    /// </summary>
    /// <param name="index">The index of the bit.</param>
    /// <returns>The value of the bit at the specified index.</returns>
    public bool GetBit(long index)
    {
        return GetBits(index, 1) > 0;
    }

    /// <summary>
    /// Sets the bit at the specified index to the specified value.
    /// </summary>
    /// <param name="index">The index of the bit.</param>
    /// <param name="value">The value to set the bit to.</param>
    public void SetBit(long index, bool value)
    {
        SetBits(index, value ? 1 : 0, 1);
    }

    /// <summary>
    /// Gets the specified number of bits starting from the specified index.
    /// </summary>
    /// <param name="index">The starting index of the bits.</param>
    /// <param name="bits">The number of bits to retrieve.</param>
    /// <returns>The retrieved bits as an integer.</returns>
    public unsafe int GetBits(long index, int bits)
    {
#if DEBUG
        if (bits > 32)
            throw new ArgumentOutOfRangeException($"{nameof(BitBuffer)} does not support accessing more then size of int (32 bits) : {bits}");
        if (bits <= 0)
            throw new ArgumentOutOfRangeException($"{nameof(BitBuffer)} cannot have zero or lower be the length of the bits : {bits}");
        if (index <0)
            throw new IndexOutOfRangeException($"{nameof(BitBuffer)} cannot index a value lower then zero : {index}");
        if (index >= length - bits)
            throw new IndexOutOfRangeException($"{nameof(BitBuffer)} cannot bit greater then the length of the bitBuffer : {index},  {bits}");
        #endif
            long byteIndex = index / BitsInByte;
        int offset = (int)(index - byteIndex * BitsInByte);

        fixed (byte* d = &data[byteIndex])
        {
            return (*((int*)d) >> offset) & ((1 << bits) - 1);
        }
    }

    /// <summary>
    /// Sets the specified number of bits starting from the specified index to the specified value.
    /// </summary>
    /// <param name="index">The starting index of the bits.</param>
    /// <param name="value">The value to set the bits to.</param>
    /// <param name="bits">The number of bits to set.</param>
    public unsafe void SetBits(long index, int value, int bits)
    {
        int mask = (1 << bits) - 1;
        int newValue = value;
        newValue &= mask;
        long byteIndex = index / BitsInByte;
        int offset = (int)(index - byteIndex * BitsInByte);

        fixed (byte* d = data)
        {
            int* l = (int*)(d + byteIndex);
            int v = *l;
            v = v & ~((mask << offset));
            v = v | (newValue << offset);
            *(int*)(d + byteIndex) = v;
        }
    }

    /// <summary>
    /// Gets the underlying byte array of the BitBuffer.
    /// </summary>
    /// <returns>The underlying byte array.</returns>
    public byte[] GetData()
    {
        return data;
    }
}