namespace NewLife.Security;

/// <summary>CRC32校验</summary>
/// <remarks>
/// Generate a table for a byte-wise 32-bit CRC calculation on the polynomial:
/// x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1.
///
/// Polynomials over GF(2) are represented in binary, one bit per coefficient,
/// with the lowest powers in the most significant bit.  Then adding polynomials
/// is just exclusive-or, and multiplying a polynomial by x is a right shift by
/// one.  If we call the above polynomial p, and represent a byte as the
/// polynomial q, also with the lowest power in the most significant bit (so the
/// byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
/// where a mod b means the remainder after dividing a by b.
///
/// This calculation is done using the shift-register method of multiplying and
/// taking the remainder.  The register is initialized to zero, and for each
/// incoming bit, x^32 is added mod p to the register if the bit is a one (where
/// x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
/// x (which is shifting right by one and adding x^32 mod p if the bit shifted
/// out is a one).  We start with the highest power (least significant bit) of
/// q and repeat for all eight bits of q.
///
/// The table is simply the CRC of all possible eight bit values.  This is all
/// the information needed to generate CRC's on data a byte at a time for all
/// combinations of CRC register values and incoming bytes.
/// </remarks>
public sealed class Crc32 //: HashAlgorithm
{
    const UInt32 CrcSeed = 0xFFFFFFFF;

    #region 数据表
    /// <summary>校验表</summary>
    public readonly static UInt32[] Table;

    static Crc32()
    {
        Table = new UInt32[256];
        const UInt32 kPoly = 0xEDB88320;
        for (UInt32 i = 0; i < 256; i++)
        {
            var r = i;
            for (var j = 0; j < 8; j++)
                if ((r & 1) != 0)
                    r = (r >> 1) ^ kPoly;
                else
                    r >>= 1;
            Table[i] = r;
        }
    }
    #endregion

    //internal static uint ComputeCrc32(uint oldCrc, byte value)
    //{
    //    return (uint)(Table[(oldCrc ^ value) & 0xFF] ^ (oldCrc >> 8));
    //}

    /// <summary>校验值</summary>
    UInt32 crc = CrcSeed;
    /// <summary>校验值</summary>
    public UInt32 Value { get { return crc ^ CrcSeed; } set { crc = value ^ CrcSeed; } }

    /// <summary>重置清零</summary>
    public Crc32 Reset() { crc = CrcSeed; return this; }

    /// <summary>添加整数进行校验</summary>
    /// <param name = "value">
    /// the byte is taken as the lower 8 bits of value
    /// </param>
    public Crc32 Update(Int32 value)
    {
        crc = Table[(crc ^ value) & 0xFF] ^ (crc >> 8);

        return this;
    }

    /// <summary>添加字节数组进行校验</summary>
    /// <param name = "buffer">
    /// The buffer which contains the data
    /// </param>
    /// <param name = "offset">
    /// The offset in the buffer where the data starts
    /// </param>
    /// <param name = "count">
    /// The number of data bytes to update the CRC with.
    /// </param>
    public Crc32 Update(Byte[] buffer, Int32 offset = 0, Int32 count = -1)
    {
        if (buffer == null) throw new ArgumentNullException(nameof(buffer));
        //if (count < 0) throw new ArgumentOutOfRangeException("count", "Count不能小于0！");
        if (count < 0) count = buffer.Length;
        if (offset < 0 || offset + count > buffer.Length) throw new ArgumentOutOfRangeException(nameof(offset));

        while (--count >= 0)
        {
            crc = Table[(crc ^ buffer[offset++]) & 0xFF] ^ (crc >> 8);
        }

        return this;
    }

    /// <summary>添加数据区进行检验</summary>
    /// <param name="buffer"></param>
    /// <returns></returns>
    public Crc32 Update(ReadOnlySpan<Byte> buffer)
    {
        for (var i = 0; i < buffer.Length; i++)
        {
            crc = Table[(crc ^ buffer[i]) & 0xFF] ^ (crc >> 8);
        }

        return this;
    }

    /// <summary>添加数据流进行校验</summary>
    /// <param name="stream"></param>
    /// <param name="count">数量</param>
    public Crc32 Update(Stream stream, Int64 count = -1)
    {
        if (stream == null) throw new ArgumentNullException(nameof(stream));
        //if (count < 0) throw new ArgumentOutOfRangeException("count", "Count不能小于0！");
        if (count <= 0) count = Int64.MaxValue;

        while (--count >= 0)
        {
            var b = stream.ReadByte();
            if (b == -1) break;

            crc = Table[(crc ^ b) & 0xFF] ^ (crc >> 8);
        }

        return this;
    }

    /// <summary>计算校验码</summary>
    /// <param name="buf"></param>
    /// <param name="offset"></param>
    /// <param name="count"></param>
    /// <returns></returns>
    public static UInt32 Compute(Byte[] buf, Int32 offset = 0, Int32 count = -1)
    {
        var crc = new Crc32();
        crc.Update(buf, offset, count);
        return crc.Value;
    }

    /// <summary>计算校验码</summary>
    /// <param name="buffer"></param>
    /// <returns></returns>
    public static UInt32 Compute(ReadOnlySpan<Byte> buffer)
    {
        var crc = new Crc32();
        crc.Update(buffer);
        return crc.Value;
    }

    /// <summary>计算数据流校验码</summary>
    /// <param name="stream"></param>
    /// <param name="count"></param>
    /// <returns></returns>
    public static UInt32 Compute(Stream stream, Int32 count = -1)
    {
        var crc = new Crc32();
        crc.Update(stream, count);
        return crc.Value;
    }

    /// <summary>计算数据流校验码，指定起始位置和字节数偏移量</summary>
    /// <remarks>
    /// 一般用于计算数据包校验码，需要回过头去开始校验，并且可能需要跳过最后的校验码长度。
    /// position小于0时，数据流从当前位置开始计算校验；
    /// position大于等于0时，数据流移到该位置开始计算校验，最后由count决定可能差几个字节不参与计算；
    /// </remarks>
    /// <param name="stream"></param>
    /// <param name="position">如果大于等于0，则表示从该位置开始计算</param>
    /// <param name="count">字节数偏移量，一般用负数表示</param>
    /// <returns></returns>
    public static UInt32 ComputeRange(Stream stream, Int64 position = -1, Int32 count = -1)
    {
        if (position >= 0)
        {
            if (count > 0) count = -count;
            count += (Int32)(stream.Position - position);
            if (count == 0) return 0;

            stream.Position = position;
        }

        var crc = new Crc32();
        crc.Update(stream, count);
        return crc.Value;
    }

    //#region 抽象实现
    ///// <summary>哈希核心</summary>
    ///// <param name="array"></param>
    ///// <param name="ibStart"></param>
    ///// <param name="cbSize"></param>
    //protected override void HashCore(byte[] array, int ibStart, int cbSize)
    //{
    //    while (--cbSize >= 0)
    //    {
    //        crc = Table[(crc ^ array[ibStart++]) & 0xFF] ^ (crc >> 8);
    //    }
    //}

    ///// <summary>最后哈希</summary>
    ///// <returns></returns>
    //protected override byte[] HashFinal() { return BitConverter.GetBytes(Value); }

    ///// <summary>初始化</summary>
    //public override void Initialize() { }
    //#endregion
}