using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

/// <summary>
/// CRC32相关工具集
/// </summary>
public partial class Utils
{
    /// <summary>
    /// key
    /// </summary>
    private static ulong[] Crc32Table;

    /// <summary>
    /// 获取指定字符串的CRC32值
    /// </summary>
    /// <param name="str">源串</param>
    /// <returns>经crc32换算后的数值</returns>
    public static ulong CRC32(string str)
    {
        GetCRC32Tale();
        byte[] buffer = System.Text.ASCIIEncoding.ASCII.GetBytes(str);
        ulong value = 0xffffffff;
        int len = buffer.Length;
        for (int i = 0; i < len; i++)
        {
            value = (value >> 8) ^ Crc32Table[(value & 0xFF) ^ buffer[i]];
        }
        return value ^ 0xffffffff;
    }

    /// <summary>
    /// 创建CRC32因子
    /// </summary>
    private static void GetCRC32Tale()
    {
        ulong Crc;
        Crc32Table = new ulong[256];
        int i, j;
        for (i = 0; i < 256; i++)
        {
            Crc = (ulong)i;
            for (j = 8; j > 0; j--)
            {
                if ((Crc & 1) == 1)
                {
                    Crc = (Crc >> 1) ^ 0xEDB88320;
                }
                else
                {
                    Crc >>= 1;
                }
            }
            Crc32Table[i] = Crc;
        }
    }
}