我在Wikipedia的Code Fragment:1 sample之后有一个基本的CRC32实现。我认为我做得对，修改了使用n位寄存器用于余数多项式而不是n + 1位用法。
我得到的结果与在线CRC32实现结果不同。在实施中我需要更改什么？
请忽略逻辑的Console.Writeline语句。
const UInt32 poly = 0x04C11DB7;
public static UInt32 GenerateCRC_32(byte[] message)
{
byte[] augmentedMsg = new byte[message.Length + 4];
message.CopyTo(augmentedMsg, 0);
UInt32 remainder = Convert.ToUInt32(augmentedMsg[0]) << 24 |
Convert.ToUInt32(augmentedMsg[1]) << 16 |
Convert.ToUInt32(augmentedMsg[2]) << 8 |
Convert.ToUInt32(augmentedMsg[3]);
for (Int32 i = 4; i < augmentedMsg.Length; i++)
{
for (int bit = 0; bit < 8; bit++)
{
UInt32 nextBit = ((UInt32)augmentedMsg[i] >> (7 - bit)) & 0x01;
if ((remainder & 0x80000000) > 0)
{
Console.WriteLine("---------------DO XOR --------------------");
Console.WriteLine(Convert.ToString(((remainder << 1) | nextBit), 2).PadLeft(32, '0'));
Console.WriteLine(Convert.ToString(poly, 2).PadLeft(32, '0'));
Console.WriteLine("------------------------------------------");
remainder = ((remainder << 1) | nextBit) ^ poly;
Console.WriteLine(Convert.ToString(remainder, 2).PadLeft(32, '0'));
Console.WriteLine("------------------------------------------");
}
else
{
remainder = (remainder << 1) | nextBit;
Console.WriteLine("--------------NO---------------------");
Console.WriteLine(Convert.ToString(remainder, 2).PadLeft(32, '0'));
Console.WriteLine("------------------------------------------");
}
}
}
Console.WriteLine(Convert.ToString(remainder, 2).PadLeft(32, '0'));
Console.WriteLine(remainder.ToString("X"));
return remainder;
}
我不是在寻找优化逻辑的最佳方法，因为我只是想尝试使用C＃来跟踪维基百科样本。
输入消息：'A'（十六进制：0x41） 输出：0x30476DC0 根据{{3}}：输出应为：0xD3D99E8B
我认为我错过了CRC的反转/初始化，但我不知道如何更改此基本实现以获得与网站结果相同的结果。
运行我的程序时的输出：
--------------NO---------------------
10000010000000000000000000000000
------------------------------------------
---------------DO XOR --------------------
00000100000000000000000000000000
00000100110000010001110110110111
------------------------------------------
00000000110000010001110110110111
------------------------------------------
--------------NO---------------------
00000001100000100011101101101110
------------------------------------------
--------------NO---------------------
00000011000001000111011011011100
------------------------------------------
--------------NO---------------------
00000110000010001110110110111000
------------------------------------------
--------------NO---------------------
00001100000100011101101101110000
------------------------------------------
--------------NO---------------------
00011000001000111011011011100000
------------------------------------------
--------------NO---------------------
00110000010001110110110111000000
------------------------------------------
00110000010001110110110111000000
十六进制的最后一行：0x30476DC0
@Mark Adler评论的后续行动：**
我修改了上面的内容如下，以下是修改（注释内联添加到代码中）：
XOR到最终值，反转XORed值
public static UInt32 GenerateCRC_32（byte [] message） { byte [] augmentedMsg = new byte [message.Length + 8]; message.CopyTo（augmentedMsg，4）; //修改为创建初始化空间
UInt32 remainder = Convert.ToUInt32(augmentedMsg[0]) << 24 |
Convert.ToUInt32(augmentedMsg[1]) << 16 |
Convert.ToUInt32(augmentedMsg[2]) << 8 |
Convert.ToUInt32(augmentedMsg[3]);
remainder = ~remainder; // Overwrite the above and initialized the register to 0xFFFFFFFF
for (Int32 i = 4; i < augmentedMsg.Length; i++)
{
byte reversedMessage = Reverse(augmentedMsg[i]); // Reversed the augmented message byte
for (int bit = 0; bit < 8; bit++)
{
UInt32 nextBit = Convert.ToUInt32(reversedMessage >> (7 - bit)) & 0x1; // Use the reversed message byte
if ((remainder & 0x80000000) > 0)
{
Console.WriteLine("---------------DO XOR --------------------");
Console.WriteLine(Convert.ToString(((remainder << 1) | nextBit), 2).PadLeft(32, '0'));
Console.WriteLine(Convert.ToString(poly32, 2).PadLeft(32, '0'));
Console.WriteLine("------------------------------------------");
remainder = Convert.ToUInt32((UInt32)((UInt32)(remainder << 1) | nextBit) ^ poly32);
Console.WriteLine(Convert.ToString(remainder, 2).PadLeft(32, '0'));
Console.WriteLine("------------------------------------------");
}
else
{
remainder = (UInt32)((UInt32)(remainder << 1) | nextBit);
Console.WriteLine("--------------NO---------------------");
Console.WriteLine(Convert.ToString(remainder, 2).PadLeft(32, '0'));
Console.WriteLine("------------------------------------------");
}
}
}
Console.WriteLine(Convert.ToString(remainder, 2).PadLeft(32, '0') + "(" + remainder.ToString("X") + ")");
remainder = (~remainder);
Console.WriteLine("XOR ^ 0xFFFFFFFF : " + Convert.ToString(remainder, 2).PadLeft(32, '0') + "(" + remainder.ToString("X") + ")");
remainder = Reverse(remainder);
Console.WriteLine("Reversed the Abv : " + Convert.ToString(remainder, 2).PadLeft(32, '0') + "(" + remainder.ToString("X") + ")");
return remainder;
}
输出：
---------------DO XOR --------------------
11111111111111111111111111111111
00000100110000010001110110110111
------------------------------------------
11111011001111101110001001001000
------------------------------------------
---------------DO XOR --------------------
11110110011111011100010010010000
00000100110000010001110110110111
------------------------------------------
11110010101111001101100100100111
------------------------------------------
---------------DO XOR --------------------
11100101011110011011001001001110
00000100110000010001110110110111
------------------------------------------
11100001101110001010111111111001
------------------------------------------
---------------DO XOR --------------------
11000011011100010101111111110010
00000100110000010001110110110111
------------------------------------------
11000111101100000100001001000101
------------------------------------------
---------------DO XOR --------------------
10001111011000001000010010001010
00000100110000010001110110110111
------------------------------------------
10001011101000011001100100111101
------------------------------------------
---------------DO XOR --------------------
00010111010000110011001001111010
00000100110000010001110110110111
------------------------------------------
00010011100000100010111111001101
------------------------------------------
--------------NO---------------------
00100111000001000101111110011011
------------------------------------------
--------------NO---------------------
01001110000010001011111100110110
------------------------------------------
--------------NO---------------------
10011100000100010111111001101100
------------------------------------------
---------------DO XOR --------------------
00111000001000101111110011011000
00000100110000010001110110110111
------------------------------------------
00111100111000111110000101101111
------------------------------------------
--------------NO---------------------
01111001110001111100001011011110
------------------------------------------
--------------NO---------------------
11110011100011111000010110111100
------------------------------------------
---------------DO XOR --------------------
11100111000111110000101101111000
00000100110000010001110110110111
------------------------------------------
11100011110111100001011011001111
------------------------------------------
---------------DO XOR --------------------
11000111101111000010110110011110
00000100110000010001110110110111
------------------------------------------
11000011011111010011000000101001
------------------------------------------
---------------DO XOR --------------------
10000110111110100110000001010010
00000100110000010001110110110111
------------------------------------------
10000010001110110111110111100101
------------------------------------------
---------------DO XOR --------------------
00000100011101101111101111001010
00000100110000010001110110110111
------------------------------------------
00000000101101111110011001111101
------------------------------------------
--------------NO---------------------
00000001011011111100110011111010
------------------------------------------
--------------NO---------------------
00000010110111111001100111110100
------------------------------------------
--------------NO---------------------
00000101101111110011001111101000
------------------------------------------
--------------NO---------------------
00001011011111100110011111010000
------------------------------------------
--------------NO---------------------
00010110111111001100111110100000
------------------------------------------
--------------NO---------------------
00101101111110011001111101000000
------------------------------------------
--------------NO---------------------
01011011111100110011111010000000
------------------------------------------
--------------NO---------------------
10110111111001100111110100000000
------------------------------------------
---------------DO XOR --------------------
01101111110011001111101000000000
00000100110000010001110110110111
------------------------------------------
01101011000011011110011110110111
------------------------------------------
--------------NO---------------------
11010110000110111100111101101110
------------------------------------------
---------------DO XOR --------------------
10101100001101111001111011011100
00000100110000010001110110110111
------------------------------------------
10101000111101101000001101101011
------------------------------------------
---------------DO XOR --------------------
01010001111011010000011011010110
00000100110000010001110110110111
------------------------------------------
01010101001011000001101101100001
------------------------------------------
--------------NO---------------------
10101010010110000011011011000010
------------------------------------------
---------------DO XOR --------------------
01010100101100000110110110000100
00000100110000010001110110110111
------------------------------------------
01010000011100010111000000110011
------------------------------------------
--------------NO---------------------
10100000111000101110000001100110
------------------------------------------
---------------DO XOR --------------------
01000001110001011100000011001100
00000100110000010001110110110111
------------------------------------------
01000101000001001101110101111011
------------------------------------------
--------------NO---------------------
10001010000010011011101011110110
------------------------------------------
---------------DO XOR --------------------
00010100000100110111010111101100
00000100110000010001110110110111
------------------------------------------
00010000110100100110100001011011
------------------------------------------
--------------NO---------------------
00100001101001001101000010110110
------------------------------------------
--------------NO---------------------
01000011010010011010000101101100
------------------------------------------
--------------NO---------------------
10000110100100110100001011011000
------------------------------------------
---------------DO XOR --------------------
00001101001001101000010110110000
00000100110000010001110110110111
------------------------------------------
00001001111001111001100000000111
------------------------------------------
--------------NO---------------------
00010011110011110011000000001110
------------------------------------------
--------------NO---------------------
00100111100111100110000000011100
------------------------------------------
00100111100111100110000000011100(279E601C)
XOR ^ 0xFFFFFFFF : 11011000011000011001111111100011(D8619FE3)
Reversed the Abv : 11000111111110011000011000011011(C7F9861B)
这不是预期的输出。我使用下面的表查找代码实现了相同的，结果与上面的完全相同（0xC7F9861B），这是错误的
public static UInt32 GenerateCRC_32_from_Table(byte[] message)
{
byte[] augmentedMsg = new byte[message.Length + 4];
message.CopyTo(augmentedMsg, 0);
UInt32 remainder = 0xFFFFFFFF;
foreach (byte msgByte in augmentedMsg)
{
byte reversedMsgByte = Reverse(msgByte);
remainder = ((remainder << 8) | Convert.ToUInt32(reversedMsgByte)) ^ crc32_table[((remainder >> 24)) & 0xFF];
}
remainder = Reverse(~remainder);
return remainder;
}
然而，如果我使用下面的代码（避免消息增加）产生了正确的结果。
public static UInt32 GenerateCRC_32_from_Table(byte[] message)
{
UInt32 remainder = 0xFFFFFFFF;
foreach (byte msgByte in message)
{
byte reversedMsgByte = Reverse(msgByte);
remainder = (remainder << 8) ^ crc32_table[((remainder >> 24) ^ Convert.ToUInt32(reversedMsgByte)) & 0xFF];
}
remainder = Reverse(~remainder);
return remainder;
}
注释中提到的Reverse（）和poly32：**
const UInt32 poly32 = 0x04C11DB7;
public static UInt32 Reverse(UInt32 message)
{
UInt32 msgReversed = 0;
for (int i = 0; i < 32; i++)
{
msgReversed = ((message & 0x80000000) >> (31 - i)) | msgReversed;
message = message << 1;
}
return msgReversed;
}
public static byte Reverse(byte message)
{
byte msgReversed = 0;
for (int i = 0; i < 8; i++)
{
msgReversed = (byte)(((byte)((byte)(message) & 0x80) >> (7 - i)) | msgReversed);
message = (byte)(message << 1);
}
return msgReversed;
}
答案 0 :(得分：5)
反映了您所指的标准CRC，即位被反转，并用0xfffffff初始化，最终的CRC与0xffffffff异或。
您正在使用未反转的位生成CRC，初始CRC为零，并且最后没有异或 -
通常为了实现位反转CRC，您将输入字节保留原样，但反映多项式并在另一个方向上移位。输入字节被送入CRC的底部而不是顶部。这也通过增强消息方法减少了要处理的总字节数。
更新更新的问题：
问题中的代码试图“扩充”消息，但后来甚至不使用扩充。相反，它使用从偏移量4开始的数据，这相当于使用偏移量为0的原始消息。
应该做的是甚至不尝试将消息，独占或消息添加到CRC的顶部而不是尝试将消息提供到CRC的底部。
此外，反转CRC，反向应用消息，然后再次反转CRC等同于不执行任何操作，而是反转多项式并在另一个方向上移位。多项式是一个常数，因此在编写代码时会完成反转，如我原来的答案中所述。 0x04c11db7撤消0xedb88320。
所以代码最终看起来像这样（在C中）：
#include <stddef.h>
#include <stdint.h>
/* CRC-32 (Ethernet, ZIP, etc.) polynomial in reversed bit order. */
#define POLY 0xedb88320
/* Compute CRC of buf[0..len-1] with initial CRC crc. This permits the
computation of a CRC by feeding this routine a chunk of the input data at a
time. The value of crc for the first chunk should be zero. */
uint32_t crc32(uint32_t crc, const unsigned char *buf, size_t len)
{
int k;
crc = ~crc;
while (len--) {
crc ^= *buf++;
for (k = 0; k < 8; k++)
crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
}
return ~crc;
}