Created
February 16, 2017 11:39
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simplest crc32 c++ implementation
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#pragma once | |
#include <stdint.h> | |
struct crc32 | |
{ | |
static void generate_table(uint32_t(&table)[256]) | |
{ | |
uint32_t polynomial = 0xEDB88320; | |
for (uint32_t i = 0; i < 256; i++) | |
{ | |
uint32_t c = i; | |
for (size_t j = 0; j < 8; j++) | |
{ | |
if (c & 1) { | |
c = polynomial ^ (c >> 1); | |
} | |
else { | |
c >>= 1; | |
} | |
} | |
table[i] = c; | |
} | |
} | |
static uint32_t update(uint32_t (&table)[256], uint32_t initial, const void* buf, size_t len) | |
{ | |
uint32_t c = initial ^ 0xFFFFFFFF; | |
const uint8_t* u = static_cast<const uint8_t*>(buf); | |
for (size_t i = 0; i < len; ++i) | |
{ | |
c = table[(c ^ u[i]) & 0xFF] ^ (c >> 8); | |
} | |
return c ^ 0xFFFFFFFF; | |
} | |
}; | |
// usage: the following code generates crc for 2 pieces of data | |
// uint32_t table[256]; | |
// crc32::generate_table(table); | |
// uint32_t crc = crc32::update(table, 0, data_piece1, len1); | |
// crc = crc32::update(table, crc, data_piece2, len2); | |
// output(crc); |
With class:
#pragma once
#include <stdint.h>
struct CRC32_s
{
void generate_table(uint32_t(&table)[256])
{
uint32_t polynomial = 0xEDB88320;
for (uint32_t i = 0; i < 256; i++)
{
uint32_t c = i;
for (size_t j = 0; j < 8; j++)
{
if (c & 1) {
c = polynomial ^ (c >> 1);
}
else {
c >>= 1;
}
}
table[i] = c;
}
}
uint32_t update(uint32_t(&table)[256], uint32_t initial, const void* buf, size_t len)
{
uint32_t c = initial ^ 0xFFFFFFFF;
const uint8_t* u = static_cast<const uint8_t*>(buf);
for (size_t i = 0; i < len; ++i)
{
c = table[(c ^ u[i]) & 0xFF] ^ (c >> 8);
}
return c ^ 0xFFFFFFFF;
}
};
class CRC32
{
private:
uint32_t table[256];
CRC32_s crc32_s;
uint32_t initial;
public:
CRC32()
: initial(0)
{
crc32_s.generate_table(table);
}
void Update(const unsigned __int8 * buf, size_t len)
{
initial = crc32_s.update(table, initial, (const void *)buf, len);
}
uint32_t GetValue() const
{
return initial;
}
};
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Interesting, the second use is handy when crc32 structs in C++.