mirror of
https://github.com/go-gitea/gitea
synced 2024-11-14 06:04:25 +00:00
90 lines
2.5 KiB
Go
90 lines
2.5 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
|
|
// Use of this source code is governed by a BSD-style
|
|
// license that can be found in the LICENSE file.
|
|
|
|
// This file contains CRC32 algorithms that are not specific to any architecture
|
|
// and don't use hardware acceleration.
|
|
//
|
|
// The simple (and slow) CRC32 implementation only uses a 256*4 bytes table.
|
|
//
|
|
// The slicing-by-8 algorithm is a faster implementation that uses a bigger
|
|
// table (8*256*4 bytes).
|
|
|
|
package crc32
|
|
|
|
// simpleMakeTable allocates and constructs a Table for the specified
|
|
// polynomial. The table is suitable for use with the simple algorithm
|
|
// (simpleUpdate).
|
|
func simpleMakeTable(poly uint32) *Table {
|
|
t := new(Table)
|
|
simplePopulateTable(poly, t)
|
|
return t
|
|
}
|
|
|
|
// simplePopulateTable constructs a Table for the specified polynomial, suitable
|
|
// for use with simpleUpdate.
|
|
func simplePopulateTable(poly uint32, t *Table) {
|
|
for i := 0; i < 256; i++ {
|
|
crc := uint32(i)
|
|
for j := 0; j < 8; j++ {
|
|
if crc&1 == 1 {
|
|
crc = (crc >> 1) ^ poly
|
|
} else {
|
|
crc >>= 1
|
|
}
|
|
}
|
|
t[i] = crc
|
|
}
|
|
}
|
|
|
|
// simpleUpdate uses the simple algorithm to update the CRC, given a table that
|
|
// was previously computed using simpleMakeTable.
|
|
func simpleUpdate(crc uint32, tab *Table, p []byte) uint32 {
|
|
crc = ^crc
|
|
for _, v := range p {
|
|
crc = tab[byte(crc)^v] ^ (crc >> 8)
|
|
}
|
|
return ^crc
|
|
}
|
|
|
|
// Use slicing-by-8 when payload >= this value.
|
|
const slicing8Cutoff = 16
|
|
|
|
// slicing8Table is array of 8 Tables, used by the slicing-by-8 algorithm.
|
|
type slicing8Table [8]Table
|
|
|
|
// slicingMakeTable constructs a slicing8Table for the specified polynomial. The
|
|
// table is suitable for use with the slicing-by-8 algorithm (slicingUpdate).
|
|
func slicingMakeTable(poly uint32) *slicing8Table {
|
|
t := new(slicing8Table)
|
|
simplePopulateTable(poly, &t[0])
|
|
for i := 0; i < 256; i++ {
|
|
crc := t[0][i]
|
|
for j := 1; j < 8; j++ {
|
|
crc = t[0][crc&0xFF] ^ (crc >> 8)
|
|
t[j][i] = crc
|
|
}
|
|
}
|
|
return t
|
|
}
|
|
|
|
// slicingUpdate uses the slicing-by-8 algorithm to update the CRC, given a
|
|
// table that was previously computed using slicingMakeTable.
|
|
func slicingUpdate(crc uint32, tab *slicing8Table, p []byte) uint32 {
|
|
if len(p) >= slicing8Cutoff {
|
|
crc = ^crc
|
|
for len(p) > 8 {
|
|
crc ^= uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
|
|
crc = tab[0][p[7]] ^ tab[1][p[6]] ^ tab[2][p[5]] ^ tab[3][p[4]] ^
|
|
tab[4][crc>>24] ^ tab[5][(crc>>16)&0xFF] ^
|
|
tab[6][(crc>>8)&0xFF] ^ tab[7][crc&0xFF]
|
|
p = p[8:]
|
|
}
|
|
crc = ^crc
|
|
}
|
|
if len(p) == 0 {
|
|
return crc
|
|
}
|
|
return simpleUpdate(crc, &tab[0], p)
|
|
}
|