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