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// Copyright 2013 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.
package crc16
// Predefined polynomials.
const (
// IBM is used by Bisync, Modbus, USB, ANSI X3.28, SIA DC-07, ...
IBM = 0xA001
// CCITT is used by X.25, V.41, HDLC FCS, XMODEM, Bluetooth, PACTOR, SD, ...
// CCITT forward is 0x8408. Reverse is 0x1021.
CCITT = 0x8408
CCITTFalse = 0x1021
// SCSI is used by SCSI
SCSI = 0xEDD1
// MBUS is used by Meter-Bus, DNP, ...
MBUS = 0x3D65
)
// Table is a 256-word table representing the polynomial for efficient processing.
type Table struct {
entries [256]uint16
reversed bool
noXOR bool
}
// IBMTable is the table for the IBM polynomial.
var IBMTable = makeTable(IBM)
// CCITTTable is the table for the CCITT polynomial.
var CCITTTable = makeTable(CCITT)
// CCITTFalseTable is the table for CCITT-FALSE.
var CCITTFalseTable = makeBitsReversedTable(CCITTFalse)
// SCSITable is the table for the SCSI polynomial.
var SCSITable = makeTable(SCSI)
// MBusTable is the tabe used for Meter-Bus polynomial.
var MBusTable = makeBitsReversedTable(MBUS)
// MakeTable returns the Table constructed from the specified polynomial.
func MakeTable(poly uint16) *Table {
return makeTable(poly)
}
// MakeBitsReversedTable returns the Table constructed from the specified polynomial.
func MakeBitsReversedTable(poly uint16) *Table {
return makeBitsReversedTable(poly)
}
// MakeTableNoXOR returns the Table constructed from the specified polynomial.
// Updates happen without XOR in and XOR out.
func MakeTableNoXOR(poly uint16) *Table {
tab := makeTable(poly)
tab.noXOR = true
return tab
}
// makeTable returns the Table constructed from the specified polynomial.
func makeBitsReversedTable(poly uint16) *Table {
t := &Table{
reversed: true,
}
width := uint16(16)
for i := uint16(0); i < 256; i++ {
crc := i << (width - 8)
for j := 0; j < 8; j++ {
if crc&(1<<(width-1)) != 0 {
crc = (crc << 1) ^ poly
} else {
crc <<= 1
}
}
t.entries[i] = crc
}
return t
}
func makeTable(poly uint16) *Table {
t := &Table{
reversed: false,
}
for i := 0; i < 256; i++ {
crc := uint16(i)
for j := 0; j < 8; j++ {
if crc&1 == 1 {
crc = (crc >> 1) ^ poly
} else {
crc >>= 1
}
}
t.entries[i] = crc
}
return t
}
func updateBitsReversed(crc uint16, tab *Table, p []byte) uint16 {
for _, v := range p {
crc = tab.entries[byte(crc>>8)^v] ^ (crc << 8)
}
return crc
}
func update(crc uint16, tab *Table, p []byte) uint16 {
crc = ^crc
for _, v := range p {
crc = tab.entries[byte(crc)^v] ^ (crc >> 8)
}
return ^crc
}
func updateNoXOR(crc uint16, tab *Table, p []byte) uint16 {
for _, v := range p {
crc = tab.entries[byte(crc)^v] ^ (crc >> 8)
}
return crc
}
// Update returns the result of adding the bytes in p to the crc.
func Update(crc uint16, tab *Table, p []byte) uint16 {
if tab.reversed {
return updateBitsReversed(crc, tab, p)
} else if tab.noXOR {
return updateNoXOR(crc, tab, p)
} else {
return update(crc, tab, p)
}
}
// Checksum returns the CRC-16 checksum of data
// using the polynomial represented by the Table.
func Checksum(data []byte, tab *Table) uint16 { return Update(0, tab, data) }
// ChecksumIBM returns the CRC-16 checksum of data
// using the IBM polynomial.
func ChecksumIBM(data []byte) uint16 { return Update(0, IBMTable, data) }
// ChecksumCCITTFalse returns the CRC-16 checksum using
// what some call the CCITT-False polynomial, which matches what is used
// by Perl Digest/CRC and Boost for example.
func ChecksumCCITTFalse(data []byte) uint16 { return Update(0xffff, CCITTFalseTable, data) }
// ChecksumCCITT returns the CRC-16 checksum of data
// using the CCITT polynomial.
func ChecksumCCITT(data []byte) uint16 { return Update(0, CCITTTable, data) }
// ChecksumSCSI returns the CRC-16 checksum of data
// using the SCSI polynomial.
func ChecksumSCSI(data []byte) uint16 { return Update(0, SCSITable, data) }
// ChecksumMBus returns the CRC-16 checksum of data
// using the MBus polynomial.
func ChecksumMBus(data []byte) uint16 { return Update(0, MBusTable, data) }
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