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Source file src/hash/crc32/crc32.go

  // Copyright 2009 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 crc32 implements the 32-bit cyclic redundancy check, or CRC-32,
  // checksum. See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for
  // information.
  //
  // Polynomials are represented in LSB-first form also known as reversed representation.
  //
  // See http://en.wikipedia.org/wiki/Mathematics_of_cyclic_redundancy_checks#Reversed_representations_and_reciprocal_polynomials
  // for information.
  package crc32
  
  import (
  	"hash"
  	"sync"
  )
  
  // The size of a CRC-32 checksum in bytes.
  const Size = 4
  
  // Predefined polynomials.
  const (
  	// IEEE is by far and away the most common CRC-32 polynomial.
  	// Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, ...
  	IEEE = 0xedb88320
  
  	// Castagnoli's polynomial, used in iSCSI.
  	// Has better error detection characteristics than IEEE.
  	// http://dx.doi.org/10.1109/26.231911
  	Castagnoli = 0x82f63b78
  
  	// Koopman's polynomial.
  	// Also has better error detection characteristics than IEEE.
  	// http://dx.doi.org/10.1109/DSN.2002.1028931
  	Koopman = 0xeb31d82e
  )
  
  // Table is a 256-word table representing the polynomial for efficient processing.
  type Table [256]uint32
  
  // This file makes use of functions implemented in architecture-specific files.
  // The interface that they implement is as follows:
  //
  //    // archAvailableIEEE reports whether an architecture-specific CRC32-IEEE
  //    // algorithm is available.
  //    archAvailableIEEE() bool
  //
  //    // archInitIEEE initializes the architecture-specific CRC3-IEEE algorithm.
  //    // It can only be called if archAvailableIEEE() returns true.
  //    archInitIEEE()
  //
  //    // archUpdateIEEE updates the given CRC32-IEEE. It can only be called if
  //    // archInitIEEE() was previously called.
  //    archUpdateIEEE(crc uint32, p []byte) uint32
  //
  //    // archAvailableCastagnoli reports whether an architecture-specific
  //    // CRC32-C algorithm is available.
  //    archAvailableCastagnoli() bool
  //
  //    // archInitCastagnoli initializes the architecture-specific CRC32-C
  //    // algorithm. It can only be called if archAvailableCastagnoli() returns
  //    // true.
  //    archInitCastagnoli()
  //
  //    // archUpdateCastagnoli updates the given CRC32-C. It can only be called
  //    // if archInitCastagnoli() was previously called.
  //    archUpdateCastagnoli(crc uint32, p []byte) uint32
  
  // castagnoliTable points to a lazily initialized Table for the Castagnoli
  // polynomial. MakeTable will always return this value when asked to make a
  // Castagnoli table so we can compare against it to find when the caller is
  // using this polynomial.
  var castagnoliTable *Table
  var castagnoliTable8 *slicing8Table
  var castagnoliArchImpl bool
  var updateCastagnoli func(crc uint32, p []byte) uint32
  var castagnoliOnce sync.Once
  
  func castagnoliInit() {
  	castagnoliTable = simpleMakeTable(Castagnoli)
  	castagnoliArchImpl = archAvailableCastagnoli()
  
  	if castagnoliArchImpl {
  		archInitCastagnoli()
  		updateCastagnoli = archUpdateCastagnoli
  	} else {
  		// Initialize the slicing-by-8 table.
  		castagnoliTable8 = slicingMakeTable(Castagnoli)
  		updateCastagnoli = func(crc uint32, p []byte) uint32 {
  			return slicingUpdate(crc, castagnoliTable8, p)
  		}
  	}
  }
  
  // IEEETable is the table for the IEEE polynomial.
  var IEEETable = simpleMakeTable(IEEE)
  
  // ieeeTable8 is the slicing8Table for IEEE
  var ieeeTable8 *slicing8Table
  var ieeeArchImpl bool
  var updateIEEE func(crc uint32, p []byte) uint32
  var ieeeOnce sync.Once
  
  func ieeeInit() {
  	ieeeArchImpl = archAvailableIEEE()
  
  	if ieeeArchImpl {
  		archInitIEEE()
  		updateIEEE = archUpdateIEEE
  	} else {
  		// Initialize the slicing-by-8 table.
  		ieeeTable8 = slicingMakeTable(IEEE)
  		updateIEEE = func(crc uint32, p []byte) uint32 {
  			return slicingUpdate(crc, ieeeTable8, p)
  		}
  	}
  }
  
  // MakeTable returns a Table constructed from the specified polynomial.
  // The contents of this Table must not be modified.
  func MakeTable(poly uint32) *Table {
  	switch poly {
  	case IEEE:
  		ieeeOnce.Do(ieeeInit)
  		return IEEETable
  	case Castagnoli:
  		castagnoliOnce.Do(castagnoliInit)
  		return castagnoliTable
  	}
  	return simpleMakeTable(poly)
  }
  
  // digest represents the partial evaluation of a checksum.
  type digest struct {
  	crc uint32
  	tab *Table
  }
  
  // New creates a new hash.Hash32 computing the CRC-32 checksum
  // using the polynomial represented by the Table.
  // Its Sum method will lay the value out in big-endian byte order.
  func New(tab *Table) hash.Hash32 {
  	if tab == IEEETable {
  		ieeeOnce.Do(ieeeInit)
  	}
  	return &digest{0, tab}
  }
  
  // NewIEEE creates a new hash.Hash32 computing the CRC-32 checksum
  // using the IEEE polynomial.
  // Its Sum method will lay the value out in big-endian byte order.
  func NewIEEE() hash.Hash32 { return New(IEEETable) }
  
  func (d *digest) Size() int { return Size }
  
  func (d *digest) BlockSize() int { return 1 }
  
  func (d *digest) Reset() { d.crc = 0 }
  
  // Update returns the result of adding the bytes in p to the crc.
  func Update(crc uint32, tab *Table, p []byte) uint32 {
  	switch tab {
  	case castagnoliTable:
  		return updateCastagnoli(crc, p)
  	case IEEETable:
  		// Unfortunately, because IEEETable is exported, IEEE may be used without a
  		// call to MakeTable. We have to make sure it gets initialized in that case.
  		ieeeOnce.Do(ieeeInit)
  		return updateIEEE(crc, p)
  	default:
  		return simpleUpdate(crc, tab, p)
  	}
  }
  
  func (d *digest) Write(p []byte) (n int, err error) {
  	switch d.tab {
  	case castagnoliTable:
  		d.crc = updateCastagnoli(d.crc, p)
  	case IEEETable:
  		// We only create digest objects through New() which takes care of
  		// initialization in this case.
  		d.crc = updateIEEE(d.crc, p)
  	default:
  		d.crc = simpleUpdate(d.crc, d.tab, p)
  	}
  	return len(p), nil
  }
  
  func (d *digest) Sum32() uint32 { return d.crc }
  
  func (d *digest) Sum(in []byte) []byte {
  	s := d.Sum32()
  	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
  }
  
  // Checksum returns the CRC-32 checksum of data
  // using the polynomial represented by the Table.
  func Checksum(data []byte, tab *Table) uint32 { return Update(0, tab, data) }
  
  // ChecksumIEEE returns the CRC-32 checksum of data
  // using the IEEE polynomial.
  func ChecksumIEEE(data []byte) uint32 {
  	ieeeOnce.Do(ieeeInit)
  	return updateIEEE(0, data)
  }
  

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