From 169adec231562565c0ee2486b1da832618b584c5 Mon Sep 17 00:00:00 2001 From: Shenghou Ma Date: Sun, 3 May 2015 17:53:19 -0400 Subject: [PATCH] hash/crc32: move reverse representation docs to an example Updates #8229. Change-Id: I3e691479d3659ed1b3ff8ebbb71b4fc03f2e67af Reviewed-on: https://go-review.googlesource.com/9680 Reviewed-by: Rob Pike --- src/hash/crc32/crc32.go | 7 ------- src/hash/crc32/example_test.go | 28 ++++++++++++++++++++++++++++ 2 files changed, 28 insertions(+), 7 deletions(-) create mode 100644 src/hash/crc32/example_test.go diff --git a/src/hash/crc32/crc32.go b/src/hash/crc32/crc32.go index 4b13c12d32..10867a79a8 100644 --- a/src/hash/crc32/crc32.go +++ b/src/hash/crc32/crc32.go @@ -8,13 +8,6 @@ // // Polynomials are represented in LSB-first form also known as reversed representation. // -// LSB-first representation is a hexadecimal number with n bits, the most -// significant bit represents the coefficient of x^0 and the least significant -// bit represents the coefficient of x^(n-1). -// -// For example, x^5 + x^2 + x^0 is (binary:10100, hexadecimal:0x14) in 5-bit LSB-first form, -// as opposed to (binary:00101, hexadecimal:0x05) in MSB-first (normal) form. -// // See http://en.wikipedia.org/wiki/Mathematics_of_cyclic_redundancy_checks#Reversed_representations_and_reciprocal_polynomials // for information. package crc32 diff --git a/src/hash/crc32/example_test.go b/src/hash/crc32/example_test.go new file mode 100644 index 0000000000..621bf83830 --- /dev/null +++ b/src/hash/crc32/example_test.go @@ -0,0 +1,28 @@ +// Copyright 2015 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_test + +import ( + "fmt" + "hash/crc32" +) + +func ExampleMakeTable() { + // In this package, the CRC polynomial is represented in reversed notation, + // or LSB-first representation. + // + // LSB-first representation is a hexadecimal number with n bits, in which the + // most significant bit represents the coefficient of x⁰ and the least significant + // bit represents the coefficient of xⁿ⁻¹ (the coefficient for xⁿ is implicit). + // + // For example, CRC32-Q, as defined by the following polynomial, + // x³²+ x³¹+ x²⁴+ x²²+ x¹⁶+ x¹⁴+ x⁸+ x⁷+ x⁵+ x³+ x¹+ x⁰ + // has the reversed notation 0b11010101100000101000001010000001, so the value + // that should be passed to MakeTable is 0xD5828281. + crc32q := crc32.MakeTable(0xD5828281) + fmt.Printf("%08x\n", crc32.Checksum([]byte("Hello world"), crc32q)) + // Output: + // 2964d064 +} -- 2.48.1