1 /* 2 * Copyright 2012 The WebRTC Project Authors. All rights reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 #include "webrtc/base/crc32.h" 12 13 #include "webrtc/base/arraysize.h" 14 15 namespace rtc { 16 17 // This implementation is based on the sample implementation in RFC 1952. 18 19 // CRC32 polynomial, in reversed form. 20 // See RFC 1952, or http://en.wikipedia.org/wiki/Cyclic_redundancy_check 21 static const uint32_t kCrc32Polynomial = 0xEDB88320; 22 static uint32_t kCrc32Table[256] = {0}; 23 24 static void EnsureCrc32TableInited() { 25 if (kCrc32Table[arraysize(kCrc32Table) - 1]) 26 return; // already inited 27 for (uint32_t i = 0; i < arraysize(kCrc32Table); ++i) { 28 uint32_t c = i; 29 for (size_t j = 0; j < 8; ++j) { 30 if (c & 1) { 31 c = kCrc32Polynomial ^ (c >> 1); 32 } else { 33 c >>= 1; 34 } 35 } 36 kCrc32Table[i] = c; 37 } 38 } 39 40 uint32_t UpdateCrc32(uint32_t start, const void* buf, size_t len) { 41 EnsureCrc32TableInited(); 42 43 uint32_t c = start ^ 0xFFFFFFFF; 44 const uint8_t* u = static_cast<const uint8_t*>(buf); 45 for (size_t i = 0; i < len; ++i) { 46 c = kCrc32Table[(c ^ u[i]) & 0xFF] ^ (c >> 8); 47 } 48 return c ^ 0xFFFFFFFF; 49 } 50 51 } // namespace rtc 52 53
