1 /* 2 * Copyright 2012 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkChecksum_DEFINED 9 #define SkChecksum_DEFINED 10 11 #include "SkTypes.h" 12 13 /** 14 * Computes a 32bit checksum from a blob of 32bit aligned data. This is meant 15 * to be very very fast, as it is used internally by the font cache, in 16 * conjuction with the entire raw key. This algorithm does not generate 17 * unique values as well as others (e.g. MD5) but it performs much faster. 18 * Skia's use cases can survive non-unique values (since the entire key is 19 * always available). Clients should only be used in circumstances where speed 20 * over uniqueness is at a premium. 21 */ 22 class SkChecksum : SkNoncopyable { 23 private: 24 /* 25 * Our Rotate and Mash helpers are meant to automatically do the right 26 * thing depending if sizeof(uintptr_t) is 4 or 8. 27 */ 28 enum { 29 ROTR = 17, 30 ROTL = sizeof(uintptr_t) * 8 - ROTR, 31 HALFBITS = sizeof(uintptr_t) * 4 32 }; 33 Mash(uintptr_t total,uintptr_t value)34 static inline uintptr_t Mash(uintptr_t total, uintptr_t value) { 35 return ((total >> ROTR) | (total << ROTL)) ^ value; 36 } 37 38 public: 39 40 /** 41 * Calculate 32-bit Murmur hash (murmur3). 42 * This should take 2-3x longer than SkChecksum::Compute, but is a considerably better hash. 43 * See en.wikipedia.org/wiki/MurmurHash. 44 * 45 * @param data Memory address of the data block to be processed. Must be 32-bit aligned. 46 * @param size Size of the data block in bytes. Must be a multiple of 4. 47 * @param seed Initial hash seed. (optional) 48 * @return hash result 49 */ 50 static uint32_t Murmur3(const uint32_t* data, size_t bytes, uint32_t seed=0) { 51 SkASSERT(SkIsAlign4(bytes)); 52 const size_t words = bytes/4; 53 54 uint32_t hash = seed; 55 for (size_t i = 0; i < words; i++) { 56 uint32_t k = data[i]; 57 k *= 0xcc9e2d51; 58 k = (k << 15) | (k >> 17); 59 k *= 0x1b873593; 60 61 hash ^= k; 62 hash = (hash << 13) | (hash >> 19); 63 hash *= 5; 64 hash += 0xe6546b64; 65 } 66 hash ^= bytes; 67 hash ^= hash >> 16; 68 hash *= 0x85ebca6b; 69 hash ^= hash >> 13; 70 hash *= 0xc2b2ae35; 71 hash ^= hash >> 16; 72 return hash; 73 } 74 75 /** 76 * Compute a 32-bit checksum for a given data block 77 * 78 * WARNING: this algorithm is tuned for efficiency, not backward/forward 79 * compatibility. It may change at any time, so a checksum generated with 80 * one version of the Skia code may not match a checksum generated with 81 * a different version of the Skia code. 82 * 83 * @param data Memory address of the data block to be processed. Must be 84 * 32-bit aligned. 85 * @param size Size of the data block in bytes. Must be a multiple of 4. 86 * @return checksum result 87 */ Compute(const uint32_t * data,size_t size)88 static uint32_t Compute(const uint32_t* data, size_t size) { 89 SkASSERT(SkIsAlign4(size)); 90 91 /* 92 * We want to let the compiler use 32bit or 64bit addressing and math 93 * so we use uintptr_t as our magic type. This makes the code a little 94 * more obscure (we can't hard-code 32 or 64 anywhere, but have to use 95 * sizeof()). 96 */ 97 uintptr_t result = 0; 98 const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(data); 99 100 /* 101 * count the number of quad element chunks. This takes into account 102 * if we're on a 32bit or 64bit arch, since we use sizeof(uintptr_t) 103 * to compute how much to shift-down the size. 104 */ 105 size_t n4 = size / (sizeof(uintptr_t) << 2); 106 for (size_t i = 0; i < n4; ++i) { 107 result = Mash(result, *ptr++); 108 result = Mash(result, *ptr++); 109 result = Mash(result, *ptr++); 110 result = Mash(result, *ptr++); 111 } 112 size &= ((sizeof(uintptr_t) << 2) - 1); 113 114 data = reinterpret_cast<const uint32_t*>(ptr); 115 const uint32_t* stop = data + (size >> 2); 116 while (data < stop) { 117 result = Mash(result, *data++); 118 } 119 120 /* 121 * smash us down to 32bits if we were 64. Note that when uintptr_t is 122 * 32bits, this code-path should go away, but I still got a warning 123 * when I wrote 124 * result ^= result >> 32; 125 * since >>32 is undefined for 32bit ints, hence the wacky HALFBITS 126 * define. 127 */ 128 if (8 == sizeof(result)) { 129 result ^= result >> HALFBITS; 130 } 131 return static_cast<uint32_t>(result); 132 } 133 }; 134 135 #endif 136