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 "SkString.h" 12 #include "SkTLogic.h" 13 #include "SkTypes.h" 14 15 // #include "SkOpts.h" 16 // It's sort of pesky to be able to include SkOpts.h here, so we'll just re-declare what we need. 17 namespace SkOpts { 18 extern uint32_t (*hash_fn)(const void*, size_t, uint32_t); 19 } 20 21 class SkChecksum : SkNoncopyable { 22 public: 23 /** 24 * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you 25 * suspect its low bits aren't well mixed. 26 * 27 * This is the Murmur3 finalizer. 28 */ 29 static uint32_t Mix(uint32_t hash) { 30 hash ^= hash >> 16; 31 hash *= 0x85ebca6b; 32 hash ^= hash >> 13; 33 hash *= 0xc2b2ae35; 34 hash ^= hash >> 16; 35 return hash; 36 } 37 38 /** 39 * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you 40 * suspect its low bits aren't well mixed. 41 * 42 * This version is 2-lines cheaper than Mix, but seems to be sufficient for the font cache. 43 */ 44 static uint32_t CheapMix(uint32_t hash) { 45 hash ^= hash >> 16; 46 hash *= 0x85ebca6b; 47 hash ^= hash >> 16; 48 return hash; 49 } 50 }; 51 52 // SkGoodHash should usually be your first choice in hashing data. 53 // It should be both reasonably fast and high quality. 54 struct SkGoodHash { 55 template <typename K> 56 SK_WHEN(sizeof(K) == 4, uint32_t) operator()(const K& k) const { 57 return SkChecksum::Mix(*(const uint32_t*)&k); 58 } 59 60 template <typename K> 61 SK_WHEN(sizeof(K) != 4, uint32_t) operator()(const K& k) const { 62 return SkOpts::hash_fn(&k, sizeof(K), 0); 63 } 64 65 uint32_t operator()(const SkString& k) const { 66 return SkOpts::hash_fn(k.c_str(), k.size(), 0); 67 } 68 }; 69 70 #endif 71
