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 "../private/SkNoncopyable.h" 12 #include "SkString.h" 13 #include "SkTLogic.h" 14 #include "SkTypes.h" 15 16 // #include "SkOpts.h" 17 // It's sort of pesky to be able to include SkOpts.h here, so we'll just re-declare what we need. 18 namespace SkOpts { 19 extern uint32_t (*hash_fn)(const void*, size_t, uint32_t); 20 } 21 22 class SkChecksum : SkNoncopyable { 23 public: 24 /** 25 * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you 26 * suspect its low bits aren't well mixed. 27 * 28 * This is the Murmur3 finalizer. 29 */ Mix(uint32_t hash)30 static uint32_t Mix(uint32_t hash) { 31 hash ^= hash >> 16; 32 hash *= 0x85ebca6b; 33 hash ^= hash >> 13; 34 hash *= 0xc2b2ae35; 35 hash ^= hash >> 16; 36 return hash; 37 } 38 39 /** 40 * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you 41 * suspect its low bits aren't well mixed. 42 * 43 * This version is 2-lines cheaper than Mix, but seems to be sufficient for the font cache. 44 */ CheapMix(uint32_t hash)45 static uint32_t CheapMix(uint32_t hash) { 46 hash ^= hash >> 16; 47 hash *= 0x85ebca6b; 48 hash ^= hash >> 16; 49 return hash; 50 } 51 }; 52 53 // SkGoodHash should usually be your first choice in hashing data. 54 // It should be both reasonably fast and high quality. 55 struct SkGoodHash { 56 template <typename K> operatorSkGoodHash57 SK_WHEN(sizeof(K) == 4, uint32_t) operator()(const K& k) const { 58 return SkChecksum::Mix(*(const uint32_t*)&k); 59 } 60 61 template <typename K> operatorSkGoodHash62 SK_WHEN(sizeof(K) != 4, uint32_t) operator()(const K& k) const { 63 return SkOpts::hash_fn(&k, sizeof(K), 0); 64 } 65 operatorSkGoodHash66 uint32_t operator()(const SkString& k) const { 67 return SkOpts::hash_fn(k.c_str(), k.size(), 0); 68 } 69 }; 70 71 #endif 72