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 /**
16 * Computes a 32bit checksum from a blob of 32bit aligned data. This is meant
17 * to be very very fast, as it is used internally by the font cache, in
18 * conjuction with the entire raw key. This algorithm does not generate
19 * unique values as well as others (e.g. MD5) but it performs much faster.
20 * Skia's use cases can survive non-unique values (since the entire key is
21 * always available). Clients should only be used in circumstances where speed
22 * over uniqueness is at a premium.
23 */
24 class SkChecksum : SkNoncopyable {
25 private:
26 /*
27 * Our Rotate and Mash helpers are meant to automatically do the right
28 * thing depending if sizeof(uintptr_t) is 4 or 8.
29 */
30 enum {
31 ROTR = 17,
32 ROTL = sizeof(uintptr_t) * 8 - ROTR,
33 HALFBITS = sizeof(uintptr_t) * 4
34 };
35
Mash(uintptr_t total,uintptr_t value)36 static inline uintptr_t Mash(uintptr_t total, uintptr_t value) {
37 return ((total >> ROTR) | (total << ROTL)) ^ value;
38 }
39
40 public:
41 /**
42 * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you
43 * suspect its low bits aren't well mixed.
44 *
45 * This is the Murmur3 finalizer.
46 */
Mix(uint32_t hash)47 static uint32_t Mix(uint32_t hash) {
48 hash ^= hash >> 16;
49 hash *= 0x85ebca6b;
50 hash ^= hash >> 13;
51 hash *= 0xc2b2ae35;
52 hash ^= hash >> 16;
53 return hash;
54 }
55
56 /**
57 * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you
58 * suspect its low bits aren't well mixed.
59 *
60 * This version is 2-lines cheaper than Mix, but seems to be sufficient for the font cache.
61 */
CheapMix(uint32_t hash)62 static uint32_t CheapMix(uint32_t hash) {
63 hash ^= hash >> 16;
64 hash *= 0x85ebca6b;
65 hash ^= hash >> 16;
66 return hash;
67 }
68
69 /**
70 * Calculate 32-bit Murmur hash (murmur3).
71 * This should take 2-3x longer than SkChecksum::Compute, but is a considerably better hash.
72 * See en.wikipedia.org/wiki/MurmurHash.
73 *
74 * @param data Memory address of the data block to be processed.
75 * @param size Size of the data block in bytes.
76 * @param seed Initial hash seed. (optional)
77 * @return hash result
78 */
79 static uint32_t Murmur3(const void* data, size_t bytes, uint32_t seed=0) {
80 // Use may_alias to remind the compiler we're intentionally violating strict aliasing,
81 // and so not to apply strict-aliasing-based optimizations.
82 typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t;
83 typedef uint8_t SK_ATTRIBUTE(may_alias) aliased_uint8_t;
84
85 // Handle 4 bytes at a time while possible.
86 const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data;
87 const size_t words = bytes/4;
88 uint32_t hash = seed;
89 for (size_t i = 0; i < words; i++) {
90 uint32_t k = safe_data[i];
91 k *= 0xcc9e2d51;
92 k = (k << 15) | (k >> 17);
93 k *= 0x1b873593;
94
95 hash ^= k;
96 hash = (hash << 13) | (hash >> 19);
97 hash *= 5;
98 hash += 0xe6546b64;
99 }
100
101 // Handle last 0-3 bytes.
102 const aliased_uint8_t* safe_tail = (const uint8_t*)(safe_data + words);
103 uint32_t k = 0;
104 switch (bytes & 3) {
105 case 3: k ^= safe_tail[2] << 16;
106 case 2: k ^= safe_tail[1] << 8;
107 case 1: k ^= safe_tail[0] << 0;
108 k *= 0xcc9e2d51;
109 k = (k << 15) | (k >> 17);
110 k *= 0x1b873593;
111 hash ^= k;
112 }
113
114 hash ^= bytes;
115 return Mix(hash);
116 }
117
118 /**
119 * Compute a 32-bit checksum for a given data block
120 *
121 * WARNING: this algorithm is tuned for efficiency, not backward/forward
122 * compatibility. It may change at any time, so a checksum generated with
123 * one version of the Skia code may not match a checksum generated with
124 * a different version of the Skia code.
125 *
126 * @param data Memory address of the data block to be processed. Must be
127 * 32-bit aligned.
128 * @param size Size of the data block in bytes. Must be a multiple of 4.
129 * @return checksum result
130 */
Compute(const uint32_t * data,size_t size)131 static uint32_t Compute(const uint32_t* data, size_t size) {
132 // Use may_alias to remind the compiler we're intentionally violating strict aliasing,
133 // and so not to apply strict-aliasing-based optimizations.
134 typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t;
135 const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data;
136
137 SkASSERT(SkIsAlign4(size));
138
139 /*
140 * We want to let the compiler use 32bit or 64bit addressing and math
141 * so we use uintptr_t as our magic type. This makes the code a little
142 * more obscure (we can't hard-code 32 or 64 anywhere, but have to use
143 * sizeof()).
144 */
145 uintptr_t result = 0;
146 const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(safe_data);
147
148 /*
149 * count the number of quad element chunks. This takes into account
150 * if we're on a 32bit or 64bit arch, since we use sizeof(uintptr_t)
151 * to compute how much to shift-down the size.
152 */
153 size_t n4 = size / (sizeof(uintptr_t) << 2);
154 for (size_t i = 0; i < n4; ++i) {
155 result = Mash(result, *ptr++);
156 result = Mash(result, *ptr++);
157 result = Mash(result, *ptr++);
158 result = Mash(result, *ptr++);
159 }
160 size &= ((sizeof(uintptr_t) << 2) - 1);
161
162 safe_data = reinterpret_cast<const aliased_uint32_t*>(ptr);
163 const aliased_uint32_t* stop = safe_data + (size >> 2);
164 while (safe_data < stop) {
165 result = Mash(result, *safe_data++);
166 }
167
168 /*
169 * smash us down to 32bits if we were 64. Note that when uintptr_t is
170 * 32bits, this code-path should go away, but I still got a warning
171 * when I wrote
172 * result ^= result >> 32;
173 * since >>32 is undefined for 32bit ints, hence the wacky HALFBITS
174 * define.
175 */
176 if (8 == sizeof(result)) {
177 result ^= result >> HALFBITS;
178 }
179 return static_cast<uint32_t>(result);
180 }
181 };
182
183 // SkGoodHash should usually be your first choice in hashing data.
184 // It should be both reasonably fast and high quality.
185
186 template <typename K>
SkGoodHash(const K & k)187 uint32_t SkGoodHash(const K& k) {
188 if (sizeof(K) == 4) {
189 return SkChecksum::Mix(*(const uint32_t*)&k);
190 }
191 return SkChecksum::Murmur3(&k, sizeof(K));
192 }
193
SkGoodHash(const SkString & k)194 inline uint32_t SkGoodHash(const SkString& k) {
195 return SkChecksum::Murmur3(k.c_str(), k.size());
196 }
197
198 #endif
199