/* | |

* Copyright 2012 Google Inc. | |

* | |

* Use of this source code is governed by a BSD-style license that can be | |

* found in the LICENSE file. | |

*/ | |

#ifndef SkChecksum_DEFINED | |

#define SkChecksum_DEFINED | |

#include "SkTypes.h" | |

/** | |

* Computes a 32bit checksum from a blob of 32bit aligned data. This is meant | |

* to be very very fast, as it is used internally by the font cache, in | |

* conjuction with the entire raw key. This algorithm does not generate | |

* unique values as well as others (e.g. MD5) but it performs much faster. | |

* Skia's use cases can survive non-unique values (since the entire key is | |

* always available). Clients should only be used in circumstances where speed | |

* over uniqueness is at a premium. | |

*/ | |

class SkChecksum : SkNoncopyable { | |

private: | |

/* | |

* Our Rotate and Mash helpers are meant to automatically do the right | |

* thing depending if sizeof(uintptr_t) is 4 or 8. | |

*/ | |

enum { | |

ROTR = 17, | |

ROTL = sizeof(uintptr_t) * 8 - ROTR, | |

HALFBITS = sizeof(uintptr_t) * 4 | |

}; | |

static inline uintptr_t Mash(uintptr_t total, uintptr_t value) { | |

return ((total >> ROTR) | (total << ROTL)) ^ value; | |

} | |

public: | |

/** | |

* uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you | |

* suspect its low bits aren't well mixed. | |

* | |

* This is the Murmur3 finalizer. | |

*/ | |

static uint32_t Mix(uint32_t hash) { | |

hash ^= hash >> 16; | |

hash *= 0x85ebca6b; | |

hash ^= hash >> 13; | |

hash *= 0xc2b2ae35; | |

hash ^= hash >> 16; | |

return hash; | |

} | |

/** | |

* Calculate 32-bit Murmur hash (murmur3). | |

* This should take 2-3x longer than SkChecksum::Compute, but is a considerably better hash. | |

* See en.wikipedia.org/wiki/MurmurHash. | |

* | |

* @param data Memory address of the data block to be processed. Must be 32-bit aligned. | |

* @param size Size of the data block in bytes. Must be a multiple of 4. | |

* @param seed Initial hash seed. (optional) | |

* @return hash result | |

*/ | |

static uint32_t Murmur3(const uint32_t* data, size_t bytes, uint32_t seed=0) { | |

// Use may_alias to remind the compiler we're intentionally violating strict aliasing, | |

// and so not to apply strict-aliasing-based optimizations. | |

typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t; | |

const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data; | |

SkASSERTF(SkIsAlign4(bytes), "Expected 4-byte multiple, got %zu", bytes); | |

const size_t words = bytes/4; | |

uint32_t hash = seed; | |

for (size_t i = 0; i < words; i++) { | |

uint32_t k = safe_data[i]; | |

k *= 0xcc9e2d51; | |

k = (k << 15) | (k >> 17); | |

k *= 0x1b873593; | |

hash ^= k; | |

hash = (hash << 13) | (hash >> 19); | |

hash *= 5; | |

hash += 0xe6546b64; | |

} | |

hash ^= bytes; | |

return Mix(hash); | |

} | |

/** | |

* Compute a 32-bit checksum for a given data block | |

* | |

* WARNING: this algorithm is tuned for efficiency, not backward/forward | |

* compatibility. It may change at any time, so a checksum generated with | |

* one version of the Skia code may not match a checksum generated with | |

* a different version of the Skia code. | |

* | |

* @param data Memory address of the data block to be processed. Must be | |

* 32-bit aligned. | |

* @param size Size of the data block in bytes. Must be a multiple of 4. | |

* @return checksum result | |

*/ | |

static uint32_t Compute(const uint32_t* data, size_t size) { | |

// Use may_alias to remind the compiler we're intentionally violating strict aliasing, | |

// and so not to apply strict-aliasing-based optimizations. | |

typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t; | |

const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data; | |

SkASSERT(SkIsAlign4(size)); | |

/* | |

* We want to let the compiler use 32bit or 64bit addressing and math | |

* so we use uintptr_t as our magic type. This makes the code a little | |

* more obscure (we can't hard-code 32 or 64 anywhere, but have to use | |

* sizeof()). | |

*/ | |

uintptr_t result = 0; | |

const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(safe_data); | |

/* | |

* count the number of quad element chunks. This takes into account | |

* if we're on a 32bit or 64bit arch, since we use sizeof(uintptr_t) | |

* to compute how much to shift-down the size. | |

*/ | |

size_t n4 = size / (sizeof(uintptr_t) << 2); | |

for (size_t i = 0; i < n4; ++i) { | |

result = Mash(result, *ptr++); | |

result = Mash(result, *ptr++); | |

result = Mash(result, *ptr++); | |

result = Mash(result, *ptr++); | |

} | |

size &= ((sizeof(uintptr_t) << 2) - 1); | |

safe_data = reinterpret_cast<const aliased_uint32_t*>(ptr); | |

const aliased_uint32_t* stop = safe_data + (size >> 2); | |

while (safe_data < stop) { | |

result = Mash(result, *safe_data++); | |

} | |

/* | |

* smash us down to 32bits if we were 64. Note that when uintptr_t is | |

* 32bits, this code-path should go away, but I still got a warning | |

* when I wrote | |

* result ^= result >> 32; | |

* since >>32 is undefined for 32bit ints, hence the wacky HALFBITS | |

* define. | |

*/ | |

if (8 == sizeof(result)) { | |

result ^= result >> HALFBITS; | |

} | |

return static_cast<uint32_t>(result); | |

} | |

}; | |

#endif |