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      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 
     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      */
     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