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 /*

  * 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:

     /**

      * 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) {

         SkASSERT(SkIsAlign4(bytes));

         const size_t words = bytes/4;

         uint32_t hash = seed;

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

             uint32_t k = data[i];

             k *= 0xcc9e2d51;

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

             k *= 0x1b873593;

             hash ^= k;

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

             hash *= 5;

             hash += 0xe6546b64;

         }

         hash ^= bytes;

         hash ^= hash >> 16;

         hash *= 0x85ebca6b;

         hash ^= hash >> 13;

         hash *= 0xc2b2ae35;

         hash ^= hash >> 16;

         return 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) {

         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*>(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);

         data = reinterpret_cast<const uint32_t*>(ptr);

         const uint32_t* stop = data + (size >> 2);

         while (data < stop) {

             result = Mash(result, *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