Move SkTHash.h to include/private.

src/core/SkChecksum.h

-/*
- * 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 "SkString.h"
-#include "SkTLogic.h"
-#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;
-    }
-
-    /**
-     * 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 version is 2-lines cheaper than Mix, but seems to be sufficient for the font cache.
-     */
-    static uint32_t CheapMix(uint32_t hash) {
-        hash ^= hash >> 16;
-        hash *= 0x85ebca6b;
-        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.
-     *  @param size Size of the data block in bytes.
-     *  @param seed Initial hash seed. (optional)
-     *  @return hash result
-     */
-    static uint32_t Murmur3(const void* 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;
-        typedef uint8_t SK_ATTRIBUTE(may_alias) aliased_uint8_t;
-
-        // Handle 4 bytes at a time while possible.
-        const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data;
-        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;
-        }
-
-        // Handle last 0-3 bytes.
-        const aliased_uint8_t* safe_tail = (const uint8_t*)(safe_data + words);
-        uint32_t k = 0;
-        switch (bytes & 3) {
-            case 3: k ^= safe_tail[2] << 16;
-            case 2: k ^= safe_tail[1] <<  8;
-            case 1: k ^= safe_tail[0] <<  0;
-                    k *= 0xcc9e2d51;
-                    k = (k << 15) | (k >> 17);
-                    k *= 0x1b873593;
-                    hash ^= k;
-        }
-
-        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);
-    }
-};
-
-// SkGoodHash should usually be your first choice in hashing data.
-// It should be both reasonably fast and high quality.
-
-template <typename K>
-uint32_t SkGoodHash(const K& k) {
-    if (sizeof(K) == 4) {
-        return SkChecksum::Mix(*(const uint32_t*)&k);
-    }
-    return SkChecksum::Murmur3(&k, sizeof(K));
-}
-
-inline uint32_t SkGoodHash(const SkString& k) {
-    return SkChecksum::Murmur3(k.c_str(), k.size());
-}
-
-#endif