skia2/include/private/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 "../private/SkNoncopyable.h"
#include "SkString.h"
#include "SkTLogic.h"
#include "SkTypes.h"

// #include "SkOpts.h"
// It's sort of pesky to be able to include SkOpts.h here, so we'll just re-declare what we need.
namespace SkOpts {
    extern uint32_t (*hash_fn)(const void*, size_t, uint32_t);
}

class SkChecksum : SkNoncopyable {
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;
    }
};

// SkGoodHash should usually be your first choice in hashing data.
// It should be both reasonably fast and high quality.
struct SkGoodHash {
    template <typename K>
    SK_WHEN(sizeof(K) == 4, uint32_t) operator()(const K& k) const {
        return SkChecksum::Mix(*(const uint32_t*)&k);
    }

    template <typename K>
    SK_WHEN(sizeof(K) != 4, uint32_t) operator()(const K& k) const {
        return SkOpts::hash_fn(&k, sizeof(K), 0);
    }

    uint32_t operator()(const SkString& k) const {
        return SkOpts::hash_fn(k.c_str(), k.size(), 0);
    }
};

#endif