src/utils/SkBitmapHasher.cpp

/*
 * Copyright 2012 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "SkBitmap.h"
#include "SkBitmapHasher.h"
#include "SkCityHash.h"
#include "SkEndian.h"
#include "SkImageEncoder.h"
#include "SkStream.h"

/**
 * Write an integer value to a stream in little-endian order.
 */
static void write_int_to_buffer(uint32_t val, SkWStream* out) {
    val = SkEndian_SwapLE32(val);
    for (size_t byte = 0; byte < 4; ++byte) {
        out->write8((uint8_t)(val & 0xff));
        val = val >> 8;
    }
}

/*static*/ bool SkBitmapHasher::ComputeDigestInternal(const SkBitmap& bitmap,
                                                      SkHashDigest *result) {
    size_t pixelBufferSize = bitmap.width() * bitmap.height() * 4;
    size_t totalBufferSize = pixelBufferSize + 2 * sizeof(uint32_t);

    SkAutoMalloc bufferManager(totalBufferSize);
    char *bufferStart = static_cast<char *>(bufferManager.get());
    SkMemoryWStream out(bufferStart, totalBufferSize);

    // start with the x/y dimensions
    write_int_to_buffer(SkToU32(bitmap.width()), &out);
    write_int_to_buffer(SkToU32(bitmap.height()), &out);

    // add all the pixel data
    SkAutoTDelete<SkImageEncoder> enc(CreateARGBImageEncoder());
    if (!enc->encodeStream(&out, bitmap, SkImageEncoder::kDefaultQuality)) {
        return false;
    }

    *result = SkCityHash::Compute64(bufferStart, totalBufferSize);
    return true;
}

/*static*/ bool SkBitmapHasher::ComputeDigest(const SkBitmap& bitmap, SkHashDigest *result) {
    if (ComputeDigestInternal(bitmap, result)) {
        return true;
    }

    // Hmm, that didn't work. Maybe if we create a new
    // kARGB_8888_Config version of the bitmap it will work better?
    SkBitmap copyBitmap;
    if (!bitmap.copyTo(&copyBitmap, SkBitmap::kARGB_8888_Config)) {
        return false;
    }
    return ComputeDigestInternal(copyBitmap, result);
}