src/libguac/hash.c

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

#include "config.h"

#include <cairo/cairo.h>

#include <stdint.h>
#include <string.h>

/*
 * Arbitrary hash function whhich maps ALL 32-bit numbers onto 24-bit numbers
 * evenly, while guaranteeing that all 24-bit numbers are mapped onto
 * themselves.
 */
unsigned int _guac_hash_32to24(unsigned int value) {

    /* Grab highest-order byte */
    unsigned int upper = value & 0xFF000000;

    /* XOR upper with lower three bytes, truncate to 24-bit */
    return
          (value & 0xFFFFFF)
        ^ (upper >> 8)
        ^ (upper >> 16)
        ^ (upper >> 24);

}

/**
 * Rotates a given 32-bit integer by N bits.
 *
 * NOTE: We probably should check for available bitops.h macros first.
 */
unsigned int _guac_rotate(unsigned int value, int amount) {

    /* amount = amount % 32 */
    amount &= 0x1F; 

    /* Return rotated amount */
    return (value >> amount) | (value << (32 - amount));

}

unsigned int guac_hash_surface(cairo_surface_t* surface) {

    /* Init to zero */
    unsigned int hash_value = 0;

    int x, y;

    /* Get image data and metrics */
    unsigned char* data = cairo_image_surface_get_data(surface);
    int width = cairo_image_surface_get_width(surface);
    int height = cairo_image_surface_get_height(surface);
    int stride = cairo_image_surface_get_stride(surface);

    for (y=0; y<height; y++) {

        /* Get current row */
        uint32_t* row = (uint32_t*) data;
        data += stride;

        for (x=0; x<width; x++) {

            /* Get color at current pixel */
            unsigned int color = *row;
            row++;

            /* Compute next hash */
            hash_value =
                _guac_rotate(hash_value, 1) ^ color ^ 0x1B872E69;

        }

    } /* end for each row */

    /* Done */
    return _guac_hash_32to24(hash_value);

}

int guac_surface_cmp(cairo_surface_t* a, cairo_surface_t* b) {

    /* Surface A metrics */
    unsigned char* data_a = cairo_image_surface_get_data(a);
    int width_a = cairo_image_surface_get_width(a);
    int height_a = cairo_image_surface_get_height(a);
    int stride_a = cairo_image_surface_get_stride(a);

    /* Surface B metrics */
    unsigned char* data_b = cairo_image_surface_get_data(b);
    int width_b = cairo_image_surface_get_width(b);
    int height_b = cairo_image_surface_get_height(b);
    int stride_b = cairo_image_surface_get_stride(b);

    int y;

    /* If core dimensions differ, just compare those. Done. */
    if (width_a != width_b) return width_a - width_b;
    if (height_a != height_b) return height_a - height_b;

    for (y=0; y<height_a; y++) {

        /* Compare row. If different, use that result. */
        int cmp_result = memcmp(data_a, data_b, width_a * 4);
        if (cmp_result != 0)
            return cmp_result;

        /* Next row */
        data_a += stride_a;
        data_b += stride_b;

    }

    /* Otherwise, same. */
    return 0;

}