geohash.c

/*
  +----------------------------------------------------------------------+
  | PHP Version 7                                                        |
  +----------------------------------------------------------------------+
  | Copyright (c) 2017 The PHP Group                                     |
  +----------------------------------------------------------------------+
  | This source file is subject to version 3.01 of the PHP license,      |
  | that is bundled with this package in the file LICENSE, and is        |
  | available through the world-wide-web at the following url:           |
  | http://www.php.net/license/3_01.txt                                  |
  | If you did not receive a copy of the PHP license and are unable to   |
  | obtain it through the world-wide-web, please send a note to          |
  | license@php.net so we can mail you a copy immediately.               |
  +----------------------------------------------------------------------+
  | Authors: Emir Beganovic <emir@php.net>                               |
  +----------------------------------------------------------------------+
*/

#include <stdlib.h>

#include "php.h"
#include "geo_lat_long.h"
#include "geohash.h"

#define MAX_LAT   90.0
#define MIN_LAT  -90.0

#define MAX_LONG  180.0
#define MIN_LONG -180.0

typedef struct interval_string {
	double high;
	double low;
} interval_struct;

static char char_map[32] = "0123456789bcdefghjkmnpqrstuvwxyz";
static size_t char_map_size = sizeof(char_map);

char *php_geo_geohash_encode(double latitude, double longitude, int precision)
{
	char            *hash;
	int              steps;
	double           coord, mid;
	int              is_even = 1;
	unsigned int     hash_char = 0;
	int              i;
	interval_struct  lat_interval = { MAX_LAT, MIN_LAT };
	interval_struct  lng_interval = { MAX_LONG, MIN_LONG };
	interval_struct *interval;

        if (precision < 0) {
            precision = 0;
        }

	hash = (char*)safe_emalloc(precision, sizeof(char), 1);

	hash[precision] = '\0';
	steps = precision * 5.0;

	for (i = 1; i <= steps; i++) {
		if (is_even) {
			interval = &lng_interval;
			coord = longitude;
		} else {
			interval = &lat_interval;
			coord = latitude;
		}

		mid = (interval->low + interval->high) / 2.0;
		hash_char = hash_char << 1;

		if (coord > mid) {
			interval->low = mid;
			hash_char |= 0x01;
		} else {
			interval->high = mid;
		}

		if (!(i % 5)) {
			hash[(i - 1) / 5] = char_map[hash_char];
			hash_char = 0;
		}

		is_even = !is_even;
	}

	return hash;
}

static unsigned int index_for_char(char c, char *string, size_t string_amount)
{
	unsigned int index = -1;
	int          i;

	for (i = 0; i < string_amount; i++) {
		if (c == string[i]) {
			index = i;
			break;
		}
	}

	return index;
}

geo_lat_long php_geo_geohash_decode(char *hash, size_t char_amount)
{
	geo_lat_long coordinate;

	if (char_amount) {
		int              charmap_index;
		double           delta;
		int              i, j;
		interval_struct  lat_interval = { MAX_LAT, MIN_LAT };
		interval_struct  lng_interval = { MAX_LONG, MIN_LONG };
		interval_struct *interval;

		int is_even = 1;

		for (i = 0; i < char_amount; i++) {
			charmap_index = index_for_char(hash[i], char_map, char_map_size);

			/* Interpret the last 5 bits of the integer */
			for (j = 0; j < 5; j++) {
				interval = is_even ? &lng_interval : &lat_interval;

				delta = (interval->high - interval->low) / 2.0;

				if ((charmap_index << j) & 0x0010) {
					interval->low += delta;
				} else {
					interval->high -= delta;
				}

				is_even = !is_even;
			}
		}

		coordinate.x = lat_interval.high - ((lat_interval.high - lat_interval.low) / 2.0);
		coordinate.y = lng_interval.high - ((lng_interval.high - lng_interval.low) / 2.0);
		coordinate.z = 0;
	}

	return coordinate;
}