geohash.c

#include <stdio.h>

#define EVEN(x) ((x % 2) == 0)

static char *base32_encoder = "0123456789bcdefghjkmnpqrstuvwxyz";

typedef struct {
        double current_guess;
        double error_margin;
        double real_value;
} BitIterator;

int next_bit(BitIterator *g) {
        g->error_margin = g->error_margin/2;
        if (g->real_value < g->current_guess) {
                g->current_guess -= g->error_margin;
                return 0;
        }
        g->current_guess += g->error_margin;
        return 1;
}

void geohash_encode(double lat, double lon, char buf[], int len) {
        int i;
        BitIterator lat_bits = {0, 90, lat};
        BitIterator lon_bits = {0, 180, lon};

        for (i = 0; i < len; i++) {
                int bits = 0;
                int j;
                for (j = 0; j < 5; j++) {
                        int bit_position = i * 5 + j;
                        int bit = next_bit(EVEN(bit_position) ?
                                        &lon_bits : &lat_bits);
                        bits |= bit << (4-j);
                }
                buf[i] = base32_encoder[bits];
        }
        buf[i] = '\0';
}

int main(void) {
        char hash[51];

        geohash_encode(42.6, -5.6, hash, 22);
        printf("%s\n", hash);

        return 0;
}