Mercator Hash Key

mercator.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

static char *base16 = "0123456789abcdef";

char *mercator_hash(double x, double y, int precision)
{
	double quad_width = 20037508.34;
	double cx = 0.0;
	double cy = 0.0;
	char bits[] = {8,4,2,1};
	char *hash = malloc(precision+1);
	int i = 0, bit = 0, ch = 0, is_x = 1;
	
	while (i < precision)
	{
		if (is_x)
		{
			quad_width /= 2.0;
			printf("[%d] resolution: %g meters\n", i, quad_width);
			
			if (x >= cx)
			{
				ch |= bits[bit];
				cx += quad_width;
			}
			else
			{
				cx -= quad_width;
			}
		}
		else
		{
			if (y >= cy)
			{
				ch |= bits[bit];
				cy += quad_width;
			}
			else
			{
				cy -= quad_width;
			}
		}
		
		is_x = !is_x;
		if (bit < 3)
		{
			bit++;
		}
		else
		{
			hash[i++] = base16[ch];
			bit = ch = 0;
		}
	}
	
	// printf("resolution: %g meters\n", quad_width);

	hash[i] = '\0';
	return hash;
}

int main(int argc, char** argv) 
{

	/* 
	 * echo "-126 45" | proj "+init=epsg:3857" 
	 */
	double x = -14026255.84;
	double y = 5621521.49;
	int precision = 12;
	char *hash;

	if ( argc == 3 )
	{
		x = atof(argv[1]);
		y = atof(argv[2]);
	}

	if ( argc == 4 )
	{
		precision = atoi(argv[3]);
	}

	hash = mercator_hash(x, y, precision);

	printf("%s\n", hash);
}