projection.c

#include <math.h>
#include "projection.h"

// http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames
void latlon2tile(double lat, double lon, int zoom, long long *x, long long *y) {
	// Must limit latitude somewhere to prevent overflow.
	// 89.9 degrees latitude is 0.621 worlds beyond the edge of the flat earth,
	// hopefully far enough out that there are few expectations about the shape.
	if (lat < -89.9) {
		lat = -89.9;
	}
	if (lat > 89.9) {
		lat = 89.9;
	}

	if (lon < -360) {
		lon = -360;
	}
	if (lon > 360) {
		lon = 360;
	}

	double lat_rad = lat * M_PI / 180;
	unsigned long long n = 1LL << zoom;

	long long llx = n * ((lon + 180) / 360);
	long long lly = n * (1 - (log(tan(lat_rad) + 1 / cos(lat_rad)) / M_PI)) / 2;

	*x = llx;
	*y = lly;
}

// http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames
void tile2latlon(long long x, long long y, int zoom, double *lat, double *lon) {
	unsigned long long n = 1LL << zoom;
	*lon = 360.0 * x / n - 180.0;
	*lat = atan(sinh(M_PI * (1 - 2.0 * y / n))) * 180.0 / M_PI;
}

unsigned long long encode(unsigned int wx, unsigned int wy) {
	long long out = 0;

	int i;
	for (i = 0; i < 32; i++) {
		long long v = ((wx >> (32 - (i + 1))) & 1) << 1;
		v |= (wy >> (32 - (i + 1))) & 1;
		v = v << (64 - 2 * (i + 1));

		out |= v;
	}

	return out;
}

void decode(unsigned long long index, unsigned *wx, unsigned *wy) {
	*wx = *wy = 0;

	int i;
	for (i = 0; i < 32; i++) {
		*wx |= ((index >> (64 - 2 * (i + 1) + 1)) & 1) << (32 - (i + 1));
		*wy |= ((index >> (64 - 2 * (i + 1) + 0)) & 1) << (32 - (i + 1));
	}
}