#include <sys/stat.h>		/* struct stat */
#include <stdlib.h>		/* malloc() */
#include <stdio.h>		/* fprintf() */
#include <math.h>		/* sqrt() */
#include "stl.h"

/* load a STL file and return a pointer to the stl_t struct */
stl_t *	stl_load(const char * filename)
{
	struct stat s;
	if (stat(filename, &s))
	{
		fprintf(stderr, "Couldn't stat '%s'\n", filename);
		return NULL;
	}
	if (s.st_size < sizeof(stl_t))
	{
		fprintf(stderr, "Size of '%s' only %d bytes!\n", filename, s.st_size);
		return NULL;
	}
	FILE *fil;
	if ((fil = fopen(filename, "rb")) == NULL)
	{
		fprintf(stderr, "Couldn't open '%s'\n", filename);
		return NULL;
	}
	stl_t * stl = (stl_t *) malloc(s.st_size);
	fread(stl, 1, s.st_size, fil);
	fclose(fil);
	if (s.st_size <
		sizeof(stl_t) + sizeof(stl_face_t) * (stl_num_faces(stl)-1))
	{
		fprintf(stderr, "Size of '%s' only %d bytes, but has %d faces!\n",
			filename, s.st_size, stl_num_faces(stl));
		free(stl);
		return NULL;
	}
	return stl;
}

/* fill vector pointed to by normal with the normal of the face */
void	stl_get_face_normal(stl_t * stl, int face, float * normal)
{
	float e1[3];
	float e2[3];
	int i;
	for (i = 0; i < 3; i++)
	{
		e1[i] = stl_face(stl, face).vertices[1][i] -
			stl_face(stl, face).vertices[0][i];
		e2[i] = stl_face(stl, face).vertices[2][i] -
			stl_face(stl, face).vertices[1][i];
	}

	/* take the cross product of the first two edges */
	normal[0] = e1[1] * e2[2] - e1[2] * e2[1];
	normal[1] = e1[2] * e2[0] - e1[0] * e2[2];
	normal[2] = e1[0] * e2[1] - e1[1] * e2[0];

	/* then normalize */
	double l;
	l = sqrt(normal[0] * normal[0] +
		 normal[1] * normal[1] +
		 normal[2] * normal[2]);
	normal[0] /= l;
	normal[1] /= l;
	normal[2] /= l;
}