vector.c

/*
 * Elite - The New Kind.
 *
 * Reverse engineered from the BBC disk version of Elite.
 * Additional material by C.J.Pinder.
 *
 * The original Elite code is (C) I.Bell & D.Braben 1984.
 * This version re-engineered in C by C.J.Pinder 1999-2001.
 *
 * email: <christian@newkind.co.uk>
 *
 */


/*
 * The original Elite code did all the vector calculations using 8-bit integers.
 *
 * Writing all the routines in C to use 8 bit ints would have been fairly pointless.
 * I have, therefore, written a new set of routines which use floating point math.
 */

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

#include "config.h"
#include "vector.h"



static Matrix start_matrix =
{
	{1.0, 0.0, 0.0},
	{0.0, 1.0, 0.0},
	{0.0, 0.0,-1.0}
};



/*
 * Multiply first matrix by second matrix.
 * Put result into first matrix.
 */


void mult_matrix (struct vector *first, struct vector *second)
{
	int i;
	Matrix rv;

	for (i = 0; i < 3; i++)
	{

		rv[i].x =	(first[0].x * second[i].x) +
				 	(first[1].x * second[i].y) +
					(first[2].x * second[i].z);

		rv[i].y =	(first[0].y * second[i].x) +
					(first[1].y * second[i].y) +
					(first[2].y * second[i].z);

		rv[i].z =	(first[0].z * second[i].x) +
					(first[1].z * second[i].y) +
					(first[2].z * second[i].z);
	}

	for (i = 0; i < 3; i++)
		first[i] = rv[i];
}




void mult_vector (struct vector *vec, struct vector *mat)
{
	double x;
	double y;
	double z;

	x = (vec->x * mat[0].x) +
		(vec->y * mat[0].y) +
		(vec->z * mat[0].z);

	y = (vec->x * mat[1].x) +
		(vec->y * mat[1].y) +
		(vec->z * mat[1].z);

	z = (vec->x * mat[2].x) +
		(vec->y * mat[2].y) +
		(vec->z * mat[2].z);

	vec->x = x;
	vec->y = y;
	vec->z = z;
}


/*
 * Calculate the dot product of two vectors sharing a common point.
 * Returns the cosine of the angle between the two vectors.
 */


double vector_dot_product (struct vector *first, struct vector *second)
{
	return (first->x * second->x) + (first->y * second->y) + (first->z * second->z);	
}



/*
 * Convert a vector into a vector of unit (1) length.
 */

struct vector unit_vector (struct vector *vec)
{
	double lx,ly,lz;
	double uni;
	struct vector res;

	lx = vec->x;
	ly = vec->y;
	lz = vec->z;

	uni = sqrt (lx * lx + ly * ly + lz * lz);

	res.x = lx / uni;
	res.y = ly / uni;
	res.z = lz / uni;
	
	return res;
}





void set_init_matrix (struct vector *mat)
{
	int i;

	for (i = 0; i < 3; i++)
		mat[i] = start_matrix[i];
}



void tidy_matrix (struct vector *mat)
{
	mat[2] = unit_vector (&mat[2]);

	if ((mat[2].x > -1) && (mat[2].x < 1))
	{
		if ((mat[2].y > -1) && (mat[2].y < 1))
		{
			mat[1].z = -(mat[2].x * mat[1].x + mat[2].y * mat[1].y) / mat[2].z;
		}
		else
		{
			mat[1].y = -(mat[2].x * mat[1].x + mat[2].z * mat[1].z) / mat[2].y;
		}
	}
	else
	{
		mat[1].x = -(mat[2].y * mat[1].y + mat[2].z * mat[1].z) / mat[2].x;
	}
	
	mat[1] = unit_vector (&mat[1]);
	

	/* xyzzy... nothing happens. :-)*/
	
	mat[0].x = mat[1].y * mat[2].z - mat[1].z * mat[2].y;
	mat[0].y = mat[1].z * mat[2].x - mat[1].x * mat[2].z;
	mat[0].z = mat[1].x * mat[2].y - mat[1].y * mat[2].x;
}