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cae-tools/poly2d/v2d_intersect.c

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/*
* v2d_intersect.c - Intersect two lines
*
* Written 2012 by Werner Almesberger
* Copyright 2012 Werner Almesberger
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*/
#include <math.h>
#include "poly2d.h"
#define EPSILON 1e-6
/*
* Solve
*
* ax+by = e
* cx+dy = f
*
* with Cramer's rule:
* http://en.wikipedia.org/wiki/Cramer's_rule
*/
static int cramer2(double a, double b, double c, double d, double e, double f,
double *x, double *y)
{
double det;
det = a*d-b*c;
if (fabs(det) < EPSILON)
return 0;
*x = (e*d-b*f)/det;
*y = (a*f-e*c)/det;
return 1;
}
int v2d_intersect(const struct v2d *a0, const struct v2d *a1,
const struct v2d *b0, const struct v2d *b1,
double *na, double *nb)
{
double ax, ay, bx, by, dx, dy;
double a, b;
ax = a1->x-a0->x;
ay = a1->y-a0->y;
bx = b1->x-b0->x;
by = b1->y-b0->y;
dx = b0->x-a0->x;
dy = b0->y-a0->y;
if (!cramer2(ax, -bx, ay, -by, dx, dy, &a, &b))
return 0;
if (na)
*na = a;
if (nb)
*nb = b;
return a >= 0 && a <= 1 && b >= 0 && b <= 1 ? 1 : -1;
}