1 files changed, 64 insertions, 0 deletions
diff --git a/lineXline.c b/lineXline.c
new file mode 100644
index 0000000..0a07513
--- /dev/null
+++ b/lineXline.c
@@ -0,0 +1,64 @@
+#include <u.h>
+#include <libc.h>
+#include <geometry.h>
+
+int
+lineXline(Point2 *r, Point2 p0, Point2 p1, Point2 p2, Point2 p3)
+{
+	Point2 dp[2], xp;
+	double m[2], c[2];
+
+	dp[0] = subpt2(p1, p0); m[0] = dp[0].y/dp[0].x; c[0] = p0.y - m[0]*p0.x;
+	fprint(2, "#1 := %gx + %g\n", m[0], c[0]);
+	dp[1] = subpt2(p3, p2); m[1] = dp[1].y/dp[1].x; c[1] = p2.y - m[1]*p2.x;
+	fprint(2, "#2 := %gx + %g\n", m[1], c[1]);
+
+	if(m[0] == m[1]){
+		if(c[0] == c[1]){	/* coincident */
+			r[0] = p0;
+			r[1] = p1;
+			return 2;
+		}
+		/* parallel */
+		return 0;
+	}
+
+	/* intersecting */
+	xp.x = (c[1]-c[0])/(m[0]-m[1]);
+	xp.y = m[0]*xp.x + c[0];
+	xp.w = 1;
+	r[0] = xp;
+	return 1;
+}
+
+void
+usage(void)
+{
+	fprint(2, "usage: %s x0 y0 x1 y1 x2 y2 x3 y3\n", argv0);
+	exits("usage");
+}
+
+void
+main(int argc, char *argv[])
+{
+	Point2 p0, p1, p2, p3, r[2];
+	int n;
+
+	GEOMfmtinstall();
+	ARGBEGIN{
+	default: usage();
+	}ARGEND
+	if(argc != 8)
+		usage();
+
+	p0.x = strtod(argv[0], nil); p0.y = strtod(argv[1], nil);
+	p1.x = strtod(argv[2], nil); p1.y = strtod(argv[3], nil);
+	p2.x = strtod(argv[4], nil); p2.y = strtod(argv[5], nil);
+	p3.x = strtod(argv[6], nil); p3.y = strtod(argv[7], nil);
+
+	n = lineXline(r, p0, p1, p2, p3);
+	while(n--)
+		print("p%d: %v\n", n, r[n]);
+
+	exits(nil);
+}