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usr.lib/libplot/gigi/arc.c

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+/*
+ * Copyright (c) 1980 Regents of the University of California.
+ * All rights reserved.  The Berkeley software License Agreement
+ * specifies the terms and conditions for redistribution.
+ */
+
+#ifndef lint
+static char sccsid[] = "@(#)arc.c 1.1 94/10/31 SMI"; /* from UCB 5.1 5/7/85 */
+#endif not lint
+
+#include "gigi.h"
+
+/* 
+ * gigi requires knowing the anlge of arc.  To do this, the triangle formula
+ *	c^2 = a^2 + b^2 - 2*a*b*cos(angle)
+ * is used where "a" and "b" are the radius of the circle and "c" is the
+ * distance between the beginning point and the end point.
+ *
+ * This gives us "angle" or angle - 180.  To find out which, draw a line from
+ * beg to center.  This splits the plane in half.  All points on one side of the
+ * plane will have the same sign when plugged into the equation for the line.
+ * Pick a point on the "right side" of the line (see program below).  If "end"
+ * has the same sign as this point does, then they are both on the same side
+ * of the line and so angle is < 180.  Otherwise, angle > 180.
+ */
+   
+#define side(x,y)	(a*(x)+b*(y)+c > 0.0 ? 1 : -1)
+
+arc(xcent,ycent,xbeg,ybeg,xend,yend)
+int xcent,ycent,xbeg,ybeg,xend,yend;
+{
+	double radius2, c2;
+	double a,b,c;
+	int angle;
+
+	/* Probably should check that this is really a circular arc.  */
+	radius2 = (xcent-xbeg)*(xcent-xbeg) + (ycent-ybeg)*(ycent-ybeg);
+	c2 = (xend-xbeg)*(xend-xbeg) + (yend-ybeg)*(yend-ybeg);
+	angle = (int) ( 180.0/PI * acos(1.0 - c2/(2.0*radius2)) + 0.5 );
+
+	a = (double) (ycent - ybeg);
+	b = (double) (xcent - xbeg);
+	c = (double) (ycent*xbeg - xcent*ybeg);
+	if (side(xbeg + (ycent-ybeg), ybeg - (xcent-xbeg)) != side(xend,yend))
+		angle += 180;
+	
+	move(xcent, ycent);
+	printf("C(A%d c)[%d,%d]", angle, xbeg, ybeg);
+}