Index: Makefile
===================================================================
--- Makefile	(nonexistent)
+++ Makefile	(revision 37159)
@@ -0,0 +1,10 @@
+CFLAGS = -Wall -g
+LDFLAGS = -lm
+
+
+main: main.o sbiarc.o
+
+main.o: main.c sbiarc.h
+
+sbiarc.o: sbiarc.c sbiarc.h
+
Index: main.c
===================================================================
--- main.c	(nonexistent)
+++ main.c	(revision 37159)
@@ -0,0 +1,12 @@
+#include "sbiarc.h"
+
+int main()
+{
+	sba_t s;
+	s.cx[0] = 10; s.cy[0] = 1; s.r[0] = 5;
+	s.cx[1] = 40; s.cy[1] = 5; s.r[1] = 9;
+	s.style = SBA_STRAIGHT;
+
+	sba_update(&s);
+
+}
Index: sbiarc.c
===================================================================
--- sbiarc.c	(nonexistent)
+++ sbiarc.c	(revision 37159)
@@ -0,0 +1,35 @@
+#include "sbiarc.h"
+
+int sba_update(sba_t *s)
+{
+	sba_calc_t vx, vy, vlen; /* vector between centers */
+	sba_calc_t vipx, vipy, vipl; /* inflection point on v */
+	sba_calc_t ipx, ipy, ipr; /* "inflection" point on the curve, where r is halfway between r[0] and r[1] */
+
+	vx = s->cx[1] - s->cx[0]; vy = s->cy[1] - s->cy[0];
+	if ((vx == 0) && (vy == 0)) {
+#warning TODO: single arc
+		abort();
+	}
+	vlen = sqrt(vx*vx + vy*vy);
+	vx /= vlen; vy /= vlen;
+
+	vipl = s->r[0] / (s->r[0] + s->r[1]);
+	vipx = s->cx[0] + vx * vipl; vipy = s->cy[0] + vy * vipl;
+
+	ipr = (s->r[0] + s->r[1])/2;
+
+	if (s->style == SBA_STRAIGHT) {
+		ipx = vipx + -vy * ipr; ipy = vipy + vx * ipr;
+	}
+	else {
+		ipx = vipx; ipy = vipy;
+	}
+
+	s->ipa[0] = atan2(ipy - s->cy[0], ipx - s->cx[0]);
+	s->ipa[1] = atan2(ipy - s->cy[1], ipx - s->cx[1]);
+
+	return 0;
+}
+
+
Index: sbiarc.h
===================================================================
--- sbiarc.h	(nonexistent)
+++ sbiarc.h	(revision 37159)
@@ -0,0 +1,23 @@
+typedef double sba_coord_t;
+typedef double sba_calc_t;
+typedef double sba_angle_t;
+
+typedef enum sba_style_e {
+	SBA_INFLEX,
+	SBA_STRAIGHT,
+	SBA_SINGLE,          /* single arc: same center point and radius */
+} sba_style_t;
+
+typedef struct {
+
+	/* public/configuration */
+	sba_coord_t cx[2], cy[2], r[2]; /* center point and radius for two arcs */
+	sba_angle_t start, delta;       /* endpoint's angle is start+delta but delta's sign matters */
+	sba_style_t style;
+
+	/* private/cache */
+	sba_angle_t ipa[2]; /* inflexion point angle from c[0] and c[1] */
+} sba_t;
+
+
+int sba_update(sba_t *s);