2 files changed, 10 insertions, 1 deletions

diff --git a/geometry.c b/geometry.c
index f2961c7..7c29f85 100644
--- a/geometry.c
+++ b/geometry.c
@@ -29,3 +29,12 @@ int
 dist_pp(LPoint const * p0, LPoint const * p1) {
     return impl_dist_pp(p0->x,p0->y,p1->x,p1->y);
 }
+
+int
+get_x_for_y(LPoint const * p0, LPoint const * p1, int y) {
+    if (p1->x!=p0->x && p1->y!=p0->y) {
+        double m = (double)(p1->y-p0->y) / (double)(p1->x-p0->x);
+        return (int)((((double)y) - (double)(p0->y) + m * (double)p0->x)/m);
+    }
+    return p1->x;
+}
diff --git a/geometry.h b/geometry.h
index 509ec7e..049fba4 100644
--- a/geometry.h
+++ b/geometry.h
@@ -15,4 +15,4 @@ typedef struct {
 // range 0..65536 (but can extend, if normale hits line outside line segment)
 int dist_pl(LPoint const * p, LLine const * l, int * offs);
 int dist_pp(LPoint const * p0, LPoint const * p1);
-
+int get_x_for_y(LPoint const * p0, LPoint const * p1, int y);