Apply some simple optimisations to LineIntersection

peterstace · Aug 2, 2021 · d09fae0 · d09fae0
1 parent 7f04f1e
commit d09fae0
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Showing 3 changed files with 16 additions and 8 deletions.
diff --git a/internal/exact/line.go b/internal/exact/line.go
@@ -27,7 +27,8 @@ func LineIntersection(lineA, lineB Line) Intersection {
 	d := sub12.Cross(sub34)
 
 	if d.Sign() == 0 {
-		if !collinear(v1, v2, v3) {
+		sub23 := v2.Sub(v3)
+		if sub12.Cross(sub23).Sign() != 0 {
 			return Intersection{Empty: true}
 		}
 
@@ -54,7 +55,7 @@ func LineIntersection(lineA, lineB Line) Intersection {
 	t := div(sub13.Cross(sub34), d)
 
 	// u := [(x2-x1)*(y1-y3)-(y2-y1)*(x1-x3)] / d
-	sub21 := v2.Sub(v1)
+	sub21 := sub12.Neg()
 	u := div(sub21.Cross(sub13), d)
 
 	if inUnitInterval(t) && inUnitInterval(u) {
@@ -64,9 +65,3 @@ func LineIntersection(lineA, lineB Line) Intersection {
 
 	return Intersection{Empty: true}
 }
-
-func collinear(v1, v2, v3 XYRat) bool {
-	sub21 := v2.Sub(v1)
-	sub32 := v3.Sub(v2)
-	return sub21.Cross(sub32).Sign() == 0
-}
diff --git a/internal/exact/real.go b/internal/exact/real.go
@@ -32,6 +32,12 @@ func add(a, b *big.Rat) *big.Rat {
 	return tmp
 }
 
+func neg(r *big.Rat) *big.Rat {
+	tmp := new(big.Rat).Set(r)
+	tmp.Neg(tmp)
+	return tmp
+}
+
 func inUnitInterval(r *big.Rat) bool {
 	return r.Sign() >= 0 && r.Cmp(one) <= 0
 }
diff --git a/internal/exact/xy.go b/internal/exact/xy.go
@@ -38,6 +38,13 @@ func (r XYRat) Add(o XYRat) XYRat {
 	}
 }
 
+func (r XYRat) Neg() XYRat {
+	return XYRat{
+		neg(r.X),
+		neg(r.Y),
+	}
+}
+
 func (r XYRat) Max(o XYRat) XYRat {
 	if r.Less(o) {
 		return o