created line.go and moved tests and funcs

line.go

@@ -0,0 +1,45 @@
+package rect
+
+import (
+	"fmt"
+	"math"
+)
+
+type line struct {
+	pt1, pt2 Point
+}
+
+func lineHelper(l line) (float64, float64, float64) {
+	a := l.pt2.Y - l.pt1.Y
+	b := l.pt1.X - l.pt2.X
+	c := a*l.pt1.X + b*l.pt1.Y
+	return a, b, c
+}
+
+func lineIntersection(l1, l2 line) (Point, error) {
+	a1, b1, c1 := lineHelper(l1)
+	a2, b2, c2 := lineHelper(l2)
+	det := a1*b2 - a2*b1
+
+	if det != 0 {
+		x := ((b2*c1 - b1*c2) / det)
+		y := ((a1*c2 - a2*c1) / det)
+		minx1, maxx1 := math.Min(l1.pt1.X, l1.pt2.X), math.Max(l1.pt1.X, l1.pt2.X)
+		miny1, maxy1 := math.Min(l1.pt1.Y, l1.pt2.Y), math.Max(l1.pt1.Y, l1.pt2.Y)
+		minx2, maxx2 := math.Min(l2.pt1.X, l2.pt2.X), math.Max(l2.pt1.X, l2.pt2.X)
+		miny2, maxy2 := math.Min(l2.pt1.Y, l2.pt2.Y), math.Max(l2.pt1.Y, l2.pt2.Y)
+		if minx1 <= x && x <= maxx1 && miny1 <= y && y <= maxy1 {
+			if minx2 <= x && x <= maxx2 && miny2 <= y && y <= maxy2 {
+				return Point{x, y}, nil
+			}
+		}
+	}
+	return Point{}, fmt.Errorf("lines do not intersect")
+}
+
+func lineOnLine(l1, l2 line) bool {
+	if onLine(l1.pt1, l1.pt2, l2.pt1) && onLine(l1.pt1, l1.pt2, l2.pt2) {
+		return true
+	}
+	return false
+}