diff --git a/archive/line-segment-intersection-try.ts b/archive/line-segment-intersection-try.ts
new file mode 100644
index 0000000..6e46a90
--- /dev/null
+++ b/archive/line-segment-intersection-try.ts
@@ -0,0 +1,62 @@
+import * as mathjs from 'mathjs';
+
+interface Range {
+    x0: number;
+    y0: number;
+    x1: number;
+    y1: number;
+}
+
+// my first attempt at this
+function lineSegmentIntersection(l0: Range, l1: Range): boolean {
+    // convert to y = mx + b
+    const m0 = mathjs.fraction(l0.y1 - l0.y0, l0.x1 - l0.x0);
+    const m1 = mathjs.fraction(l1.y1 - l1.y0, l1.x1 - l1.x0);
+
+    const b0 = mathjs.subtract(l0.y0, mathjs.multiply(m0, l0.x0));
+    const b1 = mathjs.subtract(l1.y1, mathjs.multiply(m1, l1.x1));
+
+    if (mathjs.equal(m0, m1)) {
+        if (mathjs.equal(b0, b1)) {
+            // parallel lines with equal intersect
+            return (
+                (l1.x0 >= l0.x0 && l1.x0 <= l0.x1) ||
+                (l1.x1 >= l1.x0 && l1.x1 <= l1.x1)
+            );
+        } else {
+            // parallel lines with different y-intersect will never intersect
+            return false;
+        }
+    }
+
+    // solve for l0 = l1
+    const M = mathjs.subtract(m0, m1);
+    const B = mathjs.subtract(b0, b1);
+
+    // MX + B = 0 -> X = -B/M
+    const X = mathjs.divide(mathjs.unaryMinus(B), M);
+    const X_num = mathjs.number(X as mathjs.Fraction);
+
+    const intersects =
+        X_num >= Math.min(l0.x0, l0.x1) &&
+        X_num <= Math.max(l0.x0, l0.x1) &&
+        X_num >= Math.min(l1.x0, l1.x1) &&
+        X_num <= Math.max(l1.x0, l1.x1);
+
+    // const Y = mathjs.add(mathjs.multiply(m0, X), b0);
+    // const Y_num = mathjs.number(Y as mathjs.Fraction);
+    // if (intersects) {
+    //     console.log('intersects at', X_num, Y_num);
+    // } else {
+    //     console.log('would have intersected at', X_num, Y_num, l0, l1);
+    // }
+
+    // to test
+    // useEffect(() => {
+    //     const l0 = { x0: 2, y0: 8, x1: 6, y1: 5, stroke: '' };
+    //     const l1 = { x0: 3, y0: 9, x1: 7, y1: 8, stroke: '' };
+    //     console.log('intersection:', l0, l1, lineSegmentIntersection(l0, l1));
+    // }, []);
+
+    return intersects;
+}
diff --git a/src/components/grid/grid.tsx b/src/components/grid/grid.tsx
index 76efad1..e0fd982 100644
--- a/src/components/grid/grid.tsx
+++ b/src/components/grid/grid.tsx
@@ -11,8 +11,6 @@ import {
 
 import './grid.scss';
 
-import * as mathjs from 'mathjs';
-
 import * as lodash from 'lodash';
 import { useRecoilValue } from 'recoil';
 import { drawModeState, lineColorState } from '../../atoms';
@@ -37,57 +35,24 @@ interface Line {
     stroke: string;
 }
 
+// https://en.wikipedia.org/wiki/Line%E2%80%93line_intersection
+// https://stackoverflow.com/questions/9043805/test-if-two-lines-intersect-javascript-function
 function lineSegmentIntersection(l0: Range, l1: Range): boolean {
-    // convert to y = mx + b
-    const m0 = mathjs.fraction(l0.y1 - l0.y0, l0.x1 - l0.x0);
-    const m1 = mathjs.fraction(l1.y1 - l1.y0, l1.x1 - l1.x0);
-
-    const b0 = mathjs.subtract(l0.y0, mathjs.multiply(m0, l0.x0));
-    const b1 = mathjs.subtract(l1.y1, mathjs.multiply(m1, l1.x1));
-
-    if (mathjs.equal(m0, m1)) {
-        if (mathjs.equal(b0, b1)) {
-            // parallel lines with equal intersect
-            return (
-                (l1.x0 >= l0.x0 && l1.x0 <= l0.x1) ||
-                (l1.x1 >= l1.x0 && l1.x1 <= l1.x1)
-            );
-        } else {
-            // parallel lines with different y-intersect will never intersect
-            return false;
-        }
+    const determinant =
+        (l0.x1 - l0.x0) * (l1.y1 - l1.y0) - (l1.x1 - l1.x0) * (l0.y1 - l0.y0);
+    if (determinant === 0) {
+        // full lines have no intersection (they must be parallel)
+        return false;
     }
-
-    // solve for l0 = l1
-    const M = mathjs.subtract(m0, m1);
-    const B = mathjs.subtract(b0, b1);
-
-    // MX + B = 0 -> X = -B/M
-    const X = mathjs.divide(mathjs.unaryMinus(B), M);
-    const X_num = mathjs.number(X as mathjs.Fraction);
-
-    const intersects =
-        X_num >= Math.min(l0.x0, l0.x1) &&
-        X_num <= Math.max(l0.x0, l0.x1) &&
-        X_num >= Math.min(l1.x0, l1.x1) &&
-        X_num <= Math.max(l1.x0, l1.x1);
-
-    // const Y = mathjs.add(mathjs.multiply(m0, X), b0);
-    // const Y_num = mathjs.number(Y as mathjs.Fraction);
-    // if (intersects) {
-    //     console.log('intersects at', X_num, Y_num);
-    // } else {
-    //     console.log('would have intersected at', X_num, Y_num, l0, l1);
-    // }
-
-    // to test
-    // useEffect(() => {
-    //     const l0 = { x0: 2, y0: 8, x1: 6, y1: 5, stroke: '' };
-    //     const l1 = { x0: 3, y0: 9, x1: 7, y1: 8, stroke: '' };
-    //     console.log('intersection:', l0, l1, lineSegmentIntersection(l0, l1));
-    // }, []);
-
-    return intersects;
+    const lambda =
+        ((l1.y1 - l1.y0) * (l1.x1 - l0.x0) +
+            (l1.x0 - l1.x1) * (l1.y1 - l0.y0)) /
+        determinant;
+    const gamma =
+        ((l0.y0 - l0.y1) * (l1.x1 - l0.x0) +
+            (l0.x1 - l0.x0) * (l1.y1 - l0.y0)) /
+        determinant;
+    return 0 < lambda && lambda < 1 && 0 < gamma && gamma < 1;
 }
 
 function pointsToRange(p0: Point, p1: Point): Range {
@@ -198,7 +163,10 @@ const Grid: FC = () => {
 
             setUserRedoLines([]);
 
-            if (lodash.isEqual(userStartPoint, point)) return;
+            if (lodash.isEqual(userStartPoint, point)) {
+                setUserStartPoint(null);
+                return;
+            }
             if (lodash.isNull(userStartPoint)) {
                 setUserStartPoint(point);
                 return;
@@ -295,16 +263,31 @@ const Grid: FC = () => {
     const userLineIndicatorElement = useMemo(() => {
         if (lodash.isNull(userStartPoint)) return null;
         const point = snapToGrid(mousePoint, GAP);
-        return (
-            <line
-                x1={userStartPoint.x}
-                y1={userStartPoint.y}
-                x2={point.x}
-                y2={point.y}
-                stroke={userLineColor}
-                strokeWidth="2"
-            />
-        );
+        if (userDrawMode === 'line') {
+            return (
+                <line
+                    x1={userStartPoint.x}
+                    y1={userStartPoint.y}
+                    x2={point.x}
+                    y2={point.y}
+                    stroke={userLineColor}
+                    strokeWidth="2"
+                />
+            );
+        } else {
+            console.assert(userDrawMode === 'trash');
+            return (
+                <line
+                    x1={userStartPoint.x}
+                    y1={userStartPoint.y}
+                    x2={point.x}
+                    y2={point.y}
+                    stroke="#ff0000"
+                    strokeDasharray="20,10,5,5,5,10"
+                    strokeWidth="2"
+                />
+            );
+        }
     }, [userStartPoint, mousePoint]);
     const userPointIndicatorElement = useMemo(() => {
         const point = snapToGrid(mousePoint, GAP);