feat: snap lines to horizontal/vertical

lib/components/canvas/_canvas_painter.dart

@@ -181,7 +181,8 @@ class CanvasPainter extends CustomPainter {
       case null:
         break;
       case DefaultUnistrokeNames.line:
-        final (firstPoint, lastPoint) = shape.convertToLine();
+        var (firstPoint, lastPoint) = shape.convertToLine();
+        (firstPoint, lastPoint) = ShapePen.snapLine(firstPoint, lastPoint);
         canvas.drawLine(firstPoint, lastPoint, shapePaint);
       case DefaultUnistrokeNames.rectangle:
         final rect = shape.convertToRect();

lib/data/tools/shape_pen.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
 import 'package:font_awesome_flutter/font_awesome_flutter.dart';
@@ -62,7 +63,8 @@ class ShapePen extends Pen {
         log.info('Detected unknown shape');
         return rawStroke;
       case DefaultUnistrokeNames.line:
-        final (firstPoint, lastPoint) = detectedShape.convertToLine();
+        var (firstPoint, lastPoint) = detectedShape.convertToLine();
+        (firstPoint, lastPoint) = snapLine(firstPoint, lastPoint);
         log.info('Detected line: $firstPoint -> $lastPoint');
         return Stroke(
           strokeProperties: rawStroke.strokeProperties,
@@ -104,4 +106,26 @@ class ShapePen extends Pen {
           ..isComplete = true;
     }
   }
+
+  /// Snaps a line to either horizontal or vertical
+  /// if the angle is close enough.
+  static (Offset firstPoint, Offset lastPoint) snapLine(
+    Offset firstPoint,
+    Offset lastPoint,
+  ) {
+    final dx = (lastPoint.dx - firstPoint.dx).abs();
+    final dy = (lastPoint.dy - firstPoint.dy).abs();
+    final angle = math.atan2(dy, dx);
+
+    const snapAngle = 5 * math.pi / 180; // 5 degrees
+    if (angle < snapAngle) {
+      // snap to horizontal
+      return (firstPoint, Offset(lastPoint.dx, firstPoint.dy));
+    } else if (angle > math.pi / 2 - snapAngle) {
+      // snap to vertical
+      return (firstPoint, Offset(firstPoint.dx, lastPoint.dy));
+    } else {
+      return (firstPoint, lastPoint);
+    }
+  }
 }

test/snap_line_test.dart

@@ -0,0 +1,49 @@
+import 'dart:math' as math;
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:saber/data/tools/shape_pen.dart';
+
+void main() {
+  group('ShapePen.snapLine', () {
+    test('horizontal line', () {
+      final (firstPoint, lastPoint) = ShapePen.snapLine(
+        Offset.zero,
+        const Offset(100, 1),
+      );
+      expect(firstPoint, Offset.zero);
+      expect(lastPoint, const Offset(100, 0));
+    });
+
+    test('horizontal line (under/over threshold)', () {
+      final threshold = 100 * math.tan(5 * math.pi / 180);
+      final underThreshold = ShapePen.snapLine(
+        Offset.zero,
+        Offset(100, threshold * 0.99),
+      );
+      final overThreshold = ShapePen.snapLine(
+        Offset.zero,
+        Offset(100, threshold * 1.01),
+      );
+      expect(underThreshold.$2.dy, 0);
+      expect(overThreshold.$2.dy, threshold * 1.01);
+    });
+
+    test('vertical line', () {
+      final (firstPoint, lastPoint) = ShapePen.snapLine(
+        Offset.zero,
+        const Offset(1, 100),
+      );
+      expect(firstPoint, Offset.zero);
+      expect(lastPoint, const Offset(0, 100));
+    });
+
+    test('diagonal line doesn\'t snap', () {
+      final (firstPoint, lastPoint) = ShapePen.snapLine(
+        Offset.zero,
+        const Offset(100, 101),
+      );
+      expect(firstPoint, Offset.zero);
+      expect(lastPoint, const Offset(100, 101));
+    });
+  });
+}