// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/// @docImport 'package:flutter/material.dart';
library;

import 'dart:math' as math;
import 'dart:ui' as ui show lerpDouble;

import 'package:flutter/foundation.dart';

import 'package:vector_math/vector_math_64.dart' show Matrix4;
import 'basic_types.dart';
import 'borders.dart';
import 'circle_border.dart';
import 'rounded_rectangle_border.dart';
import 'stadium_border.dart';

// Conversion from radians to degrees.
const double _kRadToDeg = 180 / math.pi;
// Conversion from degrees to radians.
const double _kDegToRad = math.pi / 180;

/// A border that fits a star or polygon-shaped border within the rectangle of
/// the widget it is applied to.
///
/// Typically used with a [ShapeDecoration] to draw a polygonal or star shaped
/// border.
///
/// {@tool dartpad}
/// This example serves both as a usage example, as well as an explorer for
/// determining the parameters to use with a [StarBorder]. The resulting code
/// can be copied and pasted into your app. A [Container] is just one widget
/// which takes a [ShapeBorder]. [Dialog]s, [OutlinedButton]s,
/// [ElevatedButton]s, etc. all can be shaped with a [ShapeBorder].
///
/// ** See code in examples/api/lib/painting/star_border/star_border.0.dart **
/// {@end-tool}
///
/// See also:
///
///  * [BorderSide], which is used to describe how the edge of the shape is
///  drawn.
class StarBorder extends OutlinedBorder {
  /// Create a const star-shaped border with the given number [points] on the
  /// star.
  const StarBorder({
    super.side,
    this.points = 5,
    double innerRadiusRatio = 0.4,
    this.pointRounding = 0,
    this.valleyRounding = 0,
    double rotation = 0,
    this.squash = 0,
  }) : assert(squash >= 0),
       assert(squash <= 1),
       assert(pointRounding >= 0),
       assert(pointRounding <= 1),
       assert(valleyRounding >= 0),
       assert(valleyRounding <= 1),
       assert(
         (valleyRounding + pointRounding) <= 1,
         'The sum of valleyRounding ($valleyRounding) and '
         'pointRounding ($pointRounding) must not exceed one.',
       ),
       assert(innerRadiusRatio >= 0),
       assert(innerRadiusRatio <= 1),
       assert(points >= 2),
       _rotationRadians = rotation * _kDegToRad,
       _innerRadiusRatio = innerRadiusRatio;

  /// Create a const polygon border with the given number of [sides].
  const StarBorder.polygon({
    super.side,
    double sides = 5,
    this.pointRounding = 0,
    double rotation = 0,
    this.squash = 0,
  }) : assert(squash >= 0),
       assert(squash <= 1),
       assert(pointRounding >= 0),
       assert(pointRounding <= 1),
       assert(sides >= 2),
       points = sides,
       valleyRounding = 0,
       _rotationRadians = rotation * _kDegToRad,
       _innerRadiusRatio = null;

  /// The number of points in this star, or sides on a polygon.
  ///
  /// This is a floating point number: if this is not a whole number, then an
  /// additional star point or corner shorter than the others will be added to
  /// finish the shape. Only whole-numbered values will yield a symmetric shape.
  /// (This enables the number of points to be animated smoothly.)
  ///
  /// For stars created with [StarBorder], this is the number of points on
  /// the star. For polygons created with [StarBorder.polygon], this is the
  /// number of sides on the polygon.
  ///
  /// Must be greater than or equal to two.
  final double points;

  /// The ratio of the inner radius of a star with the outer radius.
  ///
  /// When making a star using [StarBorder], this is the ratio of the inner
  /// radius that to the outer radius. If it is one, then the inner radius
  /// will equal the outer radius.
  ///
  /// For polygons created with [StarBorder.polygon], getting this value will
  /// return the incircle radius of the polygon (the radius of a circle
  /// inscribed inside the polygon).
  ///
  /// Defaults to 0.4 for stars, and must be between zero and one, inclusive.
  double get innerRadiusRatio {
    // Polygons are just a special case of a star where the inner radius is the
    // incircle radius of the polygon (the radius of an inscribed circle).
    return _innerRadiusRatio ?? math.cos(math.pi / points);
  }

  final double? _innerRadiusRatio;

  /// The amount of rounding on the points of stars, or the corners of polygons.
  ///
  /// This is a value between zero and one which describes how rounded the point
  /// or corner should be. A value of zero means no rounding (sharp corners),
  /// and a value of one means that the entire point or corner is a portion of a
  /// circle.
  ///
  /// Defaults to zero. The sum of [pointRounding] and [valleyRounding] must be
  /// less than or equal to one.
  final double pointRounding;

  /// The amount of rounding of the interior corners of stars.
  ///
  /// This is a value between zero and one which describes how rounded the inner
  /// corners in a star (the "valley" between points) should be. A value of zero
  /// means no rounding (sharp corners), and a value of one means that the
  /// entire corner is a portion of a circle.
  ///
  /// Defaults to zero. The sum of [pointRounding] and [valleyRounding] must be
  /// less than or equal to one. For polygons created with [StarBorder.polygon],
  /// this will always be zero.
  final double valleyRounding;

  /// The rotation in clockwise degrees around the center of the shape.
  ///
  /// The rotation occurs before the [squash] effect is applied, so that you can
  /// fine tune where the points of a star or corners of a polygon start.
  ///
  /// Defaults to zero, meaning that the first point or corner is pointing up.
  double get rotation => _rotationRadians * _kRadToDeg;
  final double _rotationRadians;

  /// How much of the aspect ratio of the attached widget to take on.
  ///
  /// If [squash] is non-zero, the border will match the aspect ratio of the
  /// bounding box of the widget that it is attached to, which can give a
  /// squashed appearance.
  ///
  /// The [squash] parameter lets you control how much of that aspect ratio this
  /// border takes on.
  ///
  /// A value of zero means that the border will be drawn with a square aspect
  /// ratio at the size of the shortest side of the bounding rectangle, ignoring
  /// the aspect ratio of the widget, and a value of one means it will be drawn
  /// with the aspect ratio of the widget. The value of [squash] has no effect
  /// if the widget is square to begin with.
  ///
  /// Defaults to zero, and must be between zero and one, inclusive.
  final double squash;

  @override
  ShapeBorder scale(double t) {
    return StarBorder(
      points: points,
      side: side.scale(t),
      rotation: rotation,
      innerRadiusRatio: innerRadiusRatio,
      pointRounding: pointRounding,
      valleyRounding: valleyRounding,
      squash: squash,
    );
  }

  ShapeBorder? _twoPhaseLerp(
    double t,
    double split,
    ShapeBorder? Function(double t) first,
    ShapeBorder? Function(double t) second,
  ) {
    // If the rectangle has square corners, then skip the extra lerp to round the corners.
    if (t < split) {
      return first(t * (1 / split));
    } else {
      t = (1 / (1.0 - split)) * (t - split);
      return second(t);
    }
  }

  @override
  ShapeBorder? lerpFrom(ShapeBorder? a, double t) {
    if (t == 0) {
      return a;
    }
    if (t == 1.0) {
      return this;
    }
    if (a is StarBorder) {
      return StarBorder(
        side: BorderSide.lerp(a.side, side, t),
        points: ui.lerpDouble(a.points, points, t)!,
        rotation: ui.lerpDouble(a._rotationRadians, _rotationRadians, t)! * _kRadToDeg,
        innerRadiusRatio: ui.lerpDouble(a.innerRadiusRatio, innerRadiusRatio, t)!,
        pointRounding: ui.lerpDouble(a.pointRounding, pointRounding, t)!,
        valleyRounding: ui.lerpDouble(a.valleyRounding, valleyRounding, t)!,
        squash: ui.lerpDouble(a.squash, squash, t)!,
      );
    }

    if (a is CircleBorder) {
      if (points >= 2.5) {
        final double lerpedPoints = ui.lerpDouble(points.round(), points, t)!;
        return StarBorder(
          side: BorderSide.lerp(a.side, side, t),
          points: lerpedPoints,
          squash: ui.lerpDouble(a.eccentricity, squash, t)!,
          rotation: rotation,
          innerRadiusRatio: ui.lerpDouble(math.cos(math.pi / lerpedPoints), innerRadiusRatio, t)!,
          pointRounding: ui.lerpDouble(1.0, pointRounding, t)!,
          valleyRounding: ui.lerpDouble(0.0, valleyRounding, t)!,
        );
      } else {
        // Have a slightly different lerp for two-pointed stars, since they get
        // kind of squirrelly with near-zero innerRadiusRatios.
        final double lerpedPoints = ui.lerpDouble(points, 2, t)!;
        return StarBorder(
          side: BorderSide.lerp(a.side, side, t),
          points: lerpedPoints,
          squash: ui.lerpDouble(a.eccentricity, squash, t)!,
          rotation: rotation,
          innerRadiusRatio: ui.lerpDouble(1, innerRadiusRatio, t)!,
          pointRounding: ui.lerpDouble(0.5, pointRounding, t)!,
          valleyRounding: ui.lerpDouble(0.5, valleyRounding, t)!,
        );
      }
    }

    if (a is StadiumBorder) {
      // Lerp from a stadium to a circle first, and from there to a star.
      final BorderSide lerpedSide = BorderSide.lerp(a.side, side, t);
      return _twoPhaseLerp(
        t,
        0.5,
        (double t) => a.lerpTo(CircleBorder(side: lerpedSide), t),
        (double t) => lerpFrom(CircleBorder(side: lerpedSide), t),
      );
    }
    if (a is RoundedRectangleBorder) {
      // Lerp from a rectangle to a stadium, then from a Stadium to a circle,
      // then from a circle to a star.
      final BorderSide lerpedSide = BorderSide.lerp(a.side, side, t);
      return _twoPhaseLerp(
        t,
        1 / 3,
        (double t) {
          return StadiumBorder(side: lerpedSide).lerpFrom(a, t);
        },
        (double t) {
          return _twoPhaseLerp(
            t,
            0.5,
            (double t) => StadiumBorder(side: lerpedSide).lerpTo(CircleBorder(side: lerpedSide), t),
            (double t) => lerpFrom(CircleBorder(side: lerpedSide), t),
          );
        },
      );
    }
    return super.lerpFrom(a, t);
  }

  @override
  ShapeBorder? lerpTo(ShapeBorder? b, double t) {
    if (t == 0) {
      return this;
    }
    if (t == 1.0) {
      return b;
    }
    if (b is StarBorder) {
      return StarBorder(
        side: BorderSide.lerp(side, b.side, t),
        points: ui.lerpDouble(points, b.points, t)!,
        rotation: ui.lerpDouble(_rotationRadians, b._rotationRadians, t)! * _kRadToDeg,
        innerRadiusRatio: ui.lerpDouble(innerRadiusRatio, b.innerRadiusRatio, t)!,
        pointRounding: ui.lerpDouble(pointRounding, b.pointRounding, t)!,
        valleyRounding: ui.lerpDouble(valleyRounding, b.valleyRounding, t)!,
        squash: ui.lerpDouble(squash, b.squash, t)!,
      );
    }
    if (b is CircleBorder) {
      // Have a slightly different lerp for two-pointed stars, since they get
      // kind of squirrelly with near-zero innerRadiusRatios.
      if (points >= 2.5) {
        final double lerpedPoints = ui.lerpDouble(points, points.round(), t)!;
        return StarBorder(
          side: BorderSide.lerp(side, b.side, t),
          points: lerpedPoints,
          squash: ui.lerpDouble(squash, b.eccentricity, t)!,
          rotation: rotation,
          innerRadiusRatio: ui.lerpDouble(innerRadiusRatio, math.cos(math.pi / lerpedPoints), t)!,
          pointRounding: ui.lerpDouble(pointRounding, 1.0, t)!,
          valleyRounding: ui.lerpDouble(valleyRounding, 0.0, t)!,
        );
      } else {
        final double lerpedPoints = ui.lerpDouble(points, 2, t)!;
        return StarBorder(
          side: BorderSide.lerp(side, b.side, t),
          points: lerpedPoints,
          squash: ui.lerpDouble(squash, b.eccentricity, t)!,
          rotation: rotation,
          innerRadiusRatio: ui.lerpDouble(innerRadiusRatio, 1, t)!,
          pointRounding: ui.lerpDouble(pointRounding, 0.5, t)!,
          valleyRounding: ui.lerpDouble(valleyRounding, 0.5, t)!,
        );
      }
    }
    if (b is StadiumBorder) {
      // Lerp to a circle first, then to a stadium.
      final BorderSide lerpedSide = BorderSide.lerp(side, b.side, t);
      return _twoPhaseLerp(
        t,
        0.5,
        (double t) => lerpTo(CircleBorder(side: lerpedSide), t),
        (double t) => b.lerpFrom(CircleBorder(side: lerpedSide), t),
      );
    }
    if (b is RoundedRectangleBorder) {
      // Lerp to a circle, and then to a stadium, then to a rounded rect.
      final BorderSide lerpedSide = BorderSide.lerp(side, b.side, t);
      return _twoPhaseLerp(
        t,
        2 / 3,
        (double t) {
          return _twoPhaseLerp(
            t,
            0.5,
            (double t) => lerpTo(CircleBorder(side: lerpedSide), t),
            (double t) =>
                StadiumBorder(side: lerpedSide).lerpFrom(CircleBorder(side: lerpedSide), t),
          );
        },
        (double t) {
          return StadiumBorder(side: lerpedSide).lerpTo(b, t);
        },
      );
    }
    return super.lerpTo(b, t);
  }

  @override
  StarBorder copyWith({
    BorderSide? side,
    double? points,
    double? innerRadiusRatio,
    double? pointRounding,
    double? valleyRounding,
    double? rotation,
    double? squash,
  }) {
    return StarBorder(
      side: side ?? this.side,
      points: points ?? this.points,
      rotation: rotation ?? this.rotation,
      innerRadiusRatio: innerRadiusRatio ?? this.innerRadiusRatio,
      pointRounding: pointRounding ?? this.pointRounding,
      valleyRounding: valleyRounding ?? this.valleyRounding,
      squash: squash ?? this.squash,
    );
  }

  @override
  Path getInnerPath(Rect rect, {TextDirection? textDirection}) {
    final Rect adjustedRect = rect.deflate(side.strokeInset);
    return _StarGenerator(
      points: points,
      rotation: _rotationRadians,
      innerRadiusRatio: innerRadiusRatio,
      pointRounding: pointRounding,
      valleyRounding: valleyRounding,
      squash: squash,
    ).generate(adjustedRect);
  }

  @override
  Path getOuterPath(Rect rect, {TextDirection? textDirection}) {
    return _StarGenerator(
      points: points,
      rotation: _rotationRadians,
      innerRadiusRatio: innerRadiusRatio,
      pointRounding: pointRounding,
      valleyRounding: valleyRounding,
      squash: squash,
    ).generate(rect);
  }

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {
    switch (side.style) {
      case BorderStyle.none:
        break;
      case BorderStyle.solid:
        final Rect adjustedRect = rect.inflate(side.strokeOffset / 2);
        final Path path = _StarGenerator(
          points: points,
          rotation: _rotationRadians,
          innerRadiusRatio: innerRadiusRatio,
          pointRounding: pointRounding,
          valleyRounding: valleyRounding,
          squash: squash,
        ).generate(adjustedRect);
        canvas.drawPath(path, side.toPaint());
    }
  }

  @override
  bool operator ==(Object other) {
    if (other.runtimeType != runtimeType) {
      return false;
    }
    return other is StarBorder &&
        other.side == side &&
        other.points == points &&
        other._innerRadiusRatio == _innerRadiusRatio &&
        other.pointRounding == pointRounding &&
        other.valleyRounding == valleyRounding &&
        other._rotationRadians == _rotationRadians &&
        other.squash == squash;
  }

  @override
  int get hashCode => side.hashCode;

  @override
  String toString() {
    return '${objectRuntimeType(this, 'StarBorder')}($side, points: $points, innerRadiusRatio: $innerRadiusRatio)';
  }
}

class _PointInfo {
  _PointInfo({
    required this.valley,
    required this.point,
    required this.valleyArc1,
    required this.pointArc1,
    required this.valleyArc2,
    required this.pointArc2,
  });

  Offset valley;
  Offset point;
  Offset valleyArc1;
  Offset pointArc1;
  Offset pointArc2;
  Offset valleyArc2;
}

class _StarGenerator {
  const _StarGenerator({
    required this.points,
    required this.innerRadiusRatio,
    required this.pointRounding,
    required this.valleyRounding,
    required this.rotation,
    required this.squash,
  }) : assert(points > 1),
       assert(innerRadiusRatio <= 1),
       assert(innerRadiusRatio >= 0),
       assert(squash >= 0),
       assert(squash <= 1),
       assert(pointRounding >= 0),
       assert(pointRounding <= 1),
       assert(valleyRounding >= 0),
       assert(valleyRounding <= 1),
       assert(pointRounding + valleyRounding <= 1);

  final double points;
  final double innerRadiusRatio;
  final double pointRounding;
  final double valleyRounding;
  final double rotation;
  final double squash;

  Path generate(Rect rect) {
    final double radius = rect.shortestSide / 2;
    final Offset center = rect.center;

    // The minimum allowed inner radius ratio. Numerical instabilities occur near
    // zero, so we just don't allow values in that range.
    const minInnerRadiusRatio = .002;

    // Map the innerRadiusRatio so that we don't get values close to zero, since
    // things get a little squirrelly there because the path thinks that the
    // length of the conicTo is small enough that it can render it as a straight
    // line, even though it will be scaled up later. This maps the range from
    // [0, 1] to [minInnerRadiusRatio, 1].
    final double mappedInnerRadiusRatio =
        (innerRadiusRatio * (1.0 - minInnerRadiusRatio)) + minInnerRadiusRatio;

    // First, generate the "points" of the star.
    final points = <_PointInfo>[];
    final double maxDiameter =
        2.0 *
        _generatePoints(
          pointList: points,
          center: center,
          radius: radius,
          innerRadius: radius * mappedInnerRadiusRatio,
        );

    // Calculate the endpoints of each of the arcs, then draw the arcs.
    final path = Path();
    _drawPoints(path, points);

    var scale = Offset(rect.width / maxDiameter, rect.height / maxDiameter);
    if (rect.shortestSide == rect.width) {
      scale = Offset(scale.dx, squash * scale.dy + (1 - squash) * scale.dx);
    } else {
      scale = Offset(squash * scale.dx + (1 - squash) * scale.dy, scale.dy);
    }
    // Scale the border so that it matches the size of the widget rectangle, so
    // that "rotation" of the shape doesn't affect how much of the rectangle it
    // covers.
    final squashMatrix = Matrix4.translationValues(rect.center.dx, rect.center.dy, 0);
    squashMatrix.multiply(Matrix4.diagonal3Values(scale.dx, scale.dy, 1));
    squashMatrix.multiply(Matrix4.rotationZ(rotation));
    squashMatrix.multiply(Matrix4.translationValues(-rect.center.dx, -rect.center.dy, 0));
    return path.transform(squashMatrix.storage);
  }

  double _generatePoints({
    required List<_PointInfo> pointList,
    required Offset center,
    required double radius,
    required double innerRadius,
  }) {
    final double step = math.pi / points;
    // Start initial rotation one step before zero.
    double angle = -math.pi / 2 - step;
    var valley = Offset(
      center.dx + math.cos(angle) * innerRadius,
      center.dy + math.sin(angle) * innerRadius,
    );

    // In order to do overall scale properly, calculate the actual radius at the
    // point, taking into account the rounding of the points and the weight of
    // the corner point. This effectively is evaluating the rational quadratic
    // bezier at the midpoint of the curve.
    Offset getCurveMidpoint(Offset a, Offset b, Offset c, Offset a1, Offset c1) {
      final double angle = _getAngle(a, b, c);
      final double w = _getWeight(angle) / 2;
      return (a1 / 4 + b * w + c1 / 4) / (0.5 + w);
    }

    double addPoint(
      double pointAngle,
      double pointStep,
      double pointRadius,
      double pointInnerRadius,
    ) {
      pointAngle += pointStep;
      final point = Offset(
        center.dx + math.cos(pointAngle) * pointRadius,
        center.dy + math.sin(pointAngle) * pointRadius,
      );
      pointAngle += pointStep;
      final nextValley = Offset(
        center.dx + math.cos(pointAngle) * pointInnerRadius,
        center.dy + math.sin(pointAngle) * pointInnerRadius,
      );
      final Offset valleyArc1 = valley + (point - valley) * valleyRounding;
      final Offset pointArc1 = point + (valley - point) * pointRounding;
      final Offset pointArc2 = point + (nextValley - point) * pointRounding;
      final Offset valleyArc2 = nextValley + (point - nextValley) * valleyRounding;

      pointList.add(
        _PointInfo(
          valley: valley,
          point: point,
          valleyArc1: valleyArc1,
          pointArc1: pointArc1,
          pointArc2: pointArc2,
          valleyArc2: valleyArc2,
        ),
      );
      valley = nextValley;
      return pointAngle;
    }

    final double remainder = points - points.truncateToDouble();
    final bool hasIntegerSides = remainder < 1e-6;
    final double wholeSides = points - (hasIntegerSides ? 0 : 1);
    for (var i = 0; i < wholeSides; i += 1) {
      angle = addPoint(angle, step, radius, innerRadius);
    }

    double valleyRadius = 0;
    double pointRadius = 0;
    final _PointInfo thisPoint = pointList[0];
    final _PointInfo nextPoint = pointList[1];

    final Offset pointMidpoint = getCurveMidpoint(
      thisPoint.valley,
      thisPoint.point,
      nextPoint.valley,
      thisPoint.pointArc1,
      thisPoint.pointArc2,
    );
    final Offset valleyMidpoint = getCurveMidpoint(
      thisPoint.point,
      nextPoint.valley,
      nextPoint.point,
      thisPoint.valleyArc2,
      nextPoint.valleyArc1,
    );
    valleyRadius = (valleyMidpoint - center).distance;
    pointRadius = (pointMidpoint - center).distance;

    // Add the final point to close the shape if there are fractional sides to
    // account for.
    if (!hasIntegerSides) {
      final double effectiveInnerRadius = math.max(valleyRadius, innerRadius);
      final double endingRadius =
          effectiveInnerRadius + remainder * (radius - effectiveInnerRadius);
      addPoint(angle, step * remainder, endingRadius, innerRadius);
    }

    // The rounding added to the valley radius can sometimes push it outside of
    // the rounding of the point, since the rounding amount can be different
    // between the points and the valleys, so we have to evaluate both the
    // valley and the point radii, and pick the largest. Also, since this value
    // is used later to determine the scale, we need to keep it finite and
    // non-zero.
    return clampDouble(math.max(valleyRadius, pointRadius), double.minPositive, double.maxFinite);
  }

  void _drawPoints(Path path, List<_PointInfo> points) {
    final Offset startingPoint = points.first.pointArc1;
    path.moveTo(startingPoint.dx, startingPoint.dy);
    final double pointAngle = _getAngle(points[0].valley, points[0].point, points[1].valley);
    final double pointWeight = _getWeight(pointAngle);
    final double valleyAngle = _getAngle(points[1].point, points[1].valley, points[0].point);
    final double valleyWeight = _getWeight(valleyAngle);

    for (var i = 0; i < points.length; i += 1) {
      final _PointInfo point = points[i];
      final _PointInfo nextPoint = points[(i + 1) % points.length];
      path.lineTo(point.pointArc1.dx, point.pointArc1.dy);
      if (pointAngle != 180 && pointAngle != 0) {
        path.conicTo(
          point.point.dx,
          point.point.dy,
          point.pointArc2.dx,
          point.pointArc2.dy,
          pointWeight,
        );
      } else {
        path.lineTo(point.pointArc2.dx, point.pointArc2.dy);
      }
      path.lineTo(point.valleyArc2.dx, point.valleyArc2.dy);
      if (valleyAngle != 180 && valleyAngle != 0) {
        path.conicTo(
          nextPoint.valley.dx,
          nextPoint.valley.dy,
          nextPoint.valleyArc1.dx,
          nextPoint.valleyArc1.dy,
          valleyWeight,
        );
      } else {
        path.lineTo(nextPoint.valleyArc1.dx, nextPoint.valleyArc1.dy);
      }
    }
    path.close();
  }

  double _getWeight(double angle) {
    return math.cos((angle / 2) % (math.pi / 2));
  }

  // Returns the included angle between points ABC in radians.
  double _getAngle(Offset a, Offset b, Offset c) {
    if (a == c || b == c || b == a) {
      return 0;
    }
    final Offset u = a - b;
    final Offset v = c - b;
    final double dot = u.dx * v.dx + u.dy * v.dy;
    final double m1 = b.dx == a.dx ? double.infinity : -u.dy / -u.dx;
    final double m2 = b.dx == c.dx ? double.infinity : -v.dy / -v.dx;
    double angle = math.atan2(m1 - m2, 1 + m1 * m2).abs();
    if (dot < 0) {
      angle += math.pi;
    }
    return angle;
  }
}
