Source code in the dart programming language

import 'dart:math';

const double EPS = 0.001;
const double EPS_SQUARE = EPS * EPS;

double side(double x1, double y1, double x2, double y2, double x, double y) {
  return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1);
}

bool naivePointInTriangle(double x1, double y1, double x2, double y2, double x3,
    double y3, double x, double y) {
  double checkSide1 = side(x1, y1, x2, y2, x, y); // >= 0;
  double checkSide2 = side(x2, y2, x3, y3, x, y); // >= 0;
  double checkSide3 = side(x3, y3, x1, y1, x, y); // >= 0;
  if (checkSide1 >= 0 && checkSide2 >= 0 && checkSide3 >= 0) {
    return true;
  } else {
    return false;
  }
}

bool pointInTriangleBoundingBox(double x1, double y1, double x2, double y2,
    double x3, double y3, double x, double y) {
  double xMin = min(x1, min(x2, x3)) - EPS;
  double xMax = max(x1, max(x2, x3)) + EPS;
  double yMin = min(y1, min(y2, y3)) - EPS;
  double yMax = max(y1, max(y2, y3)) + EPS;
  return !(x < xMin || xMax < x || y < yMin || yMax < y);
}

double distanceSquarePointToSegment(
    double x1, double y1, double x2, double y2, double x, double y) {
  double p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
  double dotProduct =
      ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength;
  if (dotProduct < 0) {
    return (x - x1) * (x - x1) + (y - y1) * (y - y1);
  } else if (dotProduct <= 1) {
    double p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y);
    return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
  } else {
    return (x - x2) * (x - x2) + (y - y2) * (y - y2);
  }
}

bool accuratePointInTriangle(double x1, double y1, double x2, double y2,
    double x3, double y3, double x, double y) {
  if (!pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)) {
    return false;
  }
  if (naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
    return true;
  }
  if (distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE) {
    return true;
  }
  if (distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE) {
    return true;
  }
  if (distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE) {
    return true;
  }
  return false;
}

void printTriangle(
    double x1, double y1, double x2, double y2, double x3, double y3) {
  print("Triangle is [($x1, $y1), ($x2, $y2), ($x3, $y3)]");
}

void test(double x1, double y1, double x2, double y2, double x3, double y3,
    double x, double y) {
  printTriangle(x1, y1, x2, y2, x3, y3);
  print("Point ($x, $y) is within triangle? ");
  if (accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
    print("true");
  } else {
    print("false");
  }
}

void main() {
  test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0);
  test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1);
  test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1);
  print('');

  test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25, 11.11111111111111,
      5.414285714285714, 14.349206349206348);
  print('');

  test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5,
      16.666666666666668, 5.414285714285714, 14.349206349206348);
  print('');
}