/**
 * Line Intersections (v1.0)
 * Mod: GoToLoop (2019/Jul/27)
 *
 * https://www.YouTube.com/watch?v=4bIsntTiKfM
 * https://GitHub.com/bit101/CodingMath/blob/master/episode32/main.js
 *
 * https://Discourse.Processing.org/t/correct-practice-line-intersect/13017/5
 * https://www.OpenProcessing.org/sketch/740740
 */

static final color BG = 0350;
static final int DIAM = 20;

final float[][] dots = {
  { 100, 100 }, { 400, 400 }, // line A coords
  { 500, 50 }, { 80, 500 }    // line B coords
};

void setup() {
  size(600, 600);
  noLoop();
  noFill();

  for (final float[] coord : dots)  println(coord);
  print("\n");
}

void draw() {
  background(BG);

  line(dots[0][X], dots[0][Y], dots[1][X], dots[1][Y]); // line A
  line(dots[2][X], dots[2][Y], dots[3][X], dots[3][Y]); // line B

  final float[] inter = lineIntersect(dots[0], dots[1], dots[2], dots[3]);
  println(inter);

  ellipse(inter[X], inter[Y], DIAM, DIAM); // intersection mark
}

static final float[] lineIntersect(
  final float[] p0, final float[] p1, // line A coords
  final float[] p2, final float[] p3) // line B coords
{
  final float
    a1 = p1[Y] - p0[Y], 
    b1 = p0[X] - p1[X], 
    c1 = a1*p0[X] + b1*p0[Y], 

    a2 = p3[Y] - p2[Y], 
    b2 = p2[X] - p3[X], 
    c2 = a2*p2[X] + b2*p2[Y], 

    d = a1*b2 - a2*b1;

  return new float[] { (b2*c1 - b1*c2) / d, (a1*c2 - a2*c1) / d };
}