Custom Code Challenge from Logement 3D: getClosestPointInsidePolygon

main.js

interface Point {
  x: number;
  y: number;
}

function getClosestPointInsidePolygon(poly: Point[], c: Point): Point {
  const fixPrecision = (x: number) => Number(x.toPrecision(12));

  let min: number = Number.MAX_SAFE_INTEGER;
  let minP: Point = { x: Number.MAX_SAFE_INTEGER, y: Number.MAX_SAFE_INTEGER };

  // Assume point i(x) is connected to i(x+1) and i(n) is connected to i(0)
  // For each line (pair of points: A & B)
  for (let i = 0; i < poly.length; i += 1) {
    const a: Point = poly[i];
    const b: Point = poly[(i + 1) % poly.length];

    // Calculate distance from given point C to A
    const dca: number = Math.sqrt((c.y - a.y) ** 2 + (c.x - a.x) ** 2);

    // Calculate distance from C to intersection of AB in perpendicular direction
    // step 1: Calculate line AB slope
    let m: number = fixPrecision((b.y - a.y) / (b.x - a.x));
    m = isFinite(m) ? m : 1; // If it's not a vertical line
    // step 2: Calculate line AB y-intercept
    const abB = fixPrecision(a.y - m * a.x);
    // step 3: Calculate perpendicular slope (pM)
    let pM: number = fixPrecision(-1 / m);
    pM = isFinite(pM) ? pM : m;
    // step 4: Calculate line CD y-intercept: b
    const cdB: number = fixPrecision(c.y - pM * c.x);
    // step 5: Calculate x value for D
    const dX: number = fixPrecision((cdB - abB) / (m - pM));
    // step 6: Calculate y value for D
    const dY: number = fixPrecision(pM * dX + cdB);
    // step 7: Calculate distance from C to D
    const dcd: number = fixPrecision(Math.sqrt((c.x - dX) ** 2 + (c.y - dY) ** 2));

    // Get min distance between dab and dcd
    const curMin = Math.min(dca, dcd);

    if (curMin < min) {
      min = curMin;
      minP = { x: dX, y: dY };
    }
  }

  return minP;
}

let poly = [{ x: 0, y: 0 }, { x: 100, y: 0 }, { x: 100, y: 100 }, { x: 0, y: 100 }];
let point = { x: 150, y: 50 };

console.log(getClosestPointInsidePolygon(poly, point));
// should return { x: 100, y: 50 }

poly = [{ x: 3, y: 2 }, { x: 7, y: 4 }];
point = { x: 8, y: -2 };

console.log(getClosestPointInsidePolygon(poly, point));
// should return { x: 5.4, y: 3.2 }

poly = [{ x: 5, y: 3 }, { x: 1, y: 4.5 }, { x: 1, y: 1.6 }, { x: 9, y: 2.9 }, { x: 8.9, y: 7 }, { x: 8, y: 7 }];
point = { x: 1, y: 3 };

console.log(getClosestPointInsidePolygon(poly, point));
// should return { x: 0.25, y: 3.75 }