Fantasy-Map-Generator/libs/simplify.js

/*
 (c) 2017, Vladimir Agafonkin
 Simplify.js, a high-performance JS polyline simplification library
 mourner.github.io/simplify-js
*/
{
  // square distance between 2 points
  function getSqDist([x1, y1], [x2, y2]) {
    const dx = x1 - x2;
    const dy = y1 - y2;

    return dx * dx + dy * dy;
  }

  // square distance from a point to a segment
  function getSqSegDist([x1, y1], [x, y], [x2, y2]) {
    let dx = x2 - x;
    let dy = y2 - y;

    if (dx !== 0 || dy !== 0) {
      const t = ((x1 - x) * dx + (y1 - y) * dy) / (dx * dx + dy * dy);

      if (t > 1) {
        x = x2;
        y = y2;
      } else if (t > 0) {
        x += dx * t;
        y += dy * t;
      }
    }

    dx = x1 - x;
    dy = y1 - y;

    return dx * dx + dy * dy;
  }
  // rest of the code doesn't care about point format

  // basic distance-based simplification
  function simplifyRadialDist(points, sqTolerance) {
    let prevPoint = points[0];
    const newPoints = [prevPoint];
    let point;

    for (let i = 1; i < points.length; i++) {
      point = points[i];
      if (!point) continue;

      if (getSqDist(point, prevPoint) > sqTolerance) {
        newPoints.push(point);
        prevPoint = point;
      }
    }

    if (prevPoint !== point) newPoints.push(point);
    return newPoints;
  }

  function simplifyDPStep(points, first, last, sqTolerance, simplified) {
    let maxSqDist = sqTolerance;
    let index = first;

    for (let i = first + 1; i < last; i++) {
      const sqDist = getSqSegDist(points[i], points[first], points[last]);

      if (sqDist > maxSqDist) {
        index = i;
        maxSqDist = sqDist;
      }
    }

    if (maxSqDist > sqTolerance) {
      if (index - first > 1) simplifyDPStep(points, first, index, sqTolerance, simplified);
      simplified.push(points[index]);
      if (last - index > 1) simplifyDPStep(points, index, last, sqTolerance, simplified);
    }
  }

  // simplification using Ramer-Douglas-Peucker algorithm
  function simplifyDouglasPeucker(points, sqTolerance) {
    const last = points.length - 1;

    const simplified = [points[0]];
    simplifyDPStep(points, 0, last, sqTolerance, simplified);
    simplified.push(points[last]);

    return simplified;
  }

  // both algorithms combined for awesome performance
  function simplify(points, tolerance, highestQuality = false) {
    if (points.length <= 2) return points;

    const sqTolerance = tolerance * tolerance;

    points = highestQuality ? points : simplifyRadialDist(points, sqTolerance);
    points = simplifyDouglasPeucker(points, sqTolerance);

    return points;
  }

  window.simplify = simplify;
}