ol-ext/src/geom/visvalingam.js

A set of cool extensions for OpenLayers (ol) in node modules structure

import ol_geom_LineString from 'ol/geom/LineString.js'
import { ol_coordinate_equal } from './GeomUtils';

(function () {

/**
 * Visvalingam polyline simplification algorithm, adapted from http://bost.ocks.org/mike/simplify/simplify.js
 * This uses the [Visvalingam–Whyatt](https://en.wikipedia.org/wiki/Visvalingam%E2%80%93Whyatt_algorithm) algorithm.
 * @param {Object} options
 *  @param {number} [area] the tolerance area for simplification
 *  @param {number} [dist] a tolerance distance for simplification
 *  @param {number} [ratio=.8] a ratio of points to keep
 *  @param {number} [minPoints=2] minimum number of points to keep
 *  @param {boolean} [keepEnds] keep line ends
 * @return { LineString } A new, simplified version of the original geometry.
 * @api
 */
ol_geom_LineString.prototype.simplifyVisvalingam = function (options) {
  var points = this.getCoordinates();
  if (options.minPoints && options.minPoints >= points.length) {
    return new ol_geom_LineString(points);
  }
  var heap = minHeap(),
      maxArea = 0,
      triangle,
      triangles = [];
  points = points.map(function (d) { return d.slice(0,2); });
  for (var i = 1, n = points.length - 1; i < n; ++i) {
    triangle = points.slice(i - 1, i + 2);
    if (triangle[1][2] = area(triangle)) {
      triangles.push(triangle);
      heap.push(triangle);
    }
  }

  for (i = 0, n = triangles.length; i < n; ++i) {
    triangle = triangles[i];
    triangle.previous = triangles[i - 1];
    triangle.next = triangles[i + 1];
  }

  while (triangle = heap.pop()) {
    // If the area of the current point is less than that of the previous point
    // to be eliminated, use the latters area instead. This ensures that the
    // current point cannot be eliminated without eliminating previously-
    // eliminated points.
    if (triangle[1][2] < maxArea) triangle[1][2] = maxArea;
    else maxArea = triangle[1][2];

    if (triangle.previous) {
      triangle.previous.next = triangle.next;
      triangle.previous[2] = triangle[2];
      update(triangle.previous);
    } else {
      triangle[0][2] = triangle[1][2];
    }

    if (triangle.next) {
      triangle.next.previous = triangle.previous;
      triangle.next[0] = triangle[0];
      update(triangle.next);
    } else {
      triangle[2][2] = triangle[1][2];
    }
  }

  function update(triangle) {
    heap.remove(triangle);
    triangle[1][2] = area(triangle);
    heap.push(triangle);
  }

  // Get area to remove
  var w = options.area;
  if (options.dist) w = options.dist * options.dist / 2;
  // If no area
  if (w === undefined || options.minPoints) {
    // Get ordered weights 
    var weights = points.map(function (d) { return d.length < 3 ? Infinity : d[2] += Math.random(); /* break ties */ });
    weights.sort(function (a, b) {
      return b - a;
    });
    if (w) {
      // Check min points
      if (weights[options.minPoints] < w) {
        w = weights[options.minPoints]
      }
    } else {
      var pointsToKeep = options.minPoints;
      // Calculate ratio
      if (!pointsToKeep) {
        var ratio = options.ratio || .8
        pointsToKeep = Math.round(points.length * ratio);
      }
      pointsToKeep = Math.min(pointsToKeep, weights.length -1);
      w = weights[pointsToKeep]
    }
  }

  var result = points.filter(function (d) {
    return d[2] > w;
  });
  if (options.keepEnds) {
    if (!ol_coordinate_equal(result[0], points[0])) result.unshift(points[0]);
    if (!ol_coordinate_equal(result[result.length-1], points[points.length-1])) result.push(points[points.length-1]);
  }

  return new ol_geom_LineString(result);
};

function compare(a, b) {
  return a[1][2] - b[1][2];
}

function area(t) {
  return Math.abs((t[0][0] - t[2][0]) * (t[1][1] - t[0][1]) - (t[0][0] - t[1][0]) * (t[2][1] - t[0][1]));
}

function minHeap() {
  var heap = {},
      array = [];

  heap.push = function() {
    for (var i = 0, n = arguments.length; i < n; ++i) {
      var object = arguments[i];
      up(object.index = array.push(object) - 1);
    }
    return array.length;
  };

  heap.pop = function() {
    var removed = array[0],
        object = array.pop();
    if (array.length) {
      array[object.index = 0] = object;
      down(0);
    }
    return removed;
  };

  heap.size = function () {
    return array.length;
  };

  heap.remove = function(removed) {
    var i = removed.index,
        object = array.pop();
    if (i !== array.length) {
      array[object.index = i] = object;
      (compare(object, removed) < 0 ? up : down)(i);
    }
    return i;
  };

  function up(i) {
    var object = array[i];
    while (i > 0) {
      var up = ((i + 1) >> 1) - 1,
          parent = array[up];
      if (compare(object, parent) >= 0) break;
      array[parent.index = i] = parent;
      array[object.index = i = up] = object;
    }
  }

  function down(i) {
    var object = array[i];
    for (;;) {
      var right = (i + 1) * 2,
          left = right - 1,
          down = i,
          child = array[down];
      if (left < array.length && compare(array[left], child) < 0) child = array[down = left];
      if (right < array.length && compare(array[right], child) < 0) child = array[down = right];
      if (down === i) break;
      array[child.index = i] = child;
      array[object.index = i = down] = object;
    }
  }

  return heap;
}
})();

export default ol_geom_LineString