import earcut from 'earcut';
import { polygon } from '@turf/helpers';

/**
 * Tesselates a {@link Feature<Polygon>} into a {@link FeatureCollection<Polygon>} of triangles
 * using [earcut](https://github.com/mapbox/earcut).
 *
 * @name tesselate
 * @param {Feature<Polygon>} poly the polygon to tesselate
 * @returns {FeatureCollection<Polygon>} a geometrycollection feature
 * @example
 * var poly = turf.polygon([[[11, 0], [22, 4], [31, 0], [31, 11], [21, 15], [11, 11], [11, 0]]]);
 * var triangles = turf.tesselate(poly);
 *
 * //addToMap
 * var addToMap = [poly, triangles]
 */
function tesselate(poly) {
  if (
    !poly.geometry ||
    (poly.geometry.type !== "Polygon" && poly.geometry.type !== "MultiPolygon")
  ) {
    throw new Error("input must be a Polygon or MultiPolygon");
  }

  var fc = { type: "FeatureCollection", features: [] };

  if (poly.geometry.type === "Polygon") {
    fc.features = processPolygon(poly.geometry.coordinates);
  } else {
    poly.geometry.coordinates.forEach(function (coordinates) {
      fc.features = fc.features.concat(processPolygon(coordinates));
    });
  }

  return fc;
}

function processPolygon(coordinates) {
  var data = flattenCoords(coordinates);
  var dim = 2;
  var result = earcut(data.vertices, data.holes, dim);

  var features = [];
  var vertices = [];

  result.forEach(function (vert, i) {
    var index = result[i];
    vertices.push([data.vertices[index * dim], data.vertices[index * dim + 1]]);
  });

  for (var i = 0; i < vertices.length; i += 3) {
    var coords = vertices.slice(i, i + 3);
    coords.push(vertices[i]);
    features.push(polygon([coords]));
  }

  return features;
}

function flattenCoords(data) {
  var dim = data[0][0].length,
    result = { vertices: [], holes: [], dimensions: dim },
    holeIndex = 0;

  for (var i = 0; i < data.length; i++) {
    for (var j = 0; j < data[i].length; j++) {
      for (var d = 0; d < dim; d++) result.vertices.push(data[i][j][d]);
    }
    if (i > 0) {
      holeIndex += data[i - 1].length;
      result.holes.push(holeIndex);
    }
  }

  return result;
}

export default tesselate;