source: node_modules/d3-contour/src/density.js@ a762898

import {blur2, max, ticks} from "d3-array";
import {slice} from "./array.js";
import constant from "./constant.js";
import Contours from "./contours.js";

function defaultX(d) {
  return d[0];
}

function defaultY(d) {
  return d[1];
}

function defaultWeight() {
  return 1;
}

export default function() {
  var x = defaultX,
      y = defaultY,
      weight = defaultWeight,
      dx = 960,
      dy = 500,
      r = 20, // blur radius
      k = 2, // log2(grid cell size)
      o = r * 3, // grid offset, to pad for blur
      n = (dx + o * 2) >> k, // grid width
      m = (dy + o * 2) >> k, // grid height
      threshold = constant(20);

  function grid(data) {
    var values = new Float32Array(n * m),
        pow2k = Math.pow(2, -k),
        i = -1;

    for (const d of data) {
      var xi = (x(d, ++i, data) + o) * pow2k,
          yi = (y(d, i, data) + o) * pow2k,
          wi = +weight(d, i, data);
      if (wi && xi >= 0 && xi < n && yi >= 0 && yi < m) {
        var x0 = Math.floor(xi),
            y0 = Math.floor(yi),
            xt = xi - x0 - 0.5,
            yt = yi - y0 - 0.5;
        values[x0 + y0 * n] += (1 - xt) * (1 - yt) * wi;
        values[x0 + 1 + y0 * n] += xt * (1 - yt) * wi;
        values[x0 + 1 + (y0 + 1) * n] += xt * yt * wi;
        values[x0 + (y0 + 1) * n] += (1 - xt) * yt * wi;
      }
    }

    blur2({data: values, width: n, height: m}, r * pow2k);
    return values;
  }

  function density(data) {
    var values = grid(data),
        tz = threshold(values),
        pow4k = Math.pow(2, 2 * k);

    // Convert number of thresholds into uniform thresholds.
    if (!Array.isArray(tz)) {
      tz = ticks(Number.MIN_VALUE, max(values) / pow4k, tz);
    }

    return Contours()
        .size([n, m])
        .thresholds(tz.map(d => d * pow4k))
      (values)
        .map((c, i) => (c.value = +tz[i], transform(c)));
  }

  density.contours = function(data) {
    var values = grid(data),
        contours = Contours().size([n, m]),
        pow4k = Math.pow(2, 2 * k),
        contour = value => {
          value = +value;
          var c = transform(contours.contour(values, value * pow4k));
          c.value = value; // preserve exact threshold value
          return c;
        };
    Object.defineProperty(contour, "max", {get: () => max(values) / pow4k});
    return contour;
  };

  function transform(geometry) {
    geometry.coordinates.forEach(transformPolygon);
    return geometry;
  }

  function transformPolygon(coordinates) {
    coordinates.forEach(transformRing);
  }

  function transformRing(coordinates) {
    coordinates.forEach(transformPoint);
  }

  // TODO Optimize.
  function transformPoint(coordinates) {
    coordinates[0] = coordinates[0] * Math.pow(2, k) - o;
    coordinates[1] = coordinates[1] * Math.pow(2, k) - o;
  }

  function resize() {
    o = r * 3;
    n = (dx + o * 2) >> k;
    m = (dy + o * 2) >> k;
    return density;
  }

  density.x = function(_) {
    return arguments.length ? (x = typeof _ === "function" ? _ : constant(+_), density) : x;
  };

  density.y = function(_) {
    return arguments.length ? (y = typeof _ === "function" ? _ : constant(+_), density) : y;
  };

  density.weight = function(_) {
    return arguments.length ? (weight = typeof _ === "function" ? _ : constant(+_), density) : weight;
  };

  density.size = function(_) {
    if (!arguments.length) return [dx, dy];
    var _0 = +_[0], _1 = +_[1];
    if (!(_0 >= 0 && _1 >= 0)) throw new Error("invalid size");
    return dx = _0, dy = _1, resize();
  };

  density.cellSize = function(_) {
    if (!arguments.length) return 1 << k;
    if (!((_ = +_) >= 1)) throw new Error("invalid cell size");
    return k = Math.floor(Math.log(_) / Math.LN2), resize();
  };

  density.thresholds = function(_) {
    return arguments.length ? (threshold = typeof _ === "function" ? _ : Array.isArray(_) ? constant(slice.call(_)) : constant(_), density) : threshold;
  };

  density.bandwidth = function(_) {
    if (!arguments.length) return Math.sqrt(r * (r + 1));
    if (!((_ = +_) >= 0)) throw new Error("invalid bandwidth");
    return r = (Math.sqrt(4 * _ * _ + 1) - 1) / 2, resize();
  };

  return density;
}