text: import {range} from "d3-array";
  import {abs, ceil, epsilon} from "./math";
  
  function graticuleX(y0, y1, dy) {
    var y = range(y0, y1 - epsilon, dy).concat(y1);
    return function(x) { return y.map(function(y) { return [x, y]; }); };
  }
  
  function graticuleY(x0, x1, dx) {
    var x = range(x0, x1 - epsilon, dx).concat(x1);
    return function(y) { return x.map(function(x) { return [x, y]; }); };
  }
  
  export default function() {
    var x1, x0, X1, X0,
        y1, y0, Y1, Y0,
        dx = 10, dy = dx, DX = 90, DY = 360,
        x, y, X, Y,
        precision = 2.5;
  
    function graticule() {
      return {type: "MultiLineString", coordinates: lines()};
    }
  
    function lines() {
      return range(ceil(X0 / DX) * DX, X1, DX).map(X)
          .concat(range(ceil(Y0 / DY) * DY, Y1, DY).map(Y))
          .concat(range(ceil(x0 / dx) * dx, x1, dx).filter(function(x) { return abs(x % DX) > epsilon; }).map(x))
          .concat(range(ceil(y0 / dy) * dy, y1, dy).filter(function(y) { return abs(y % DY) > epsilon; }).map(y));
    }
  
    graticule.lines = function() {
      return lines().map(function(coordinates) { return {type: "LineString", coordinates: coordinates}; });
    };
  
    graticule.outline = function() {
      return {
        type: "Polygon",
        coordinates: [
          X(X0).concat(
          Y(Y1).slice(1),
          X(X1).reverse().slice(1),
          Y(Y0).reverse().slice(1))
        ]
      };
    };
  
    graticule.extent = function(_) {
      if (!arguments.length) return graticule.extentMinor();
      return graticule.extentMajor(_).extentMinor(_);
    };
  
    graticule.extentMajor = function(_) {
      if (!arguments.length) return [[X0, Y0], [X1, Y1]];
      X0 = +_[0][0], X1 = +_[1][0];
      Y0 = +_[0][1], Y1 = +_[1][1];
      if (X0 > X1) _ = X0, X0 = X1, X1 = _;
      if (Y0 > Y1) _ = Y0, Y0 = Y1, Y1 = _;
      return graticule.precision(precision);
    };
  
    graticule.extentMinor = function(_) {
      if (!arguments.length) return [[x0, y0], [x1, y1]];
      x0 = +_[0][0], x1 = +_[1][0];
      y0 = +_[0][1], y1 = +_[1][1];
      if (x0 > x1) _ = x0, x0 = x1, x1 = _;
      if (y0 > y1) _ = y0, y0 = y1, y1 = _;
      return graticule.precision(precision);
    };
  
    graticule.step = function(_) {
      if (!arguments.length) return graticule.stepMinor();
      return graticule.stepMajor(_).stepMinor(_);
    };
  
    graticule.stepMajor = function(_) {
      if (!arguments.length) return [DX, DY];
      DX = +_[0], DY = +_[1];
      return graticule;
    };
  
    graticule.stepMinor = function(_) {
      if (!arguments.length) return [dx, dy];
      dx = +_[0], dy = +_[1];
      return graticule;
    };
  
    graticule.precision = function(_) {
      if (!arguments.length) return precision;
      precision = +_;
      x = graticuleX(y0, y1, 90);
      y = graticuleY(x0, x1, precision);
      X = graticuleX(Y0, Y1, 90);
      Y = graticuleY(X0, X1, precision);
      return graticule;
    };
  
    return graticule
        .extentMajor([[-180, -90 + epsilon], [180, 90 - epsilon]])
        .extentMinor([[-180, -80 - epsilon], [180, 80 + epsilon]]);
  }
  