src/geo/graticule.js

import "../arrays/range";
import "../math/abs";
import "../math/trigonometry";
import "geo";

d3.geo.graticule = 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 d3.range(Math.ceil(X0 / DX) * DX, X1, DX).map(X)
        .concat(d3.range(Math.ceil(Y0 / DY) * DY, Y1, DY).map(Y))
        .concat(d3.range(Math.ceil(x0 / dx) * dx, x1, dx).filter(function(x) { return abs(x % DX) > ε; }).map(x))
        .concat(d3.range(Math.ceil(y0 / dy) * dy, y1, dy).filter(function(y) { return abs(y % DY) > ε; }).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.minorExtent();
    return graticule.majorExtent(_).minorExtent(_);
  };

  graticule.majorExtent = 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.minorExtent = 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.minorStep();
    return graticule.majorStep(_).minorStep(_);
  };

  graticule.majorStep = function(_) {
    if (!arguments.length) return [DX, DY];
    DX = +_[0], DY = +_[1];
    return graticule;
  };

  graticule.minorStep = function(_) {
    if (!arguments.length) return [dx, dy];
    dx = +_[0], dy = +_[1];
    return graticule;
  };

  graticule.precision = function(_) {
    if (!arguments.length) return precision;
    precision = +_;
    x = d3_geo_graticuleX(y0, y1, 90);
    y = d3_geo_graticuleY(x0, x1, precision);
    X = d3_geo_graticuleX(Y0, Y1, 90);
    Y = d3_geo_graticuleY(X0, X1, precision);
    return graticule;
  };

  return graticule
      .majorExtent([[-180, -90 + ε], [180, 90 - ε]])
      .minorExtent([[-180, -80 - ε], [180, 80 + ε]]);
};

function d3_geo_graticuleX(y0, y1, dy) {
  var y = d3.range(y0, y1 - ε, dy).concat(y1);
  return function(x) { return y.map(function(y) { return [x, y]; }); };
}

function d3_geo_graticuleY(x0, x1, dx) {
  var x = d3.range(x0, x1 - ε, dx).concat(x1);
  return function(y) { return x.map(function(x) { return [x, y]; }); };
}