var π = Math.PI,
    τ = 2 * π,
    radians = π / 180,
    degrees = 180 / π;

var width = 960,
    height = 960,
    scale = width * .45;

var solar = solarCalculator([-122, 37]),
    start = d3.time.year.utc.floor(new Date),
    end = d3.time.year.utc.offset(start, 1);

var projection = d3.geo.projection(flippedStereographic)
    .scale(scale)
    .clipAngle(130)
    .rotate([0, -90])
    .translate([width / 2 + .5, height / 2 + .5])
    .precision(.1);

var path = d3.geo.path()
    .projection(projection);

var svg = d3.select('body').append('svg')
    .attr('class', 'sun')
    .attr('width', width)
    .attr('height', height);

svg.append('path')
    .datum(d3.geo.circle().origin([0, 90]).angle(90))
    .attr('class', 'horizon')
    .attr('d', path);

svg.append('path')
    .datum(d3.geo.graticule())
    .attr('class', 'graticule')
    .attr('d', path);

var ticksAzimuth = svg.append('g')
    .attr('class', 'ticks ticks--azimuth');

ticksAzimuth.selectAll('line')
    .data(d3.range(360))
  .enter().append('line')
    .each(function(d) {
      var p0 = projection([d, 0]),
          p1 = projection([d, d % 10 ? -1 : -2]);

      d3.select(this)
          .attr('x1', p0[0])
          .attr('y1', p0[1])
          .attr('x2', p1[0])
          .attr('y2', p1[1]);
    });

ticksAzimuth.selectAll('text')
    .data(d3.range(0, 360, 10))
  .enter().append('text')
    .each(function(d) {
      var p = projection([d, -4]);

      d3.select(this)
          .attr('x', p[0])
          .attr('y', p[1]);
    })
    .attr('dy', '.35em')
    .text(function(d) {
      return d ===   0 ? 'N' :
             d ===  90 ? 'E' :
             d === 180 ? 'S' :
             d === 270 ? 'W' :
             d + '°';
    });

svg.append('g')
    .attr('class', 'ticks ticks--elevation')
  .selectAll('text')
    .data(d3.range(10, 91, 10))
  .enter().append('text')
    .each(function(d) {
      var p = projection([0, d]);

      d3.select(this)
          .attr('x', p[0])
          .attr('y', p[1]);
    })
    .attr('dy', '.35em')
    .text(function(d) { return d + '°'; });

function analemmaForHour(h) {
  function pos(d) {
    return solar.position(d3.time.hour.utc.offset(d, h));
  }
  return {
    type: 'LineString',
    coordinates: d3.time.days.utc(start, end).map(pos)
  };
}

var analemmas = svg.insert('g', '.sphere')
    .attr('class', 'analemmas')
  .selectAll('path')
    .data(d3.range(24))
  .enter().append('path')
    .datum(analemmaForHour)
    .attr('d', path);

function setLocation(lat, lng) {
  solar = solarCalculator([lng, lat]);
  analemmas.remove();
  analemmas = svg.select('g.analemmas')
    .selectAll('path')
      .data(d3.range(24))
    .enter().append('path')
      .datum(analemmaForHour)
      .attr('d', path);
}

function flippedStereographic(λ, φ)  {
  var cosλ = Math.cos(λ),
      cosφ = Math.cos(φ),
      k = 1 / (1 + cosλ * cosφ);
  return [
    k * cosφ * Math.sin(λ),
    -k * Math.sin(φ)
  ];
}