Geo rotation

[chrisuehlinger]

(Note: Now that some of the common structure has been laid out, I've published 4 orthogonal branches. These can be reviewed and merged in any order, whenever you see fit. There might be slight merge conflicts around index.js, but that should be all.)

This branch addresses Issue #13 , d3.geoRotation.

I followed your advice and swapped out equirectangular, which successfully solved the circular dependency. I'll look into a similar solution for getting the references to spherical out of d3.geoCircle.

[mbostock]

Math.asin and d3_asin aren’t exactly the same: the latter is more robust to floating point error when the input value is slightly greater than +1 or slightly less than -1.

var pi = Math.PI,
    halfPi = pi / 2;

function asin(x) {
  return x > 1 ? halfPi : x < -1 ? -halfPi : Math.asin(x);
}

[mbostock]

I’d also recommend pulling out constants that can be used in multiple places for brevity. At the minimum, in math.js:

export var pi = Math.PI;
export var halfPi = pi / 2;
export var tau = pi * 2;

Probably also:

export var sin = Math.sin;
export var cos = Math.cos;
export var atan2 = Math.atan2;

[mbostock]

I’ll do this.