Multi-stop color gradient (AMD module)

gradient.js

// Color interpolation
// Usage:
// var myGradient = Gradient([
//   [50,  "#9999ff"],
//   [100, "#99ff99"],
//   [150, "#ff9999"]
// ]);
// myGradient(75); // returns "#99cccc"

define([], function () {

  function Piecewise(xs, ys) {
    return function(x) {
      //out of bounds
      if (x <= xs[0]) return ys[0];
      if (x >= xs[xs.length - 1]) return ys[xs.length - 1];
      //bisect
      var lo = 0, hi = xs.length - 1;
      while (hi - lo > 1) {
        var mid = (lo + hi) >> 1;
        if (x < xs[mid]) hi = mid;
        else lo = mid;
      }
      //project
      return ys[lo] + (ys[hi] - ys[lo]) / (xs[hi] - xs[lo]) * (x - xs[lo]);
    };
  }

  function formatHexPart(number) {
    var result = Math.round(number).toString(16);
    while (result.length < 2) result = "0" + result;
    return result.substr(0, 2);
  }

  function Gradient(stops) {
    var xs = [];
    var ys_r = [];
    var ys_g = [];
    var ys_b = [];
    stops.forEach(function (stop) {
      xs.push(stop[0]);
      ys_r.push(parseInt(stop[1].substr(1, 2), 16));
      ys_g.push(parseInt(stop[1].substr(3, 2), 16));
      ys_b.push(parseInt(stop[1].substr(5, 2), 16));
    });
    pw_r = Piecewise(xs, ys_r);
    pw_g = Piecewise(xs, ys_g);
    pw_b = Piecewise(xs, ys_b);
    return function (x) {
      return "#" + formatHexPart(pw_r(x)) + 
                   formatHexPart(pw_g(x)) + 
                   formatHexPart(pw_b(x));
    };
  }

  return Gradient;

});