src/interpolate/transform.js

import "../math/transform";
import "number";

d3.interpolateTransform = d3_interpolateTransform;

function d3_interpolateTransform(a, b) {
  var s = [], // string constants and placeholders
      q = [], // number interpolators
      n,
      A = d3.transform(a),
      B = d3.transform(b),
      ta = A.translate,
      tb = B.translate,
      ra = A.rotate,
      rb = B.rotate,
      wa = A.skew,
      wb = B.skew,
      ka = A.scale,
      kb = B.scale;

  if (ta[0] != tb[0] || ta[1] != tb[1]) {
    s.push("translate(", null, ",", null, ")");
    q.push({i: 1, x: d3_interpolateNumber(ta[0], tb[0])}, {i: 3, x: d3_interpolateNumber(ta[1], tb[1])});
  } else if (tb[0] || tb[1]) {
    s.push("translate(" + tb + ")");
  } else {
    s.push("");
  }

  if (ra != rb) {
    if (ra - rb > 180) rb += 360; else if (rb - ra > 180) ra += 360; // shortest path
    q.push({i: s.push(s.pop() + "rotate(", null, ")") - 2, x: d3_interpolateNumber(ra, rb)});
  } else if (rb) {
    s.push(s.pop() + "rotate(" + rb + ")");
  }

  if (wa != wb) {
    q.push({i: s.push(s.pop() + "skewX(", null, ")") - 2, x: d3_interpolateNumber(wa, wb)});
  } else if (wb) {
    s.push(s.pop() + "skewX(" + wb + ")");
  }

  if (ka[0] != kb[0] || ka[1] != kb[1]) {
    n = s.push(s.pop() + "scale(", null, ",", null, ")");
    q.push({i: n - 4, x: d3_interpolateNumber(ka[0], kb[0])}, {i: n - 2, x: d3_interpolateNumber(ka[1], kb[1])});
  } else if (kb[0] != 1 || kb[1] != 1) {
    s.push(s.pop() + "scale(" + kb + ")");
  }

  n = q.length;
  return function(t) {
    var i = -1, o;
    while (++i < n) s[(o = q[i]).i] = o.x(t);
    return s.join("");
  };
}