source: node_modules/d3-interpolate/src/transform/index.js@ e4c61dd

import number from "../number.js";
import {parseCss, parseSvg} from "./parse.js";

function interpolateTransform(parse, pxComma, pxParen, degParen) {

  function pop(s) {
    return s.length ? s.pop() + " " : "";
  }

  function translate(xa, ya, xb, yb, s, q) {
    if (xa !== xb || ya !== yb) {
      var i = s.push("translate(", null, pxComma, null, pxParen);
      q.push({i: i - 4, x: number(xa, xb)}, {i: i - 2, x: number(ya, yb)});
    } else if (xb || yb) {
      s.push("translate(" + xb + pxComma + yb + pxParen);
    }
  }

  function rotate(a, b, s, q) {
    if (a !== b) {
      if (a - b > 180) b += 360; else if (b - a > 180) a += 360; // shortest path
      q.push({i: s.push(pop(s) + "rotate(", null, degParen) - 2, x: number(a, b)});
    } else if (b) {
      s.push(pop(s) + "rotate(" + b + degParen);
    }
  }

  function skewX(a, b, s, q) {
    if (a !== b) {
      q.push({i: s.push(pop(s) + "skewX(", null, degParen) - 2, x: number(a, b)});
    } else if (b) {
      s.push(pop(s) + "skewX(" + b + degParen);
    }
  }

  function scale(xa, ya, xb, yb, s, q) {
    if (xa !== xb || ya !== yb) {
      var i = s.push(pop(s) + "scale(", null, ",", null, ")");
      q.push({i: i - 4, x: number(xa, xb)}, {i: i - 2, x: number(ya, yb)});
    } else if (xb !== 1 || yb !== 1) {
      s.push(pop(s) + "scale(" + xb + "," + yb + ")");
    }
  }

  return function(a, b) {
    var s = [], // string constants and placeholders
        q = []; // number interpolators
    a = parse(a), b = parse(b);
    translate(a.translateX, a.translateY, b.translateX, b.translateY, s, q);
    rotate(a.rotate, b.rotate, s, q);
    skewX(a.skewX, b.skewX, s, q);
    scale(a.scaleX, a.scaleY, b.scaleX, b.scaleY, s, q);
    a = b = null; // gc
    return function(t) {
      var i = -1, n = q.length, o;
      while (++i < n) s[(o = q[i]).i] = o.x(t);
      return s.join("");
    };
  };
}

export var interpolateTransformCss = interpolateTransform(parseCss, "px, ", "px)", "deg)");
export var interpolateTransformSvg = interpolateTransform(parseSvg, ", ", ")", ")");