import {namespace} from "d3-selection";

function attrInterpolate(name, i) {
  return function(t) {
    this.setAttribute(name, i.call(this, t));
  };
}

function attrInterpolateNS(fullname, i) {
  return function(t) {
    this.setAttributeNS(fullname.space, fullname.local, i.call(this, t));
  };
}

function attrTweenNS(fullname, value) {
  var t0, i0;
  function tween() {
    var i = value.apply(this, arguments);
    if (i !== i0) t0 = (i0 = i) && attrInterpolateNS(fullname, i);
    return t0;
  }
  tween._value = value;
  return tween;
}

function attrTween(name, value) {
  var t0, i0;
  function tween() {
    var i = value.apply(this, arguments);
    if (i !== i0) t0 = (i0 = i) && attrInterpolate(name, i);
    return t0;
  }
  tween._value = value;
  return tween;
}

export default function(name, value) {
  var key = "attr." + name;
  if (arguments.length < 2) return (key = this.tween(key)) && key._value;
  if (value == null) return this.tween(key, null);
  if (typeof value !== "function") throw new Error;
  var fullname = namespace(name);
  return this.tween(key, (fullname.local ? attrTweenNS : attrTween)(fullname, value));
}