Performance of map!()

[Chris via Digitalmars-d]

I replaced a range that was similar to map with map and the 
performance dropped by ~0.5 msec.

The range I used previously is based on Adam's D Cookbook. It is 
consistently faster than map.

private struct Transformer(alias agent, R) if (isInputRange!R) {

  private R r;
  this (R r) {
    this.r = r;
  }

  static if (isInfinite!R) {
    enum bool empty = false;
  }
  else {
    @property bool empty() {
      return r.empty;
    }
  }

  void popFront() {
    r.popFront();

  }
  @property ref auto front() {
    return agent(r.front);
  }

  static if (isForwardRange!R) {
    @property auto save() {
      return Transformer!(agent, R)(r.save);
    }
  }

  static if (isBidirectionalRange!R) {
    @property auto back() {
      return agent(r.back);
    }
    void popBack() {
      r.popBack();
    }
  }

  static if (isRandomAccessRange!R) {
    auto opIndex(size_t i) {
      return agent(r[i]);
    }
  }

  static if (hasLength!R) {
    @property auto length() {
      return r.length;
    }
  }
}