Safari4 で switch～case を速くする

Safari4 の switch ～ case 文は、型を統一することで実行速度が大きく変化します。

これを

switch (a) {
case "a":
case 1:
}

こうしましょう。

switch (a) {
case "a":
case "1":   // 1 → "1":
}

これだけで、2倍速です。

http://pigs.sourceforge.jp/blog/20100111/20100111.htm

Browser	switch	switch2	ratio
Firefox3.0.17	548ms	516ms	110%
Firefox3.6RC1(jit)	78ms	78ms	100%
IE6	1640ms	1640ms	100%
IE8	1234ms	1235ms	100%
Chrome4.0.266(dev)(jit)	200ms	198ms	100%
Safari4.0.4(jit)	208ms	100ms	210%
Opera10.10	812ms	766ms	110%
Opera10.50 pre-alpha(jit)	67ms	62ms	110%

項目が増えると、さらに速度差は開きます。

http://pigs.sourceforge.jp/blog/20100111/more.htm

Browser	mix	string	ratio
Safari4.0.4(jit)	2866ms	136ms	2100%

ざっくり 21倍速です。

ベンチマーク

<!doctype html><html><head>
<title>bench: switch vs Hash</title><script>
window.onload = function() {
  var hash = { color: 1, backgroundColor: 1, "background-color": 1,
               top: 2, marginTop: 2, "margin-top": 2, 3: 3 },
      ary = ("color,backgroundColor,background-color," +
             "top,marginTop,margin-top,3").split(",");

  document.body.innerHTML += navigator.userAgent + "<p>" +
      [ switchBench(100000, ary),
        switchBench2(100000, ary), "" ].join("ms</p><p>") + "</p>";

  function switchBench(loop, ary) {
    var now = +new Date;
    var rv = 0, n = 0, i = 0, j, jz = ary.length;

    for (; i < loop; ++i) {
      for (j = 0; j < jz; ++j) {
        v = ary[j];
        switch (ary[j]) {
        case "color":
        case "backgroundColor":
        case "background-color":
            n = 1;
            break;
        case "top":
        case "marginTop":
        case "margin-top":
            n = 2;
            break;
        case 3:       // ここ
            n = 3;
            break;
        default:
            n = 0;
        }
        rv += n;
      }
    }
    return (+new Date) - now;
  }

  function switchBench2(loop, ary) {
    var now = +new Date;
    var rv = 0, n = 0, i = 0, j, jz = ary.length;

    for (; i < loop; ++i) {
      for (j = 0; j < jz; ++j) {
        v = ary[j];
        switch (ary[j]) {
        case "color":
        case "backgroundColor":
        case "background-color":
            n = 1;
            break;
        case "top":
        case "marginTop":
        case "margin-top":
            n = 2;
            break;
        case "3":  // ここ
            n = 3;
            break;
        default:
            n = 0;
        }
        rv += n;
      }
    }
    return (+new Date) - now;
  }
}
</script></head><body></body></html>