Performance improvements for NaturalSort

A combination of using isset() instead of count() or strlen(), caching the
chunkify function, and replacing is_numeric() with some comparisons

lib/private/naturalsort.php

@@ -12,6 +12,7 @@ namespace OC;
 class NaturalSort {
 	private static $instance;
 	private $collator;
+	private $cache = array();
 
 	/**
 	 * Split the given string in chunks of numbers and strings
@@ -21,13 +22,15 @@ class NaturalSort {
 	private function naturalSortChunkify($t) {
 		// Adapted and ported to PHP from
 		// http://my.opera.com/GreyWyvern/blog/show.dml/1671288
+		if (isset($this->cache[$t])) {
+			return $this->cache[$t];
+		}
 		$tz = array();
 		$x = 0;
 		$y = -1;
 		$n = null;
-		$length = strlen($t);
 
-		while ($x < $length) {
+		while (isset($t[$x])) {
 			$c = $t[$x];
 			// only include the dot in strings
 			$m = ((!$n && $c === '.') || ($c >= '0' && $c <= '9'));
@@ -40,6 +43,7 @@ class NaturalSort {
 			$tz[$y] .= $c;
 			$x++;
 		}
+		$this->cache[$t] = $tz;
 		return $tz;
 	}
 
@@ -75,14 +79,13 @@ class NaturalSort {
 		// instead of ["test.txt", "test (2).txt"]
 		$aa = self::naturalSortChunkify($a);
 		$bb = self::naturalSortChunkify($b);
-		$alen = count($aa);
-		$blen = count($bb);
 
-		for ($x = 0; $x < $alen && $x < $blen; $x++) {
+		for ($x = 0; isset($aa[$x]) && isset($bb[$x]); $x++) {
 			$aChunk = $aa[$x];
 			$bChunk = $bb[$x];
 			if ($aChunk !== $bChunk) {
-				if (is_numeric($aChunk) && is_numeric($bChunk)) {
+				// test first character (character comparison, not number comparison)
+				if ($aChunk[0] >= '0' && $aChunk[0] <= '9' && $bChunk[0] >= '0' && $bChunk[0] <= '9') {
 					$aNum = (int)$aChunk;
 					$bNum = (int)$bChunk;
 					return $aNum - $bNum;
@@ -90,7 +93,7 @@ class NaturalSort {
 				return self::getCollator()->compare($aChunk, $bChunk);
 			}
 		}
-		return $alen - $blen;
+		return count($aa) - count($bb);
 	}
 
 	/**