fs: replace regexp with function

Replacing the path separator-finding regexp with a custom function
results in a measurable improvement in performance.

PR-URL: #10789
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/fs.js

@@ -1478,12 +1478,6 @@ fs.unwatchFile = function(filename, listener) {
 };
 
 
-// Regexp that finds the next portion of a (partial) path
-// result is [base_with_slash, base], e.g. ['somedir/', 'somedir']
-const nextPartRe = isWindows ?
-  /(.*?)(?:[/\\]+|$)/g :
-  /(.*?)(?:[/]+|$)/g;
-
 // Regex to find the device root, including trailing slash. E.g. 'c:\\'.
 const splitRootRe = isWindows ?
   /^(?:[a-zA-Z]:|[\\/]{2}[^\\/]+[\\/][^\\/]+)?[\\/]*/ :
@@ -1500,6 +1494,21 @@ function encodeRealpathResult(result, options) {
   }
 }
 
+// Finds the next portion of a (partial) path, up to the next path delimiter
+var nextPart;
+if (isWindows) {
+  nextPart = function nextPart(p, i) {
+    for (; i < p.length; ++i) {
+      const ch = p.charCodeAt(i);
+      if (ch === 92/*'\'*/ || ch === 47/*'/'*/)
+        return i;
+    }
+    return -1;
+  };
+} else {
+  nextPart = function nextPart(p, i) { return p.indexOf('/', i); };
+}
+
 fs.realpathSync = function realpathSync(p, options) {
   options = getOptions(options, {});
   handleError((p = getPathFromURL(p)));
@@ -1544,12 +1553,18 @@ fs.realpathSync = function realpathSync(p, options) {
   // NB: p.length changes.
   while (pos < p.length) {
     // find the next part
-    nextPartRe.lastIndex = pos;
-    var result = nextPartRe.exec(p);
+    var result = nextPart(p, pos);
     previous = current;
-    current += result[0];
-    base = previous + result[1];
-    pos = nextPartRe.lastIndex;
+    if (result === -1) {
+      var last = p.slice(pos);
+      current += last;
+      base = previous + last;
+      pos = p.length;
+    } else {
+      current += p.slice(pos, result + 1);
+      base = previous + p.slice(pos, result);
+      pos = result + 1;
+    }
 
     // continue if not a symlink
     if (knownHard[base] || (cache && cache.get(base) === base)) {
@@ -1658,12 +1673,18 @@ fs.realpath = function realpath(p, options, callback) {
     }
 
     // find the next part
-    nextPartRe.lastIndex = pos;
-    var result = nextPartRe.exec(p);
+    var result = nextPart(p, pos);
     previous = current;
-    current += result[0];
-    base = previous + result[1];
-    pos = nextPartRe.lastIndex;
+    if (result === -1) {
+      var last = p.slice(pos);
+      current += last;
+      base = previous + last;
+      pos = p.length;
+    } else {
+      current += p.slice(pos, result + 1);
+      base = previous + p.slice(pos, result);
+      pos = result + 1;
+    }
 
     // continue if not a symlink
     if (knownHard[base]) {