Uri: use List.unmodifiable

sdk/lib/core/uri.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * A parsed URI, such as a URL.
  *
  * **See also:**
@@ skipping 1000 matching lines @@
   /**
    * Returns the URI path split into its segments. Each of the
    * segments in the returned list have been decoded. If the path is
    * empty the empty list will be returned. A leading slash `/` does
    * not affect the segments returned.
    *
    * The returned list is unmodifiable and will throw [UnsupportedError] on any
    * calls that would mutate it.
    */
   List<String> get pathSegments {
-    if (_pathSegments == null) {
-      var pathToSplit = !path.isEmpty && path.codeUnitAt(0) == _SLASH
-                        ? path.substring(1)
-                        : path;
-      _pathSegments = new UnmodifiableListView(
-        pathToSplit == "" ? const<String>[]
-                          : pathToSplit.split("/")
-                                       .map(Uri.decodeComponent)
-                                       .toList(growable: false));
-    }
-    return _pathSegments;
+    var result = _pathSegments;
+    if (result != null) return result;
+
+    var pathToSplit = !path.isEmpty && path.codeUnitAt(0) == _SLASH

[Lasse Reichstein Nielsen, 2015/11/04 06:54:56]

Alternatively (I have no idea if it's better performance/code-size), you can
use:
  path.startsWith("/")
or at least use .isNotEmpty instead of !...isEmpty.

Also consider whether it's better to split the original string and remove the
first element of the resulting list instead of copying almost all of the path. 
You are doing a map below, so you can also do a "skip(1)" first in that case.

Basically:
  int skipCount = path.startsWith("/") ? 1 : 0;
...
      path.split("/").skip(skipCount).map(Uri.decodeComponent));

(It will be an efficient-length iterable, so List.unmodifiable is efficient).

[sra1, 2015/11/04 22:29:03]

I measured a small (5%) degradation for this pattern, mostly small because all
the work is in decodeComponent.

a.startsWith(b) does   b == a.substring(0, b.length), so the current form is
more efficient.

split returns a JS Array. SubListIterable tends to be used on a polymorphic list
implementation, making all the operations slower (See similar issue with
ListIterable https://github.com/dart-lang/sdk/issues/22671).

+        ? path.substring(1)
+        : path;
+    result = pathToSplit == ""

[Lasse Reichstein Nielsen, 2015/11/04 06:54:56]

Parenthesize the condition (or change to .isEmpty).

(And I know it's not your code originally, but now you are here - after this
it's your name on git blame :)

[sra1, 2015/11/04 22:29:03]

Done.

+        ? const<String>[]
+        : new List<String>.unmodifiable(
+              pathToSplit.split("/").map(Uri.decodeComponent));
+    _pathSegments = result;
+    return result;
   }
 
   /**
    * Returns the URI query split into a map according to the rules
    * specified for FORM post in the [HTML 4.01 specification section 17.13.4]
    * (http://www.w3.org/TR/REC-html40/interact/forms.html#h-17.13.4
    * "HTML 4.01 section 17.13.4"). Each key and value in the returned map
    * has been decoded. If there is no query the empty map is returned.
    *
    * Keys in the query string that have no value are mapped to the
@@ skipping 1569 matching lines @@
       0xafff,   // 0x30 - 0x3f  1111111111110101
                 //              @ABCDEFGHIJKLMNO
       0xffff,   // 0x40 - 0x4f  1111111111111111
                 //              PQRSTUVWXYZ    _
       0x87ff,   // 0x50 - 0x5f  1111111111100001
                 //               abcdefghijklmno
       0xfffe,   // 0x60 - 0x6f  0111111111111111
                 //              pqrstuvwxyz   ~
       0x47ff];  // 0x70 - 0x7f  1111111111100010
 }