Fix Path.relativeTo and add support for relative paths as input.

sdk/lib/io/path_impl.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.io;
 
 class _Path implements Path {
   final String _path;
   final bool isWindowsShare;
 
@@ skipping 42 matching lines @@
     var basePath = base.toString();
     if (base.isAbsolute && _path.startsWith(basePath) &&
         base.isWindowsShare == isWindowsShare) {
       if (_path == basePath) return new Path('.');
       if (base.hasTrailingSeparator) {
         return new Path(_path.substring(basePath.length));
       }
       if (_path[basePath.length] == '/') {
         return new Path(_path.substring(basePath.length + 1));
       }
-    } else if (base.isAbsolute && isAbsolute &&
+    } else if (base.isAbsolute == isAbsolute &&
                base.isWindowsShare == isWindowsShare) {
       List<String> baseSegments = base.canonicalize().segments();
       List<String> pathSegments = canonicalize().segments();
+      if (baseSegments.length == 1 && baseSegments[0] == '.') {
+        baseSegments = [];
+      }
+      if (pathSegments.length == 1 && pathSegments[0] == '.') {
+        pathSegments = [];
+      }
       int common = 0;
       int length = min(pathSegments.length, baseSegments.length);
       while (common < length && pathSegments[common] == baseSegments[common]) {

[Bob Nystrom, 2013/01/16 18:42:38]

I believe this will do the wrong thing if you have drive letters that differ by
case or slash style.

         common++;
       }
-      final sb = new StringBuffer();
+      final segments = new List<String>();
 
-      for (int i = common + 1; i < baseSegments.length; i++) {
-        sb.add('../');
+      if (common < baseSegments.length && baseSegments[common] == '..') {
+        throw new ArgumentError(
+            "Invalid case of Path.relativeTo(base):\n"
+            "  Base path has more '..'s than path does."
+            "  Arguments: $_path.relativeTo($base)");

[Bob Nystrom, 2013/01/16 18:42:38]

Throwing an ArgumentError here is a trip-wire for the user. They can't
programmatically prevent this unless they reimplement the same logic that this
method contains, and they shouldn't catch it because it's an Error.

       }
-      if (base.hasTrailingSeparator) {
-        sb.add('../');
+      for (int i = common; i < baseSegments.length; i++) {
+        segments.add('..');
       }
-      for (int i = common; i < pathSegments.length - 1; i++) {
-        sb.add('${pathSegments[i]}/');
+      for (int i = common; i < pathSegments.length; i++) {
+        segments.add('${pathSegments[i]}');
       }
-      sb.add('${pathSegments.last}');
+      if (segments.isEmpty) {
+        segments.add('.');
+      }
       if (hasTrailingSeparator) {
-        sb.add('/');
+        segments.add('');
       }
-      return new Path(sb.toString());
+      return new Path(Strings.join(segments, '/'));
     }
-    throw new UnimplementedError(
-      "Unimplemented case of Path.relativeTo(base):\n"
-      "  Only absolute paths are handled at present.\n"
+    throw new ArgumentError(
+      "Invalid case of Path.relativeTo(base):\n"
+      "  Path and base must both be relative, or both absolute.\n"
       "  Arguments: $_path.relativeTo($base)");

[Bob Nystrom, 2013/01/16 18:42:38]

Ditto here. This will give users a bad day.

pkg/path (like Python and Ruby) handle this by assuming a relative path is
relative to the cwd if that larger context is needed.

   }
 
   Path join(Path further) {
     if (further.isAbsolute) {
       throw new ArgumentError(
           "Path.join called with absolute Path as argument.");
     }
     if (isEmpty) {
       return further.canonicalize();
     }
@@ skipping 150 matching lines @@
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     var dirPath = (pos > 0) ? _path.substring(0, pos) : '/';
     return new _Path._internal(dirPath, isWindowsShare);
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }