Adding some asset-related utilities to code_transformers

pkg/code_transformers/lib/assets.dart

Index: pkg/code_transformers/lib/assets.dart
diff --git a/pkg/code_transformers/lib/assets.dart b/pkg/code_transformers/lib/assets.dart
new file mode 100644
index 0000000000000000000000000000000000000000..8fabaf6afd187f31c729bc9704e739be2b853156
--- /dev/null
+++ b/pkg/code_transformers/lib/assets.dart
@@ -0,0 +1,99 @@
+// Copyright (c) 2014, 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.
+
+/// Common methods used by transfomers for dealing with asset IDs.
+library code_transformers.assets;
+
+import 'dart:math' show min, max;
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+import 'package:source_maps/span.dart' show Span;
+
+/// Create an [AssetId] for a [url] seen in the [source] asset. By default this
+/// is used to resolve relative urls that occur in HTML assets, including
+/// cross-package urls of the form "packages/foo/bar.html". Dart "package:"
+/// urls are not resolved unless [source] is Dart file (has a .dart extension).
+// TODO(sigmund): delete once this is part of barback (dartbug.com/12610)
+AssetId uriToAssetId(AssetId source, String url, TransformLogger logger,
+    Span span, {bool errorOnAbsolute: true}) {
+  if (url == null || url == '') return null;
+  var uri = Uri.parse(url);
+  var urlBuilder = path.url;
+  if (uri.host != '' || uri.scheme != '' || urlBuilder.isAbsolute(url)) {
+    if (source.extension == '.dart' && uri.scheme == 'package') {
+      var index = uri.path.indexOf('/');
+      if (index != -1) {
+        return new AssetId(uri.path.substring(0, index),
+            'lib${uri.path.substring(index)}');
+      }
+    }
+
+    if (errorOnAbsolute) {
+      logger.error('absolute paths not allowed: "$url"', span: span);
+    }
+    return null;
+  }
+
+  var targetPath = urlBuilder.normalize(
+      urlBuilder.join(urlBuilder.dirname(source.path), url));
+  var segments = urlBuilder.split(targetPath);
+  var sourceSegments = urlBuilder.split(source.path);
+  assert (sourceSegments.length > 0);
+  var topFolder = sourceSegments[0];
+  var entryFolder = topFolder != 'lib' && topFolder != 'asset';
+
+  // Find the first 'packages/'  or 'assets/' segment:
+  var packagesIndex = segments.indexOf('packages');
+  var assetsIndex = segments.indexOf('assets');
+  var index = (packagesIndex >= 0 && assetsIndex >= 0)
+      ? min(packagesIndex, assetsIndex)
+      : max(packagesIndex, assetsIndex);
+  if (index > -1) {
+    if (entryFolder) {
+      // URLs of the form "packages/foo/bar" seen under entry folders (like
+      // web/, test/, example/, etc) are resolved as an asset in another
+      // package. 'packages' can be used anywhere, there is no need to walk up
+      // where the entrypoint file was.
+      return _extractOtherPackageId(index, segments, logger, span);
+    } else if (index == 1 && segments[0] == '..') {
+      // Relative URLs of the form "../../packages/foo/bar" in an asset under
+      // lib/ or asset/ are also resolved as an asset in another package, but we
+      // check that the relative path goes all the way out where the packages
+      // folder lives (otherwise the app would not work in Dartium). Since
+      // [targetPath] has been normalized, "packages" or "assets" should be at
+      // index 1.
+      return _extractOtherPackageId(1, segments, logger, span);
+    } else {
+      var prefix = segments[index];
+      var fixedSegments = [];
+      fixedSegments.addAll(sourceSegments.map((_) => '..'));
+      fixedSegments.addAll(segments.sublist(index));
+      var fixedUrl = urlBuilder.joinAll(fixedSegments);
+      logger.error('Invalid url to reach to another package: $url. Path '
+          'reaching to other packages must first reach up all the '
+          'way to the $prefix folder. For example, try changing the url above '
+          'to: $fixedUrl', span: span);
+      return null;
+    }
+  }
+
+  // Otherwise, resolve as a path in the same package.
+  return new AssetId(source.package, targetPath);
+}
+
+AssetId _extractOtherPackageId(int index, List segments,
+    TransformLogger logger, Span span) {
+  if (index >= segments.length) return null;
+  var prefix = segments[index];
+  if (prefix != 'packages' && prefix != 'assets') return null;
+  var folder = prefix == 'packages' ? 'lib' : 'asset';
+  if (segments.length < index + 3) {
+    logger.error("incomplete $prefix/ path. It should have at least 3 "
+        "segments $prefix/name/path-from-name's-$folder-dir", span: span);
+    return null;
+  }
+  return new AssetId(segments[index + 1],
+      path.url.join(folder, path.url.joinAll(segments.sublist(index + 2))));
+}