pkg/utf: fixed layout, added todos, updated docs and homepage pubspec links

pkg/utf/lib/src/utf_16_code_unit_decoder.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.
+
+library utf.utf_16_code_unit_decoder;
+
+import 'constants.dart';
+import 'list_range.dart';
+
+/**
+ * An Iterator<int> of codepoints built on an Iterator of UTF-16 code units.
+ * The parameters can override the default Unicode replacement character. Set
+ * the replacementCharacter to null to throw an ArgumentError
+ * rather than replace the bad value.
+ */
+class Utf16CodeUnitDecoder implements Iterator<int> {
+  // TODO(kevmoo): should this field be private?
+  final ListRangeIterator utf16CodeUnitIterator;
+  final int replacementCodepoint;
+  int _current = null;
+
+  Utf16CodeUnitDecoder(List<int> utf16CodeUnits, [int offset = 0, int length,
+      int this.replacementCodepoint =
+      UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]) :
+      utf16CodeUnitIterator =
+          (new ListRange(utf16CodeUnits, offset, length)).iterator;
+
+  Utf16CodeUnitDecoder.fromListRangeIterator(
+      ListRangeIterator this.utf16CodeUnitIterator,
+      int this.replacementCodepoint);
+
+  Iterator<int> get iterator => this;
+
+  int get current => _current;
+
+  bool moveNext() {
+    _current = null;
+    if (!utf16CodeUnitIterator.moveNext()) return false;
+
+    int value = utf16CodeUnitIterator.current;
+    if (value < 0) {
+      if (replacementCodepoint != null) {
+        _current = replacementCodepoint;
+      } else {
+        throw new ArgumentError(
+            "Invalid UTF16 at ${utf16CodeUnitIterator.position}");
+      }
+    } else if (value < UNICODE_UTF16_RESERVED_LO ||
+        (value > UNICODE_UTF16_RESERVED_HI && value <= UNICODE_PLANE_ONE_MAX)) {
+      // transfer directly
+      _current = value;
+    } else if (value < UNICODE_UTF16_SURROGATE_UNIT_1_BASE &&
+        utf16CodeUnitIterator.moveNext()) {
+      // merge surrogate pair
+      int nextValue = utf16CodeUnitIterator.current;
+      if (nextValue >= UNICODE_UTF16_SURROGATE_UNIT_1_BASE &&
+          nextValue <= UNICODE_UTF16_RESERVED_HI) {
+        value = (value - UNICODE_UTF16_SURROGATE_UNIT_0_BASE) << 10;
+        value += UNICODE_UTF16_OFFSET +
+            (nextValue - UNICODE_UTF16_SURROGATE_UNIT_1_BASE);
+        _current = value;
+      } else {
+        if (nextValue >= UNICODE_UTF16_SURROGATE_UNIT_0_BASE &&
+           nextValue < UNICODE_UTF16_SURROGATE_UNIT_1_BASE) {
+          utf16CodeUnitIterator.backup();
+        }
+        if (replacementCodepoint != null) {
+          _current = replacementCodepoint;
+        } else {
+          throw new ArgumentError(
+              "Invalid UTF16 at ${utf16CodeUnitIterator.position}");
+        }
+      }
+    } else if (replacementCodepoint != null) {
+      _current = replacementCodepoint;
+    } else {
+      throw new ArgumentError(
+          "Invalid UTF16 at ${utf16CodeUnitIterator.position}");
+    }
+    return true;
+  }
+}
+