Added MappedListIterable/Iterator to implement map() on Lists.

sdk/lib/collection_dev/iterable.dart

 // Copyright (c) 2011, 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.collection.dev;
 
 typedef T _Transformation<S, T>(S value);
 
 class MappedIterable<S, T> extends Iterable<T> {
   final Iterable<S> _iterable;
   // TODO(ahe): Restore type when feature is implemented in dart2js
   // checked mode. http://dartbug.com/7733
   final /* _Transformation<S, T> */ _f;
 
   MappedIterable(this._iterable, T this._f(S element));
 
   Iterator<T> get iterator => new MappedIterator<S, T>(_iterable.iterator, _f);
 
   // Length related functions are independent of the mapping.
   int get length => _iterable.length;
   bool get isEmpty => _iterable.isEmpty;
 }
 
+
 class MappedIterator<S, T> extends Iterator<T> {
   T _current;
   final Iterator<S> _iterator;
   // TODO(ahe): Restore type when feature is implemented in dart2js
   // checked mode. http://dartbug.com/7733
   final /* _Transformation<S, T> */ _f;
 
   MappedIterator(this._iterator, T this._f(S element));
 
   bool moveNext() {
     if (_iterator.moveNext()) {
       _current = _f(_iterator.current);
       return true;
     } else {
       _current = null;
       return false;
     }
   }
 
   T get current => _current;
 }
 
+/** Specialized alternative to [MappedIterable] for mapped [List]s. */
+class MappedListIterable<S, T> extends Iterable<T> {
+  final Iterable<S> _list;
+  /**
+   * Start index of the part of the list to map.
+   *
+   * Allows mapping only a sub-list of an existing list.
+   *
+   * Used to implement lazy skip/take on a [MappedListIterable].
+   */
+  final int _start;
+
+  /**
+   * End index of the part of the list to map.
+   *
+   * If null, always use the length of the list.
+   */
+  final int _end;
+
+  // TODO(ahe): Restore type when feature is implemented in dart2js
+  // checked mode. http://dartbug.com/7733
+  final /* _Transformation<S, T> */ _f;
+
+  MappedListIterable(this._list, T this._f(S element), this._start, this._end) {
+    if (_end != null && _end < _start) {
+      throw new ArgumentException("End ($end) before start ($start)");
+    }
+  }
+
+  /** The start index, limited to the current length of the list. */
+  int get _startIndex {
+    if (_start <= _list.length) return _start;
+    return _list.length;
+  }
+
+  /** The end index, if given, limited to the current length of the list. */
+  int get _endIndex {
+    if (_end == null || _end > _list.length) return _list.length;
+    return _end;
+  }
+
+  Iterator<T> get iterator =>
+      new MappedListIterator<S, T>(_list, _f, _startIndex, _endIndex);
+
+  void forEach(void action(T element)) {
+    int length = _list.length;
+    for (int i = _startIndex, n = _endIndex; i < n; i++) {
+      action(_f(_list[i]));
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+  }
+
+  bool get isEmpty => _startIndex == _endIndex;
+
+  int get length => _endIndex - _startIndex;
+
+  T get first {
+    int start = _startIndex;
+    if (start == _endIndex) {
+      throw new StateError("No elements");
+    }
+    return _f(_list.elementAt(start));
+  }
+
+  T get last {
+    int end = _endIndex;
+    if (end == _startIndex) {
+      throw new StateError("No elements");
+    }
+    return _f(_list.elementAt(end - 1));
+  }
+
+  T get single {
+    int start = _startIndex;
+    int end = _endIndex;
+    if (start != end - 1) {
+      if (start == end) {
+        throw new StateError("No elements");
+      }
+      throw new StateError("Too many elements");
+    }
+    return _f(_list[start]);
+  }
+
+  T elementAt(int index) {
+    index += _startIndex;
+    if (index >= _endIndex) {
+      throw new StateError("No matching element");
+    }
+    return _f(_list.elementAt(index));
+  }
+
+  bool contains(T element) {
+    int length = _list.length;
+    for (int i = _startIndex, n = _endIndex; i < n; i++) {

[floitsch, 2013/02/01 15:00:36]

for another CL: in theory we could say min(_endIndex, _list.length)
If it's written correctly it should help the VM.

[Lasse Reichstein Nielsen, 2013/02/01 15:13:57]

The problem is that min is in dart:math, and I don't want to import that just
for two "min" calls.

It should really be on num in some way, even if only index.restrictToRange(0,
_list.length)

[floitsch, 2013/02/01 15:23:40]

the "min" was just to illustrate. It should be something like:
int length = _list.length;
int endIndex = _endIndex;
if (length < endIndex) endIndex = length;

something like this. Ideally in a way that the VM can propagate.

[Lasse Reichstein Nielsen, 2013/02/04 09:30:16]

If we can see a difference in performance after profiling, I'm willing to
rewrite _endIndex. Or wait for the VM to get better at analyzing.

+      if (_f(_list[i]) == element) {
+        return true;
+      }
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    return false;
+  }
+
+  bool every(bool test(T element)) {
+    int length = _list.length;
+    for (int i = _startIndex, n = _endIndex; i < n; i++) {
+      if (!test(_f(_list[i]))) return false;
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    return true;
+  }
+
+  bool any(bool test(T element)) {
+    int length = _list.length;
+    for (int i = _startIndex, n = _endIndex; i < n; i++) {
+      if (test(_f(_list[i]))) return true;
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    return false;
+  }
+
+  T firstMatching(bool test(T element), { T orElse() }) {
+    int length = _list.length;
+    for (int i = _startIndex, n = _endIndex; i < n; i++) {
+      T value = _f(_list[i]);
+      if (test(value)) return value;
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    if (orElse != null) return orElse();
+    throw new StateError("No matching element");
+  }
+
+  T lastMatching(bool test(T element), { T orElse() }) {
+    int length = _list.length;
+    for (int i = _endIndex - 1, start = _startIndex; i >= start; i++) {
+      T value = _f(_list[i]);
+      if (test(value)) return value;
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    if (orElse != null) return orElse();
+    throw new StateError("No matching element");
+  }
+
+  T singleMatching(bool test(T element)) {
+    int length = _list.length;
+    T match;
+    bool matchFound = false;
+    for (int i = _startIndex, n = _endIndex; i < n; i++) {
+      T value = _f(_list[i]);
+      if (test(value)) {
+        if (matchFound) {
+          throw new StateError("More than one matching element");
+        }
+        matchFound = true;
+        match = value;
+      }
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    if (matchFound) return match;
+    throw new StateError("No matching element");
+  }
+
+  int min([int compare(E a, E b)]) {
+    if (compare == null) {
+      var defaultCompare = Comparable.compare;
+      compare = defaultCompare;
+    }
+    int length = _list.length;
+    int start = _startIndex;
+    int end = _endIndex;
+    if (start == end) return null;
+    int value = _f(_list[start]);
+    if (_list.length != length) {
+      throw new ConcurrentModificationError(_list);
+    }
+    for (int i = start + 1; i < end; i++) {
+      int nextValue = _f(_list[i]);
+      if (compare(value, nextValue) > 0) {
+        value = nextValue;
+      }
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    return value;
+  }
+
+  int max([int compare(E a, E b)]) {
+    if (compare == null) {
+      var defaultCompare = Comparable.compare;
+      compare = defaultCompare;
+    }
+    int length = _list.length;
+    int start = _startIndex;
+    int end = _endIndex;
+    if (start == end) return null;
+    int value = _f(_list[start]);
+    if (_list.length != length) {
+      throw new ConcurrentModificationError(_list);
+    }
+    for (int i = start + 1; i < end; i++) {
+      int nextValue = _f(_list[i]);
+      if (compare(value, nextValue) < 0) {
+        value = nextValue;
+      }
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    return value;
+  }
+
+  String join([String separator]) {
+    int start = _startIndex;
+    int end = _endIndex;
+    if (start == end) return "";
+    StringBuffer buffer = new StringBuffer("${_f(_list[start])}");
+    if (_list.length != length) {
+      throw new ConcurrentModificationError(_list);
+    }
+    for (int i = start + 1; i < end; i++) {
+      buffer.add(separator);
+      buffer.add("${_f(_list[i])}");
+      if (_list.length != length) {
+        throw new ConcurrentModificationError(_list);
+      }
+    }
+    return buffer.toString();
+  }
+
+  Iterable<T> where(bool test(T element)) => super.where(test);
+
+  Iterable map(f(T element)) {
+    return new MappedListIterable(_list, (S v) => f(_f(v)), _start, _end);
+  }
+
+  Iterable mappedBy(f(T element)) => map(f);
+
+  reduce(var initialValue, combine(var previousValue, T element)) {
+    return _list.reduce(initialValue, (v, S e) => combine(v, _f(e)));
+  }
+
+  Iterable<T> skip(int count) {
+    int start = _startIndex + count;
+    if (_end != null && start >= _end) {
+      return new EmptyIterable<T>();
+    }
+    return new MappedListIterable(_list, _f, start, _end);
+  }
+
+  Iterable<T> skipWhile(bool test(T element)) => super.skipWhile(test);
+
+  Iterable<T> take(int count)  {
+    int newEnd = _start + count;
+    if (_end == null || newEnd < _end)  {
+      return new MappedListIterable(_list, _f, _start, newEnd);
+    }
+    // Equivalent to "this".
+    return new MappedListIterable(_list, _f, _start, _end);
+  }
+
+  Iterable<T> takeWhile(bool test(T element)) => super.takeWhile(test);
+
+  List<T> toList() {
+    List<T> result = new List<T>();
+    forEach(result.add);
+    return result;
+  }
+
+  Set<T> toSet() {
+    List result = new Set<T>();
+    forEach(result.add);
+    return result;
+  }
+}
+
+/**
+ * Iterator for [MappedListIterable].
+ *
+ * A list iterator that iterates over (a sublist of) a list and
+ * returns the values transformed by a function.
+ *
+ * As a list iterator, it throws if the length of the list has
+ * changed during iteration.
+ */
+class MappedListIterator<S, T> implements Iterator<T> {
+  List<S> _list;
+  // TODO(ahe): Restore type when feature is implemented in dart2js
+  // checked mode. http://dartbug.com/7733
+  final /* _Transformation<S, T> */ _f;
+  final int _endIndex;
+  final int _length;
+  int _index;
+  T _current;
+
+  MappedListIterator(List<S> list, this._f, int start, this._endIndex)
+      : _list = list, _length = list.length, _index = start;
+
+  T get current => _current;
+
+  bool moveNext() {
+    if (_list.length != _length) {
+      throw new ConcurrentModificationError(_list);
+    }
+    if (_index >= _endIndex) {
+      _current = null;
+      return false;
+    }
+    _current = _f(_list[_index]);
+    _index++;
+    return true;
+  }
+}
+
 typedef bool _ElementPredicate<E>(E element);
 
 class WhereIterable<E> extends Iterable<E> {
   final Iterable<E> _iterable;
   // TODO(ahe): Restore type when feature is implemented in dart2js
   // checked mode. http://dartbug.com/7733
   final /* _ElementPredicate */ _f;
 
   WhereIterable(this._iterable, bool this._f(E element));
 
@@ skipping 161 matching lines @@
       _hasSkipped = true;
       while (_iterator.moveNext()) {
         if (!_f(_iterator.current)) return true;
       }
     }
     return _iterator.moveNext();
   }
 
   E get current => _iterator.current;
 }