Avoid extra duplication of substrings during string.replaceAll.

runtime/lib/string_patch.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.
 
 patch class String {
   /* patch */ factory String.fromCharCodes(Iterable<int> charCodes,
                                            [int start = 0, int end]) {
     return _StringBase.createFromCharCodes(charCodes, start, end);
   }
 
@@ skipping 22 matching lines @@
                                                    {String defaultValue})
       native "String_fromEnvironment";
 }
 
 
 /**
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
 class _StringBase {
+  // Constants used by replaceAll encoding of matches.
+  static const int _lengthSize = 11;
+  static const int _maxLengthValue = (1 << _lengthSize) - 1;
+  static const int _maxStartValue = (1 << (30 - _lengthSize)) - 1;

[siva, 2015/01/22 00:08:17]

what are '11' and '30' here? can they be given descriptive names.

[Lasse Reichstein Nielsen, 2015/01/22 08:25:38]

11 is semi-arbitrarily chosen as the size of the length in the encoding of a
slice, so _lengthSize is the descriptive name.
I'll rename it to _lengthBits to make it clear what the unit is.

The 30 is a safe limit of available bits in a negative smi.
I'll give it a name.

+  static const int _lengthMask = _maxLengthValue;
 
   factory _StringBase._uninstantiable() {
     throw new UnsupportedError(
         "_StringBase can't be instaniated");
   }
 
   Type get runtimeType => String;
 
   int get hashCode native "String_getHashCode";
 
@@ skipping 427 matching lines @@
     Iterator iterator =
         startIndex == 0 ? pattern.allMatches(this).iterator
                         : pattern.allMatches(this, startIndex).iterator;
     if (!iterator.moveNext()) return this;
     Match match = iterator.current;
     return "${this.substring(0, match.start)}"
            "$replacement"
            "${this.substring(match.end)}";
   }
 
+
+  static int _addReplaceSlice(List matches, int start, int end) {
+    int length = end - start;
+    if (length > 0) {
+      if (length <= _maxLengthValue && start <= _maxStartValue) {
+        matches.add(-((start << _lengthSize) | length));
+      } else {
+        matches.add(start);
+        matches.add(end);
+      }
+    }
+    return length;
+  }
+
   String replaceAll(Pattern pattern, String replacement) {
-    if (pattern is! Pattern) {
-      throw new ArgumentError("${pattern} is not a Pattern");
+    if (pattern == null) throw new ArgumentError.notNull("pattern");
+    if (replacement == null) throw new ArgumentError.notNull("replacement");
+    List matches = [];
+    int length = 0;
+    int replacementLength = replacement.length;
+    int startIndex = 0;
+    if (replacementLength == 0) {
+      int count = 0;
+      for (Match match in pattern.allMatches(this)) {
+        length += _addReplaceSlice(matches, startIndex, match.start);
+        startIndex = match.end;
+      }
+    } else {
+      for (Match match in pattern.allMatches(this)) {
+        length += _addReplaceSlice(matches, startIndex, match.start);
+        matches.add(replacement);
+        length += replacementLength;
+        startIndex = match.end;
+      }
     }
-    if (replacement is! String) {
-      throw new ArgumentError(
-          "${replacement} is not a String or Match->String function");
+    length += _addReplaceSlice(matches, startIndex, this.length);
+    bool replacementIsOneByte = (replacement is _OneByteString) ||
+                                (replacement is _ExternalOneByteString);
+    if (replacementIsOneByte && length < 500) {

[siva, 2015/01/22 00:08:17]

document this magic constant as
static const int _xxxxxxValue = 500;
in this class.

[Lasse Reichstein Nielsen, 2015/01/22 08:25:38]

Done:   
static const int _maxJoinReplaceOneByteStringLength = 500;

I guess it could be shorter, but not without loosing precision :)

+      // TODO(lrn): Is there a cut-off point, or is runtime always faster?
+      bool thisIsOneByte = (this is _OneByteString) ||
+                           (this is _ExternalOneByteString);
+      if (replacementIsOneByte && thisIsOneByte) {
+        return _joinReplaceAllOneByteResult(this, matches, length);
+      }
     }
-    StringBuffer buffer = new StringBuffer();
+    return _joinReplaceAllResult(this, matches, length,
+                                 replacementIsOneByte);
+  }
+
+  /**
+   * As [_joinReplaceAllResult], but knowing that the result
+   * is always a [_OneByteString].
+   */
+  static String _joinReplaceAllOneByteResult(String base,
+                                             List matches,
+                                             int length) {
+    _OneByteString result = _OneByteString._allocate(length);
+    int writeIndex = 0;
+    for (int i = 0; i < matches.length; i++) {
+      var entry = matches[i];
+      if (entry is _Smi) {
+        int sliceStart = entry;
+        int sliceEnd;
+        if (sliceStart < 0) {
+          int bits = -sliceStart;
+          int sliceLength = bits & _lengthMask;
+          sliceStart = bits >> _lengthSize;
+          sliceEnd = sliceStart + sliceLength;
+        } else {
+          i++;
+          // This function should only be called with valid matches lists.
+          // If the list is short, or sliceEnd is not an integer, one of
+          // the next few lines will throw anyway.
+          assert(i < matches.length);
+          sliceEnd = matches[i];
+        }
+        for (int j = sliceStart; j < sliceEnd; j++) {
+          result._setAt(writeIndex++, base.codeUnitAt(j));
+        }
+      } else {
+        // Replacement is a one-byte string.
+        String replacement = entry;
+        for (int j = 0; j < replacement.length; j++) {
+          result._setAt(writeIndex++, replacement.codeUnitAt(j));
+        }
+      }
+    }
+    assert(writeIndex == length);
+    return result;
+  }
+
+  /**
+   * Combine the results of a [replaceAll] match into a new string.
+   *
+   * The [matches] lists contains Smi index pairs representing slices of
+   * [base] and [String]s to be put in between the slices.
+   *
+   * The total [length] of the resulting string is known, as is
+   * whether the replacement strings are one-byte strings.
+   * If they are, then we have to check the base string slices to know
+   * whether the result must be a one-byte string.
+   */
+  static String _joinReplaceAllResult(String base, List matches, int length,
+                                      bool replacementStringsAreOneByte)
+      native "StringBase_joinReplaceAllResult";
+
+  String replaceAllMapped(Pattern pattern, String replace(Match match)) {
+    if (pattern == null) throw new ArgumentError.notNull("pattern");
+    if (replace == null) throw new ArgumentError.notNull("replace");
+    List matches = [];
+    int length = 0;
     int startIndex = 0;
+    bool replacementStringsAreOneByte = true;
     for (Match match in pattern.allMatches(this)) {
-      buffer..write(this.substring(startIndex, match.start))
-            ..write(replacement);
+      length += _addReplaceSlice(matches, startIndex, match.start);
+      String replacement = replace(match).toString();
+      matches.add(replacement);
+      length += replacement.length;
+      replacementStringsAreOneByte = replacementStringsAreOneByte &&
+          (replacement is _OneByteString ||
+           replacement is _ExternalOneByteString);
       startIndex = match.end;
     }
-    return (buffer..write(this.substring(startIndex))).toString();
-  }
-
-  String replaceAllMapped(Pattern pattern, String replace(Match match)) {
-    return splitMapJoin(pattern, onMatch: replace);
+    length += _addReplaceSlice(matches, startIndex, this.length);
+    if (replacementStringsAreOneByte && length < 500) {
+      bool thisIsOneByte = (this is _OneByteString) ||
+                           (this is _ExternalOneByteString);
+      if (thisIsOneByte) {
+        return _joinReplaceAllOneByteResult(this, matches, length);
+      }
+    }
+    return _joinReplaceAllResult(this, matches, length,
+                                 replacementStringsAreOneByte);
   }
 
   static String _matchString(Match match) => match[0];
   static String _stringIdentity(String string) => string;
 
   String _splitMapJoinEmptyString(String onMatch(Match match),
                                   String onNonMatch(String nonMatch)) {
     // Pattern is the empty string.
     StringBuffer buffer = new StringBuffer();
     int length = this.length;
@@ skipping 555 matching lines @@
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }