New, simpler and faster line splitter.

lib/src/rule_set.dart

+// Copyright (c) 2015, 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 dart_style.src.rule_set;
+
+import 'rule/rule.dart';
+
+/// An optimized data structure for storing a set of values for some rules.
+///
+/// This conceptually behaves roughly like a `Map<Rule, int>`, but is much
+/// faster since it avoids hashing. Instead, it assumes the line splitter has
+/// provided an ordered list of [Rule]s and that each rule's [index] field has
+/// been set to point to the rule in the list.
+///
+/// Internally, this then just stores the values in a sparse list whose indices
+/// are the indices of the rules.
+class RuleSet {
+  List<int> _values;
+
+  RuleSet(int numRules) : this._(new List(numRules));
+
+  RuleSet._(this._values);
+
+  /// Returns `true` of [rule] is bound in this set.
+  bool contains(Rule rule) => _values[rule.index] != null;
+
+  /// Gets the bound value for [rule] or `0` if it is not bound.
+  int getValue(Rule rule) {
+    var value = _values[rule.index];
+    if (value != null) return value;
+
+    return 0;
+  }
+
+  /// Invokes [callback] for each rule in [rules] with the rule's value, which
+  /// will be `null` if it is not bound.
+  void forEach(List<Rule> rules, callback(Rule rule, int value)) {
+    var i = 0;
+    for (var rule in rules) {
+      var value = _values[i];
+      if (value != null) callback(rule, value);
+      i++;
+    }
+  }
+
+  /// Creates a new [RuleSet] with the same bound rule values as this one.
+  RuleSet clone() => new RuleSet._(_values.toList(growable: false));
+
+  /// Binds [rule] to [value] then checks to see if that violates any
+  /// constraints.
+  ///
+  /// Returns `true` if all constraints between the bound rules are valid. Even
+  /// if not, this still modifies the [RuleSet].
+  ///
+  /// If an unbound rule gets constrained to `-1` (meaning it must split, but
+  /// can split any way it wants), invokes [onSplitRule] with it.
+  bool tryBind(List<Rule> rules, Rule rule, int value, onSplitRule(Rule rule)) {
+    _values[rule.index] = value;
+
+    // Test this rule against the other rules being bound.
+    for (var other in rules) {
+      if (rule == other) continue;
+
+      var otherValue = _values[other.index];
+      var constraint = rule.constrain(value, other);
+
+      if (otherValue == null) {
+        // The other rule is unbound, so see if we can constrain it eagerly to
+        // a value now.
+        if (constraint == -1) {
+          // If we know the rule has to split and there's only one way it can,
+          // just bind that.
+          if (other.numValues == 2) {
+            if (!tryBind(rules, other, 1, onSplitRule)) return false;
+          } else {
+            onSplitRule(other);
+          }
+        } else if (constraint != null) {
+          // Bind the other rule to its value and recursively propagate its
+          // constraints.
+          if (!tryBind(rules, other, constraint, onSplitRule)) return false;
+        }
+      } else {
+        // It's already bound, so see if the new rule's constraint disallows
+        // that value.
+        if (constraint == -1) {
+          if (otherValue == 0) return false;
+        } else if (constraint != null) {
+          if (otherValue != constraint) return false;
+        }
+
+        // See if the other rule's constraint allows us to use this value.
+        constraint = other.constrain(otherValue, rule);
+        if (constraint == -1) {
+          if (value == 0) return false;
+        } else if (constraint != null) {
+          if (value != constraint) return false;
+        }
+      }
+    }
+
+    return true;
+  }
+
+  String toString() =>
+      _values.map((value) => value == null ? "?" : value).join(" ");
+}
+
+/// For each chunk, this tracks if it has been split and, if so, what the
+/// chosen column is for the following line.
+///
+/// Internally, this uses a list where each element corresponds to the column
+/// of the chunk at that index in the chunk list, or `null` if that chunk did
+/// not split. This had about a 10% perf improvement over using a [Set] of
+/// splits.
+class SplitSet {
+  List<int> _columns;
+
+  /// The cost of the solution that led to these splits.
+  int get cost => _cost;
+  int _cost;
+
+  /// Creates a new empty split set for a line with [numChunks].
+  SplitSet(int numChunks) : _columns = new List(numChunks - 1);
+
+  /// Marks the line after chunk [index] as starting at [column].
+  void add(int index, int column) {
+    _columns[index] = column;
+  }
+
+  /// Returns `true` if the chunk at [splitIndex] should be split.
+  bool shouldSplitAt(int index) =>
+      index < _columns.length && _columns[index] != null;
+
+  /// Gets the zero-based starting column for the chunk at [index].
+  int getColumn(int index) => _columns[index];
+
+  /// Sets the resulting [cost] for the splits.
+  ///
+  /// This can only be called once.
+  void setCost(int cost) {
+    assert(_cost == null);
+    _cost = cost;
+  }
+
+  String toString() {
+    var result = [];
+    for (var i = 0; i < _columns.length; i++) {
+      if (_columns[i] != null) {
+        result.add("$i:${_columns[i]}");
+      }
+    }
+
+    return result.join(" ");
+  }
+}