Refactor cassowary so it uses imports rather than parts.

Also misc cleanup:
 - reorder members to be more consistent and fit the style guide
 - remove use of _Pair
 - made Variable.applyUpdate and Variable.owner public
 - added docs to Priority, tweaked the code a bit
 - added some docs to Result
 - removed the internal-error Result (replaced with asserts)
 - removed unused Results
 - made Result const
 - merged some files together since they had used privates a lot

I'm sorry this is completely unreviewable. I did the move from `lib/*`
to `lib/src/*` first, then did the `part`-to-`import` change, and then
found out how many of the files involved privates, which I wasn't
expecting. I can redo this as multiple commits if that would make it
easier to review.

examples/layers/rendering/autolayout.dart

@@ -27,8 +27,7 @@ class _MyAutoLayoutDelegate extends AutoLayoutDelegate {
       // The widths of the first and the third boxes should be equal
       p1.width.equals(p3.width),
 
-      // The width of the second box should be twice as much as that of the first
-      // and third
+      // The width of the first box should be twice as much as that of the second
       p1.width.equals(p2.width * al.cm(2.0)),
 
       // The height of the three boxes should be equal to that of the container

examples/layers/widgets/autolayout.dart

@@ -26,8 +26,7 @@ class _MyAutoLayoutDelegate extends AutoLayoutDelegate {
       // The widths of the first and the third boxes should be equal
       p1.width.equals(p3.width),
 
-      // The width of the second box should be twice as much as that of the first
-      // and third
+      // The width of the first box should be twice as much as that of the second
       p1.width.equals(p2.width * al.cm(2.0)),
 
       // The height of the three boxes should be equal to that of the container

packages/cassowary/lib/cassowary.dart

@@ -3,22 +3,23 @@
 // found in the LICENSE file.
 
 /// An implementation of the Cassowary constraint solving algorithm in Dart.
+///
+/// This is used by the [RenderAutoLayout] render object in the rendering
+/// library and by the [AutoLayout] widget in the widget library.
+///
+/// See also:
+///
+/// * <https://en.wikipedia.org/wiki/Cassowary_(software)>
+/// * <https://constraints.cs.washington.edu/solvers/cassowary-tochi.pdf>
 library cassowary;
 
-import 'dart:collection';
-import 'dart:math';
-
-part 'constraint.dart';
-part 'expression.dart';
-part 'term.dart';
-part 'variable.dart';
-part 'equation_member.dart';
-part 'constant_member.dart';
-part 'solver.dart';
-part 'symbol.dart';
-part 'row.dart';
-part 'utils.dart';
-part 'result.dart';
-part 'parser_exception.dart';
-part 'param.dart';
-part 'priority.dart';
+export 'src/constraint.dart';
+export 'src/expression.dart';
+export 'src/term.dart';
+export 'src/equation_member.dart';
+export 'src/constant_member.dart';
+export 'src/solver.dart';
+export 'src/result.dart';
+export 'src/parser_exception.dart';
+export 'src/param.dart';
+export 'src/priority.dart';

packages/cassowary/lib/equation_member.dart

-// Copyright (c) 2015 The Chromium Authors. 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 cassowary;
-
-abstract class _EquationMember {
-  Expression asExpression();
-
-  bool get isConstant;
-
-  double get value;
-
-  Constraint operator >=(_EquationMember m) => asExpression() >= m;
-
-  Constraint operator <=(_EquationMember m) => asExpression() <= m;
-
-  Constraint equals(_EquationMember m) => asExpression().equals(m);
-
-  Expression operator +(_EquationMember m) => asExpression() + m;
-
-  Expression operator -(_EquationMember m) => asExpression() - m;
-
-  Expression operator *(_EquationMember m) => asExpression() * m;
-
-  Expression operator /(_EquationMember m) => asExpression() / m;
-}

packages/cassowary/lib/result.dart

-// Copyright (c) 2015 The Chromium Authors. 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 cassowary;
-
-class Result {
-  final String message;
-  final bool error;
-
-  const Result(this.message, { bool isError: true }) : error = isError;
-
-  static final Result success = const Result("Success", isError: false);
-  static final Result unimplemented = const Result("Unimplemented");
-  static final Result duplicateConstraint =
-      const Result("Duplicate Constraint");
-  static final Result unsatisfiableConstraint =
-      const Result("Unsatisfiable Constraint");
-  static final Result unknownConstraint =
-      const Result("Unknown Constraint");
-  static final Result duplicateEditVariable =
-      const Result("Duplicate Edit Variable");
-  static final Result badRequiredStrength =
-      const Result("Bad Required Strength");
-  static final Result unknownEditVariable =
-      const Result("Unknown Edit Variable");
-  static final Result internalSolverError =
-      const Result("Internal Solver Error");
-}

packages/cassowary/lib/row.dart

-// Copyright (c) 2015 The Chromium Authors. 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 cassowary;
-
-class _Row {
-  final Map<_Symbol, double> cells;
-  double constant = 0.0;
-
-  _Row(this.constant) : this.cells = new Map<_Symbol, double>();
-  _Row.fromRow(_Row row)
-      : this.cells = new Map<_Symbol, double>.from(row.cells),
-        this.constant = row.constant;
-
-  double add(double value) => constant += value;
-
-  void insertSymbol(_Symbol symbol, [double coefficient = 1.0]) {
-    double val = cells[symbol] ?? 0.0;
-
-    if (_nearZero(val + coefficient)) {
-      cells.remove(symbol);
-    } else {
-      cells[symbol] = val + coefficient;
-    }
-  }
-
-  void insertRow(_Row other, [double coefficient = 1.0]) {
-    constant += other.constant * coefficient;
-    other.cells.forEach((_Symbol s, double v) => insertSymbol(s, v * coefficient));
-  }
-
-  void removeSymbol(_Symbol symbol) {
-    cells.remove(symbol);
-  }
-
-  void reverseSign() {
-    constant = -constant;
-    cells.forEach((_Symbol s, double v) => cells[s] = -v);
-  }
-
-  void solveForSymbol(_Symbol symbol) {
-    assert(cells.containsKey(symbol));
-    double coefficient = -1.0 / cells[symbol];
-    cells.remove(symbol);
-    constant *= coefficient;
-    cells.forEach((_Symbol s, double v) => cells[s] = v * coefficient);
-  }
-
-  void solveForSymbols(_Symbol lhs, _Symbol rhs) {
-    insertSymbol(lhs, -1.0);
-    solveForSymbol(rhs);
-  }
-
-  double coefficientForSymbol(_Symbol symbol) => cells[symbol] ?? 0.0;
-
-  void substitute(_Symbol symbol, _Row row) {
-    double coefficient = cells[symbol];
-
-    if (coefficient == null) {
-      return;
-    }
-
-    cells.remove(symbol);
-    insertRow(row, coefficient);
-  }
-
-  @override
-  String toString() {
-    StringBuffer buffer = new StringBuffer();
-
-    buffer.write(constant);
-
-    cells.forEach((_Symbol symbol, double value) {
-      buffer.write(" + " + value.toString() + " * " + symbol.toString());
-    });
-
-    return buffer.toString();
-  }
-}

packages/cassowary/lib/constant_member.dart → packages/cassowary/lib/src/constant_member.dart

@@ -2,21 +2,27 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'expression.dart';
+import 'equation_member.dart';
 
-class ConstantMember extends _EquationMember {
+class ConstantMember extends EquationMember {
+  /// Creates a [ConstantMember] object.
+  ///
+  /// The [cm] convenience method may be a more convenient way to create a
+  /// [ConstantMember] object.
   ConstantMember(this.value);
 
   @override
-  final double value;
+  Expression asExpression() => new Expression([], this.value);
 
   @override
-  bool get isConstant => true;
+  final double value;
 
   @override
-  Expression asExpression() => new Expression([], this.value);
+  bool get isConstant => true;
 }
 
-ConstantMember cm(double value) {
-  return new ConstantMember(value);
-}
+/// Creates a [ConstantMember].
+///
+/// This is a convenience method to make cassowary expressions less verbose.
+ConstantMember cm(double value) => new ConstantMember(value);

packages/cassowary/lib/constraint.dart → packages/cassowary/lib/src/constraint.dart

@@ -2,16 +2,23 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'priority.dart';
+import 'expression.dart';
 
-enum Relation { equalTo, lessThanOrEqualTo, greaterThanOrEqualTo, }
+enum Relation {
+  equalTo,
+  lessThanOrEqualTo,
+  greaterThanOrEqualTo,
+}
 
 class Constraint {
+  Constraint(this.expression, this.relation);
+
   final Relation relation;
+
   final Expression expression;
-  double priority = Priority.required;
 
-  Constraint(this.expression, this.relation);
+  double priority = Priority.required;
 
   Constraint operator |(double p) => this..priority = p;
 
@@ -34,9 +41,8 @@ class Constraint {
 
     buffer.write(' | priority = $priority');
 
-    if (priority == Priority.required) {
+    if (priority == Priority.required)
       buffer.write(' (required)');
-    }
 
     return buffer.toString();
   }

packages/cassowary/lib/src/equation_member.dart

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'expression.dart';
+import 'constraint.dart';
+
+/// Base class for the various parts of cassowary equations.
+abstract class EquationMember {
+  Expression asExpression();
+
+  bool get isConstant;
+
+  double get value;
+
+  Constraint operator >=(EquationMember m) => asExpression() >= m;
+
+  Constraint operator <=(EquationMember m) => asExpression() <= m;
+
+  Constraint equals(EquationMember m) => asExpression().equals(m);
+
+  Expression operator +(EquationMember m) => asExpression() + m;
+
+  Expression operator -(EquationMember m) => asExpression() - m;
+
+  Expression operator *(EquationMember m) => asExpression() * m;
+
+  Expression operator /(EquationMember m) => asExpression() / m;
+}

packages/cassowary/lib/expression.dart → packages/cassowary/lib/src/expression.dart

@@ -2,10 +2,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'constant_member.dart';
+import 'constraint.dart';
+import 'equation_member.dart';
+import 'param.dart';
+import 'parser_exception.dart';
+import 'term.dart';
+
+class _Multiplication {
+  const _Multiplication(this.multiplier, this.multiplicand);
+  final Expression multiplier;
+  final double multiplicand;
+}
 
-class Expression extends _EquationMember {
+class Expression extends EquationMember {
   Expression(this.terms, this.constant);
+
   Expression.fromExpression(Expression expr)
     : this.terms = new List<Term>.from(expr.terms),
       this.constant = expr.constant;
@@ -14,6 +26,9 @@ class Expression extends _EquationMember {
 
   final double constant;
 
+  @override
+  Expression asExpression() => this;
+
   @override
   bool get isConstant => terms.length == 0;
 
@@ -21,9 +36,21 @@ class Expression extends _EquationMember {
   double get value => terms.fold(constant, (double value, Term term) => value + term.value);
 
   @override
-  Expression asExpression() => this;
+  Constraint operator >=(EquationMember value) {
+    return _createConstraint(value, Relation.greaterThanOrEqualTo);
+  }
+
+  @override
+  Constraint operator <=(EquationMember value) {
+    return _createConstraint(value, Relation.lessThanOrEqualTo);
+  }
 
-  Constraint _createConstraint(_EquationMember /* rhs */ value, Relation relation) {
+  @override
+  Constraint equals(EquationMember value) {
+    return _createConstraint(value, Relation.equalTo);
+  }
+
+  Constraint _createConstraint(EquationMember /* rhs */ value, Relation relation) {
     if (value is ConstantMember) {
       return new Constraint(
         new Expression(new List<Term>.from(terms), constant - value.value),
@@ -60,22 +87,7 @@ class Expression extends _EquationMember {
   }
 
   @override
-  Constraint operator >=(_EquationMember value) {
-    return _createConstraint(value, Relation.greaterThanOrEqualTo);
-  }
-
-  @override
-  Constraint operator <=(_EquationMember value) {
-    return _createConstraint(value, Relation.lessThanOrEqualTo);
-  }
-
-  @override
-  Constraint equals(_EquationMember value) {
-    return _createConstraint(value, Relation.equalTo);
-  }
-
-  @override
-  Expression operator +(_EquationMember m) {
+  Expression operator +(EquationMember m) {
     if (m is ConstantMember)
       return new Expression(new List<Term>.from(terms), constant + m.value);
 
@@ -100,7 +112,7 @@ class Expression extends _EquationMember {
   }
 
   @override
-  Expression operator -(_EquationMember m) {
+  Expression operator -(EquationMember m) {
     if (m is ConstantMember)
       return new Expression(new List<Term>.from(terms), constant - m.value);
 
@@ -126,49 +138,22 @@ class Expression extends _EquationMember {
     return null;
   }
 
-  _EquationMember _applyMultiplicand(double m) {
-    List<Term> newTerms = terms.fold(
-      new List<Term>(),
-      (List<Term> list, Term term) {
-        return list..add(new Term(term.variable, term.coefficient * m));
-      }
-    );
-    return new Expression(newTerms, constant * m);
-  }
-
-  _Pair<Expression, double> _findMulitplierAndMultiplicand(_EquationMember m) {
-    // At least on of the the two members must be constant for the resulting
-    // expression to be linear
-
-    if (!this.isConstant && !m.isConstant)
-      return null;
-
-    if (this.isConstant)
-      return new _Pair<Expression, double>(m.asExpression(), this.value);
-
-    if (m.isConstant)
-      return new _Pair<Expression, double>(this.asExpression(), m.value);
-
-    assert(false);
-    return null;
-  }
-
   @override
-  _EquationMember operator *(_EquationMember m) {
-    _Pair<Expression, double> args = _findMulitplierAndMultiplicand(m);
+  EquationMember operator *(EquationMember m) {
+    _Multiplication args = _findMulitplierAndMultiplicand(m);
 
     if (args == null) {
       throw new ParserException(
         'Could not find constant multiplicand or multiplier',
-        <_EquationMember>[this, m]
+        <EquationMember>[this, m]
       );
     }
 
-    return args.first._applyMultiplicand(args.second);
+    return args.multiplier._applyMultiplicand(args.multiplicand);
   }
 
   @override
-  _EquationMember operator /(_EquationMember m) {
+  EquationMember operator /(EquationMember m) {
     if (!m.isConstant) {
       throw new ParserException(
           'The divisor was not a constant expression', [this, m]);
@@ -178,6 +163,32 @@ class Expression extends _EquationMember {
     return this._applyMultiplicand(1.0 / m.value);
   }
 
+  _Multiplication _findMulitplierAndMultiplicand(EquationMember m) {
+    // At least one of the the two members must be constant for the resulting
+    // expression to be linear
+
+    if (!this.isConstant && !m.isConstant)
+      return null;
+
+    if (this.isConstant)
+      return new _Multiplication(m.asExpression(), this.value);
+
+    if (m.isConstant)
+      return new _Multiplication(this.asExpression(), m.value);
+    assert(false);
+    return null;
+  }
+
+  EquationMember _applyMultiplicand(double m) {
+    List<Term> newTerms = terms.fold(
+      new List<Term>(),
+      (List<Term> list, Term term) {
+        return list..add(new Term(term.variable, term.coefficient * m));
+      }
+    );
+    return new Expression(newTerms, constant * m);
+  }
+
   @override
   String toString() {
     StringBuffer buffer = new StringBuffer();

packages/cassowary/lib/param.dart → packages/cassowary/lib/src/param.dart

@@ -2,12 +2,41 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'equation_member.dart';
+import 'expression.dart';
+import 'term.dart';
 
-class Param extends _EquationMember {
+class Variable {
+  static int _total = 0;
+
+  Variable(this.value) : _tick = _total++;
+
+  final int _tick;
+
+  double value;
+
+  String name;
+
+  Param get owner => _owner;
+  Param _owner;
+
+  bool applyUpdate(double updated) {
+    bool res = updated != value;
+    value = updated;
+    return res;
+  }
+
+  String get debugName => name ?? 'variable$_tick';
+
+  @override
+  String toString() => debugName;
+}
+
+class Param extends EquationMember {
   Param([double value = 0.0]) : variable = new Variable(value) {
     variable._owner = this;
   }
+
   Param.withContext(dynamic context, [double value = 0.0])
     : variable = new Variable(value),
       context = context {
@@ -18,6 +47,9 @@ class Param extends _EquationMember {
 
   dynamic context;
 
+  @override
+  Expression asExpression() => new Expression(<Term>[new Term(variable, 1.0)], 0.0);
+
   @override
   bool get isConstant => false;
 
@@ -26,7 +58,4 @@ class Param extends _EquationMember {
 
   String get name => variable.name;
   void set name(String name) { variable.name = name; }
-
-  @override
-  Expression asExpression() => new Expression(<Term>[new Term(variable, 1.0)], 0.0);
 }

packages/cassowary/lib/parser_exception.dart → packages/cassowary/lib/src/parser_exception.dart

@@ -2,16 +2,19 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'equation_member.dart';
 
 class ParserException implements Exception {
-  final String message;
-  List<_EquationMember> members;
   ParserException(this.message, this.members);
 
+  final String message;
+
+  List<EquationMember> members;
+
   @override
   String toString() {
-    if (message == null) return "Error while parsing constraint or expression";
-    return "Error: '$message' while trying to parse constraint or expression";
+    if (message == null)
+      return 'Error while parsing constraint or expression';
+    return 'Error: "$message" while trying to parse constraint or expression';
   }
 }

packages/cassowary/lib/priority.dart → packages/cassowary/lib/src/priority.dart

@@ -2,14 +2,35 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'dart:math';
 
+/// Utility functions for managing cassowary priorities.
+///
+/// Priorities in cassowary expressions are internally expressed as a number
+/// between 0 and 1,000,000,000. These numbers can be created by using the
+/// [Priority.create] static method.
 class Priority {
+  /// The priority level that, by convention, is the highest allowed priority level (1,000,000,000).
   static final double required = create(1e3, 1e3, 1e3);
+
+  /// A priority level that is below the [required] level but still near it (1,000,000).
   static final double strong = create(1.0, 0.0, 0.0);
+
+  /// A priority level logarithmically in the middle of [strong] and [weak] (1,000).
   static final double medium = create(0.0, 1.0, 0.0);
+
+  /// A priority level that, by convention, is the lowest allowed priority level (1).
   static final double weak = create(0.0, 0.0, 1.0);
 
+  /// Computes a priority level by combining three numbers in the range 0..1000.
+  ///
+  /// The first number is a multiple of [strong].
+  ///
+  /// The second number is a multiple of [medium].
+  ///
+  /// The third number is a multiple of [weak].
+  ///
+  /// By convention, at least one of these numbers should be equal to or greater than 1.
   static double create(double a, double b, double c) {
     double result = 0.0;
     result += max(0.0, min(1e3, a)) * 1e6;
@@ -17,8 +38,4 @@ class Priority {
     result += max(0.0, min(1e3, c));
     return result;
   }
-
-  static double clamp(double value) {
-    return max(0.0, min(required, value));
-  }
 }

packages/cassowary/lib/src/result.dart

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'solver.dart';
+
+/// Return values used by methods on the cassowary [Solver].
+class Result {
+  const Result._(this.message, { bool isError: true }) : error = isError;
+
+  /// The human-readable string associated with this result.
+  ///
+  /// This message is typically brief and intended for developers to help debug
+  /// erroneous expressions.
+  final String message;
+
+  /// Whether this [Result] represents an error (true) or not (false).
+  final bool error;
+
+  static const Result success =
+      const Result._('Success', isError: false);
+
+  static const Result duplicateConstraint =
+      const Result._('Duplicate constraint');
+
+  static const Result unsatisfiableConstraint =
+      const Result._('Unsatisfiable constraint');
+
+  static const Result unknownConstraint =
+      const Result._('Unknown constraint');
+
+  static const Result duplicateEditVariable =
+      const Result._('Duplicate edit variable');
+
+  static const Result badRequiredStrength =
+      const Result._('Bad required strength');
+
+  static const Result unknownEditVariable =
+      const Result._('Unknown edit variable');
+}

packages/cassowary/lib/solver.dart → packages/cassowary/lib/src/solver.dart

@@ -2,7 +2,148 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'dart:collection';
+
+import 'constraint.dart';
+import 'expression.dart';
+import 'priority.dart';
+import 'result.dart';
+import 'param.dart';
+import 'term.dart';
+
+enum _SymbolType { invalid, external, slack, error, dummy, }
+
+class _Symbol {
+  const _Symbol(this.type, this.tick);
+  final _SymbolType type;
+  final int tick;
+
+  @override
+  String toString() {
+    String typeString = 'unknown';
+    switch (type) {
+      case _SymbolType.invalid:
+        typeString = 'i';
+        break;
+      case _SymbolType.external:
+        typeString = 'v';
+        break;
+      case _SymbolType.slack:
+        typeString = 's';
+        break;
+      case _SymbolType.error:
+        typeString = 'e';
+        break;
+      case _SymbolType.dummy:
+        typeString = 'd';
+        break;
+    }
+    return '$typeString$tick';
+  }
+}
+
+class _Tag {
+  _Tag(this.marker, this.other);
+  _Tag.fromTag(_Tag tag)
+      : this.marker = tag.marker,
+        this.other = tag.other;
+  _Symbol marker;
+  _Symbol other;
+}
+
+class _EditInfo {
+  _Tag tag;
+  Constraint constraint;
+  double constant;
+}
+
+bool _isValidNonRequiredPriority(double priority) {
+  return (priority >= 0.0 && priority < Priority.required);
+}
+
+typedef Result _SolverBulkUpdate(dynamic item);
+
+bool _nearZero(double value) {
+  const double epsilon = 1.0e-8;
+  return value < 0.0 ? -value < epsilon : value < epsilon;
+}
+
+class _Row {
+  _Row(this.constant) : this.cells = new Map<_Symbol, double>();
+
+  _Row.fromRow(_Row row)
+      : this.cells = new Map<_Symbol, double>.from(row.cells),
+        this.constant = row.constant;
+
+  final Map<_Symbol, double> cells;
+
+  double constant = 0.0;
+
+  double add(double value) => constant += value;
+
+  void insertSymbol(_Symbol symbol, [double coefficient = 1.0]) {
+    double val = cells[symbol] ?? 0.0;
+
+    if (_nearZero(val + coefficient)) {
+      cells.remove(symbol);
+    } else {
+      cells[symbol] = val + coefficient;
+    }
+  }
+
+  void insertRow(_Row other, [double coefficient = 1.0]) {
+    constant += other.constant * coefficient;
+    other.cells.forEach((_Symbol s, double v) => insertSymbol(s, v * coefficient));
+  }
+
+  void removeSymbol(_Symbol symbol) {
+    cells.remove(symbol);
+  }
+
+  void reverseSign() {
+    constant = -constant;
+    cells.forEach((_Symbol s, double v) => cells[s] = -v);
+  }
+
+  void solveForSymbol(_Symbol symbol) {
+    assert(cells.containsKey(symbol));
+    double coefficient = -1.0 / cells[symbol];
+    cells.remove(symbol);
+    constant *= coefficient;
+    cells.forEach((_Symbol s, double v) => cells[s] = v * coefficient);
+  }
+
+  void solveForSymbols(_Symbol lhs, _Symbol rhs) {
+    insertSymbol(lhs, -1.0);
+    solveForSymbol(rhs);
+  }
+
+  double coefficientForSymbol(_Symbol symbol) => cells[symbol] ?? 0.0;
+
+  void substitute(_Symbol symbol, _Row row) {
+    double coefficient = cells[symbol];
+
+    if (coefficient == null) {
+      return;
+    }
+
+    cells.remove(symbol);
+    insertRow(row, coefficient);
+  }
+
+  @override
+  String toString() {
+    StringBuffer buffer = new StringBuffer();
+
+    buffer.write(constant);
+
+    cells.forEach((_Symbol symbol, double value) {
+      buffer.write(" + " + value.toString() + " * " + symbol.toString());
+    });
+
+    return buffer.toString();
+  }
+}
 
 class Solver {
   final Map<Constraint, _Tag> _constraints = new Map<Constraint, _Tag>();
@@ -25,9 +166,8 @@ class Solver {
   }
 
   Result addConstraint(Constraint constraint) {
-    if (_constraints.containsKey(constraint)) {
+    if (_constraints.containsKey(constraint))
       return Result.duplicateConstraint;
-    }
 
     _Tag tag = new _Tag(new _Symbol(_SymbolType.invalid, 0),
         new _Symbol(_SymbolType.invalid, 0));
@@ -45,9 +185,8 @@ class Solver {
     }
 
     if (subject.type == _SymbolType.invalid) {
-      if (!_addWithArtificialVariableOnRow(row)) {
+      if (!_addWithArtificialVariableOnRow(row))
         return Result.unsatisfiableConstraint;
-      }
     } else {
       row.solveForSymbol(subject);
       _substitute(subject, row);
@@ -68,9 +207,8 @@ class Solver {
 
   Result removeConstraint(Constraint constraint) {
     _Tag tag = _constraints[constraint];
-    if (tag == null) {
+    if (tag == null)
       return Result.unknownConstraint;
-    }
 
     tag = new _Tag.fromTag(tag);
     _constraints.remove(constraint);
@@ -81,16 +219,11 @@ class Solver {
     if (row != null) {
       _rows.remove(tag.marker);
     } else {
-      _Pair<_Symbol, _Row> rowPair = _leavingRowPairForMarkerSymbol(tag.marker);
-
-      if (rowPair == null) {
-        return Result.internalSolverError;
-      }
+      _Symbol leaving = _leavingSymbolForMarkerSymbol(tag.marker);
+      assert(leaving != null);
 
-      _Symbol leaving = rowPair.first;
-      row = rowPair.second;
-      _Row removed = _rows.remove(rowPair.first);
-      assert(removed != null);
+      row = _rows.remove(leaving);
+      assert(row != null);
       row.solveForSymbols(leaving, tag.marker);
       _substitute(tag.marker, row);
     }
@@ -110,21 +243,19 @@ class Solver {
   }
 
   Result addEditVariable(Variable variable, double priority) {
-    if (_edits.containsKey(variable)) {
+    if (_edits.containsKey(variable))
       return Result.duplicateEditVariable;
-    }
 
-    if (!_isValidNonRequiredPriority(priority)) {
+    if (!_isValidNonRequiredPriority(priority))
       return Result.badRequiredStrength;
-    }
 
     Constraint constraint = new Constraint(
-        new Expression([new Term(variable, 1.0)], 0.0), Relation.equalTo);
+      new Expression([new Term(variable, 1.0)], 0.0),
+      Relation.equalTo
+    );
     constraint.priority = priority;
 
-    if (addConstraint(constraint) != Result.success) {
-      return Result.internalSolverError;
-    }
+    assert(addConstraint(constraint) == Result.success);
 
     _EditInfo info = new _EditInfo();
     info.tag = _constraints[constraint];
@@ -149,8 +280,7 @@ class Solver {
     if (info == null)
       return Result.unknownEditVariable;
 
-    if (removeConstraint(info.constraint) != Result.success)
-      return Result.internalSolverError;
+    assert(removeConstraint(info.constraint) == Result.success);
 
     _edits.remove(variable);
     return Result.success;
@@ -161,9 +291,8 @@ class Solver {
   }
 
   Result suggestValueForVariable(Variable variable, double value) {
-    if (!_edits.containsKey(variable)) {
+    if (!_edits.containsKey(variable))
       return Result.unknownEditVariable;
-    }
 
     _suggestValueForEditInfoWithoutDualOptimization(_edits[variable], value);
 
@@ -179,8 +308,8 @@ class Solver {
 
       double updatedValue = row == null ? 0.0 : row.constant;
 
-      if (variable._applyUpdate(updatedValue) && variable._owner != null) {
-        dynamic context = variable._owner.context;
+      if (variable.applyUpdate(updatedValue) && variable.owner != null) {
+        dynamic context = variable.owner.context;
         if (context != null)
           updates.add(context);
       }
@@ -293,34 +422,30 @@ class Solver {
 
   _Symbol _chooseSubjectForRow(_Row row, _Tag tag) {
     for (_Symbol symbol in row.cells.keys) {
-      if (symbol.type == _SymbolType.external) {
+      if (symbol.type == _SymbolType.external)
         return symbol;
     }
-    }
 
     if (tag.marker.type == _SymbolType.slack ||
         tag.marker.type == _SymbolType.error) {
-      if (row.coefficientForSymbol(tag.marker) < 0.0) {
+      if (row.coefficientForSymbol(tag.marker) < 0.0)
         return tag.marker;
     }
-    }
 
     if (tag.other.type == _SymbolType.slack ||
         tag.other.type == _SymbolType.error) {
-      if (row.coefficientForSymbol(tag.other) < 0.0) {
+      if (row.coefficientForSymbol(tag.other) < 0.0)
         return tag.other;
     }
-    }
 
     return new _Symbol(_SymbolType.invalid, 0);
   }
 
   bool _allDummiesInRow(_Row row) {
     for (_Symbol symbol in row.cells.keys) {
-      if (symbol.type != _SymbolType.dummy) {
+      if (symbol.type != _SymbolType.dummy)
         return false;
     }
-    }
     return true;
   }
 
@@ -342,23 +467,20 @@ class Solver {
     _Row foundRow = _rows[artificial];
     if (foundRow != null) {
       _rows.remove(artificial);
-      if (foundRow.cells.isEmpty) {
+      if (foundRow.cells.isEmpty)
         return success;
-      }
 
       _Symbol entering = _anyPivotableSymbol(foundRow);
-      if (entering.type == _SymbolType.invalid) {
+      if (entering.type == _SymbolType.invalid)
         return false;
-      }
 
       foundRow.solveForSymbols(artificial, entering);
       _substitute(entering, foundRow);
       _rows[entering] = foundRow;
     }
 
-    for (_Row row in _rows.values) {
+    for (_Row row in _rows.values)
       row.removeSymbol(artificial);
-    }
     _objective.removeSymbol(artificial);
     return success;
   }
@@ -366,19 +488,13 @@ class Solver {
   Result _optimizeObjectiveRow(_Row objective) {
     while (true) {
       _Symbol entering = _enteringSymbolForObjectiveRow(objective);
-      if (entering.type == _SymbolType.invalid) {
+      if (entering.type == _SymbolType.invalid)
         return Result.success;
-      }
 
-      _Pair<_Symbol, _Row> leavingPair = _leavingRowForEnteringSymbol(entering);
-
-      if (leavingPair == null) {
-        return Result.internalSolverError;
-      }
+      _Symbol leaving = _leavingSymbolForEnteringSymbol(entering);
+      assert(leaving != null);
 
-      _Symbol leaving = leavingPair.first;
-      _Row row = leavingPair.second;
-      _rows.remove(leavingPair.first);
+      _Row row = _rows.remove(leaving);
       row.solveForSymbols(leaving, entering);
       _substitute(entering, row);
       _rows[entering] = row;
@@ -389,38 +505,28 @@ class Solver {
     Map<_Symbol, double> cells = objective.cells;
 
     for (_Symbol symbol in cells.keys) {
-      if (symbol.type != _SymbolType.dummy && cells[symbol] < 0.0) {
+      if (symbol.type != _SymbolType.dummy && cells[symbol] < 0.0)
         return symbol;
     }
-    }
 
     return new _Symbol(_SymbolType.invalid, 0);
   }
 
-  _Pair<_Symbol, _Row> _leavingRowForEnteringSymbol(_Symbol entering) {
+  _Symbol _leavingSymbolForEnteringSymbol(_Symbol entering) {
     double ratio = double.MAX_FINITE;
-    _Pair<_Symbol, _Row> result = new _Pair<_Symbol, _Row>(null, null);
-
+    _Symbol result;
     _rows.forEach((_Symbol symbol, _Row row) {
       if (symbol.type != _SymbolType.external) {
         double temp = row.coefficientForSymbol(entering);
-
         if (temp < 0.0) {
           double tempRatio = -row.constant / temp;
-
           if (tempRatio < ratio) {
             ratio = tempRatio;
-            result.first = symbol;
-            result.second = row;
+            result = symbol;
           }
         }
       }
     });
-
-    if (result.first == null || result.second == null) {
-      return null;
-    }
-
     return result;
   }
 
@@ -431,12 +537,10 @@ class Solver {
         _infeasibleRows.add(first);
       }
     });
-
     _objective.substitute(symbol, row);
-    if (_artificial != null) {
+    if (_artificial != null)
       _artificial.substitute(symbol, row);
   }
-  }
 
   _Symbol _anyPivotableSymbol(_Row row) {
     for (_Symbol symbol in row.cells.keys) {
@@ -466,43 +570,34 @@ class Solver {
     }
   }
 
-  _Pair<_Symbol, _Row> _leavingRowPairForMarkerSymbol(_Symbol marker) {
+  _Symbol _leavingSymbolForMarkerSymbol(_Symbol marker) {
     double r1 = double.MAX_FINITE;
     double r2 = double.MAX_FINITE;
 
-    _Pair<_Symbol, _Row> first, second, third;
+    _Symbol first, second, third;
 
     _rows.forEach((_Symbol symbol, _Row row) {
       double c = row.coefficientForSymbol(marker);
-
-      if (c == 0.0) {
+      if (c == 0.0)
         return;
-      }
-
       if (symbol.type == _SymbolType.external) {
-        third = new _Pair<_Symbol, _Row>(symbol, row);
+        third = symbol;
       } else if (c < 0.0) {
         double r = -row.constant / c;
         if (r < r1) {
           r1 = r;
-          first = new _Pair<_Symbol, _Row>(symbol, row);
+          first = symbol;
         }
       } else {
         double r = row.constant / c;
         if (r < r2) {
           r2 = r;
-          second = new _Pair<_Symbol, _Row>(symbol, row);
+          second = symbol;
         }
       }
     });
 
-    if (first != null) {
-      return first;
-    }
-    if (second != null) {
-      return second;
-    }
-    return third;
+    return first ?? second ?? third;
   }
 
   void _suggestValueForEditInfoWithoutDualOptimization(
@@ -551,9 +646,7 @@ class Solver {
       if (row != null && row.constant < 0.0) {
         _Symbol entering = _dualEnteringSymbolForRow(row);
 
-        if (entering.type == _SymbolType.invalid) {
-          return Result.internalSolverError;
-        }
+        assert(entering.type != _SymbolType.invalid);
 
         _rows.remove(leaving);
 
@@ -630,25 +723,3 @@ class Solver {
     return buffer.toString();
   }
 }
-
-class _Tag {
-  _Symbol marker;
-  _Symbol other;
-
-  _Tag(this.marker, this.other);
-  _Tag.fromTag(_Tag tag)
-      : this.marker = tag.marker,
-        this.other = tag.other;
-}
-
-class _EditInfo {
-  _Tag tag;
-  Constraint constraint;
-  double constant;
-}
-
-bool _isValidNonRequiredPriority(double priority) {
-  return (priority >= 0.0 && priority < Priority.required);
-}
-
-typedef Result _SolverBulkUpdate(dynamic item);

packages/cassowary/lib/term.dart → packages/cassowary/lib/src/term.dart

@@ -2,23 +2,26 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-part of cassowary;
+import 'equation_member.dart';
+import 'expression.dart';
+import 'param.dart';
 
-class Term extends _EquationMember {
+class Term extends EquationMember {
   Term(this.variable, this.coefficient);
 
   final Variable variable;
+
   final double coefficient;
 
   @override
-  bool get isConstant => false;
+  Expression asExpression() =>
+      new Expression([new Term(this.variable, this.coefficient)], 0.0);
 
   @override
-  double get value => coefficient * variable.value;
+  bool get isConstant => false;
 
   @override
-  Expression asExpression() =>
-      new Expression([new Term(this.variable, this.coefficient)], 0.0);
+  double get value => coefficient * variable.value;
 
   @override
   String toString() {

packages/cassowary/lib/symbol.dart

-// Copyright (c) 2015 The Chromium Authors. 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 cassowary;
-
-enum _SymbolType { invalid, external, slack, error, dummy, }
-
-class _Symbol {
-  final _SymbolType type;
-  final int tick;
-
-  _Symbol(this.type, this.tick);
-
-  @override
-  String toString() {
-    String typeString = 'unknown';
-    switch (type) {
-      case _SymbolType.invalid:
-        typeString = 'i';
-        break;
-      case _SymbolType.external:
-        typeString = 'v';
-        break;
-      case _SymbolType.slack:
-        typeString = 's';
-        break;
-      case _SymbolType.error:
-        typeString = 'e';
-        break;
-      case _SymbolType.dummy:
-        typeString = 'd';
-        break;
-    }
-    return '$typeString$tick';
-  }
-}

packages/cassowary/lib/utils.dart

-// Copyright (c) 2015 The Chromium Authors. 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 cassowary;
-
-bool _nearZero(double value) {
-  const double epsilon = 1.0e-8;
-  return value < 0.0 ? -value < epsilon : value < epsilon;
-}
-
-class _Pair<X, Y> {
-  X first;
-  Y second;
-  _Pair(this.first, this.second);
-}

packages/cassowary/lib/variable.dart

-// Copyright (c) 2015 The Chromium Authors. 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 cassowary;
-
-class Variable {
-  double value;
-  String name;
-
-  Param _owner;
-
-  final int _tick;
-  static int _total = 0;
-
-  Variable(this.value) : _tick = _total++;
-
-  bool _applyUpdate(double updated) {
-    bool res = updated != value;
-    value = updated;
-    return res;
-  }
-
-  String get debugName => name ?? 'variable$_tick';
-
-  @override
-  String toString() => debugName;
-}