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/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;
-}