Constraints can be setup directly from non-expression via operator overrides

packages/cassowary/lib/cassowary.dart

@@ -10,3 +10,4 @@ part 'term.dart';
 part 'variable.dart';
 part 'equation_member.dart';
 part 'constant_member.dart';
+part 'solver.dart';

packages/cassowary/lib/constant_member.dart

@@ -8,6 +8,14 @@ class ConstantMember extends EquationMember {
   double value = 0.0;
   ConstantMember(this.value);
 
+  Expression _asExpression() => new Expression([], this.value);
+
+  Constraint operator >=(EquationMember m) => _asExpression() >= m;
+
+  Constraint operator <=(EquationMember m) => _asExpression() <= m;
+
+  operator ==(EquationMember m) => _asExpression() == m;
+
   Expression operator +(EquationMember m) {
     if (m is ConstantMember) {
       return new Expression([], this.value + m.value);

packages/cassowary/lib/expression.dart

@@ -12,18 +12,44 @@ class Expression extends EquationMember {
 
   Expression(this.terms, this.constant);
 
-  Constraint _createConstraint(double value, Relation relation) {
+  Constraint _createConstraint(
+      EquationMember /* rhs */ value, Relation relation) {
+    if (value is ConstantMember) {
+      return new Constraint(new Expression(
+          new List.from(this.terms), this.constant - value.value), relation);
+    }
+
+    if (value is Variable) {
+      var newTerms = new List<Term>.from(this.terms)
+        ..add(new Term(value, -1.0));
+      return new Constraint(new Expression(newTerms, this.constant), relation);
+    }
+
+    if (value is Term) {
+      var newTerms = new List<Term>.from(this.terms)
+        ..add(new Term(value.variable, -value.coefficient));
+      return new Constraint(new Expression(newTerms, this.constant), relation);
+    }
+
+    if (value is Expression) {
+      var newTerms = value.terms.fold(new List<Term>.from(this.terms),
+          (list, t) => list..add(new Term(t.variable, -t.coefficient)));
       return new Constraint(
-        new Expression(this.terms, this.constant + value), relation);
+          new Expression(newTerms, this.constant - value.constant), relation);
+    }
+
+    assert(false);
+    return null;
   }
 
-  Constraint operator >=(double value) =>
-      _createConstraint(-value, Relation.greaterThanOrEqualTo);
+  Constraint operator >=(EquationMember value) =>
+      _createConstraint(value, Relation.greaterThanOrEqualTo);
 
-  Constraint operator <=(double value) =>
-      _createConstraint(-value, Relation.lessThanOrEqualTo);
+  Constraint operator <=(EquationMember value) =>
+      _createConstraint(value, Relation.lessThanOrEqualTo);
 
-  operator ==(double value) => _createConstraint(-value, Relation.equalTo);
+  operator ==(EquationMember value) =>
+      _createConstraint(value, Relation.equalTo);
 
   Expression operator +(EquationMember m) {
     if (m is ConstantMember) {

packages/cassowary/lib/solver.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 Solver {
+  bool addConstraint(Constraint c) {
+    return false;
+  }
+
+  bool removeContraint(Constraint c) {
+    return false;
+  }
+
+  Solver operator <<(Constraint c) => this..addConstraint(c);
+}

packages/cassowary/lib/term.dart

@@ -11,6 +11,15 @@ class Term extends EquationMember {
 
   Term(this.variable, this.coefficient);
 
+  Expression _asExpression() =>
+      new Expression([new Term(this.variable, this.coefficient)], 0.0);
+
+  Constraint operator >=(EquationMember m) => _asExpression() >= m;
+
+  Constraint operator <=(EquationMember m) => _asExpression() <= m;
+
+  operator ==(EquationMember m) => _asExpression() == m;
+
   Expression operator +(EquationMember m) {
     if (m is ConstantMember) {
       return new Expression([this], m.value);

packages/cassowary/lib/variable.dart

@@ -8,6 +8,14 @@ class Variable extends EquationMember {
   double value = 0.0;
   Variable(this.value);
 
+  Expression _asExpression() => new Expression([new Term(this, 1.0)], 0.0);
+
+  Constraint operator >=(EquationMember m) => _asExpression() >= m;
+
+  Constraint operator <=(EquationMember m) => _asExpression() <= m;
+
+  operator ==(EquationMember m) => _asExpression() == m;
+
   Expression operator +(EquationMember m) {
     if (m is ConstantMember) {
       return new Expression([new Term(this, 1.0)], m.value);

packages/cassowary/test/cassowary_test.dart

@@ -194,7 +194,7 @@ void main() {
     var left = new Variable(10.0);
     var right = new Variable(100.0);
 
-    var c = right - left >= 25.0;
+    var c = right - left >= CM(25.0);
     expect(c is Constraint, true);
   });
 
@@ -244,17 +244,17 @@ void main() {
     var left = new Variable(2.0);
     var right = new Variable(10.0);
 
-    var c1 = right - left >= 20.0;
+    var c1 = right - left >= CM(20.0);
     expect(c1 is Constraint, true);
     expect(c1.expression.constant, -20.0);
     expect(c1.relation, Relation.greaterThanOrEqualTo);
 
-    var c2 = (right - left == 30.0) as Constraint;
+    var c2 = (right - left == CM(30.0)) as Constraint;
     expect(c2 is Constraint, true);
     expect(c2.expression.constant, -30.0);
     expect(c2.relation, Relation.equalTo);
 
-    var c3 = right - left <= 30.0;
+    var c3 = right - left <= CM(30.0);
     expect(c3 is Constraint, true);
     expect(c3.expression.constant, -30.0);
     expect(c3.relation, Relation.lessThanOrEqualTo);
@@ -264,10 +264,79 @@ void main() {
     var left = new Variable(2.0);
     var right = new Variable(10.0);
 
-    var c = (right - left >= 200.0) | 750.0;
+    var c = (right - left >= CM(200.0)) | 750.0;
     expect(c is Constraint, true);
     expect(c.expression.terms.length, 2);
     expect(c.expression.constant, -200.0);
     expect(c.priority, 750.0);
   });
+
+  test('solver', () {
+    var s = new Solver();
+
+    var left = new Variable(2.0);
+    var right = new Variable(100.0);
+
+    var c1 = right - left >= CM(200.0);
+    var c2 = right + left >= CM(0.0);
+
+    // TODO: Add assertions for this
+    s << c1 << c2;
+  });
+
+  test('constraint_complex', () {
+    var e = new Variable(200.0) - new Variable(100.0);
+
+    // Constant
+    var c1 = e >= CM(50.0);
+    expect(c1 is Constraint, true);
+    expect(c1.expression.terms.length, 2);
+    expect(c1.expression.constant, -50.0);
+
+    // Variable
+    var c2 = e >= new Variable(2.0);
+    expect(c2 is Constraint, true);
+    expect(c2.expression.terms.length, 3);
+    expect(c2.expression.constant, 0.0);
+
+    // Term
+    var c3 = e >= new Term(new Variable(2.0), 1.0);
+    expect(c3 is Constraint, true);
+    expect(c3.expression.terms.length, 3);
+    expect(c3.expression.constant, 0.0);
+
+    // Expression
+    var c4 = e >= new Expression([new Term(new Variable(2.0), 1.0)], 20.0);
+    expect(c4 is Constraint, true);
+    expect(c4.expression.terms.length, 3);
+    expect(c4.expression.constant, -20.0);
+  });
+
+  test('constraint_complex_non_exprs', () {
+    // Constant
+    var c1 = CM(100.0) >= CM(50.0);
+    expect(c1 is Constraint, true);
+    expect(c1.expression.terms.length, 0);
+    expect(c1.expression.constant, 50.0);
+
+    // Variable
+    var c2 = new Variable(100.0) >= new Variable(2.0);
+    expect(c2 is Constraint, true);
+    expect(c2.expression.terms.length, 2);
+    expect(c2.expression.constant, 0.0);
+
+    // Term
+    var t = new Term(new Variable(100.0), 1.0);
+    var c3 = t >= new Term(new Variable(2.0), 1.0);
+    expect(c3 is Constraint, true);
+    expect(c3.expression.terms.length, 2);
+    expect(c3.expression.constant, 0.0);
+
+    // Expression
+    var e = new Expression([t], 0.0);
+    var c4 = e >= new Expression([new Term(new Variable(2.0), 1.0)], 20.0);
+    expect(c4 is Constraint, true);
+    expect(c4.expression.terms.length, 2);
+    expect(c4.expression.constant, -20.0);
+  });
 }