Make constraint priority setup more expressive

packages/cassowary/lib/cassowary.dart

@@ -4,6 +4,8 @@
 
 library cassowary;
 
+import 'dart:math';
+
 part 'constraint.dart';
 part 'expression.dart';
 part 'term.dart';
@@ -17,3 +19,4 @@ part 'utils.dart';
 part 'result.dart';
 part 'parser_exception.dart';
 part 'param.dart';
+part 'priority.dart';

packages/cassowary/lib/constraint.dart

@@ -9,19 +9,9 @@ enum Relation { equalTo, lessThanOrEqualTo, greaterThanOrEqualTo, }
 class Constraint {
   final Relation relation;
   final Expression expression;
-  final bool required;
+  double priority = Priority.required;
 
-  static const double requiredPriority = 1000.0;
-  double _priority = requiredPriority - 1.0;
-
-  Constraint(this.expression, this.relation) : this.required = false;
-  Constraint.Required(this.expression, this.relation) : this.required = true {
-    this.priority = requiredPriority;
-  }
-
-  double get priority => required ? requiredPriority : _priority;
-  set priority(double p) => _priority =
-      required ? requiredPriority : p.clamp(0.0, requiredPriority - 1.0);
+  Constraint(this.expression, this.relation);
 
   Constraint operator |(double p) => this..priority = p;
 
@@ -31,7 +21,7 @@ class Constraint {
 
     switch (relation) {
       case Relation.equalTo:
-        buffer.write(" <= 0 ");
+        buffer.write(" == 0 ");
         break;
       case Relation.greaterThanOrEqualTo:
         buffer.write(" >= 0 ");
@@ -43,7 +33,7 @@ class Constraint {
 
     buffer.write(" | priority = ${priority}");
 
-    if (required) {
+    if (priority == Priority.required) {
       buffer.write(" (required)");
     }
 

packages/cassowary/lib/priority.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 Priority {
+  static final double required = create(1e3, 1e3, 1e3);
+  static final double strong = create(1.0, 0.0, 0.0);
+  static final double medium = create(0.0, 1.0, 0.0);
+  static final double weak = create(0.0, 0.0, 1.0);
+
+  static double create(double a, double b, double c) {
+    double result = 0.0;
+    result += max(0.0, min(1e3, a)) * 1e6;
+    result += max(0.0, min(1e3, b)) * 1e3;
+    result += max(0.0, min(1e3, c));
+    return result;
+  }
+
+  static double clamp(double value) {
+    return max(0.0, min(required, value));
+  }
+}

packages/cassowary/lib/row.dart

@@ -16,9 +16,9 @@ class _Row {
   double add(double value) => constant += value;
 
   void insertSymbol(_Symbol symbol, [double coefficient = 1.0]) {
-    double val = _elvis(cells[symbol], 0.0) + coefficient;
+    double val = _elvis(cells[symbol], 0.0);
 
-    if (_nearZero(val)) {
+    if (_nearZero(val + coefficient)) {
       cells.remove(symbol);
     } else {
       cells[symbol] = val + coefficient;

packages/cassowary/lib/solver.dart

@@ -221,7 +221,7 @@ class Solver {
           tag.marker = slack;
           row.insertSymbol(slack, coefficient);
 
-          if (!constraint.required) {
+          if (constraint.priority < Priority.required) {
             _Symbol error = new _Symbol(_SymbolType.error, tick++);
             tag.other = error;
             row.insertSymbol(error, -coefficient);
@@ -230,7 +230,7 @@ class Solver {
         }
         break;
       case Relation.equalTo:
-        if (!constraint.required) {
+        if (constraint.priority < Priority.required) {
           _Symbol errPlus = new _Symbol(_SymbolType.error, tick++);
           _Symbol errMinus = new _Symbol(_SymbolType.error, tick++);
           tag.marker = errPlus;
@@ -605,5 +605,5 @@ class _EditInfo {
 }
 
 bool _isValidNonRequiredPriority(double priority) {
-  return (priority >= 0.0 && priority < Constraint.requiredPriority);
+  return (priority >= 0.0 && priority < Priority.required);
 }

packages/cassowary/lib/variable.dart

@@ -17,5 +17,5 @@ class Variable {
 
   String get debugName => _elvis(name, "variable${_tick}");
 
-  String toString() => "${debugName}(=${value})";
+  String toString() => debugName;
 }