Fix for intermittent failure for multi term expressions. The cause is…

… Dictionary is not Ordered and Terms must be evaluated in insertion order.

Package.swift

@@ -7,8 +7,14 @@ let cassowary = Package(
   products: [
     .library(name: "cassowary", type: .dynamic, targets: ["Cassowary"]),
   ],
+  dependencies: [
+    .package(url: "https://github.com/apple/swift-collections.git",
+             .upToNextMinor(from: "1.0.0")),
+  ],
   targets: [
-    .target(name: "Cassowary", dependencies: []),
+    .target(name: "Cassowary", dependencies: [
+        .product(name: "OrderedCollections", package: "swift-collections")
+    ]),
     .testTarget(name: "CassowaryTests", dependencies: ["Cassowary"]),
   ]
 )

Sources/Cassowary/Expression.swift

@@ -1,6 +1,9 @@
 // Copyright © 2019 Saleem Abdulrasool <compnerd@compnerd.org>.
 // SPDX-License-Identifier: BSD-3-Clause
 
+@_implementationOnly
+import OrderedCollections
+
 public struct Expression {
   public let terms: [Term]
   public let constant: Double
@@ -38,7 +41,7 @@ extension Expression: Equatable {
 }
 
 internal func reduce(_ expression: Expression) -> Expression {
-  var vars: [Variable:Double] = [:]
+  var vars: OrderedDictionary<Variable, Double> = [:]
   for term in expression.terms {
     vars[term.variable] = vars[term.variable, default: 0.0] + term.coefficient
   }

Sources/Cassowary/Row.swift

@@ -1,8 +1,12 @@
 // Copyright © 2019 Saleem Abdulrasool <compnerd@compnerd.org>.
 // SPDX-License-Identifier: BSD-3-Clause
 
+@_implementationOnly
+import OrderedCollections
+
 internal class Row {
-  public private(set) var cells: [Symbol:Double] = [:]
+  // Must be in insertion order
+  public private(set) var cells: OrderedDictionary<Symbol, Double> = [:]
   public private(set) var constant: Double
 
   public init() {

Tests/CassowaryTests/CassowaryTests.swift

@@ -218,4 +218,25 @@ final class CassowaryTests: XCTestCase {
     XCTAssertEqual(x_l.value, 80)
     XCTAssertEqual(x_r.value, 100)
   }
+
+  // Terms must be evaluated in insertion order
+  func testMultiTermExpressionOrder() throws {
+    let solver: Solver = Solver()
+
+    let c: Variable = Variable("c")
+    let a: Variable = Variable("a")
+    let b: Variable = Variable("b")
+
+    try solver.add(variable: c, strength: .strong)
+    try solver.add(constraint: a >= 0, .strong)
+    try solver.add(constraint: b >= a, .strong)
+    try solver.add(constraint: b - a == c, .required)
+    try solver.suggest(value: 100, for: c)
+
+    solver.update()
+
+    XCTAssertEqual(a.value, 0)
+    XCTAssertEqual(b.value, 100)
+    XCTAssertEqual(c.value, 100)
+  }
 }