Add strategy pattern: "Reservation cargo spaces".

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.21.0
+- Add strategy pattern: "Reservation cargo spaces".
+
 ## 0.20.0
 - Add "Conceptual Dialog Factory" example.
 

README.md

@@ -20,7 +20,7 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [ ] **State**
     - [ ] **Template Method**
     - [ ] **Visitor**
-    - [ ] **Strategy**
+    - [ ] **Strategy** [[Reservation cargo spaces](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/strategy/reservation_cargo_spaces)]
 - [ ] **Structural**
     - [x] **Adapter** - [[Text Graphics](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/adapter/text_graphics)] [[Square Round conflict](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/adapter/square_round_conflict)] [[Flutter Adapter](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/adapter/flutter_adapter)] 
     - [x] **Bridge** - [[Remote Device Control](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/bridge/devices_remote_control)] [[Clock](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/bridge/clock)] 

patterns/strategy/reservation_cargo_spaces/README.md

@@ -0,0 +1,43 @@
+# Memento pattern
+Strategy is a behavioral design pattern that lets you define a family of algorithms, put each of 
+them into a separate class, and make their objects interchangeable.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/strategy).
+
+## About example: Reservation cargo spaces. 
+This example is taken from the "Blue" book **"Domain-Driven Design" - Eric Evans. Chapter Once**.
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/166560051-6e392b01-6777-4eb1-ae20-fcd643b248ef.png)
+
+### Client code:
+```dart
+void main() {
+  final overbookingPolicy = OverbookingPolicy();
+  final app = Application(overbookingPolicy);
+  final voyage = Voyage();
+
+  try {
+    app.makeBooking(Cargo(1000), voyage);
+    app.makeBooking(Cargo(500), voyage);
+    app.makeBooking(Cargo(800), voyage); // error
+  } catch (e) {
+    print(e);
+  }
+}
+
+class Application {
+  void makeBooking(Cargo cargo, Voyage voyage) {
+    if (overbookingPolicy.isAllowed(cargo, voyage)) {
+      voyage.addCargo(cargo, confirmation);
+    } 
+  }
+}
+```
+
+**Output:**
+```
+add Cargo(1000.0) to voyage.
+add Cargo(500.0) to voyage.
+The weight of the cargo exceeds the permissible norm.
+```

patterns/strategy/reservation_cargo_spaces/application/application.dart

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+import '../partners/cargo.dart';
+import '../partners/voyage.dart';
+import '../policy/overbooking_policy.dart';
+import 'order_confirmation_sequence.dart';
+
+class Application {
+  final OverbookingPolicy overbookingPolicy;
+  final orderConfirmationSequence = OrderConfirmationSequence();
+
+  Application(this.overbookingPolicy);
+
+  void makeBooking(Cargo cargo, Voyage voyage) {
+    if (overbookingPolicy.isAllowed(cargo, voyage)) {
+      final confirmation = orderConfirmationSequence.next();
+      voyage.addCargo(cargo, confirmation);
+    } else {
+      throw 'The weight of the cargo exceeds the permissible norm.';
+    }
+  }
+}

patterns/strategy/reservation_cargo_spaces/application/order_confirmation_sequence.dart

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+class OrderConfirmationSequence {
+  var _index = 0;
+
+  int next() => _index++;
+}

patterns/strategy/reservation_cargo_spaces/main.dart

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+import 'application/application.dart';
+import 'partners/cargo.dart';
+import 'partners/voyage.dart';
+import 'policy/overbooking_policy.dart';
+
+void main() {
+  final overbookingPolicy = OverbookingPolicy();
+  final app = Application(overbookingPolicy);
+  final voyage = Voyage();
+
+  try {
+    app.makeBooking(Cargo(1000), voyage);
+    app.makeBooking(Cargo(500), voyage);
+    app.makeBooking(Cargo(800), voyage); // error
+  } catch (e) {
+    print(e);
+  }
+}

patterns/strategy/reservation_cargo_spaces/partners/cargo.dart

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+class Cargo {
+  final double size;
+
+  Cargo(this.size);
+}

patterns/strategy/reservation_cargo_spaces/partners/voyage.dart

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+import 'cargo.dart';
+
+class Voyage {
+  final _cargo = <int, Cargo>{};
+
+  double get capacity => 2000.0;
+
+  double bookedCargoSize() {
+    return _cargo.values.fold(0, (prev, cargo) => prev + cargo.size);
+  }
+
+  void addCargo(Cargo cargo, int confirmation) {
+    _cargo.putIfAbsent(confirmation, () => cargo);
+    print('Add Cargo(${cargo.size}) to voyage.');
+  }
+}

patterns/strategy/reservation_cargo_spaces/policy/overbooking_policy.dart

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+import '../partners/cargo.dart';
+import '../partners/voyage.dart';
+
+class OverbookingPolicy {
+  static const allowableRedundancy = 1.1;
+
+  bool isAllowed(Cargo cargo, Voyage voyage) {
+    final maxBooking = voyage.capacity * allowableRedundancy;
+    final futureWeight = voyage.bookedCargoSize() + cargo.size;
+
+    return futureWeight < maxBooking;
+  }
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.20.0
+version: 0.21.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue