Add examples for package manager and spreadsheet (#13)

The examples help me to understand how to define the API. Later, they
can be used for tests and for documentation.

See pull request #13

Sources/SwiftDependencyGraphs/DependencyGraph.swift

@@ -8,22 +8,10 @@ public struct DependencyGraph<V> where V: Hashable, V: Identifiable {
 
   // For efficiency, two hashsets are maintained.
 
-  /// Incoming edges of a vertex are directed edges that the vertex is the destination.
-  ///
-  /// The edge `v --> w` (or `(v, w)`) is an incoming edge of `w`, but not of `v`.
-  /// In this case, `w` is a key and `v` its value.
-  ///
-  /// - Note: Informally speaking, `v` is adjacent to `w`, or a neighbour of `w`, and
-  /// `w` is targeted by `v`'s arrow.
+  /// The dictionary maps the edge `v --> w` as `[w: v]`. `w` is the key and `v` the value.
   public internal(set) var incomingEdges: [V.ID: OrderedSet<V>] = [:]
 
-  /// Outgoing edges of a vertex are directed edges that the vertex is the origin.
-  ///
-  /// The edge `v --> w` (or `(v, w)`) is an outgoing edge of `v`, but not of `w`.
-  /// In this case, `v` is a key and `w` its value.
-  ///
-  /// - Note: Informally speaking, `v` is adjacent to `w`, or a neighbour of `w`, and
-  /// its arrow points to `w`.
+  /// The dictionary maps the edge `v --> w` as `[v: w]`. `v` is the key and `w` the value.
   public internal(set) var outgoingEdges: [V.ID: OrderedSet<V>] = [:]
 
   func contains(vertex: V) -> Bool {

Tests/Examples/PackageManager/Package.swift

@@ -0,0 +1,5 @@
+struct Package: Identifiable, Hashable {
+  let id: String
+  let name: String
+  let author: String
+}

Tests/Examples/PackageManager/PackageGraph.swift

@@ -0,0 +1,25 @@
+import DependencyGraphs
+
+// A -> B means that A depends on B
+typealias PackageGraph = DependencyGraph<Package>
+
+extension PackageGraph {
+  func findAllDependencies(of package: Package) -> [Package] {
+    // Traverse the graph starting from package
+    // and collect vertices visited
+    []
+  }
+
+  // This should be in `DependencyGraph`.
+  func insert(package: Package, withDependencies dependencies: [Package]) {
+    // Call `insert` method in `DependencyGraph`.
+    // `insert` also inserts the dependencies and checks
+    // if circular dependencies would be introduced.
+  }
+
+  func findDependents(of package: Package) -> [Package] {
+    // Traverse graph in reverse direction
+    // and collect packages.
+    []
+  }
+}

Tests/Examples/SpreadSheet/Cell.swift

@@ -0,0 +1,5 @@
+struct Cell: Identifiable, Hashable {
+  let id: Coordinate
+  let computedValue: Int
+  let expression: String
+}

Tests/Examples/SpreadSheet/Coordinate.swift

@@ -0,0 +1,4 @@
+struct Coordinate: Hashable {
+  let row: Int
+  let column: Int
+}

Tests/Examples/SpreadSheet/SpreadSheet.swift

@@ -0,0 +1,36 @@
+import DependencyGraphs
+
+// A -> B means that A depends on B
+typealias SpreadSheet = DependencyGraph<Cell>
+
+extension SpreadSheet {
+  // This should be in `DependencyGraph`.
+  func insert(cell: Cell, withDependencies dependencies: [Cell]) {
+    // Call `insert` method in `DependencyGraph`.
+    // `inserts` also inserts the dependencies of `cell`
+    // and checks if circular dependencies would be introduced.
+  }
+
+  func update(cell: Cell, with newExpression: String) {
+    compute(expression: newExpression)
+    // Call `update` method in `DependencyGraph`
+    // to update `expression` and `computedValue` for `cell`.
+    // `update` also updates the dependencies of `cell`
+    // and checks if circular dependencies would be introduced.
+    // We need to recompute values of cells that depend on `cell`.
+    // Traverse graph with dfs starting from `cell`
+    // and update their values.
+  }
+
+  func compute(expression: String) -> Double {
+    // Parse expression and get cell names C it depends on.
+    // Find C in `vertices` dictionary.
+    // We don't need to recompute value of C.
+    // Compute expression.
+    0.0
+  }
+
+  // This should be in `DependencyGraph`.
+  // There is no domain specific logic here.
+  func remove(cell: Cell) {}
+}