Add Identifier (#159)

Author: jsbean

Sources/DataStructures/Identifier.swift

@@ -0,0 +1,46 @@
+//
+//  Identifier.swift
+//  DataStructures
+//
+//  Created by James Bean on 9/1/18.
+//
+
+/// A type-safe identifier.
+///
+/// An `Identifier` is merely a wrapper around an `Int`, but with the added type-safety of the
+/// generic type parameter `Subject`.
+///
+///     let truck: Identifier<Truck> = 0
+///     let friend: Identifier<Friend> = 0
+///
+/// The `Subject` is not used within the type, but it exists to ensure that things like this don't
+/// compile:
+///
+///     truck == friend
+///     // Binary operator '==' cannot be applied to operands of type 'Identifier<Truck>' and
+///     // 'Identifier<Friend>'
+///
+public struct Identifier <Subject> {
+
+    let value: Int
+
+    // MARK: - Initializers
+
+    /// Creates an `Identifier` with the given `value`.
+    public init(_ value: Int) {
+        self.value = value
+    }
+}
+
+extension Identifier: ExpressibleByIntegerLiteral {
+
+    // MARK: - ExpressibleByIntgerLiteral
+
+    /// Creates an `Identifier` with the given `value`.
+    public init(integerLiteral value: Int) {
+        self.value = value
+    }
+}
+
+extension Identifier: Equatable { }
+extension Identifier: Hashable { }