Add Abelian protocol, make AdditiveGroup default implementations public (#222)

Author: jsbean

Diff for: Sources/Algebra/Abelian.swift

@@ -0,0 +1,9 @@
+//
+//  Abelian.swift
+//  Algebra
+//
+//  Created by James Bean on 9/6/19.
+//
+
+/// Interface defining a `Group` which is also commutative.
+public protocol Abelian: Group { }

Diff for: Sources/Algebra/AdditiveGroup.swift

@@ -5,19 +5,27 @@
 //  Created by Benjamin Wetherfield on 10/11/2018.
 //
 
+/// The requirements for a type to display the behaviors of an `AdditiveGroup`.
+///
+/// The  `AdditiveGroup` builds upon the `AdditiveMonoid` with an `inverse` operation.
 public protocol AdditiveGroup: Additive, Invertible {
-    
+
+    /// - Returns: The additive inverse of this `AdditiveGroup` element.
     static prefix func - (_ element: Self) -> Self
+
+    /// - Returns: The difference between two `AdditiveGroup` elements.
     static func - (lhs: Self, rhs: Self) -> Self
 }
 
 extension AdditiveGroup {
-    
-    static prefix func - (_ element: Self) -> Self {
+
+    /// - Returns: The additive inverse of this `AdditiveGroup` element.
+    public static prefix func - (_ element: Self) -> Self {
         return element.inverse
     }
-    
-    static func - (lhs: Self, rhs: Self) -> Self {
+
+    /// - Returns: The difference between two `AdditiveGroup` elements.
+    public static func - (lhs: Self, rhs: Self) -> Self {
         return lhs + -rhs
     }
 }

Diff for: Tests/DataStructuresPerformanceTests/GraphPerformanceTests/GraphPerformanceTests.swift

@@ -50,7 +50,7 @@ class GraphPerformanceTests: XCTestCase {
         XCTAssert(benchmark.performance(is: .constant))
     }
 
-    #warning("FIXME: Assess why the testEdgeFromSourceInCompleteGraph benchmark test files. It is currently disabled.")
+    // FIXME: Assess why the testEdgeFromSourceInCompleteGraph benchmark test files
     func DISABLED_testEdgeFromSourceInCompleteGraph_O_n() {
         let benchmark = Benchmark.mutating(
             testPoints: Scale.small,

Diff for: Tests/DataStructuresTests/MatrixTests.swift

@@ -12,11 +12,9 @@ import DataStructures
 class MatrixTests: XCTestCase {
 
     func testInit() {
-
         let amountRows = 2
         let amountColumns = 3
-        var matrix = Matrix(height: amountRows, width: amountColumns, initial: 0)
-
+        let matrix = Matrix(height: amountRows, width: amountColumns, initial: 0)
         for row in 0 ..< amountRows {
             for column in 0 ..< amountColumns {
                 XCTAssertEqual(matrix[row, column], 0)