Example UnitTests for a Stack in Swift

Stack.swift

import Foundation

public struct Stack<Element> {
    private var storage = [Element]()

    public init() {}

    public init(_ elements: [Element]) {
        storage = elements
    }

    public mutating func push(_ element: Element) {
        storage.append(element)
    }

    @discardableResult
    public mutating func pop() -> Element? {
        storage.popLast()
    }

    public func peek() -> Element? {
        storage.last
    }

    public var isEmpty: Bool {
        peek() == nil
    }
}

extension Stack: CustomStringConvertible {
    public var description: String {
        """
        --- top ---
        \(storage.map { "\($0)" }.reversed().joined(separator: "\n"))
        -----------
        """
    }
}

extension Stack: ExpressibleByArrayLiteral {
    public init(arrayLiteral elements: Element...) {
        storage = elements
    }
}

StackTests.swift

import XCTest
@testable import DataStructuresAndAlgorithms

class StackTests: XCTestCase {

    func test_init_stack_is_empty() {
        var sut = Stack<Int>()

        XCTAssertNil(sut.pop())
    }

    func test_push_LIFO() {
        var sut = Stack<Int>()

        sut.push(1)
        sut.push(2)
        sut.push(3)

        XCTAssertEqual(3, sut.pop())
        XCTAssertEqual(2, sut.pop())
        XCTAssertEqual(1, sut.pop())
        XCTAssertNil(sut.pop())
    }

    func test_description() {
        var sut = Stack<Int>()

        sut.push(1)
        sut.push(2)

        let stackDescription =
            """
            --- top ---
            2
            1
            -----------
            """

        XCTAssertEqual(sut.description, stackDescription)
    }

    func test_peek_isEmpty_empty_stack() {
        let sut = Stack<Int>()

        XCTAssertNil(sut.peek())
        XCTAssertTrue(sut.isEmpty)
    }

    func test_peek_isEmpty_not_empty_stack() {
        var sut = Stack<Int>()

        sut.push(1)
        sut.push(2)

        XCTAssertEqual(sut.peek(), 2)
        XCTAssertFalse(sut.isEmpty)
    }

    func test_init_from_array() {
        let elements = [1, 2, 3]
        var sut = Stack(elements)

        XCTAssertFalse(sut.isEmpty)
        XCTAssertEqual(sut.peek(), 3)

        sut.push(4)
        XCTAssertEqual(sut.peek(), 4)
    }

    func test_init_array_literal() {
        var sut: Stack = [1, 2, 3]
        XCTAssertFalse(sut.isEmpty)
        XCTAssertEqual(sut.peek(), 3)

        sut.push(4)
        XCTAssertEqual(sut.peek(), 4)
    }
}