Test Triangle Type (exercise from Kent Beck's "Test-Driven Development by Example")

TriangleType.swift

//
//  TriangleTest.swift
//  TriangleTest
//
//  Created by Stanislaw Pankevich on 24/01/16.
//  Copyright © 2016 Stanislaw Pankevich. All rights reserved.
//

import XCTest

enum TriangleType {
    case Equilateral
    case Isosceles
    case Scalene

    case NotWellFormed
}

func triangleType(a: Int, _ b: Int, _ c: Int) -> TriangleType {
    if (a <= 0 || b <= 0 || c <= 0) {
        return .NotWellFormed
    }

    if (a == b && b == c) {
        return .Equilateral
    }

    if (a == b || b == c || a == c) {
        return .Isosceles
    }

    return .Scalene
}

class TriangleTest: XCTestCase {

    func testEquilateral() {
        let type = triangleType(1, 1, 1)

        XCTAssert(type == .Equilateral)
    }

    func testScalene() {
        let type = triangleType(1, 2, 3)

        XCTAssert(type == .Scalene)
    }

    // Isosceles: 3 cases

    func testIsoscelesAandB() {
        let type = triangleType(2, 2, 1)

        XCTAssert(type == .Isosceles)
    }

    func testIsoscelesBandC() {
        let type = triangleType(2, 1, 2)

        XCTAssert(type == .Isosceles)
    }

    func testIsoscelesAandC() {
        let type = triangleType(2, 1, 2)

        XCTAssert(type == .Isosceles)
    }

    // First parameter validation

    func testMalformedIfFirstParameterIsZero() {
        let type = triangleType(0, 1, 1)

        XCTAssert(type == .NotWellFormed)
    }

    func testMalformedIfFirstParameterIsNegative() {
        let type = triangleType(-1, 1, 1)

        XCTAssert(type == .NotWellFormed)
    }

    // Second parameter validation

    func testMalformedIfSecondParameterIsZero() {
        let type = triangleType(1, 0, 1)

        XCTAssert(type == .NotWellFormed)
    }

    func testMalformedIfSecondParameterIsNegative() {
        let type = triangleType(1, -2, 1)

        XCTAssert(type == .NotWellFormed)
    }

    // Third parameter validation

    func testMalformedIfThirdParameterIsZero() {
        let type = triangleType(1, 1, 0)

        XCTAssert(type == .NotWellFormed)
    }

    func testMalformedIfThirdParameterIsNegative() {
        let type = triangleType(1, 1, -3)

        XCTAssert(type == .NotWellFormed)
    }
}

You forgot to check if sum of any two sides is bigger than the third one. For instance, a 5:1:2 triangle is according to your program scalene, whilst in fact it is malformed.