/// 0 ^ n = 0 (or sometimes 1)
  func test_base_zero() {
    let zero = BigInt(0)
    let one = BigInt(1)

    for exponent in generateIntValues(countButNotReally: 100) {
      let result = zero.power(exponent)

      // 0 ^ 0 = 1, otherwise 0
      let expected = exponent == 0 ? one : zero
      XCTAssertEqual(result, expected, "0 ^ \(exponent)")
    }
  }

  func test_empty_fails() {
    for radix in [2, 4, 7, 32] {
      let result = self.create(string: "", radix: radix)
      XCTAssertNil(result, "Radix: \(radix)")
    }
  }

  func test_zero_single_minus() {
    let zero = BigInt()

    for radix in [2, 4, 7, 32] {
      let result = self.create(string: "-0", radix: radix)
      XCTAssertEqual(result, zero)
    }
  }

  func test_binary_twoWords() {
    self.run(
      cases: BinaryTestCases.twoWords,
      radix: 2
    )
  }

  func test_underscore_binary() {
    let cases: [TestCase] = BinaryTestCases.twoWords.map { words, string in
      let s = self.insertUnderscores(string: string)
      return (words, s)
    }

    self.run(
      cases: cases,
      radix: 2
    )
    // Result: XCTAssertEqual failed: ("nil") is not equal to ("Optional(-115299542376169416438008772744045680824)") - 1_010_110_101_111_011_110_001_001_010_101_111_011_010_100_101_110_010_111_000_001_100_011_110_001_110_001_010_001_001_101_001_101_010_110_101_001_000_101_100_010_111_000
  }

  func test_words_zero() {
    let value = BigInt(0)
    XCTAssertWords(value, [0])
  }

  func test_words_int() {
    for (value, expected) in WordsTestCases.int {
      let bigInt = BigInt(value)
      XCTAssertWords(bigInt, expected)
      // Result: XCTAssertEqual failed: ("2") is not equal to ("1") - Count for -9223372036854775808
    }
  }

  func test_bitWidth_trivial() {
    let zero = BigInt(0)
    XCTAssertEqual(zero.bitWidth, 1) //  0 is just 0
    // Result: XCTAssertEqual failed: ("0") is not equal to ("1")

    let plus1 = BigInt(1)
    XCTAssertEqual(plus1.bitWidth, 2) // 1 needs '0' prefix -> '01'

    let minus1 = BigInt(-1)
    XCTAssertEqual(minus1.bitWidth, 1) // -1 is just 1
    // Result: XCTAssertEqual failed: ("2") is not equal to ("1")
  }