String Calculator Kata

README.md

String Calculator

This is my first implementation of this String Calculator Code Kata.

It was written as TDD with Guard driving Minitest.

string_calculator.rb

module StringCalculator
  def add
    raise_if_negatives_present
    return digits_less_than_or_equal_to_1000.reduce 0, :+
  end

  private

  def digits
    @digits ||= gsub(/[\n]/, delimiter).split(delimiter).map { |item| item.to_i }
  end

  def digits_less_than_or_equal_to_1000
    digits.select { |x| x <= 1000 }
  end

  def negatives
    @negatives ||= digits.select { |x| x < 0 }
  end

  def raise_if_negatives_present
    raise "Cannot have negative numbers: #{negatives}" if negatives.any?
  end

  def delimiter
    @delimiter ||= self[0,2] == '//' ? self[2,1] : ','
  end
end

test_string_calculator.rb

# coding: utf-8
require "minitest/autorun"
require "./string_calculator"

class String
  include StringCalculator
end

class TestStringCalculator < Minitest::Test
  def test_empty_string_returns_zero
    assert_equal 0, "".add
  end

  def test_single_character_numeric_string_returns_number
    assert_equal 4, "4".add
  end

  def test_two_character_numerics_string_returns_number
    assert_equal 14, "14".add
  end

  def test_three_character_numeric_string_returns_number
    assert_equal 123, "123".add
  end

  def test_adds_two_comma_delimited_single_numbers
    assert_equal 3, "1,2".add
  end

  def test_adds_larger_single_digit_comma_delimited_numbers
    assert_equal 12, "7,5".add
  end

  def test_adds_collection_of_comma_delimited_numbers
    assert_equal 7, "1,2,4".add
  end

  def test_handles_newlines_as_delimiters
    assert_equal 3, "1\n2".add
  end

  def test_handles_mixed_delimiters
    assert_equal 9, "1,6\n2".add
  end

  def test_handles_alternate_delimiters
    assert_equal 6, "//;\n1;2;3".add
  end

  def test_raises_error_with_negative_number
    assert_raises { "-1".add }
  end

  def test_raises_error_with_negative_numbers_and_includes_negatives_in_message
    error = assert_raises(RuntimeError) {"-1,-7".add }
    regex = /Cannot have negative numbers\: \[-1, -7\]/
    assert_match regex, error.message
  end

  def test_includes_all_negative_numbers_in_raised_error_message
    error = assert_raises(RuntimeError) {"-1,-7,9,-27".add }
    regex = /Cannot have negative numbers\: \[-1, -7, -27\]/
    assert_match regex, error.message
  end

  def test_ignores_single_numbers_greater_than_1000
    assert_equal 0, "1001".add
  end

  def test_ignores_numbers_greater_than_1000_in_expressions
    assert_equal 5, "1001,5".add
  end
end