String Calculator Tabula Rasa

Contained herein are implementations of the String Calculator Kata in several languages. It is used as a live coding exercise during interviews to explore a candidate's approach to test-driven development, incremental design, and refactoring.

The kata itself was created by Roy Osherove. The full problem statement lives at https://osherove.com/tdd-kata-1.

Before You Start

• Try not to read ahead.
• Do one task at a time. The trick is to learn to work incrementally.
• Only test for correct inputs — there is no need to test for invalid inputs for this kata.
• Remember to refactor after each passing test.

The Kata

1. Create a simple String calculator with a method add(numberString) that returns an integer.
   • The method can take 0, 1, or 2 numbers and will return their sum. An empty string returns 0. For example: "", "1", or "1,2".
   • Start with the empty-string case, then one number, then two.
2. Allow the add method to handle an unknown amount of numbers.
3. Allow the add method to handle newlines between numbers in addition to commas. For example, "1\n2,3" should equal 6.
4. Support different delimiters. To change a delimiter, the beginning of the string contains a line of the form "//[delimiter]\n". For example, "//;\n1;2" should return 3. The first line is optional; all previous scenarios should still work.
5. Calling add with a negative number should throw an exception containing all of the negatives that were passed in.

Stop here if you are a beginner. Continue if you finished the steps above in under 30 minutes.

6. Numbers bigger than 1000 should be ignored: 2 + 1001 = 2.
7. Delimiters can be of any length, given in the form "//[delimiter]\n". For example, "//[***]\n1***2***3" should return 6.
8. Allow multiple delimiters, given as "//[delim1][delim2]\n". For example, "//[*][%]\n1*2%3" should return 6.
9. Make sure multi-character delimiters also work when there are multiple of them.

Implementations

• js/ — JavaScript with QUnit. Open start.html for the empty starter or index.html for a worked example.
• php/ — PHP with PHPUnit. Run rake from the php/ directory.
• python/ — Python 3 with unittest. Run python -m unittest from the python/ directory.

Credit

Based on the String Calculator TDD Kata by Roy Osherove.