This is an example solution.

Make sure to try solving the kata yourself first and check out the example solution afterwards. The provided solution is of course only an example of what your solution could look like.

Tax ID Kata

A coding kata that focuses on encapsulation and domain-driven design.

Design Goals

• If I create or receive a tax ID object, it is just that: A tax ID, not a string or something else.
• The creation of the object fails if it would be based on ill-formed input (user input, file input, network response, etc.).
• As the caller/user of the object, I don't know and don't want to know what is inside of it (strings, numbers, algorithms, rules). I can use it "as is".
• A tax ID can be converted to a human-readable string representation, but the resulting string is not itself the tax ID.

Discuss with your peers: What is the main purpose of any ID?

Reveal answer

An ID's main (and often only) purpose is to provide a way to check two entities for identity (from Latin: "this entity"/"this being"). Often an ID object's only method is equals(), thus fulfilling its only purpose. An identity is inherently immutable.

An entity's identity is a domain concept, and often separate from the technical ID that might be used e.g. by a database.

Requirements

• A well-formed tax ID consists of exactly 11 digits.
• The first digit must not be 0.
• In the first 10 digits, one digit occurs either 2 or 3 times, but never 3 times in a row. The other digits are unique.
• The last digit is a check digit which is calculated from the first 10 digits according to the ISO/IEC 7064, MOD 11,10 standard:
  • Start with a "product" of 10.
  • For each of the first 10 digits, update the product as follows:

    sum := (current_digit + product) MOD 10;
    if sum = 0 then sum := 10;
    product := (sum * 2) MOD 11;
    

  • At the end the check digit is the result of (11 - product) MOD 10.

Some examples of well-formed IDs (last digit is check digit):

11345678904
13345678906
12445678907
12345698991

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