Syllabus Lesson 45 of 141 · Delete the problem
Delete the problem

When nothing can go

Why it matters Subtraction is a strong habit, and a strong habit misfires. Some code is exactly as long as the problem, and every line is load-bearing.

Support agents can refund a card charge. The payments team gives four rules:

  1. The refund amount is more than zero.
  2. The refund currency matches the charge currency.
  3. All refunds on the charge, including this one, add up to no more than the charge.
  4. The refund is made within 180 days of the charge date (day 180 is fine, day 181 is not).

A colleague suggests dropping the sum of earlier refunds in rule 3, "because the payment provider would refuse an over-refund anyway". Another suggests a small rule engine so new rules can be registered without touching the function. Decide for yourself which, if either, is right.

Your turn

Write refund_ok(charge, earlier, cents, currency, today): charge is a dict with "cents", "currency" and "date" (a date); earlier is a list of cents already refunded on it; cents and currency describe the new refund; today is a date. Return True only if all four rules hold. Rule: your whole code may have at most 90 syntax nodes (code_size).

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