Code Breaker

Level: Intermediate 30–60 min

Concepts: AlgorithmsStringsValidationEdge Cases

Solutions: C# | TypeScript | Python

Build the scoring engine for a Mastermind-style code-breaking game. Given a secret code and a guess, return feedback indicating exact matches and partial matches.

Requirements

The secret code and guess are both 4-digit strings using digits 1–6.

Feedback Rules

  • Exact match (+) — correct digit in the correct position
  • Partial match (-) — correct digit but in the wrong position
  • No match — digit not in the secret code

Priority

  • Exact matches are evaluated first
  • A digit consumed by an exact match cannot also count as a partial match
  • Feedback string lists all + symbols before - symbols

Output

Return a feedback string:

  • "++" means 2 exact matches
  • "+-" means 1 exact match and 1 partial match
  • "" means no matches at all

Test Cases

SecretGuessFeedbackExplanation
"1234""5678"""No matches
"1234""1578""+"1 exact (position 1)
"1234""1234""++++"All exact
"1234""4321""----"All partial, none exact
"1234""1243""++-"Wait — see below
"1234""1243""++--"Positions 1,2 exact; 3,4 partial
"1234""2135""+--"Position 2 exact; 1,3 partial
"1124""5167""+"Position 2 exact; only one 1 available
"1122""2211""----"All partial, none in correct position
"1111""1112""+++"3 exact; 2 is not a 1
"1111""2111""+++"Positions 2,3,4 exact; 2 not in code

The Tricky Case

Secret "1124", Guess "5167":

  • Position 1: 5 vs 1 → no match
  • Position 2: 1 vs 1 → exact match (+)
  • Position 3: 6 vs 2 → no match
  • Position 4: 7 vs 4 → no match

The 1 in position 1 of the guess might seem like a partial match, but the secret’s 1 at position 2 is already consumed by the exact match. Result: "+"

Bonus

  • Support configurable code length (4, 5, or 6 digits)
  • Support configurable digit range (1–6, 1–8, etc.)
  • Build a full game loop: generate random secret, accept guesses, track attempt count
  • Add a isWon() check when feedback is all +

Hint

Process exact matches first in a separate pass. Mark matched positions as “consumed” in both the secret and guess. Then do a second pass for partial matches, only considering unconsumed positions. This two-pass approach handles all the tricky duplicate-digit cases cleanly.

Reference Walkthrough

Reference implementations in C#, TypeScript, and Python live at tddbuddy-reference-katas/code-breaker. This is an F2 (light builder) kata: two primary entities (Secret, Guess) each get a small test-folder builder so four-peg codes read as a single fluent line, a typed Peg enum closes the 1–6 domain at compile time, and Feedback carries both the structured counts (exactMatches, colorMatches) and the canonical "+"/"-" string.

Scope note — feedback engine only. The reference is scoped to scoring a guess against a secret. Random secret generation, the full game loop with attempt tracking, configurable code length, and a configurable peg range are all out of scope for this reference and listed as stretch goals in the repo README. Those responsibilities introduce a SecretGenerator collaborator and a Game aggregate — F3 territory, not F2.

This kata ships in Agent Full-Bake mode at middle gear, the F2 (light builder) tier. See the repo’s Gears section for why middle gear is the deliberate choice.