Imagine we have a list of jobs, each represented by a character. Because certain jobs must be done before others, a job may have a dependency on another job. For example, a may depend on b, meaning the final sequence of jobs should place b before a. If a has no dependency, the position of a in the final sequence does not matter.

The goal of the kata is to parse the job dependency structure and produce a sequence of jobs in the order that observes their dependency chain.

Start with a method that accepts a single string argument and returns a string which represents the ordered sequence of jobs (since each job is a single character). We’ll refine the algorithm by evolving the requirements with each step, just like the string calculator kata.

Step 1 – Empty String

Given you’re passed an empty string (no jobs), the result should be an empty sequence.

Step 2 – Single Job

Given the following job structure:

a => The result should be a sequence consisting of a single job a.

Step 3 – Multiple Jobs

Given the following job structure:

a => b => c => The result should be a sequence containing all three jobs abc in no significant order.

Step 4 – Multiple Jobs, Single Dependency

Given the following job structure:

a => b => c c => The result should be a sequence that positions c before b, containing all three jobs abc.

Step 5 – Multiple Jobs, Multiple Dependencies

Given the following job structure:

a => b => c c => f d => a e => b f => The result should be a sequence that positions f before c, c before b, b before e and a before d containing all six jobs abcdef.

Step 6 – Multiple Jobs, Self Referencing Dependency

Given the following job structure:

a => b => c => c The result should be an error stating that jobs can’t depend on themselves.

Step 7 – Multiple Jobs, Circular Dependency Chain

Given the following job structure:

a => b => c c => f d => a e => f => b The result should be an error stating that jobs can’t have circular dependencies.