factorial interview project

factorial.md

Factorial

Methodology

• TDD
  • focus on red, green, refactor
• Start small, add complexity
• Limit scope at each step
• Use any language

Phases

Zero - testbed

• Setup a project in your language of choice.
• Add an empty method/class/module/function.
• Add an empty test.
• Assert that true equals false. Run the test, it should fail, demonstrating that the testbed is working.

One - function

• Create a function that returns the factorial of the input provided.
• happy path behavior
  • 1! == 1
  • 2! == 2
  • 5! == 120
  • 8! == 40320
• edge case
  • 0! == 1

Two - arbitrary precision

• The factorial function should handle very large calculations with exact precision
• Test cases
  • 10! == 3628800
  • 20! == 2432902008176640000
  • 100! == 9.3 * 10^157

Three - input validation

• Some inputs don't make sense. Negative numbers should result in an error. No inputs above 100 should be accepted. If the language is weakly typed, non-integer inputs should not be accepted.
• Test cases
  • (-10)! == error
  • 105! == error
  • (hello)! == error
  • (3.03)! == error

Four - webservice

• The function should be exposed as a webservice. The input will be a query parameter and the output will be in json form.
• Acceptance tests
  • GET http://localhost/api/factorial?input=5 -->> 200 {"output":"120"}
  • GET http://localhost/api/factorial?input=100 -->> 200 {"output":"93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000"}
  • GET http://localhost/api/factorial?input= -->> 400
  • GET http://localhost/api/factorial?input=hello -->> 400

Five - ui

• The webservice should be wrapped in a simple page and presented to users
• ui/ux design will be collaborative
• Controls
  • title
  • header
  • input
  • button
  • result