Introduction

Your revolutionary My Barista app is getting a lot of traction, and users are demanding a new feature: coffee recommendations.
You decided to implement it as a backend service serving HTTP/JSON API, using your last fancy "hello world" NestJS app for starters. There is no need to worry about a persistent database at this point—startup world is full of risk, and you are ready to embrace it by relying on in-memory storage only.

To bring any value to your wonderful users, you need to provide endpoints to:

• Rate coffee brewed today
• List previous ratings and previously rated coffee types
• Receive recommendations of coffee types to brew today

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App requirements

Your app has to satisfy the requirements of 4 HTTP requests described below:

Endpoint to rate coffee type

• Request: POST /ratings
• Request body: JSON in form of object:

  { "coffeeType": "coffee type to rate", "starRating": "3/5" }

• HTTP status for success: 201 Created
• HTTP status for validation error: 400 Bad Request
  • Validation of coffeeType property:
    • Must be present in the request body
  • Validation of starRating property:
    • Must be present in the request body
    • Must satisfy the format "1/5", where the first integer is a number of stars given (min 1, max 5), and the second integer is always 5
    • No extra whitespaces or other number formats (e.g., "5/5" is correct, but "5.0 / 5.0" is not)

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Endpoint to list rated coffee types

• Request: GET /ratings/coffee-types
• HTTP status for success: 200 OK
• Response body: JSON in form of array

  ["some rated coffee type", "another rated coffee type"]

• Business logic:
  • All coffee types rated with POST /ratings should be included in the response body
  • The resulting array should be empty ([]) if no coffee type was rated yet
  • The resulting array should contain no duplicates

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Endpoint to obtain rating of previously rated coffee type

• Request: GET /ratings
• Request URL query param: coffeeType
• Response body: JSON in form of object

  { "coffeeType": "previously rated coffee type", "starRating": "3/5" }

• HTTP status for success: 200 OK
• HTTP status for validation error: 400 Bad Request
• HTTP status when requesting a coffee type that was not rated yet: 400 Bad Request
• Validation of coffeeType query param:
  • Must be present in the URL
• Business logic:
  • Rating given via POST /ratings can be retrieved using GET /ratings
  • Returned rating should be the most recent one if multiple ratings exist for the same coffee type
  • An error response should be returned if requesting the rating of a coffee type that was not rated yet, with the response body in the form:

    { "coffeeType": "not rated yet coffee type taken from query param" }

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Endpoint to recommend a coffee type for today

• Request: GET /recommendation
• Response body for success: JSON in form of object

  { "coffeeType": "recommended coffee type" }

  • HTTP status for success: 200 OK
• Response body for empty result: JSON in form of object

  { "message": "NO_RECOMMENDATIONS_AVAILABLE" }

  • HTTP status for empty result: 200 OK
• Business logic:
  • The recommended coffee type is calculated based on the most recent ratings only
  • The most recently rated coffee type is not included in the recommendation
  • Only coffee types rated with 4 or 5 stars ("4/5" and "5/5") are considered good enough to be recommended
  • Among those, the oldest rated coffee type is recommended
  • If no such coffee type exists, a response with the message "NO_RECOMMENDATIONS_AVAILABLE" is returned

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