Interview Test - Fee Calculation

The test

The requirement was to build a fee calculator that - given a monetary amount and a term (the contractual duration of the loan, expressed as a number of months) - will produce an appropriate fee for a loan, based on a fee structure and a set of rules described below. A general contract for this functionality is defined in the interface FeeCalculator.

Implement solution such that it fulfills the requirements.

• The fee structure does not follow a formula.
• Values in between the breakpoints should be interpolated linearly between the lower bound and upper bound that they fall between.
• The number of breakpoints, their values, or storage might change.
• The term can be either 12 or 24 (number of months), you can also assume values will always be within this set.
• The fee should be rounded up such that fee + loan amount is an exact multiple of 5.
• The minimum amount for a loan is 1,000 PLN, and the maximum is 20,000 PLN.
• You can assume values will always be within this range but they may be any value up to 2 decimal places.

Example inputs/outputs:

Loan amount	Term	Fee
11,500 PLN	24 months	460 PLN
19,250 PLN	12 months	385 PLN

Fee Structure and definitions

The fee structure doesn't follow a particular algorithm and it is possible that the same fee will be applicable for different amounts. Example definitions of loan values and corresponding fees can be seen in fixtures/ directory. Definitions can be changed.

Installation and how to run

A database or any other external dependency is not required for this test.

composer install
# run the code with 2 input parameters
php solution.php --term 24 --amount 2750
# run tests
./vendor/bin/phpunit tests/

Developer notes

• this code requires PHP 8.2 and Composer version 2.5
• Fee definitions for given ranges are in "fixtures" dir. It can be modified to any number of ranges with any fee values
• I have provided multiple unit tests, that confirm if the calculations are ok in a few different ways
  • by checking the fee value
  • by checking the sum of loan+fee
  • by checking if loan+fee are multiple of 5
  • by checking the "out of range" scenario
• there is no Struct in PHP, but to create a contract between findNearFeeRanges() function, that returns some data structure, and calculateFee() that requires it, I have created NearFeeRangesStruct that simulates something similar to Struct (see this RFC: https://wiki.php.net/rfc/structs)
• to be a little fancy, I have used PHP8.2 new feature of Constructor Property Promotion ( RFC: https://wiki.php.net/rfc/constructor_promotion ). It is in src/Model/NearFeeRangesStruct.php:
• ranges of 1,000~20,000 are not fixed, but automaticaly set from the ranges definitions
• this code features Clean Code by:
  • Meaningful and Descriptive Variable Names
  • Proper Indentation and Formatting (https://prettier.io/ for VSCode )
  • DRY ( sunrise.php used for starting solution.php and for tests)
  • Keep Functions and Methods Short
  • Use Comments Sparingly and Purposefully
  • Consistent Coding Style
  • Follow PSR Standards ( PSR-1 & PSR-2 & PSR-4 autoloading )
  • Unit Testing
  • Avoid Global Variables
• this code features SOLID principles
  • Single Responsibility Principle - separate class for loading data from file, seperate that delivers Fee definitions to calculator
  • Open/Closed Principle - eg: FeeDefinitionsInterface
  • Liskov Substitution Principle - CsvFeeDefinitionsRepository and JsonFeeDefinitionsRepository
  • Interface Segregation Principl: FeeDefinitionsInterface, FeeDefinitionsStorageInterface
  • Dependency Inversion Principle: Repository pattern for FeeDefinitionsRepositoryInterface, and specific class of CsvFeeDefinitionsRepository and JsonFeeDefinitionsRepository
• I ran this code on MacOS with PHP installed from brew, and composer installed locally. If you run into trouble starting this code, please let me know