Kata PHP String Calculator

This classic kata guides you step by step through the implementation of a calculator that receives a String as input. It is a good exercise on refactoring and incremental implementation. It is also a good candidate for practising TDD.

First step

Create a function add that takes a String and returns a String:

String add(String number)

    The method can take 0, 1 or 2 numbers separated by comma, and returns their sum.
    An empty string will return “0”.
    Example of inputs: "", "1", "1.1,2.2".

Many numbers

Allow the add method to handle an unknow number of arguments.

Newline as separator

Allow the add method to handle newlines as separators:

    "1\n2,3" should return "6".
    "175.2,\n35" is invalid and should return the message "Number expected but '\n' found at position 6."

Missing number in last position

Don’t allow the input to end in a separator.

"1,3," is invalid and should return the message Number expected but EOF found.

Custom separators

Allow the add method to handle a different delimiter. To change the delimiter, the beginning of the input will contain a separate line that looks like this:

//[delimiter]\n[numbers]

    "//;\n1;2" should return "3"
    "//|\n1|2|3" should return "6"
    "//sep\n2sep3" should return "5"
    "//|\n1|2,3" is invalid and should return the message "'|' expected but ',' found at position 3."

All existing scenarios should work as before.

Negative numbers

Calling add with negative numbers will return the message "Negative not allowed : " listing all negative numbers that were in the list of numbers.

    "-1,2" is invalid and should return the message "Negative not allowed : -1"
    "2,-4,-5" is invalid and should return the message "Negative not allowed : -4, -5"

Multiple errors

Calling add with multiple errors will return all error messages separated by newlines.

"-1,,2" is invalid and return the message "Negative not allowed : -1\nNumber expected but ',' found at position 3."

Errors management

Introduce an internal add function returning a number instead of a String, and test many solutions for the error messages.

Exception
maybe and monad approch
POSIX return code with message managemement
tuple with error struct like in Go
etc.

Other operations

Write a function for multiply with same rules