Challenge #4: 🧮 Decipher the Santa PIN

The elves have found the encrypted code that protects the door to Santa’s workshop 🔐. The PIN has 4 digits, and it is hidden inside blocks like these:

[1++][2-][3+][<]

Write a function that deciphers the PIN from the code.

The code is made up of blocks between brackets [...] and each block generates one digit of the PIN.

A normal block has the form [nOP...], where n is a number (0-9) and after it there can be a list of (optional) operations.

The operations are applied in order to the number and are:

• + adds 1
• - subtracts 1

The result is always a digit (mod 10 arithmetic), for example 9 + 1 → 0 and 0 - 1 → 9.

There is also the special block [<], which repeats the digit from the previous block.

If in the end there are fewer than 4 digits, you must return null.

🧩 Examples

decodeSantaPin('[1++][2-][3+][<]')
// "3144"

decodeSantaPin('[9+][0-][4][<]')
// "0944"

decodeSantaPin('[1+][2-]')
// null (only 2 digits)