Grok 4 vs Codex Mini

# Challenge \#4: 🧮 Decipher the Santa PIN 





MEDIUM

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

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

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

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