617 B
617 B
Let's look ate the diagonal numbers of our 5x5 grid:
| n | info |
|---|---|
| 25 | starting number |
| 21 | = 25 - 4 |
| 17 | = 21 - 4 |
| 13 | = 17 - 4 |
| 9 | = 13 - 4 |
| 7 | = 9 - 2 |
| 5 | = 7 - 2 |
| 3 | = 5 - 2 |
| 1 | = 3 - 2 |
You can probably see the pattern here. The rest of the algorithm is simply loop from 1001^2 to 1, subtracting the right amount each round and in the end summing up all numbers.
I have this little code to calculate the sum of stack values until a zero is encountered:
>+\:#<_$
Perhaps it's useful for someone else.