AoC 2021 day 9 (dc)

#!/usr/bin/dc -finput

z sn                                # lines of input
0 sw                                # width of lines

[1+] S1                             # inc
[ d2*1+2%* ] s|                     # abs
[ d l|x d 0=1 / ] sS                # sign

# load array, find maximum width line of input
[ dZ sw ] sW

[
    dZ lw<W                         # num -> (max width < len(num), set new width) num
    z :g                            # store line in grid
    z 0<L                           # loop until stack empty
] sL
lLx

# add top and bottom borders
lw2+ 10r^1- d 0:g ln1+:g

# add borders to left and right
[
    dd                              # i -> i i i
    ;g lw 10r^ 9*+ 10*9+            # g(i) i i -> ((g(i) + 10^x * 9) * 10 + 9) i i
    r:g                             # (g(i) = number with 9 borders) i
    1- d 0<L                        # i--, loop while > 0
] sL
ln lLx

0 ss

[s. _1] sZ                          # replace top of stack with -1 (so it adds zero)
[ ;gr 10r^/ 10% ] s=                # y x -> digit x of g(y)

[
    lx ly l=x dddd 0r               # g(y,x) five times, insert sum of signs pos 2
    lx1+ ly l=x - lSx+ r            # take sign of diff for dir, add to sum of signs
    lx1- ly l=x - lSx+ r            # ditto
    lx ly1+ l=x - lSx+ r            # ditto
    lx ly1- l=x - lSx+              # ditto
    _4!=Z 1+ ls+ ss                 # if sum of signs isn't -4, make it add zero to score

    lx1- dsx 0<X
] sX

[
    lw sx lXx
    ly1- dsy 0<Y
] sY
ln sy lYx

[Part 1: ]n lsp