i:0:"i/6test.in"
m:~^i.*|:                       / on map
s:(-1 0;,/&"^"=i)               / starting (dir;pos)
b:+&"#"=i                       / oBstructions
w:{(@[y;0;|-1 1*];+\y)@^x?+/y}  / walk one step
P:?*|+-1_m w[b]\s               / guard's path on the map (deduped)
#P                              / part 1: length of the path

mv:{(y,,z;w[x;z])}              / move[obstacles;trace;position] -> (tr';pos')
ok:{(^x?y)&m y}.                / ok[tr;pos] (on map and hasn't looped)
lp:{m@*|ok(mv[b,,x].)/(!0;s)}   / obstacle here loops?
+/lp'P                          / part 2: number of loops

A nasty imperative day...

For some reason, I was dead set against using mutation for part 2, which means I inadvertantly ended up emulating a state monad in K. Do not do this. Just use state.

This can be sped up, for example by starting each loop check when the guard hits the obstacle, and not from the very state.

~/aoc24k/6.html