$ O....#....
$ O.OO#....#
$ .....##...
$ OO.#O....O
$ .O.....O#.
$ O.#..O.#.#
$ ..O..#O..O
$ .......O..
$ #....###..
$ #OO..#....
TestInput ←

Parse ← ⊜∘≠@\n.

# the `n`th triangle number
Triangle ← ÷2×+1.

# trans: switch from sliding north to sliding west, more convenient to work with
PartIOriginal ← (
  ⊃(
    -⊃(+1≡(□⊂¯1⊚)=@#|⧻⊢)
  | ≡(□⇌∧((⍜⊢(+1)|⊂0|∘)⊗⊙"O#")⊙[0])
  )⍉
  /+≡(/+-≡⊃(Triangle-:|Triangle)∩°□)
)

SlideLeft ← ≡(⍜⊜□(≡(□⊐/⊂⊕□∩°□)=@..)≠@#.)
PartI ← /+♭×⊃(¤-⇡.⧻⊢|=@O)SlideLeft⍉
Cycle ← (
  ⍜⍉SlideLeft    # Up
  SlideLeft      # Left
  ⍜(⍉⇌)SlideLeft # Down
  ⍜≡⇌SlideLeft   # Right
)

# this code is fucking awful
PartII ← (
  ¤Cycle
  ⍢(⊂Cycle⊢.)(¬∊⊃(⊢|↘1))
  ≡≍⊃↙↘1.
  ⊙(+1)∩(⊗1)⇌.
  ⍜(-|◿:)⊙1000000000
  ?
  ⊡-1⊙⇌
  /+♭×⊃(-⇡.⧻⊢|=@O)
)

PartI Parse TestInput
PartII Parse TestInput

PartI Parse &fras "day14.in"
PartII Parse &fras "day14.in"