$ 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⍉

PartI Parse TestInput

PartI Parse &fras "day14.in"