# a‿b Sub x: the b-length subarray starting at a
Sub ⇐ {a‿b𝕊𝕩:b↑a↓𝕩}

# e Split xs: split xs on occurrences of e (default 0)
Split ⇐ {(⊏ ∾ 1⊸↓¨∘(1⊸↓)) Sub⟜𝕩¨ (» ⋈¨ ⊢-») /w=𝕩∾(w←𝕨⊣0)}

# n Chunk xs: split into chunks of length n (the last one might be smaller)
Chunk ⇐ {(⌊𝕨÷˜↕≠𝕩)⊔𝕩}

Day1 ⇐ (⊑ ⋈ (+´3⊸↑))∘(∨+´¨)∘Split∘(•BQN⎊0¨)∘•FLines
# ⟨ 72602 207410 ⟩

Day2 ⇐ {
  B ← ⊣ (A ← {1+𝕩+3×3|1+𝕩-𝕨}) {3|𝕨+𝕩-1}
  +´ (A⋈B)´∘{"AX"-˜0‿2⊏𝕩}¨ •FLines 𝕩
}
# ⟨ 17189 13490 ⟩

Day3 ⇐ {
  Prio ← ⊢ - ≤⟜'Z'◶"`&"
  A ← +´∘∾ {(Prio¨×∊⟜𝕩)𝕨}○⍷´∘(<˘ 2‿∘⊸⥊)¨
  B ← +´∘∾ {a‿b‿c:(Prio¨a)×b∧○(a⊸∊)c}¨∘(3⊸Chunk)∘(⍷¨)
  (A⋈B) •FLines 𝕩
}
# ⟨ 7683 2488 ⟩

Day4 ⇐ {
  Line ← •BQN '‿'⍟(⊑∊⟜"-,")¨ # hehe
  F ← {∨´∧´˘≥´⎉1(3⥊2)⥊𝕩⊏˜𝕨-'a'}
  A ← "cabddbac"⊸F, B ← "cabcacda"⊸F
  +´ (A⋈B)∘Line¨ •FLines 𝕩
}
# ⟨ 536 845 ⟩

Day5 ⇐ {
  inp ← •FLines 𝕩
  Start ← {(𝕨⊸=⊑⎊0)¨⊸/ 𝕩}
  stks ← ' '⊸≠⊸/¨ <˘⍉> (1⊸⊑¨ 4⊸Chunk)¨ '[' Start inp
  todo ← •BQN⚇1 (1‿3‿5⊸⊏ ' '⊸Split)¨ 'm' Start inp
  _Do1 ← {
    n‿s‿t ← 𝕩 - 0‿1‿1
    l‿r ← n (↑⋈↓) s⊑𝕨
    (𝔽 l)⊸∾⌾(t⊸⊑) r⌾(s⊸⊑) 𝕨
  }
  _Do ← {⊑¨ 𝕨𝔽_Do1˜´⌽𝕩}, A ← ⌽_Do, B ← ⊢_Do
  stks (A⋈B) todo
}
# ⟨ "QNNTGTPFN" "GGNPJBTTR" ⟩

Day6 ⇐ {{𝕨+ ⊑/ ≡⟜⍷˘ 𝕨↕𝕩}⟜(•FChars 𝕩)¨ 4‿14}
# ⟨ 1929 3298 ⟩

Day8 ⇐ {
  _rots ← {F‿x←𝔽‿𝕩, >{F⌾𝕏x}¨⟨⊢,⍉,⌽˘,⍉⌽⟩}
  A ← {+´⥊ ∨˝ ∧´∘(⊑>1⊸↓)¨ 𝕩}
  B ← {⌈´⥊ ×˝ {1⊑⎊(1-˜≠𝕩) (/⊢≥⊑) 𝕩}¨ 𝕩}
  (A⋈B) (¯1↓↓)˘_rots '0'-˜ >•FLines 𝕩
}
# ⟨ 1843 180000 ⟩

Day10 ⇐ {
  Line ← 0∾ ·•BQN⎊⟨⟩ 5↓⊢
  A ← (+´ 19⊸⊏∘⍉) (⊢×1+↕∘≠⌾⥊)
  B ← ⊏⟜" █" (1≥⟜|-)⟜(↕≠)˘
  (A⋈B) ∘‿40⥊ +` 1»∾Line¨ •FLines 𝕩
}
# ⟨ 13060 "FJUBULRZ" ⟩
# (but the string is a big banner)

Day18 ⇐ { # part 1 only :[
  grid  ← (↕∘> 1+⌈˝)⊸∊ •BQN¨ '‿'⍟(=⟜',')⚇0 •FLines 𝕩
  touch ← +´⥊ «‿» {∧⟜(𝕎⎉𝕩) grid}⌜ 1+↕3
  +´⥊ (6×grid)-touch
}
# 3470