# 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)´∘{a‿s‿b:⟨a-'A',b-'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 ⟩