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# Observation: don't need to handle cases like:
# -S-
# |
# because S only connects to two sides
# 15/16 segments not including S (16/17 including S) -> return 8
$ 7-F7-
$ .FJ|7
$ SJLL7
$ |F--J
$ LJ.LJ
TestInput ←
$ FF7FSF7F7F7F7F7F---7
$ L|LJ||||||||||||F--J
$ FL-7LJLJ||||||LJL-77
$ F--JF--7||LJLJ7F7FJ-
$ L---JF-JLJ.||-FJLJJ7
$ |F|F-JF---7F7-L7L|7|
$ |FFJF7L7F-JF7|JL---7
$ 7-L-JL7||F7|L7F-7F7|
$ L.L7LFJ|||||FJL7||LJ
$ L7JLJL-JLJLJL--JLJ.L
TestInputII ←
DirectionMap ← ⊏⊙[¯1_0 1_0]_[0_¯1 0_1]_[0_¯1 1_0]_[0_¯1 ¯1_0]_[0_1 1_0]_[0_1 ¯1_0]_[0_0 0_0]⊗:"|-7JFL."
Parse ← ⊜∘≠@\n.
ConstructPath ← (
⊢⊚⌕@S.
▽≡/×
⊢⊃(▽≡(/↥♭)⌕:|⋅¤)+∵DirectionMap⊃(⊡|≡↯2|:⊙⋅∘)⊙,
⍢(
⊃(
⊡⊃(⊗1¬≡≍|∘)⊓(+⊃(DirectionMap⊡|↯2)|¤⊢⇌)⊙:
| ⊂:
| ⋅⋅∘
)
)(¬≍⊙⊢)
)
# Determinant of a 2x2 matrix (useful for finding turning number - 1 ccw; `1 cw)
Determinant ← /-/×⍜⊢⇌
# given a direction and shape 2 vector, rotate it (1 ccw, `1 cw)
Rot ← ⇌⍜⊡¯±+1
PartI ← ⌊÷2⧻⋅⊙;ConstructPath
PartII ← (
⋅⊙;ConstructPath
⊃(
≡/-◫2⊂:⊢.
÷4/+≡Determinant◫2⊂:⊢..
≡Rot¤
| ∘
| ¬°⊚ # mask
)
±°⊚⊂⊃(+|+↻¯1)
?
⊃×⋅∘ # apply mask
⊃(⊙∘|↯:0△) # set up stack structure: TOP curr mask old BOT
⍢(
⊙⊙;
⊃(
×±/+≡↻[¯1_0 0_¯1 1_0 0_1 0_0]¤
| ⋅∘
| ∘
)
)(¬≍⊙⋅∘)
/+/+⊙⋅;
)
PartI Parse TestInput
PartII Parse TestInputII
PartI Parse &fras "day10.in"
PartII Parse &fras "day10.in"