day ten! flood fill wasn't too hard to implement

day10.ua

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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.
+  ▽≡/×≥0.+[¯1_0 0_¯1 1_0 0_1]¤.
+  ⊢⊃(▽≡(/↥♭)⌕:|⋅¤)+∵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"