Day 12 solution.

p12.dyalog

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+⎕IO←0 ⋄ ⎕PP←17
+s←15↓⊃p←'#'=⊃⎕NGET'p12.txt'1 ⍝ initial state
+u←↑5↑¨2↓p ⋄ v←⊃∘⌽¨2↓p ⍝ patterns and replacements
+f←{v[u⍳⍉0 ¯1↓(5,5+≢⍵)⍴0 0 0 0,⍵]} ⍝ next generation
+g←{+/(⍸⍵)-0.5×(≢⍵)-≢s} ⍝ score function
+g f⍣20⊢s ⍝ part 1
+t←f⍣{≡/(⊢-⌊/)∘⍸¨⍺⍵}s ⍝ iterate until pattern stabilises
+(g t)+((g f t)-g t)×50E9-0.25×(≢t)-≢s ⍝ part 2

p12.txt

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+initial state: .#..##..#.....######.....#....####.##.#.#...#...##.#...###..####.##.##.####..######......#..##.##.##
+
+#.... => .
+.##.# => #
+#..## => .
+....# => .
+###.# => #
+...#. => #
+#...# => #
+#.### => .
+.#... => #
+...## => .
+..### => .
+####. => .
+##.## => .
+..##. => .
+.#.## => #
+#..#. => #
+..... => .
+#.#.. => .
+##.#. => #
+.#### => #
+##### => .
+#.##. => #
+.#..# => #
+##... => .
+..#.# => #
+##..# => #
+.###. => .
+.#.#. => #
+#.#.# => #
+###.. => .
+.##.. => .
+..#.. => .