Unsatisfying day 12.

answers/12.txt

@@ -0,0 +1,2 @@
+2823
+2900000001856

data/12.txt

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

day12.nim

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+import math, sequtils, strscans, strutils, tables
+
+type
+    State = tuple[start: int, plants: string]
+
+const
+    iterations = 50_000_000_000
+
+func score(state: State): int =
+    for i, c in state.plants:
+        if c == '#':
+            result += i + state.start
+
+
+func iterate(state: State, rules: ref Table[string, char]): State =
+    let
+        input = "...." & state.plants & "...."
+        start = state.start - 2
+
+    var
+        output = ""
+        left = -1
+        
+    for i in 0..input.len - 5:
+        let val = rules[input.substr(i, i + 4)]
+        output &= val
+        if left == -1 and val == '#':
+            left = i - 1
+
+    (start + left, output.substr(left).strip(leading=false, chars={'.'}))
+
+
+proc solve(initialState: State, rules: ref Table[string, char]) =
+    var state = initialState
+
+    for i in 1..iterations:
+        var nextState = iterate(state, rules)
+
+        if i == 20:
+            echo nextState.score
+
+        if state.plants == nextState.plants:
+            # Same pattern of plants, they're just wandering off somewhere.
+            let
+                difference = nextState.score - state.score
+                remaining = iterations - i
+                score = nextState.score + difference * remaining
+            echo score
+            return
+
+        state = nextState
+
+    # If we reach here then the pattern didn't repeat, and it's
+    # probably taken an *awfully* long time.
+    echo state.score
+
+
+var
+    rules = newTable[string, char]()
+    lines = readFile("data/12.txt").strip.splitlines
+    initialState: State = (0, lines[0].strip.substr(15))
+
+for line in lines[2..lines.len-1]:
+    if line.strip.len > 0:
+        rules.add(line.substr(0,4), line.substr(9)[0])
+
+solve(initialState, rules)