commit 17f2fd0f76b5f2b8528c1dc14b5b381383350371 · dunkirk.sh/aoc-2025

+94
nix/03/solution.nix
···

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       let

     

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         input = builtins.readFile ../../shared/03/input.txt;

     

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         banks = builtins.filter (s: builtins.isString s && s != "") (builtins.split "\n" input);

     

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         # Helper to convert string to list of digit ints

     

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         stringToDigits = str:

     

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           let

     

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             len = builtins.stringLength str;

     

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             indices = builtins.genList (i: i) len;

     

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           in map (i: builtins.fromJSON (builtins.substring i 1 str)) indices;

     

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         # Part 1: Find highest pair (largestI * 10 + jVal) for each bank

     

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         part1 =

     

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           let

     

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             processBank = bank:

     

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               let

     

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                 digits = stringToDigits bank;

     

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                 L = builtins.length digits;

     

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                 # For each position j, find the largest digit before it and form pair

     

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                 processPair = state: j:

     

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                   let

     

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                     jVal = builtins.elemAt digits j;

     

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                     # Find largest digit in digits[0..j-1]

     

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                     largestI = builtins.foldl' 

     

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                       (max: i: let iVal = builtins.elemAt digits i; in if iVal > max then iVal else max)

     

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                       (-1)

     

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                       (builtins.genList (i: i) j);

     

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                     pair = largestI * 10 + jVal;

     

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                     newHighest = if pair > state.highest then pair else state.highest;

     

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                   in { highest = newHighest; };

     

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                 # Process j from 1 to L-1

     

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                 result = builtins.foldl' 

     

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                   processPair 

     

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                   { highest = -1; }

     

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                   (builtins.genList (i: i + 1) (L - 1));

     

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               in result.highest;

     

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             jolts = map processBank banks;

     

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           in builtins.foldl' builtins.add 0 jolts;

     

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         # Part 2: Monotonic stack to find best K-digit number

     

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         part2 =

     

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           let

     

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             K = 12;

     

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             processBank = bank:

     

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               let

     

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                 digits = stringToDigits bank;

     

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                 L = builtins.length digits;

     

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                 # Process each digit with monotonic stack approach

     

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                 processDigit = state: j:

     

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                   let

     

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                     jVal = builtins.elemAt digits j;

     

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                     remaining = L - j;

     

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                     # Pop smaller tail digits if we can still reach K

     

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                     popSmaller = stack:

     

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                       let

     

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                         stackLen = builtins.length stack;

     

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                         canPop = stackLen > 0 && 

     

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                                  (builtins.elemAt stack (stackLen - 1)) < jVal &&

     

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                                  stackLen - 1 + remaining >= K;

     

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                       in

     

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                       if canPop then

     

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                         popSmaller (builtins.genList (i: builtins.elemAt stack i) (stackLen - 1))

     

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                       else

     

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                         stack;

     

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                     newStack = popSmaller state.stack;

     

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                     stackLen = builtins.length newStack;

     

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                     # Add current digit if we still need more

     

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                     finalStack = if stackLen < K then newStack ++ [jVal] else newStack;

     

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                   in { stack = finalStack; };

     

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                 # Process all digits

     

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                 result = builtins.foldl' 

     

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                   processDigit 

     

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                   { stack = []; }

     

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                   (builtins.genList (i: i) L);

     

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                 # Convert stack to number

     

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                 stackToNum = builtins.foldl' (acc: d: acc * 10 + d) 0 result.stack;

     

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               in stackToNum;

     

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             jolts = map processBank banks;

     

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           in builtins.foldl' builtins.add 0 jolts;

     

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       in {

     

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         inherit part1 part2;

     

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       }