commit 17f2fd0f76b5f2b8528c1dc14b5b381383350371 · dunkirk.sh/aoc-2025

+94

nix/03/solution.nix

···

       1
       +
       let

     

       2
       +
         input = builtins.readFile ../../shared/03/input.txt;

     

       3
       +
         banks = builtins.filter (s: builtins.isString s && s != "") (builtins.split "\n" input);

     

       4
       +
         

     

       5
       +
         # Helper to convert string to list of digit ints

     

       6
       +
         stringToDigits = str:

     

       7
       +
           let

     

       8
       +
             len = builtins.stringLength str;

     

       9
       +
             indices = builtins.genList (i: i) len;

     

       10
       +
           in map (i: builtins.fromJSON (builtins.substring i 1 str)) indices;

     

       11
       +
         

     

       12
       +
         # Part 1: Find highest pair (largestI * 10 + jVal) for each bank

     

       13
       +
         part1 =

     

       14
       +
           let

     

       15
       +
             processBank = bank:

     

       16
       +
               let

     

       17
       +
                 digits = stringToDigits bank;

     

       18
       +
                 L = builtins.length digits;

     

       19
       +
                 

     

       20
       +
                 # For each position j, find the largest digit before it and form pair

     

       21
       +
                 processPair = state: j:

     

       22
       +
                   let

     

       23
       +
                     jVal = builtins.elemAt digits j;

     

       24
       +
                     # Find largest digit in digits[0..j-1]

     

       25
       +
                     largestI = builtins.foldl' 

     

       26
       +
                       (max: i: let iVal = builtins.elemAt digits i; in if iVal > max then iVal else max)

     

       27
       +
                       (-1)

     

       28
       +
                       (builtins.genList (i: i) j);

     

       29
       +
                     pair = largestI * 10 + jVal;

     

       30
       +
                     newHighest = if pair > state.highest then pair else state.highest;

     

       31
       +
                   in { highest = newHighest; };

     

       32
       +
                 

     

       33
       +
                 # Process j from 1 to L-1

     

       34
       +
                 result = builtins.foldl' 

     

       35
       +
                   processPair 

     

       36
       +
                   { highest = -1; }

     

       37
       +
                   (builtins.genList (i: i + 1) (L - 1));

     

       38
       +
               in result.highest;

     

       39
       +
             

     

       40
       +
             jolts = map processBank banks;

     

       41
       +
           in builtins.foldl' builtins.add 0 jolts;

     

       42
       +
         

     

       43
       +
         # Part 2: Monotonic stack to find best K-digit number

     

       44
       +
         part2 =

     

       45
       +
           let

     

       46
       +
             K = 12;

     

       47
       +
             

     

       48
       +
             processBank = bank:

     

       49
       +
               let

     

       50
       +
                 digits = stringToDigits bank;

     

       51
       +
                 L = builtins.length digits;

     

       52
       +
                 

     

       53
       +
                 # Process each digit with monotonic stack approach

     

       54
       +
                 processDigit = state: j:

     

       55
       +
                   let

     

       56
       +
                     jVal = builtins.elemAt digits j;

     

       57
       +
                     remaining = L - j;

     

       58
       +
                     

     

       59
       +
                     # Pop smaller tail digits if we can still reach K

     

       60
       +
                     popSmaller = stack:

     

       61
       +
                       let

     

       62
       +
                         stackLen = builtins.length stack;

     

       63
       +
                         canPop = stackLen > 0 && 

     

       64
       +
                                  (builtins.elemAt stack (stackLen - 1)) < jVal &&

     

       65
       +
                                  stackLen - 1 + remaining >= K;

     

       66
       +
                       in

     

       67
       +
                       if canPop then

     

       68
       +
                         popSmaller (builtins.genList (i: builtins.elemAt stack i) (stackLen - 1))

     

       69
       +
                       else

     

       70
       +
                         stack;

     

       71
       +
                     

     

       72
       +
                     newStack = popSmaller state.stack;

     

       73
       +
                     stackLen = builtins.length newStack;

     

       74
       +
                     

     

       75
       +
                     # Add current digit if we still need more

     

       76
       +
                     finalStack = if stackLen < K then newStack ++ [jVal] else newStack;

     

       77
       +
                   in { stack = finalStack; };

     

       78
       +
                 

     

       79
       +
                 # Process all digits

     

       80
       +
                 result = builtins.foldl' 

     

       81
       +
                   processDigit 

     

       82
       +
                   { stack = []; }

     

       83
       +
                   (builtins.genList (i: i) L);

     

       84
       +
                 

     

       85
       +
                 # Convert stack to number

     

       86
       +
                 stackToNum = builtins.foldl' (acc: d: acc * 10 + d) 0 result.stack;

     

       87
       +
               in stackToNum;

     

       88
       +
             

     

       89
       +
             jolts = map processBank banks;

     

       90
       +
           in builtins.foldl' builtins.add 0 jolts;

     

       91
       +
       

     

       92
       +
       in {

     

       93
       +
         inherit part1 part2;

     

       94
       +
       }