commit eaf214bd11a1e340299eb4e3b7643485c9d932df · dunkirk.sh/aoc-2025

+115
nix/06/solution.nix
···

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       let

     

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         input = builtins.readFile ../../shared/06/input.txt;

     

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

     

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         # Helper to get character at position or space if out of bounds

     

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         charAt = str: idx: 

     

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           if idx < 0 || idx >= builtins.stringLength str 

     

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           then " " 

     

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           else builtins.substring idx 1 str;

     

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       +
         

     

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         # Find whitespace columns (columns that are all spaces/tabs in data rows)

     

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         dataRows = builtins.genList (i: builtins.elemAt lines i) (builtins.length lines - 1);

     

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         maxLen = builtins.foldl' (max: row: 

     

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           let len = builtins.stringLength row; 

     

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           in if len > max then len else max

     

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         ) 0 dataRows;

     

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       +
         

     

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         # Find columns that are all whitespace

     

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         splitCols = builtins.filter (col: col != null) (builtins.genList (i:

     

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           let

     

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             allWS = builtins.foldl' (acc: row:

     

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               acc && (let ch = charAt row i; in ch == " " || ch == "	")

     

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             ) true dataRows;

     

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           in if allWS then i else null

     

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         ) maxLen);

     

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         # Split each row at whitespace columns  

     

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         splitRow = row:

     

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           let

     

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             rowLen = builtins.stringLength row;

     

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             splitImpl = cuts: startPos: acc:

     

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               if builtins.length cuts == 0

     

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               then acc ++ [(builtins.substring startPos (rowLen - startPos) row)]

     

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               else

     

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                 let

     

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                   cut = builtins.head cuts;

     

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                   restCuts = builtins.tail cuts;

     

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                   end = if cut + 1 > rowLen then rowLen else cut + 1;

     

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                   segment = builtins.substring startPos (end - startPos) row;

     

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                 in splitImpl restCuts end (acc ++ [segment]);

     

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           in splitImpl splitCols 0 [];

     

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         # Split all lines (including operator row)

     

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         segmentedRows = builtins.map splitRow lines;

     

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         # Transpose to get columns (problems)

     

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         numProblems = builtins.length (builtins.head segmentedRows);

     

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         problems = builtins.genList (colIdx:

     

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           builtins.map (row: builtins.elemAt row colIdx) segmentedRows

     

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         ) numProblems;

     

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         # Fast trim - just remove spaces and tabs

     

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         # Extract just digits from a string for numbers

     

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         extractNum = str:

     

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           let

     

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             cleaned = builtins.replaceStrings [" " "	"] ["" ""] str;

     

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           in if builtins.stringLength cleaned == 0 then 0 else builtins.fromJSON cleaned;

     

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         # Extract operator

     

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         extractOp = str:

     

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           let

     

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             cleaned = builtins.replaceStrings [" " "	"] ["" ""] str;

     

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           in cleaned;

     

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         # Part 1: Normal left-to-right evaluation

     

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         part1 = builtins.foldl' (total: problem:

     

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           let

     

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             lastIdx = builtins.length problem - 1;

     

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             operator = extractOp (builtins.elemAt problem lastIdx);

     

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             nums = builtins.map (s: extractNum s) (builtins.genList (i: builtins.elemAt problem i) lastIdx);

     

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             value = 

     

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               if operator == "*"

     

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               then builtins.foldl' (acc: n: acc * n) 1 nums

     

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               else builtins.foldl' (acc: n: acc + n) 0 nums;

     

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           in total + value

     

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         ) 0 problems;

     

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         # Part 2: Cepheid (vertical) reading

     

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         part2 = builtins.foldl' (total: problem:

     

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           let

     

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             lastIdx = builtins.length problem - 1;

     

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             operator = extractOp (builtins.elemAt problem lastIdx);

     

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             numStrs = builtins.genList (i: builtins.elemAt problem i) lastIdx;

     

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       +
             

     

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             # Find max width

     

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             maxWidth = builtins.foldl' (max: s:

     

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               let len = builtins.stringLength s;

     

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               in if len > max then len else max

     

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             ) 0 numStrs;

     

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             # Read vertically from right to left

     

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             cephNums = builtins.filter (n: n != null) (builtins.genList (colR:

     

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               let

     

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                 # Build digits string from this column (right to left)

     

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                 idx = maxWidth - colR;

     

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                 digitsStr = builtins.concatStringsSep "" (builtins.filter (ch: ch != " " && ch != "	") (builtins.map (s:

     

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                   if idx >= 0 && idx < builtins.stringLength s

     

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                   then builtins.substring idx 1 s

     

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                   else " "

     

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                 ) numStrs));

     

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               in if builtins.stringLength digitsStr > 0

     

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                  then builtins.fromJSON digitsStr

     

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                  else null

     

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             ) (maxWidth + 1));

     

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             value =

     

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               if operator == "*"

     

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               then builtins.foldl' (acc: n: acc * n) 1 cephNums

     

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               else builtins.foldl' (acc: n: acc + n) 0 cephNums;

     

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           in total + value

     

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         ) 0 problems;

     

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       in {

     

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         inherit part1 part2;

     

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       }