commit 9f664fa8aa2d4e17243b3984626c2ec95286dd90 · dunkirk.sh/aoc-2025

+87
nix/04/solution.nix
···

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       let

     

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         input = builtins.readFile ../../shared/04/input.txt;

     

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

     

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         # Parse input into grid of booleans (true = @, false = .)

     

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         paperMap = builtins.map (line:

     

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           let

     

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             len = builtins.stringLength line;

     

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

     

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           in builtins.map (i:

     

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             let ch = builtins.substring i 1 line;

     

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             in if ch == "@" then true else false

     

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           ) indices

     

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         ) lines;

     

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         # Count accessible papers and return new map

     

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         accessiblePapers = map:

     

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           let

     

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             numRows = builtins.length map;

     

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       +
             

     

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             # Process all cells in one pass using foldl'

     

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             result = builtins.foldl' (state: rowIdx:

     

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               let

     

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                 rowData = builtins.elemAt map rowIdx;

     

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                 numCols = builtins.length rowData;

     

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                 rowResult = builtins.foldl' (rowState: colIdx:

     

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                   let

     

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                     cell = builtins.elemAt rowData colIdx;

     

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                   in

     

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                     if !cell then {

     

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                       accessible = rowState.accessible;

     

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                       newRow = rowState.newRow ++ [false];

     

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                     }

     

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                     else

     

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                       let

     

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                         # Check all 8 neighbors inline

     

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                         check = r: c:

     

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                           if r < 0 || c < 0 || r >= numRows then false

     

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                           else

     

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                             let rd = builtins.elemAt map r;

     

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                             in if c >= builtins.length rd then false

     

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                                else builtins.elemAt rd c;

     

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                         fullAdj = 

     

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                           (if check (rowIdx - 1) colIdx then 1 else 0) +

     

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                           (if check (rowIdx + 1) colIdx then 1 else 0) +

     

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                           (if check rowIdx (colIdx + 1) then 1 else 0) +

     

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                           (if check rowIdx (colIdx - 1) then 1 else 0) +

     

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                           (if check (rowIdx - 1) (colIdx + 1) then 1 else 0) +

     

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                           (if check (rowIdx + 1) (colIdx + 1) then 1 else 0) +

     

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                           (if check (rowIdx - 1) (colIdx - 1) then 1 else 0) +

     

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                           (if check (rowIdx + 1) (colIdx - 1) then 1 else 0);

     

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                         isAccessible = fullAdj < 4;

     

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                       in {

     

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                         accessible = rowState.accessible + (if isAccessible then 1 else 0);

     

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                         newRow = rowState.newRow ++ [(if isAccessible then false else true)];

     

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                       }

     

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                 ) { accessible = 0; newRow = []; } (builtins.genList (i: i) numCols);

     

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               in {

     

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                 accessible = state.accessible + rowResult.accessible;

     

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                 newMap = state.newMap ++ [rowResult.newRow];

     

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               }

     

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             ) { accessible = 0; newMap = []; } (builtins.genList (i: i) numRows);

     

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           in result;

     

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         # Part 1: Single iteration

     

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         part1 = (accessiblePapers paperMap).accessible;

     

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         # Part 2: Iterate until no more accessible papers

     

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         part2 =

     

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           let

     

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             iterate = state:

     

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               let

     

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                 result = accessiblePapers state.map;

     

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               in

     

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                 if result.accessible == 0 then state.total

     

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                 else iterate {

     

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                   map = result.newMap;

     

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                   total = state.total + result.accessible;

     

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                 };

     

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           in iterate { map = paperMap; total = 0; };

     

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       in {

     

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         inherit part1 part2;

     

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       }