copy pasting the rules from last year’s thread:

Rules: no spoilers.

The other rules are made up aswe go along.

Share code by link to a forge, home page, pastebin (Eric Wastl has one here) or code section in a comment.

  • @ArchiteuthisOP
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    25 days ago
    10 commentary

    Yeah basically if you were doing DFS and forgot to check if you’d already visited the next node you were solving for pt2, since the rule about the next node always having a value of +1 compared to the current one was already preventing cyclic paths.

    10 Code

    Hardly a groundbreaking implementation but I hadn’t posted actual code in a while so

    (* F# - file reading code and other boilerplate omited *)
    
    let mapAllTrails (matrix : int array2d) =
        let rowCount = matrix |> Array2D.length1
        let colCount = matrix |> Array2D.length2
    
        let rec search (current:int*int) (visited: HashSet<int*int>) (path: (int*int) list) : (int*int) list list= 
            let (row,col) = current
            let currentValue = matrix.[row,col]
    
            // Remove to solve for 10-2
            visited.Add (row,col) |> ignore
    
            // If on a 9 return the complete path
            if currentValue = 9 then [List.append path [row,col] ]
            // Otherwise filter for eligible neihboring cells and continue search
            else                    
                [ row-1, col;row, col-1; row, col+1; row+1,col]
                |> List.filter (fun (r,c) -> 
                    not (visited.Contains(r,c))
                    && r >= 0 && c>=0 && r < rowCount && c < colCount
                    && matrix.[r,c]-currentValue = 1 )
                |> List.collect (fun next ->
                    [row,col] 
                    |> List.append path  
                    |> search next visited)
    
        // Find starting cells, i.e. contain 0
        matrix
        |> Global.matrixIndices
        |> Seq.filter (fun (row,col) -> matrix.[row,col] = 0)
        // Find all trails starting from those cells and flatten the result
        |> Seq.collect (fun trailhead -> search trailhead (HashSet<int*int>()) [])
        
    
    "./input.example"
    |> Common.parse 
    |> mapAllTrails
    |> Seq.length
    |> Global.shouldBe 81
    
    "./input.actual"
    |> Common.parse 
    |> mapAllTrails
    |> Seq.length
    |> printfn "The sum total of trail rankings is %d"
    
    • @gerikson
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      45 days ago
      re: 10 commentary

      apparently “everyone” did this. Me too

      • @zogwarg
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        English
        14 days ago
        re:10

        Mwahaha I’m just lazy and did are “unique” (single word dropped for part 2) of start/end pairs.

        #!/usr/bin/env jq -n -R -f
        
        ([
             inputs/ "" | map(tonumber? // -1) | to_entries
         ] | to_entries | map( # '.' = -1 for handling examples #
             .key as $y | .value[]
           | .key as $x | .value   | { "\([$x,$y])":[[$x,$y],.] }
        )|add) as $grid | #           Get indexed grid          #
        
        [
          ($grid[]|select(last==0)) | [.] |    #   Start from every '0' head
          recurse(                             #
            .[-1][1] as $l |                   # Get altitude of current trail
            (                                  #
              .[-1][0]                         #
              | ( .[0] = (.[0] + (1,-1)) ),    #
                ( .[1] = (.[1] + (1,-1)) )     #
            ) as $np |                         #   Get all possible +1 steps
            if $grid["\($np)"][1] != $l + 1 then
              empty                            #     Drop path if invalid
            else                               #
            . += [ $grid["\($np)"] ]           #     Build path if valid
            end                                #
          ) | select(last[1]==9)               #   Only keep complete trails
            | . |= [first,last]                #      Only Keep start/end
        ]
        
        # Get score = sum of unique start/end pairs.
        | group_by(first) | map(unique|length) | add