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Advent of Code 2024 - Historian goes looking for history in all the wrong places (adventofcode.com)

submitted 3 weeks ago by Architeuthis@awful.systems to c/notawfultech@awful.systems

54 comments fedilink hide all child comments

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.

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[–] Architeuthis@awful.systems 2 points 2 weeks ago (1 children)

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@awful.systems 4 points 2 weeks ago (1 children)

re: 10 commentary

apparently "everyone" did this. Me too

[–] zogwarg@awful.systems 1 points 2 weeks 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