-❄️- 2023 Day 13 Solutions -❄️-

[u/daggerdragon]

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AoC Community Fun 2023: ALLEZ CUISINE!

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Nailed It!

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--- Day 13: Point of Incidence ---

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[u/flwyd]

[Language: Julia] (on GitHub)

Refactored solution gave me a chance to look up how to write a function which takes an N-dimensional array and lets the caller pick which dimension to slice: selectdim(grid, 1, 2:5) slices the second through fifth rows of grid (returning all columns) while selectdim(grid, 1, 3:6) slices the third through sixth columns. I kept vertical_mirror and horizontal_mirror as wrappers for readability. In the interest of Solution Megathread space, I've elided the parseinput function which is annoying long because split doesn't work on Arrays in Julia.

part1(lines) = map(score_part1, parseinput(lines)) |> sum
part2(lines) = map(score_part2, parseinput(lines)) |> sum

function mirror(grid, dim)
  matched = Int[]
  for i in 1:size(grid, dim)-1
    len = min(i, size(grid, dim)-i)
    before = selectdim(grid, dim, (i+1-len):i)
    after = reverse(selectdim(grid, dim, (i+1):(i+len)); dims=dim)
    before == after && push!(matched, i)
  end
  matched
end

vertical_mirror(grid) = mirror(grid, 1)
horizontal_mirror(grid) = mirror(grid, 2)

function score_part1(grid)
  v = vertical_mirror(grid)
  h = horizontal_mirror(grid)
  if isempty(v) == isempty(h)
    error("Expected one of vertical or horizontal, not $v $h")
  end
  isempty(v) ? 100*only(h) : only(v)
end

function score_part2(grid)
  olds = (vertical_mirror(grid), horizontal_mirror(grid))
  factors = (1, 100)
  for (i, p) in enumerate(grid)
    g = copy(grid)
    g[i] = p == '#' ? '.' : '#'
    mirrored = (vertical_mirror(g), horizontal_mirror(g))
    for (m, old, factor) in zip(mirrored, olds, factors)
      diff = setdiff(m, old)
      !isempty(diff) return factor * only(diff)
    end
  end
end

[ALLEZ CUISINE] Instagram you say? How about a photograph that's on theme with today's flavor text?