My solution first checks whether the presents can fit into a region based on area alone. Turns out that eliminated more than half of the regions. I then started to design in my head a brute-force algorithm to check the other regions. Before implementing it, though, I thought that I should check my solution on the Advent of Code site. I thought that I would get “this is not correct; it is too high”, but it turned out to be the correct answer.
defmodule Day12 do
def part1(input) do
{presents, regions} = parse(input)
regions
|> Enum.reject(fn region ->
presents_fit?(region, presents)
end)
|> Enum.count
end
defp presents_fit?(region, presents) do
num_units = Enum.map(presents, fn {index, [shape | _]} ->
Enum.reduce(shape, [], &(&1++&2))
|> Enum.count(&(&1 === ?\#))
|> then(&{index, &1})
end)
|> Map.new
{{width, height}, qs} = region
area = width * height
qs
|> Enum.with_index
|> Enum.map(fn {q, index} ->
q * Map.get(num_units, index)
end)
|> Enum.sum
|> then(fn needed ->
if needed > area do
# Can't possibly fit.
true
else
# Might fit; must check.
brute_force_solution(region, presents)
end
end)
end
defp brute_force_solution(_region, _presents) do
# Turns out that for all inputs the presents will always fit.
false
end
defp rot(grid) do
Enum.zip_with(grid, &Enum.reverse/1)
end
defp hflip(grid) do
Enum.map(grid, &Enum.reverse/1)
end
defp vflip(grid) do
Enum.reverse(grid)
end
defp transpose(list) do
Enum.zip_with(list, &Function.identity/1)
end
defp parse(input) do
{presents, [regions]} = Enum.split(input, length(input) - 1)
presents = Enum.map(presents, fn present ->
[index | grid] = String.split(present, "\n", trim: true)
grid = Enum.map(grid, &String.to_charlist/1)
grids = Enum.flat_map_reduce(1..4, grid, fn _, grid ->
{[grid, hflip(grid), vflip(grid)], rot(grid)}
end)
|> elem(0)
|> Enum.uniq
index = index
|> String.trim(":")
|> String.to_integer
if true do
IO.puts("#{index}:")
grids
|> transpose
|> Enum.each(fn grid ->
Enum.each(grid, &IO.write("#{&1} "))
IO.puts ""
end)
IO.puts ""
end
{index, grids}
end)
regions = regions
|> String.split("\n", trim: true)
|> Enum.map(fn region ->
[size, qs] = String.split(region, ": ")
size = size
|> String.split("x")
|> Enum.map(&String.to_integer(&1))
|> List.to_tuple
qs = String.split(qs, " ")
|> Enum.map(&String.to_integer(&1))
{size, qs}
end)
|> Enum.sort
{presents, regions}
end
end
. Anyway thanks.
