From c4da223576f4fb940e4c854b23cea283a6398f0b Mon Sep 17 00:00:00 2001
From: Alexandre Jesus <adbjesus@gmail.com>
Date: Sun, 8 Dec 2024 21:15:19 +0000
Subject: Day 8
---
src/day08.exs | 134 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 134 insertions(+)
create mode 100644 src/day08.exs
diff --git a/src/day08.exs b/src/day08.exs
new file mode 100644
index 0000000..0d312e1
--- /dev/null
+++ b/src/day08.exs
@@ -0,0 +1,134 @@
+defmodule Day08 do
+ defmodule Grid do
+ defstruct [:grid, :height, :width]
+
+ def from_data(data) do
+ grid = data
+ |> String.split("\n", trim: true)
+ |> Enum.map(& &1 |> String.codepoints() |> :array.from_list())
+ |> :array.from_list()
+
+ height = :array.size(grid)
+
+ width = :array.size(:array.get(0, grid))
+
+ %Grid{grid: grid, height: height, width: width}
+ end
+
+ def at(grid, i, j) do
+ :array.get(j, :array.get(i, grid.grid))
+ end
+
+ def in?(grid, i, j) do
+ i >= 0 and i < grid.height and j >= 0 and j < grid.width
+ end
+
+ def find_elements(grid, regex) do
+ for i <- 0..grid.height-1,
+ j <- 0..grid.width-1,
+ v = at(grid, i, j) do
+ if String.match?(v, regex) do
+ {v, i, j}
+ end
+ end
+ |> Enum.filter(& &1 != nil)
+ end
+ end
+
+ def part1(data) do
+ grid = data
+ |> Grid.from_data()
+
+ antennas = grid
+ |> Grid.find_elements(~r/[[:alnum:]]/)
+ |> Enum.group_by(fn {k, _, _} -> k end, fn {_, i, j} -> {i, j} end)
+
+ antennas
+ |> Enum.map(fn {_, a} -> antinodes([], a) end)
+ |> Enum.flat_map(& &1)
+ |> Enum.uniq()
+ |> Enum.filter(fn {i, j} -> Grid.in?(grid, i, j) end)
+ |> Enum.count()
+ end
+
+ defp antinodes(acc, [a | rest]) do
+ rest
+ |> Enum.reduce(acc, fn b, acc -> antinodes(acc, a, b) end)
+ |> antinodes(rest)
+ end
+
+ defp antinodes(acc, []), do: acc
+
+ defp antinodes(acc, {ai, aj}, {bi, bj}) do
+ di = abs(ai - bi)
+ dj = abs(aj - bj)
+ [{min(ai, bi) - di, if aj < bj do aj - dj else aj + dj end} |
+ [{max(ai, bi) + di, if aj < bj do bj + dj else bj - dj end} |
+ acc]]
+ end
+
+ def part2(data) do
+ grid = data
+ |> Grid.from_data()
+
+ antennas = grid
+ |> Grid.find_elements(~r/[[:alnum:]]/)
+ |> Enum.group_by(fn {k, _, _} -> k end, fn {_, i, j} -> {i, j} end)
+
+ antennas
+ |> Enum.map(fn {_, a} -> antinodes2([], a, grid) end)
+ |> Enum.flat_map(& &1)
+ |> Enum.uniq()
+ |> Enum.count()
+
+ end
+
+ defp antinodes2(acc, [a | rest], grid) do
+ rest
+ |> Enum.reduce(acc, fn b, acc -> antinodes2(acc, a, b, grid) end)
+ |> antinodes2(rest, grid)
+ end
+
+ defp antinodes2(acc, [], _), do: acc
+
+ defp antinodes2(acc, {ai, aj}, {bi, bj}, grid) do
+ di = abs(ai - bi)
+ dj = abs(aj - bj)
+
+ cond do
+ di == 0 ->
+ acc
+ |> antinodes2(ai, aj, 0, -1, grid)
+ |> antinodes2(ai, aj, 0, 1, grid)
+ dj == 0 ->
+ acc
+ |> antinodes2(ai, aj, -1, 0, grid)
+ |> antinodes2(ai, aj, 1, 0, grid)
+ true ->
+ g = Integer.gcd(di, dj)
+ di = div(di, g)
+ dj = div(dj, g)
+ acc
+ |> antinodes2(ai, aj, -di, if aj < bj do -dj else dj end, grid)
+ |> antinodes2(ai, aj, di, if aj < bj do dj else -dj end, grid)
+ end
+ end
+
+ defp antinodes2(acc, i, j, di, dj, grid) do
+ if Grid.in?(grid, i, j) do
+ antinodes2([{i, j} | acc], i+di, j+dj, di, dj, grid)
+ else
+ acc
+ end
+ end
+end
+
+data = IO.read(:stdio, :eof)
+
+{time1 , ans1} = :timer.tc(fn -> Day08.part1(data) end)
+IO.puts("Time : #{time1 / 1000000}")
+IO.puts("Answer: #{ans1}")
+
+{time2 , ans2} = :timer.tc(fn -> Day08.part2(data) end)
+IO.puts("Time : #{time2 / 1000000}")
+IO.puts("Answer: #{ans2}")
--
cgit v1.2.3