Day 10. Easier than yesterday I think

.gitignore

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+*.txt
+main
+.DS_Store

day10/main.exs

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+defmodule Day10 do
+  def run do
+    map =
+      File.read!("data.txt")
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.map(fn el -> String.graphemes(el) |> Enum.map(&String.to_integer/1) end)
+
+    height = length(map)
+    width = length(hd(map))
+
+    trailheads =
+      Enum.concat(map)
+      |> Enum.with_index()
+      |> Enum.filter(fn {el, _} -> el == 0 end)
+      |> Enum.map(fn {_, idx} -> {div(idx, height), rem(idx, width)} end)
+
+    paths1 =
+      Enum.reduce(trailheads, 0, fn th, acc ->
+        (walk(th, map) |> List.flatten() |> Enum.uniq() |> length) + acc
+      end)
+
+    dbg(paths1)
+
+    # same as part one, just don't uniq() them
+    paths2 =
+      Enum.reduce(trailheads, 0, fn th, acc ->
+        (walk(th, map) |> List.flatten() |> length) + acc
+      end)
+
+    dbg(paths2)
+  end
+
+  defp walk(coord, map) do
+    value = element_at(coord, map)
+
+    if value == 9 do
+      coord
+    else
+      # find the coords of all neighbors
+      neighbors(coord)
+      # filter by whether the element is 1 greater
+      |> Enum.filter(fn c -> element_at(c, map) == value + 1 end)
+      # add this path to the list. it'll be empty and then flattened away if it doesn't reach the top
+      |> Enum.reduce([], fn el, acc -> [walk(el, map) | acc] end)
+    end
+  end
+
+  defp neighbors({x, y}) do
+    [{-1, 0}, {0, 1}, {1, 0}, {0, -1}]
+    |> Enum.map(fn {dx, dy} -> {x + dx, y + dy} end)
+    |> Enum.reject(fn {x2, y2} -> x2 < 0 or y2 < 0 end)
+  end
+
+  defp element_at({x, y}, map) do
+    row = Enum.at(map, x)
+    if is_nil(row), do: nil, else: Enum.at(row, y)
+  end
+end
+
+Day10.run()