Day 5 Part 1

2020/day5/day5.ex

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+defmodule Day5 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp half_range({lo, hi}, half) do
+    halfway = lo + div(hi + 1 - lo, 2)
+    lower = {lo, halfway - 1}
+    upper = {halfway, hi}
+
+    case half do
+      :lower -> lower
+      :upper -> upper
+    end
+  end
+
+  defp parse_seat(line) do
+    %{row: {r, r}, col: {c, c}} =
+      String.codepoints(line)
+      |> Enum.reduce(%{row: {0, 127}, col: {0, 7}}, fn char, %{row: row, col: col} ->
+        case char do
+          "F" -> %{row: half_range(row, :lower), col: col}
+          "B" -> %{row: half_range(row, :upper), col: col}
+          "L" -> %{row: row, col: half_range(col, :lower)}
+          "R" -> %{row: row, col: half_range(col, :upper)}
+        end
+      end)
+
+    {r, c}
+  end
+
+  defp seat_id({r, c}) do
+    r * 8 + c
+  end
+
+  def part1 do
+    read_input()
+    |> String.split()
+    |> Enum.map(&parse_seat/1)
+    |> Enum.map(&seat_id/1)
+    |> Enum.max()
+  end
+end
+
+Day5.part1() |> IO.inspect()