2023 day 6

lib/2023_day6.ex

@@ -0,0 +1,74 @@
+defmodule Mix.Tasks.Day6 do
+  use Mix.Task
+
+  def parse(input, :part1) do
+    input
+    |> Enum.map(fn s ->
+      String.split(s, " ")
+      |> Enum.reject(fn s -> s === "" or s === "Time:" or s === "Distance:" end)
+    end)
+    |> Enum.map(fn i ->
+      i
+      |> Enum.map(fn s ->
+        s |> String.trim() |> String.to_integer()
+      end)
+    end)
+  end
+
+  def parse(input, :part2) do
+    input
+    |> Enum.map(fn s ->
+      String.split(s, " ")
+      |> Enum.reject(fn s -> s === "" or s === "Time:" or s === "Distance:" end)
+    end)
+    |> Enum.map(fn u ->
+      u
+      |> Enum.join("")
+      |> String.trim()
+      |> String.to_integer()
+    end)
+  end
+
+  def distance_traveled_in_n_seconds(seconds, duration) do
+    seconds * (duration - seconds)
+  end
+
+  def polynomial_roots(a, b, c) do
+    [
+      (-b + :math.sqrt(b ** 2 - 4 * a * c)) / (2 * a),
+      (-b - :math.sqrt(b ** 2 - 4 * a * c)) / (2 * a)
+    ]
+  end
+
+  def solve([time, distance], p) when p === :part2 do
+    # number of negative integer solutions to the polynomial
+    # x^2 - tx + d < 0
+    # where x is time spent holding button, t is the duration of the race (seconds), and d is the distance to beat
+    roots = polynomial_roots(1, -time, distance)
+    max = roots |> Enum.max()
+    min = roots |> Enum.min()
+    floor(max) - floor(min)
+  end
+
+  def solve([times, distances], p) when p === :part1 do
+    for {time, index} <- times |> Enum.with_index(),
+        seconds <- 1..(time - 1),
+        distance_traveled_in_n_seconds(seconds, time) > distances |> Enum.at(index),
+        reduce: %{} do
+      acc ->
+        acc |> Map.update(time, [seconds], fn u -> [seconds | u] end)
+    end
+    |> Map.values()
+    |> Enum.reduce(
+      1,
+      fn a, acc ->
+        acc * (a |> Enum.count())
+      end
+    )
+  end
+
+  def run(args) do
+    part = if args |> Enum.at(0) == "1", do: :part1, else: :part2
+    IO.stream() |> parse(part) |> solve(part) |> IO.puts()
+  end
+end