From 550b652b45bdef5c88053d435fe8bf3b6542201a Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Sat, 2 Sep 2023 03:08:50 +0900
Subject: [PATCH] Add 2015 Day4

---
 2015/day4.exs | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 2015/day4.exs

diff --git a/2015/day4.exs b/2015/day4.exs
new file mode 100644
index 0000000..6e630f9
--- /dev/null
+++ b/2015/day4.exs
@@ -0,0 +1,50 @@
+defmodule Day4 do
+  def part1(input), do: mine(input, 1, "00000")
+
+  def mine(input, nonce, prefix) do
+    hash = :crypto.hash(:md5, input <> Integer.to_string(nonce)) |> Base.encode16()
+    if String.starts_with?(hash, prefix), do: nonce, else: mine(input, nonce + 1, prefix)
+  end
+
+  def part2(input), do: mine(input, 1, "000000")
+
+  def input do
+    with [input] <- System.argv() do
+      input
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{result}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day4.exs input")
+  end
+end
+
+Day4.run()