From c29f204df06bc11887479f1665fee70c06887b6b Mon Sep 17 00:00:00 2001
From: Justin Wernick <j.wernick@eyeo.com>
Date: Tue, 24 Sep 2024 22:35:38 +0200
Subject: Day 11

---
 2015/day11.exs | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 2015/day11.exs

(limited to '2015/day11.exs')

diff --git a/2015/day11.exs b/2015/day11.exs
new file mode 100644
index 0000000..1801b00
--- /dev/null
+++ b/2015/day11.exs
@@ -0,0 +1,87 @@
+start = String.trim(File.read!("inputs/day11.txt"))
+
+startCharlist = String.to_charlist(start)
+startNums = Enum.reverse(startCharlist)
+
+defmodule A do
+  def incrementPassword([head | tail]) do
+    if decreasingStraight?(tail) do
+      simpleIncrementPassword([head | tail])
+    else
+      [second | _] = tail
+
+      if head < second && second < ?z && !(second >= ?h && second <= ?n) do
+        [second + 1 | tail]
+      else
+        [?a | simpleIncrementPassword(tail)]
+      end
+    end
+  end
+
+  def simpleIncrementPassword([?z | tail]) do
+    [?a | simpleIncrementPassword(tail)]
+  end
+
+  def simpleIncrementPassword([?h | tail]) do
+    [?j | tail]
+  end
+
+  def simpleIncrementPassword([?n | tail]) do
+    [?p | tail]
+  end
+
+  def simpleIncrementPassword([?k | tail]) do
+    [?m | tail]
+  end
+
+  def simpleIncrementPassword([head | tail]) do
+    [head + 1 | tail]
+  end
+
+  def decreasingStraight?([]) do
+    false
+  end
+
+  def decreasingStraight?([_]) do
+    false
+  end
+
+  def decreasingStraight?([_, _]) do
+    false
+  end
+
+  def decreasingStraight?([a, b, c | _]) when b === a - 1 and c === a - 2 do
+    true
+  end
+
+  def decreasingStraight?([_ | tail]) do
+    decreasingStraight?(tail)
+  end
+
+  def twoDifferentPairs?(list) do
+    differentPairs =
+      Enum.zip(list, Enum.drop(list, 1))
+      |> Enum.flat_map(fn
+        {a, a} -> [a]
+        {_, _} -> []
+      end)
+      |> Enum.dedup()
+      |> Enum.count()
+
+    differentPairs >= 2
+  end
+end
+
+validPasswords =
+  Stream.iterate(startNums, &A.incrementPassword(&1))
+  |> Stream.filter(fn password ->
+    A.decreasingStraight?(password) &&
+      A.twoDifferentPairs?(password)
+  end)
+
+nextPasswords =
+  Enum.take(validPasswords, 2)
+  |> Enum.map(&Enum.reverse(&1))
+  |> Enum.join("\n")
+
+IO.puts("New passwords:\n#{nextPasswords}")
-- 
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