Day 5 with two linked lists. Really fast.

I fully blame betseg for this, we had an hour of discussion whether you
could make this O(n). Turns out, you can.

day5_doublelinked.exs

@@ -0,0 +1,53 @@
+defmodule DayFive do
+  def part_one(input) do
+    shortened = fully_react_new(input)
+    # IO.puts shortened
+    (String.length shortened) - 1
+  end
+
+  def fully_react(input) do
+    replacer = ~r/Aa|Bb|Cc|Dd|Ee|Ff|Gg|Hh|Ii|Jj|Kk|Ll|Mm|Nn|Oo|Pp|Qq|Rr|Ss|Tt|Uu|Ww|Vv|Xx|Yy|Zz|aA|bB|cC|dD|eE|fF|gG|hH|iI|jJ|kK|lL|mM|nN|oO|pP|qQ|rR|sS|tT|uU|wW|vV|xX|yY|zZ/
+    replace_forever(input, replacer, "")
+  end
+
+
+  def replace_forever(string, pattern, replacement) do
+    if Regex.match?(pattern, string) do
+      replace_forever(Regex.replace(pattern, string, replacement), pattern, replacement)
+    else
+      string
+    end
+  end
+
+  def react([<< char1 >> , << char2 >> | tail], rev) when char1 == char2 - 32 or char2 == char1 - 32, do: react(tail, rev)
+  def react([<< char1 >> | tail], [<< char2 >> | rev]) when char1 == char2 - 32 or char2 == char1 - 32, do: react(tail, rev)
+  def react([char | tail], rev), do: react(tail, [char | rev])
+  def react("", rev), do: rev
+  def react([], rev), do: rev
+
+  def fully_react_new(input) do
+    # IO.puts "mark"
+    String.reverse(Enum.join(react(String.codepoints(input), [])))
+  end
+
+  #def fully_react_new(input, acc) when input == acc, do: acc
+  #def fully_react_new(input, _acc) do
+    # IO.puts("#{length _acc} #{length input}")
+    #  fully_react_new(react(input), input)
+  #end
+
+  def part_two(input) do
+    chars = Enum.map(?a..?z, &(<< &1 >>))
+    [{_char, poly} | _ ] = Enum.map(chars, fn char -> {char, String.replace(input, char, "")} end)
+        |> Enum.map(fn {char, poly} -> {char, String.replace(poly, String.upcase(char), "") } end)
+        |> Enum.map(fn {char, poly} -> {char, fully_react_new(poly)} end)
+        |> Enum.sort_by(fn {_char, poly} -> String.length(poly) end)
+    # IO.puts poly
+    String.length(poly) - 1
+  end
+end
+
+{:ok, content} = File.read "input5.txt"
+
+IO.puts "Part 1: #{DayFive.part_one(content)}"
+IO.puts "Part 2: #{DayFive.part_two(content)}"