From c8fa4449b27bd900b9818473f53d8165782dc99e Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sun, 13 Dec 2020 23:32:42 -0800
Subject: [PATCH] Day 14 Part 2

---
 2020/day14/day14.ex | 113 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 87 insertions(+), 26 deletions(-)

diff --git a/2020/day14/day14.ex b/2020/day14/day14.ex
index c388f74..3b7a5b1 100644
--- a/2020/day14/day14.ex
+++ b/2020/day14/day14.ex
@@ -6,10 +6,10 @@ defmodule Day14 do
 
   defp test_input do
     """
-    mask = XXXXXXXXXXXXXXXXXXXXXXXXXXXXX1XXXX0X
-    mem[8] = 11
-    mem[7] = 101
-    mem[8] = 0
+    mask = 000000000000000000000000000000X1001X
+    mem[42] = 100
+    mask = 00000000000000000000000000000000X0XX
+    mem[26] = 1
     """
   end
 
@@ -31,28 +31,15 @@ defmodule Day14 do
     end)
   end
 
-  defp pow(_base, 0) do
-    1
-  end
-
-  defp pow(_base, exp) when exp < 0 do
-    raise ArithmeticError, "negative exponents not supported"
-  end
-
-  defp pow(base, exp) when exp > 0 do
-    List.duplicate(base, exp)
-    |> Enum.reduce(1, &(&1 * &2))
-  end
-
   defp parse_mask(mask) do
     String.codepoints(mask)
     |> Enum.reverse()
     |> Enum.with_index()
     |> Enum.reduce([], fn {char, idx}, ops ->
       case char do
-        "X" -> ops
-        "0" -> [{:and, Bitwise.bnot(pow(2, idx))} | ops]
-        "1" -> [{:or, pow(2, idx)} | ops]
+        "X" -> [{:float, idx} | ops]
+        "0" -> [{:unset, idx} | ops]
+        "1" -> [{:set, idx} | ops]
       end
     end)
   end
@@ -61,12 +48,16 @@ defmodule Day14 do
     num
   end
 
-  defp apply_mask([{:and, val} | rest], num) do
-    apply_mask(rest, Bitwise.band(val, num))
+  defp apply_mask([{:float, _} | rest], num) do
+    apply_mask(rest, num)
   end
 
-  defp apply_mask([{:or, val} | rest], num) do
-    apply_mask(rest, Bitwise.bor(val, num))
+  defp apply_mask([{:unset, idx} | rest], num) do
+    apply_mask(rest, Bitwise.band(num, Bitwise.bnot(Bitwise.bsl(1, idx))))
+  end
+
+  defp apply_mask([{:set, idx} | rest], num) do
+    apply_mask(rest, Bitwise.bor(num, Bitwise.bsl(1, idx)))
   end
 
   defp run([], {_, memory}) do
@@ -93,10 +84,80 @@ defmodule Day14 do
     |> Enum.sum()
   end
 
+  def part2 do
+    read_input()
+    |> parse_program()
+    |> run_v2({nil, %{}})
+    |> Enum.map(fn {_, val} -> val end)
+    |> Enum.sum()
+  end
+
+  defp run_v2([], {_, memory}) do
+    memory
+  end
+
+  defp run_v2([{:mask, mask} | rest], {_, memory}) do
+    run_v2(rest, {mask, memory})
+  end
+
+  defp run_v2([{:mem, addr, value} | rest], {mask, memory}) do
+    addrs = expand_float(mask, [apply_mask_v2(mask, addr)])
+
+    memory =
+      Enum.reduce(addrs, memory, fn addr, mem ->
+        Map.put(mem, addr, value)
+      end)
+
+    run_v2(rest, {mask, memory})
+  end
+
+  defp expand_float([], addrs) do
+    addrs
+  end
+
+  defp expand_float([{:float, idx} | rest], addrs) do
+    addrs =
+      Enum.reduce(addrs, [], fn addr, expanded ->
+        [
+          apply_mask([{:unset, idx}], addr),
+          apply_mask([{:set, idx}], addr)
+        ] ++ expanded
+      end)
+
+    expand_float(rest, addrs)
+  end
+
+  defp expand_float([{:set, _} | rest], addrs) do
+    expand_float(rest, addrs)
+  end
+
+  defp expand_float([{:unset, _} | rest], addrs) do
+    expand_float(rest, addrs)
+  end
+
+  defp apply_mask_v2([], num) do
+    num
+  end
+
+  defp apply_mask_v2([{:float, _} | rest], num) do
+    apply_mask_v2(rest, num)
+  end
+
+  defp apply_mask_v2([{:unset, _} | rest], num) do
+    apply_mask_v2(rest, num)
+  end
+
+  defp apply_mask_v2([{:set, idx} | rest], num) do
+    apply_mask_v2(rest, Bitwise.bor(num, Bitwise.bsl(1, idx)))
+  end
+
   def debug do
-    nil
+    mask = parse_mask("000000000000000000000000000000X1001X")
+    addr = 42
+    expand_float(mask, [apply_mask_v2(mask, addr)])
   end
 end
 
 Day14.part1() |> IO.inspect()
-Day14.debug() |> IO.inspect()
+Day14.part2() |> IO.inspect()
+# Day14.debug() |> IO.inspect()