From c489f6e77ae9d8b56398fe9ca652ddc8f40b6dde Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Thu, 26 Dec 2024 20:48:14 -0700
Subject: [PATCH] Day 22 again. FINALLY. Just needed a map

---
 day22/main.exs | 96 +++++++++++---------------------------------------
 1 file changed, 21 insertions(+), 75 deletions(-)

diff --git a/day22/main.exs b/day22/main.exs
index b111b4d..bfdee2d 100644
--- a/day22/main.exs
+++ b/day22/main.exs
@@ -13,89 +13,44 @@ defmodule Day22 do
 
     dbg(part1)
 
-    # lines = [123, 123]
-
-    {prices, diffs} =
-      Enum.reduce(lines, {[], []}, fn secret, {prices_acc, diffs_acc} ->
-        {_, prices, diffs} =
-          Enum.reduce(1..2000, {secret, [price(secret)], []}, fn _, {s, prices, diffs} ->
+    # build a map of all 4-digit sequences + list index to prices
+    map =
+      Enum.with_index(lines)
+      |> Enum.reduce(%{}, fn {secret, idx}, map ->
+        {_, prices, diffs, map2} =
+          Enum.reduce(1..2000, {secret, [price(secret)], [], map}, fn _, {s, prices, diffs, m} ->
             new_secret = evolve(s)
             price = price(new_secret)
             diff = price - hd(prices)
-            {new_secret, [price | prices], [diff | diffs]}
+            new_diff = [diff | diffs]
+            sec = Enum.slice(new_diff, 0, 4)
+            new_map = Map.put_new(m, {sec, idx}, price)
+            {new_secret, [price | prices], new_diff, new_map}
           end)
 
-        {[prices | prices_acc], [diffs | diffs_acc]}
-      end)
-
-    dbg({prices, diffs})
-
-    # max length of all price lists
-    max = Enum.map(prices, &length/1) |> Enum.max()
-
-    # sort the lists by price, keeping their original index
-    sorted_by_price_with_index =
-      Enum.map(prices, fn prices ->
-        prices |> Enum.with_index() |> Enum.sort(fn {p1, _}, {p2, _} -> p1 >= p2 end)
+        map2
       end)
-      |> Enum.with_index()
 
-    # dbg(sorted_by_price_with_index)
+    # for every possible sequence (there's only like 19 ^ 4 or something)
+    all = for a <- -9..9, b <- -9..9, c <- -9..9, d <- -9..9, do: [a, b, c, d]
 
-    # go from 0 up to the max list length of the lists sorted by price, highest to lowest
-    # grab the 4-sequence of each list
-    # check each sequence against each list, add up the prices
-    # if that price is greater than the max acc price, that's the new winning sequence
     part2 =
-      Enum.reduce(0..(max - 1), 0, fn index, acc ->
-        dbg({"checking all lists at index", index})
-        dbg({"current acc", acc})
-
-        res =
-          Enum.reduce(sorted_by_price_with_index, {0, []}, fn {price_list, list_index},
-                                                              {max_price, max_seq} ->
-            {price, original_index} = Enum.at(price_list, index)
-            # dbg({diffs, index})
-            seq = Enum.at(diffs, list_index) |> Enum.slice(original_index, 4)
-            # dbg({price, seq})
-
-            if length(seq) == 4 do
-              max_price = score_seq(prices, diffs, seq)
-              # dbg({"max_price", max_price})
-
-              {max_price, seq}
-            else
-              {max_price, max_seq}
+      Enum.reduce(all, {0, []}, fn sec, {max, max_sec} ->
+        # and for every buyer, get the price at this sequence
+        val =
+          Enum.reduce(0..length(lines), 0, fn idx, acc ->
+            case Map.get(map, {sec, idx}, nil) do
+              nil -> acc
+              val -> val + acc
             end
           end)
 
-        if res > acc do
-          dbg({"new high", res})
-          res
-        else
-          acc
-        end
+        if val > max, do: {val, sec}, else: {max, max_sec}
       end)
 
     dbg(part2)
   end
 
-  defp score_seq(prices, diffs, seq) do
-    # dbg({"score seq", seq})
-
-    res =
-      Enum.zip(prices, diffs)
-      |> Enum.reduce(0, fn {p, d}, acc ->
-        case index_of_sublist(d, seq) do
-          nil -> acc
-          idx -> Enum.at(p, idx) + acc
-        end
-      end)
-
-    res
-    # dbg(res)
-  end
-
   defp evolve(secret) do
     step1 = secret |> Kernel.*(64) |> mix(secret) |> prune()
     step2 = step1 |> div(32) |> mix(step1) |> prune()
@@ -113,15 +68,6 @@ defmodule Day22 do
   defp price(num) do
     Integer.digits(num) |> Enum.take(-1) |> Integer.undigits()
   end
-
-  # finds the index of a sequence in a list, nil if it isn't in there
-  defp index_of_sublist(parent, sublist) do
-    Enum.reduce_while(0..(length(parent) - length(sublist)), nil, fn idx, acc ->
-      if Enum.slice(parent, idx, length(sublist)) == sublist,
-        do: {:halt, idx},
-        else: {:cont, acc}
-    end)
-  end
 end
 
 Day22.run()