From 397d69c2ba8bb5c4136a6637f515e69ca69b5d95 Mon Sep 17 00:00:00 2001
From: Sloane Perrault <sloane.perrault@gmail.com>
Date: Wed, 21 Sep 2022 09:19:53 -0400
Subject: [PATCH] refactor 2021 day 7

---
 2021/lib/2021/7.ex | 42 ++++++++++++++++++++++--------------------
 1 file changed, 22 insertions(+), 20 deletions(-)

diff --git a/2021/lib/2021/7.ex b/2021/lib/2021/7.ex
index 16b986c..df71cbb 100644
--- a/2021/lib/2021/7.ex
+++ b/2021/lib/2021/7.ex
@@ -3,32 +3,34 @@ import AOC
 aoc 2021, 7 do
   use AOCHelpers
 
-  def p1 do
-    crabs = input_number_list()
+  def crabs(), do: input_number_list()
 
-    max = Enum.max(crabs)
+  def p1, do: cost_of_most_efficient_position(&Function.identity/1)
 
-    for position <- 1..max do
-      for crab <- crabs do
-        abs(crab - position)
-      end
-      |> Enum.sum()
-    end
-    |> Enum.min()
+  def p2, do: cost_of_most_efficient_position(&(1 / 2 * &1 * (&1 + 1)))
+
+  def total_fuel_cost_for_position(crab_freqs, pos, cost_fun) do
+    crab_freqs
+    |> Enum.reduce(0, fn {crab_pos, num_crab}, partial_cost ->
+      linear_distance = abs(crab_pos - pos)
+
+      partial_cost + num_crab * cost_fun.(linear_distance)
+    end)
   end
 
-  def p2 do
-    crabs = input_number_list()
+  def cost_of_most_efficient_position(cost_fun) when is_function(cost_fun) do
+    crab_positions = input_number_list()
 
-    max = Enum.max(crabs)
+    max_crab_position = Enum.max(crab_positions)
+    crab_frequencies = Enum.frequencies(crab_positions)
 
-    for position <- 1..max do
-      for crab <- crabs do
-        n = abs(crab - position)
-        1 / 2 * n * (n + 1)
-      end
-      |> Enum.sum()
-    end
+    crab_frequencies
+    |> cost_of_most_efficient_position(1..max_crab_position, cost_fun)
+  end
+
+  def cost_of_most_efficient_position(crab_freqs, range, cost_fun) do
+    range
+    |> Enum.map(&total_fuel_cost_for_position(crab_freqs, &1, cost_fun))
     |> Enum.min()
     |> trunc()
   end