From a5579093468eeaad610e59e0aded5be3a7cccb93 Mon Sep 17 00:00:00 2001
From: Caleb Webber <caleb@codingthemsoftly.com>
Date: Sat, 1 Mar 2025 21:48:42 -0500
Subject: [PATCH] add solution for problem 23

---
 problems/23/solution.exs | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)
 create mode 100644 problems/23/solution.exs

diff --git a/problems/23/solution.exs b/problems/23/solution.exs
new file mode 100644
index 0000000..0d11334
--- /dev/null
+++ b/problems/23/solution.exs
@@ -0,0 +1,38 @@
+defmodule AmicableNumbers do
+  def divisors(n) do
+    [ 1 |
+    (for i <- 2..floor(:math.sqrt(n))//1,
+      j = div(n, i),
+      j == n / i do
+        if i !== j, do: [i,j], else: [i]
+    end |> Enum.flat_map(&(&1))) ]
+  end
+end
+
+defmodule Abundance do
+  def is_abundant?(n) do
+    cache = Process.get(:abundance, %{})
+
+    case cache |> Map.get(n) do
+      nil ->
+        sum = AmicableNumbers.divisors(n) |> Enum.sum()
+        is? = sum > n
+        Process.put(:abundance, Map.put(cache, n, is?))
+        is?
+      is? -> is? 
+    end
+  end
+end
+
+# sum of all numbers that can be expressed as sum of two abundant numbers
+sum = for n <- 1..28123,
+  mid = div(n, 2),
+  Enum.any?(1..mid, &(Abundance.is_abundant?(&1) and Abundance.is_abundant?(n - &1))),
+  reduce: 0
+  do
+    s -> s + n
+end
+
+(((28123*28124)/2) - sum)
+|> IO.puts()
+