add problem 25 solution

lib/euler/algebra.ex

@@ -23,6 +23,27 @@ defmodule Euler.Algebra do
   Returns the sum of the digits of `n`.
   """
   def digit_sum(n), do: digit_sum(n, 0)
+
+  def fib(0), do: 1
+  def fib(1), do: 1
+  def fib(n) do
+    cache = Process.get(:fib, %{})
+    case Map.get(cache, n, nil) do
+      nil ->
+        value = fib(n - 1) + fib(n - 2)
+        cache = Process.get(:fib, %{}) |> Map.put_new(n, value)
+        Process.put(:fib, cache)
+        value
+      m -> m
+    end
+  end
+
+  def num_digits(0, acc), do: acc
+  def num_digits(n, acc), do: num_digits(div(n,10),acc+1)
+  def num_digits(0), do: 1
+  def num_digits(n) do
+    num_digits(n, 0)
+  end
 end
 
 

solutions/25/solution.exs

@@ -0,0 +1,9 @@
+import Euler.Algebra, only: [fib: 1, num_digits: 1]
+
+Stream.iterate(1, &(&1 + 1))
+|> Stream.map(&{&1, fib(&1)})
+|> Stream.filter(&num_digits(elem(&1, 1)) >= 1000)
+|> Enum.take(1)
+|> Enum.map(&elem(&1, 0) + 1)
+|> Enum.at(0)
+|> IO.puts()