lab02 q2

labs/02/primes.exs

@@ -0,0 +1,42 @@
+defmodule Primes do
+  defp sieve([], acc, _) do
+    Enum.reverse(acc)
+  end
+
+  defp sieve([h | t], acc, n) do
+    if h * h > n do
+      Enum.reverse(acc) ++ [h | t]
+    else
+      sieve(
+        Enum.filter(t, fn x -> rem(x, h) != 0 end),
+        [h | acc],
+        n)
+    end
+  end
+
+  def primes(n) do
+    l = Enum.to_list(2..n)
+    sieve(l, [], n)
+  end
+
+  defp largest_permutation_set(p) do
+    p |> Enum.group_by(fn x -> x
+      |> Integer.to_string()
+      |> String.to_charlist()
+      |> Enum.sort() end)
+      |> Map.values()
+      |> Enum.max_by(&length/1)
+  end
+
+  def run(n, d) do
+    p = primes(n)
+    p = Enum.filter(p, fn x ->
+      String.length(Integer.to_string(x)) >= d
+    end)
+
+    IO.puts(length(p))
+    IO.puts(length(largest_permutation_set(p)))
+  end
+end
+
+Primes.run(1_000_000, 6);