restructure + lecutre 07

lectures/07/sieve.ml

@@ -0,0 +1,16 @@
+let isqrt n = n |> float_of_int |> sqrt |> int_of_float;;
+
+let sieve n =
+  let is_prime = Array.make n true in
+  for i = 2 to isqrt n do
+    if is_prime.(i) then
+      let j = ref i in
+      while i * !j < n do
+        is_prime.(i * !j) <- false;
+        incr j
+      done
+  done;
+  is_prime.(0) <- false;
+  is_prime.(1) <- false;
+  is_prime |> Array.to_list |> List.mapi (fun i b -> (i, b)) |>
+  List.filter_map (fun (i, b) -> if b then Some i else None);;