Add pow10, log10i and digits10 functions

These are used by a couple of solutions now.

lib/aoc.ml

@@ -66,3 +66,18 @@ module Grid = struct
       (String.sub grid.grid (idx + 1) (length grid - idx - 1));
     { grid with grid = Buffer.contents builder }
 end
+
+let log10i i =
+  let rec impl acc = function 0 -> acc | x -> impl (acc + 1) (x / 10) in
+  assert (i > 0);
+  impl ~-1 i
+
+let digits10 = function
+  | 0 -> 1
+  | n when n > 0 -> 1 + log10i n
+  | n (* when n < 0 *) -> 1 + log10i (-n)
+
+let pow10 n =
+  let rec impl acc = function 0 -> acc | x -> impl (acc * 10) (x - 1) in
+  assert (n >= 0);
+  impl 1 n