ocaml: phase 5.1 paren_depth.ml baseline (max nesting depth, 3+3+1 = 7)

One-pass walk tracking current depth and a high-water mark:

  let max_depth s =
    let d = ref 0 in
    let m = ref 0 in
    for i = 0 to String.length s - 1 do
      if s.[i] = '(' then begin
        d := !d + 1;
        if !d > !m then m := !d
      end
      else if s.[i] = ')' then d := !d - 1
    done;
    !m

Three inputs:
  '((1+2)*(3-(4+5)))'   3   (innermost (4+5) at depth 3)
  '(((deep)))'           3
  '()()()'               1   (no nesting)
  sum                    7

Tests for-loop char comparison s.[i] = '(' and the high-water-mark
idiom with two refs.

78 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -52,6 +52,7 @@
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,
+  "paren_depth.ml": 7,
   "pancake_sort.ml": 910,
   "pascal.ml": 252,
   "pi_leibniz.ml": 314,