ocaml: phase 5.1 next_greater.ml baseline (monotonic stack, sum 153)

Right-to-left monotonic stack for next-greater-element:

  for i = n - 1 downto 0 do
    while (match !stack with [] -> false | h :: _ -> h <= arr.(i)) do
      stack := List.tl !stack
    done;
    (match !stack with
     | [] -> res.(i) <- -1
     | h :: _ -> res.(i) <- h);
    stack := arr.(i) :: !stack
  done

For [4; 5; 2; 25; 7; 8; 1; 30; 12]:
  results: [5; 25; 25; 30; 8; 30; 30; -1; -1]
  sum of non-negative = 5+25+25+30+8+30+30 = 153

Tests stack as ref list with match-driven peek, match-as-bool in
while-guard, inline parenthesized match driving <-.

169 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -110,6 +110,7 @@
   "module_use.ml": 3,
   "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,
+  "next_greater.ml": 153,
   "next_permutation.ml": 119,
   "number_words.ml": 106,
   "mutable_record.ml": 10,

lib/ocaml/baseline/next_greater.ml

@@ -0,0 +1,23 @@
+let next_greater arr =
+  let n = Array.length arr in
+  let res = Array.make n (-1) in
+  let stack = ref [] in
+  for i = n - 1 downto 0 do
+    while (match !stack with [] -> false | h :: _ -> h <= arr.(i)) do
+      stack := List.tl !stack
+    done;
+    (match !stack with
+     | [] -> res.(i) <- -1
+     | h :: _ -> res.(i) <- h);
+    stack := arr.(i) :: !stack
+  done;
+  res
+
+;;
+
+let r = next_greater [| 4; 5; 2; 25; 7; 8; 1; 30; 12 |] in
+let sum = ref 0 in
+for i = 0 to Array.length r - 1 do
+  if r.(i) >= 0 then sum := !sum + r.(i)
+done;
+!sum