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

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-10 Phase 5.1 — next_greater.ml baseline (monotonic stack
+  for next-greater-element over [4;5;2;25;7;8;1;30;12], sum of
+  successors = 153). Right-to-left scan with a stack that pops
+  elements ≤ current. Result: [5,25,25,30,8,30,30,-1,-1]; sum of
+  non-negative entries = 5+25+25+30+8+30+30 = 153. Tests stack
+  encoded as `ref list` with match-driven peek, `match … with []
+  -> false | h :: _ -> …` boolean coercion in `while` guard,
+  inline match-statement on rhs of `<-` (paren-wrapped for parse).
+  169 baseline programs total.
 - 2026-05-10 Phase 5.1 — fenwick_tree.ml baseline (Binary Indexed
   Tree over [1;3;5;7;9;11;13;15], total + after = 228). Initial
   prefix_sum n = 64; after +100 at index 1, prefix_sum n = 164;