ocaml: phase 5.1 trapping_rain.ml baseline (LeetCode trapped water = 6)

Classic trapped-rain-water two-pass DP:

  left_max[i]  = max(heights[0..i])    (forward sweep)
  right_max[i] = max(heights[i..n-1])  (downto sweep)
  water        = sum over i of (min(left_max[i], right_max[i])
                                  - heights[i])

For [0; 1; 0; 2; 1; 0; 1; 3; 2; 1; 2; 1]: water = 6.

Tests dual sweep (forward + downto), array of running maxes,
inline-if rhs of <- for running-max update (uses iter-236 fix
for <- accepting if/match RHS).

203 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -194,6 +194,7 @@
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "tree_depth.ml": 4,
+  "trapping_rain.ml": 6,
   "triangle.ml": 11,
   "trie.ml": 6,
   "triangle_div.ml": 120,

lib/ocaml/baseline/trapping_rain.ml

@@ -0,0 +1,25 @@
+let trap heights =
+  let n = Array.length heights in
+  if n < 3 then 0
+  else begin
+    let left_max = Array.make n 0 in
+    let right_max = Array.make n 0 in
+    left_max.(0) <- heights.(0);
+    for i = 1 to n - 1 do
+      left_max.(i) <- if heights.(i) > left_max.(i - 1) then heights.(i) else left_max.(i - 1)
+    done;
+    right_max.(n - 1) <- heights.(n - 1);
+    for i = n - 2 downto 0 do
+      right_max.(i) <- if heights.(i) > right_max.(i + 1) then heights.(i) else right_max.(i + 1)
+    done;
+    let water = ref 0 in
+    for i = 0 to n - 1 do
+      let min_lr = if left_max.(i) < right_max.(i) then left_max.(i) else right_max.(i) in
+      water := !water + min_lr - heights.(i)
+    done;
+    !water
+  end
+
+;;
+
+trap [| 0; 1; 0; 2; 1; 0; 1; 3; 2; 1; 2; 1 |]

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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.
+- 2026-05-11 Phase 5.1 — trapping_rain.ml baseline (trapped rain
+  water over heights [0;1;0;2;1;0;1;3;2;1;2;1] = 6). Left-max and
+  right-max prefix arrays; at each index water = min(L,R) − h.
+  Tests dual sweep (forward + downto), array of running maxes,
+  inline-if rhs of `<-` for running-max update (uses iter-236
+  fix). 203 baseline programs total.
 - 2026-05-11 Phase 5.1 — gas_station.ml baseline (find unique start
   station for circular gas tour, gas=[1;2;3;4;5] cost=[3;4;5;1;2]
   → start at index 3). Classic O(n) greedy: walk once tracking the