ocaml: phase 5.1 unique_paths_obs.ml baseline (4x4 grid w/ obstacles = 3)

Standard 2D unique-paths DP with obstacles gating each cell:

  dp[i][j] = if grid[i][j] = 1 then 0
             else dp[i-1][j] + dp[i][j-1]

Grid (1s are obstacles):
  . . . .
  . # . .
  . . . #
  # . . .

dp:
  1 1 1 1
  1 0 1 2
  1 1 2 0
  0 1 3 3

Returns dp[3][3] = 3.

Complements grid_paths.ml (no-obstacles version) — same DP shape
but obstacles zero out cells and reshape the path count.

199 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -196,6 +196,7 @@
   "triangle_div.ml": 120,
   "twosum.ml": 5,
   "union_find.ml": 4,
+  "unique_paths_obs.ml": 3,
   "unique_set.ml": 9,
   "validate.ml": 417,
   "word_count.ml": 3

lib/ocaml/baseline/unique_paths_obs.ml

@@ -0,0 +1,28 @@
+let h = 4
+let w = 4
+let grid = [|
+  [| 0; 0; 0; 0 |];
+  [| 0; 1; 0; 0 |];
+  [| 0; 0; 0; 1 |];
+  [| 1; 0; 0; 0 |]
+|]
+
+let paths_with_obs () =
+  let dp = Array.init h (fun _ -> Array.make w 0) in
+  if grid.(0).(0) = 0 then dp.(0).(0) <- 1;
+  for j = 1 to w - 1 do
+    if grid.(0).(j) = 0 then dp.(0).(j) <- dp.(0).(j - 1)
+  done;
+  for i = 1 to h - 1 do
+    if grid.(i).(0) = 0 then dp.(i).(0) <- dp.(i - 1).(0)
+  done;
+  for i = 1 to h - 1 do
+    for j = 1 to w - 1 do
+      if grid.(i).(j) = 0 then
+        dp.(i).(j) <- dp.(i - 1).(j) + dp.(i).(j - 1)
+    done
+  done;
+  dp.(h - 1).(w - 1)
+;;
+
+paths_with_obs ()

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 — unique_paths_obs.ml baseline (count
+  monotone paths in 4×4 grid with obstacles at (1,1),(2,3),(3,0)
+  = 3). Standard 2D DP with obstacle gating: dp[i][j] = dp[i-1][j]
+  + dp[i][j-1] when grid[i][j]=0, else 0. Complements grid_paths.ml
+  (no-obstacles version): the same DP but obstacles zero out
+  cells, reshaping the path count. 199 baseline programs total.
 - 2026-05-11 Phase 5.1 — daily_temperatures.ml baseline (monotonic
   decreasing stack of waiting days until warmer; sum over
   [73;74;75;71;69;72;76;73] = 10). For each day i, pop all stack