From 3ea8967571a201a5f6fb397afa3e2fbb65a5dd0e Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sun, 10 May 2026 21:57:42 +0000
Subject: [PATCH] ocaml: phase 5.1 flood_fill.ml baseline (largest grid
 component = 7)

Recursive 4-way flood fill from every unvisited 1-cell:

  let rec flood visited r c =
    if r < 0 || r >= h || c < 0 || c >= w then 0
    else if visited.(r).(c) || grid.(r).(c) = 0 then 0
    else begin
      visited.(r).(c) <- true;
      1 + flood visited (r - 1) c
        + flood visited (r + 1) c
        + flood visited r (c - 1)
        + flood visited r (c + 1)
    end

Grid (1s shown as #, 0s as .):
  # # . # #
  # . . . #
  . . # . .
  # # # # .
  . . . # #

Largest component: {(2,2),(3,0),(3,1),(3,2),(3,3),(4,3),(4,4)} = 7.

Bounds check r >= 0 must short-circuit before visited/grid reads;
relies on the && / || fix from iter 242.

157 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/flood_fill.ml | 38 ++++++++++++++++++++++++++++++++
 plans/ocaml-on-sx.md             |  8 +++++++
 3 files changed, 47 insertions(+)
 create mode 100644 lib/ocaml/baseline/flood_fill.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index d1aa3488..26f54877 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -73,6 +73,7 @@
   "fizz_classifier.ml": 540,
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
+  "flood_fill.ml": 7,
   "floyd_warshall.ml": 9,
   "lis.ml": 6,
   "list_ops.ml": 30,
diff --git a/lib/ocaml/baseline/flood_fill.ml b/lib/ocaml/baseline/flood_fill.ml
new file mode 100644
index 00000000..bc2a7e25
--- /dev/null
+++ b/lib/ocaml/baseline/flood_fill.ml
@@ -0,0 +1,38 @@
+let h = 5
+let w = 5
+
+let grid = [|
+  [| 1; 1; 0; 1; 1 |];
+  [| 1; 0; 0; 0; 1 |];
+  [| 0; 0; 1; 0; 0 |];
+  [| 1; 1; 1; 1; 0 |];
+  [| 0; 0; 0; 1; 1 |]
+|]
+
+let rec flood visited r c =
+  if r < 0 || r >= h || c < 0 || c >= w then 0
+  else if visited.(r).(c) || grid.(r).(c) = 0 then 0
+  else begin
+    visited.(r).(c) <- true;
+    1 + flood visited (r - 1) c
+      + flood visited (r + 1) c
+      + flood visited r (c - 1)
+      + flood visited r (c + 1)
+  end
+
+let largest_component () =
+  let visited = Array.init h (fun _ -> Array.make w false) in
+  let best = ref 0 in
+  for r = 0 to h - 1 do
+    for c = 0 to w - 1 do
+      if grid.(r).(c) = 1 && not visited.(r).(c) then begin
+        let s = flood visited r c in
+        if s > !best then best := s
+      end
+    done
+  done;
+  !best
+
+;;
+
+largest_component ()
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 0646c2ec..e2a86a11 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,14 @@ _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 — flood_fill.ml baseline (largest connected
+  component in 5×5 grid = 7). Recursive 4-direction flood from
+  every unvisited 1-cell; bounds check short-circuits before the
+  visited/grid reads (relies on iter-242 fix). The maximal
+  component in the test grid spans {(2,2), (3,0), (3,1), (3,2),
+  (3,3), (4,3), (4,4)} = 7 cells. Tests recursive function with
+  4-way self-call, in-place visited array mutation, nested
+  short-circuited bounds + content guards. 157 baseline programs total.
 - 2026-05-10 Phase 5.1 — next_permutation.ml baseline (count
   permutations of [1;2;3;4;5] via Narayana's algorithm = 119).
   Standard in-place algorithm: find largest i with a.(i) < a.(i+1),