From b3d5da53610cfec61aec473aeb08d22d5402d20e Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 10:07:12 +0000
Subject: [PATCH] ocaml: phase 5.1 kadane.ml baseline (max subarray sum = 6)

Kadane's algorithm in O(n):

  let max_subarray xs =
    let max_so_far = ref min_int in
    let cur = ref 0 in
    List.iter (fun x ->
      cur := max x (!cur + x);
      max_so_far := max !max_so_far !cur
    ) xs;
    !max_so_far

For [-2;1;-3;4;-1;2;1;-5;4] the optimal subarray is [4;-1;2;1] = 6.

Exercises min_int (iter 94), max as global, ref / ! / :=, and
List.iter with two side-effecting steps in one closure body.

46 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/kadane.ml     | 12 ++++++++++++
 plans/ocaml-on-sx.md             |  5 +++++
 3 files changed, 18 insertions(+)
 create mode 100644 lib/ocaml/baseline/kadane.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index f0724929..5e4f9195 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -21,6 +21,7 @@
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "json_pretty.ml": 24,
+  "kadane.ml": 6,
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
diff --git a/lib/ocaml/baseline/kadane.ml b/lib/ocaml/baseline/kadane.ml
new file mode 100644
index 00000000..49774818
--- /dev/null
+++ b/lib/ocaml/baseline/kadane.ml
@@ -0,0 +1,12 @@
+let max_subarray xs =
+  let max_so_far = ref min_int in
+  let cur = ref 0 in
+  List.iter (fun x ->
+    cur := max x (!cur + x);
+    max_so_far := max !max_so_far !cur
+  ) xs;
+  !max_so_far
+
+;;
+
+max_subarray [-2; 1; -3; 4; -1; 2; 1; -5; 4]
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index e263bf85..aabe7b2e 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,11 @@ _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.
+- 2026-05-09 Phase 5.1 — kadane.ml baseline (Kadane's max subarray
+  sum = 6). Classic O(n) algorithm using two refs and `max`. For
+  [-2;1;-3;4;-1;2;1;-5;4] the optimal subarray is [4;-1;2;1] = 6.
+  Exercises `min_int`, `max`, ref/!/:=, and List.iter with multiple
+  side-effecting steps in one closure body. 46 baseline programs total.
 - 2026-05-09 Phase 5.1 — pascal.ml baseline (Pascal's triangle row
   10 middle = C(10, 5) = 252). next_row prepends 1, walks adjacent
   pairs (x, y) emitting x+y, appends a final 1. row n iterates