From a91ff62730e0e540fdabb5e9b6ede3f2712cd8e3 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 10:40:49 +0000
Subject: [PATCH] ocaml: phase 5.1 bsearch.ml baseline (binary search, position
 sum = 7)

Iterative binary search on a sorted int array:

  let bsearch arr target =
    let n = Array.length arr in
    let lo = ref 0 and hi = ref (n - 1) in
    let found = ref (-1) in
    while !lo <= !hi && !found = -1 do
      let mid = (!lo + !hi) / 2 in
      if arr.(mid) = target then found := mid
      else if arr.(mid) < target then lo := mid + 1
      else hi := mid - 1
    done;
    !found

For [1;3;5;7;9;11;13;15;17;19;21]:
  bsearch a 13   = 6
  bsearch a 5    = 2
  bsearch a 100  = -1
  sum            = 7

Exercises Array.of_list + arr.(i) + multi-let 'let lo = ... and
hi = ...' + while + multi-arm if/else if/else.

49 baseline programs total.
---
 lib/ocaml/baseline/bsearch.ml    | 16 ++++++++++++++++
 lib/ocaml/baseline/expected.json |  1 +
 plans/ocaml-on-sx.md             |  7 +++++++
 3 files changed, 24 insertions(+)
 create mode 100644 lib/ocaml/baseline/bsearch.ml

diff --git a/lib/ocaml/baseline/bsearch.ml b/lib/ocaml/baseline/bsearch.ml
new file mode 100644
index 00000000..fb2df41f
--- /dev/null
+++ b/lib/ocaml/baseline/bsearch.ml
@@ -0,0 +1,16 @@
+let bsearch arr target =
+  let n = Array.length arr in
+  let lo = ref 0 and hi = ref (n - 1) in
+  let found = ref (-1) in
+  while !lo <= !hi && !found = -1 do
+    let mid = (!lo + !hi) / 2 in
+    if arr.(mid) = target then found := mid
+    else if arr.(mid) < target then lo := mid + 1
+    else hi := mid - 1
+  done;
+  !found
+
+;;
+
+let a = Array.of_list [1;3;5;7;9;11;13;15;17;19;21] in
+bsearch a 13 + bsearch a 5 + bsearch a 100
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index cecf44fb..b97ffddf 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -7,6 +7,7 @@
   "bfs.ml": 6,
   "btree.ml": 39,
   "brainfuck.ml": 75,
+  "bsearch.ml": 7,
   "caesar.ml": 215,
   "calc.ml": 13,
   "closures.ml": 315,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 63949809..5a516692 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,13 @@ _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 — bsearch.ml baseline (binary search,
+  position-sum 6 + 2 + (-1) = 7). Iterative bsearch on a sorted int
+  array using two index refs `lo`/`hi` and a sentinel `found = -1`.
+  while loop runs until `lo > hi` or found set. For [1;3;...;21]:
+  position of 13 = 6, 5 = 2, 100 = -1. Exercises Array.of_list +
+  arr.(i) + multi-let `let lo = ... and hi = ...` + while + multi-arm
+  if/else if. 49 baseline programs total.
 - 2026-05-09 Phase 5.1 — palindrome.ml baseline (two-pointer
   palindrome check, 4 of 6 inputs are palindromes). is_palindrome
   walks from both ends meeting in the middle, returning false on