From 7de014cd7565c4152e7c7d61d66ea5868f84dcad Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 13:30:46 +0000
Subject: [PATCH] ocaml: phase 5.1 hailstone.ml baseline (Collatz length from
 27 = 111 steps)

Iterative Collatz / hailstone sequence:

  let collatz_length n =
    let m = ref n in
    let count = ref 0 in
    while !m > 1 do
      if !m mod 2 = 0 then m := !m / 2
      else m := 3 * !m + 1;
      count := !count + 1
    done;
    !count

27 is the famous 'long-running' Collatz starter. Reaches a peak of
9232 mid-sequence and takes 111 steps to bottom out at 1.

64 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/hailstone.ml  | 13 +++++++++++++
 plans/ocaml-on-sx.md             |  6 ++++++
 3 files changed, 20 insertions(+)
 create mode 100644 lib/ocaml/baseline/hailstone.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index c6706fe0..65062630 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -28,6 +28,7 @@
   "frequency.ml": 5,
   "gcd_lcm.ml": 60,
   "grep_count.ml": 3,
+  "hailstone.ml": 111,
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
diff --git a/lib/ocaml/baseline/hailstone.ml b/lib/ocaml/baseline/hailstone.ml
new file mode 100644
index 00000000..4f9e3f9b
--- /dev/null
+++ b/lib/ocaml/baseline/hailstone.ml
@@ -0,0 +1,13 @@
+let collatz_length n =
+  let m = ref n in
+  let count = ref 0 in
+  while !m > 1 do
+    if !m mod 2 = 0 then m := !m / 2
+    else m := 3 * !m + 1;
+    count := !count + 1
+  done;
+  !count
+
+;;
+
+collatz_length 27
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 9eee200c..face6a09 100644
--- a/plans/ocaml-on-sx.md
+++ b/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.
+- 2026-05-09 Phase 5.1 — hailstone.ml baseline (Collatz length,
+  starting from 27 → 111 steps to reach 1). Iterative while-loop
+  applies `n / 2` if even, `3n + 1` if odd, counting steps. 27 is
+  the famous "long-running" Collatz starter that produces 111
+  iterations and a peak value of 9232 mid-sequence. 64 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — twosum.ml baseline (LeetCode #1, hashtable
   one-pass, index-sum 1+3+1 = 5). Walks list with List.iteri,
   checking if `target - x` is already in the hashtable; if yes, the