From 63901931c41c55419ccbc390997e8b96469deef8 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sun, 10 May 2026 02:05:09 +0000
Subject: [PATCH] ocaml: phase 5.1 tail_factorial.ml baseline (12! via
 tail-recursion = 479001600)
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Single-helper tail-recursive loop threading an accumulator:

  let factorial n =
    let rec go n acc =
      if n <= 1 then acc
      else go (n - 1) (n * acc)
    in
    go n 1

  factorial 12 = 479_001_600

Companion to factorial.ml (10! = 3628800 via doubly-recursive
style); same answer-shape, different evaluator stress: this version
has constant stack depth.

130 baseline programs total — milestone.
---
 lib/ocaml/baseline/expected.json     |  1 +
 lib/ocaml/baseline/tail_factorial.ml | 10 ++++++++++
 plans/ocaml-on-sx.md                 |  6 ++++++
 3 files changed, 17 insertions(+)
 create mode 100644 lib/ocaml/baseline/tail_factorial.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 7fa0f1b5..3d81952e 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -114,6 +114,7 @@
   "simpson_int.ml": 10000,
   "stable_unique.ml": 46,
   "subseq_check.ml": 3,
+  "tail_factorial.ml": 479001600,
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
   "word_freq.ml": 8,
diff --git a/lib/ocaml/baseline/tail_factorial.ml b/lib/ocaml/baseline/tail_factorial.ml
new file mode 100644
index 00000000..3526cbcc
--- /dev/null
+++ b/lib/ocaml/baseline/tail_factorial.ml
@@ -0,0 +1,10 @@
+let factorial n =
+  let rec go n acc =
+    if n <= 1 then acc
+    else go (n - 1) (n * acc)
+  in
+  go n 1
+
+;;
+
+factorial 12
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index cfebefa3..969de697 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.
+- 2026-05-10 Phase 5.1 — tail_factorial.ml baseline (12! via tail
+  recursion = 479001600). Single-helper tail-recursive loop
+  threading an accumulator. Companion to factorial.ml (10! via
+  doubly-recursive style); same answer-shape, different evaluator
+  stress (tail-call optimisation if any, or pure constant stack
+  depth otherwise). 130 baseline programs total — milestone.
 - 2026-05-10 Phase 5.1 — zerosafe.ml baseline (Option-chained safe
   division, sum 10 + (-1) + (-1) + 20 = 28). safe_div returns None
   on division by zero; safe_chain stitches two divisions, propagating