From a98d683e60d255aea1772e6f6d2c129b51b6b0f4 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 14:40:22 +0000
Subject: [PATCH] ocaml: phase 5.1 group_consec.ml baseline (group consecutive
 equals, 5*10+3 = 53)

Inner 'collect cur acc tail' walks the tail while head matches
'cur', accumulating into 'acc'. Returns (rev acc, remaining) on
first mismatch. Outer 'group' recurses on the remaining list.

  group [1;1;2;2;2;3;1;1;4]
   = [[1;1]; [2;2;2]; [3]; [1;1]; [4]]

  List.length groups        = 5
  List.length (gs.(1))      = 3
  5 * 10 + 3                = 53

Tests nested recursion (inner aux + outer recursion), tuple
destructuring 'let (g, tail) = ...' inside the outer match arm,
and List.nth.

69 baseline programs total.
---
 lib/ocaml/baseline/expected.json   |  1 +
 lib/ocaml/baseline/group_consec.ml | 18 ++++++++++++++++++
 plans/ocaml-on-sx.md               |  8 ++++++++
 3 files changed, 27 insertions(+)
 create mode 100644 lib/ocaml/baseline/group_consec.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 208228be..019643c9 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,
+  "group_consec.ml": 53,
   "hailstone.ml": 111,
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
diff --git a/lib/ocaml/baseline/group_consec.ml b/lib/ocaml/baseline/group_consec.ml
new file mode 100644
index 00000000..690f0271
--- /dev/null
+++ b/lib/ocaml/baseline/group_consec.ml
@@ -0,0 +1,18 @@
+let rec group xs =
+  match xs with
+  | [] -> []
+  | x :: rest ->
+    let rec collect cur acc tail =
+      match tail with
+      | [] -> (List.rev acc, [])
+      | y :: ys ->
+        if y = cur then collect cur (y :: acc) ys
+        else (List.rev acc, y :: ys)
+    in
+    let (g, tail) = collect x [x] rest in
+    g :: group tail
+
+;;
+
+let gs = group [1;1;2;2;2;3;1;1;4] in
+List.length gs * 10 + List.length (List.nth gs 1)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 09bcf0a2..c36c914a 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.
+- 2026-05-09 Phase 5.1 — group_consec.ml baseline (group consecutive
+  equals into sublists, 5*10 + 3 = 53). Inner `collect cur acc
+  tail` walks while head matches `cur`, accumulates into `acc`,
+  returns `(rev acc, remaining)` on first mismatch. Outer `group`
+  recurses on the remaining list. [1;1;2;2;2;3;1;1;4] →
+  [[1;1];[2;2;2];[3];[1;1];[4]] (5 groups, second group has 3
+  elements). Sum = 53. Tests nested recursion + tuple destructuring
+  in let-binding. 69 baseline programs total.
 - 2026-05-09 Phase 5.1 — zigzag.ml baseline (interleave two lists,
   sum 1..10 = 55). One-liner that swaps the lists on every recursive
   call: `match xs with [] -> ys | x :: xs' -> x :: zigzag ys xs'`.