From 073ea44fdbd52f9059ec715a8fdce3f74f4c490c Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 10:31:22 +0000
Subject: [PATCH] ocaml: phase 5.1 palindrome.ml baseline (two-pointer check,
 4/6 inputs match)

Two-pointer palindrome check:

  let is_palindrome s =
    let n = String.length s in
    let rec check i j =
      if i >= j then true
      else if s.[i] <> s.[j] then false
      else check (i + 1) (j - 1)
    in
    check 0 (n - 1)

Tests on six strings:
  racecar = true
  hello   = false
  abba    = true
  ''      = true (vacuously, i >= j on entry)
  'a'     = true
  'ab'    = false

Sum = 4.

Uses s.[i] <> s.[j] (string-get + structural inequality), recursive
2-arg pointer advancement, and a multi-clause if/else if/else for
the three cases.

48 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/palindrome.ml | 17 +++++++++++++++++
 plans/ocaml-on-sx.md             |  7 +++++++
 3 files changed, 25 insertions(+)
 create mode 100644 lib/ocaml/baseline/palindrome.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 6e0e9145..cecf44fb 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -31,6 +31,7 @@
   "newton_sqrt.ml": 1414,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
+  "palindrome.ml": 4,
   "pascal.ml": 252,
   "pi_leibniz.ml": 314,
   "pretty_table.ml": 64,
diff --git a/lib/ocaml/baseline/palindrome.ml b/lib/ocaml/baseline/palindrome.ml
new file mode 100644
index 00000000..6286e886
--- /dev/null
+++ b/lib/ocaml/baseline/palindrome.ml
@@ -0,0 +1,17 @@
+let is_palindrome s =
+  let n = String.length s in
+  let rec check i j =
+    if i >= j then true
+    else if s.[i] <> s.[j] then false
+    else check (i + 1) (j - 1)
+  in
+  check 0 (n - 1)
+
+;;
+
+(if is_palindrome "racecar" then 1 else 0) +
+(if is_palindrome "hello" then 1 else 0) +
+(if is_palindrome "abba" then 1 else 0) +
+(if is_palindrome "" then 1 else 0) +
+(if is_palindrome "a" then 1 else 0) +
+(if is_palindrome "ab" then 1 else 0)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 0d6bb898..63949809 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 — 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
+  first mismatch. Tests on six strings (racecar ✓, hello ✗, abba ✓,
+  "" ✓, "a" ✓, "ab" ✗) sum to 4. Uses `s.[i] <> s.[j]` (string-get
+  + structural inequality) and recursive 2-arg pointer advancement.
+  48 baseline programs total.
 - 2026-05-09 Phase 5.1 — coin_change.ml baseline (min coin DP, 67¢
   with [1;5;10;25] → 6 coins). Bottom-up DP: `dp[i]` = min coins
   for amount i. For each amount 1..target, iterate coins and