ocaml: phase 5.1 palindrome_sum.ml baseline (sum of 3-digit palindromes = 49500)

let palindrome_sum lo hi =
    let total = ref 0 in
    for n = lo to hi do
      if is_pal n then total := !total + n
    done;
    !total

  palindrome_sum 100 999 = 49500

There are 90 three-digit palindromes (form aba; 9 choices for a, 10
for b). Average value 550, sum 49500.

Companion to palindrome.ml (predicate-only) and paren_depth.ml.

127 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -89,6 +89,7 @@
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,
+  "palindrome_sum.ml": 49500,
   "paren_depth.ml": 7,
   "partition.ml": 3025,
   "pancake_sort.ml": 910,

lib/ocaml/baseline/palindrome_sum.ml

@@ -0,0 +1,19 @@
+let is_pal n =
+  let s = string_of_int n in
+  let len = String.length s in
+  let p = ref true in
+  for i = 0 to len / 2 - 1 do
+    if s.[i] <> s.[len - 1 - i] then p := false
+  done;
+  !p
+
+let palindrome_sum lo hi =
+  let total = ref 0 in
+  for n = lo to hi do
+    if is_pal n then total := !total + n
+  done;
+  !total
+
+;;
+
+palindrome_sum 100 999

plans/ocaml-on-sx.md

@@ -407,6 +407,10 @@ _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 — palindrome_sum.ml baseline (sum of 3-digit
+  palindromes = 49500). 90 palindromes between 100 and 999 (form
+  aba; 9 choices for a, 10 for b). Sum = 49500 = 90 * 550 (mean
+  is 550). 127 baseline programs total.
 - 2026-05-10 Phase 5.1 — triangle_div.ml baseline (first triangle
   number with > 10 divisors = 120). PE12 with target 10. T(15) = 120
   has 16 divisors {1,2,3,4,5,6,8,10,12,15,20,24,30,40,60,120} —