ocaml: phase 5.1 anagram_check.ml baseline (char-frequency array, 2/4 anagrams)

to_counts builds a 256-slot int array of character frequencies:

  let to_counts s =
    let counts = Array.make 256 0 in
    for i = 0 to String.length s - 1 do
      let c = Char.code s.[i] in
      counts.(c) <- counts.(c) + 1
    done;
    counts

same_counts compares two arrays element-by-element via for loop +
bool ref. is_anagram composes them.

Four pairs:
  listen ~ silent       true
  hello !~ world        false
  anagram ~ nagaram     true
  abc !~ abcd           false (length differs)
  sum                   2

Exercises Array.make + arr.(i) + arr.(i) <- v + nested for loops +
Char.code + s.[i].

57 baseline programs total.

lib/ocaml/baseline/anagram_check.ml

@@ -0,0 +1,23 @@
+let to_counts s =
+  let counts = Array.make 256 0 in
+  for i = 0 to String.length s - 1 do
+    let c = Char.code s.[i] in
+    counts.(c) <- counts.(c) + 1
+  done;
+  counts
+
+let same_counts a b =
+  let result = ref true in
+  for i = 0 to 255 do
+    if a.(i) <> b.(i) then result := false
+  done;
+  !result
+
+let is_anagram s t = same_counts (to_counts s) (to_counts t)
+
+;;
+
+(if is_anagram "listen" "silent" then 1 else 0) +
+(if is_anagram "hello" "world" then 1 else 0) +
+(if is_anagram "anagram" "nagaram" then 1 else 0) +
+(if is_anagram "abc" "abcd" then 1 else 0)

lib/ocaml/baseline/expected.json

@@ -1,5 +1,6 @@
 {
   "ackermann.ml": 125,
+  "anagram_check.ml": 2,
   "anagrams.ml": 3,
   "bag.ml": 3,
   "bigint_add.ml": 28,

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 — anagram_check.ml baseline (char-frequency
+  array, 2/4 pairs are anagrams). to_counts builds a 256-slot int
+  array of character frequencies. same_counts compares two arrays
+  element-by-element. is_anagram = same_counts on both. listen ~
+  silent ✓, hello ≠ world, anagram ~ nagaram ✓, abc ≠ abcd → 2.
+  Exercises Array.make + arr.(i) + arr.(i) <- v + nested for loops
+  + Char.code + s.[i]. 57 baseline programs total.
 - 2026-05-09 Phase 5.1 — exception_user.ml baseline (user-defined
   exception with int payload, 4+5+7+10 = 26). Defines `exception
   Negative of int`, `safe_sqrt` raises it on negative input, and