ocaml: phase 5.1 frequency.ml baseline + Format module alias (+2 tests, 498 total)

frequency.ml exercises the recently-added Hashtbl.iter / fold +
Hashtbl.find_opt + s.[i] indexing + for-loop together: build a
char-count table for 'abracadabra' then take the max via
Hashtbl.fold. Expected = 5 (a x 5). Total 25 baseline programs.

Format module added as a thin alias of Printf — sprintf, printf, and
asprintf all delegate to Printf.sprintf. The dynamic runtime doesn't
distinguish boxes/breaks, so format strings work the same as in
Printf and most Format-using OCaml programs now compile.

lib/ocaml/baseline/expected.json

@@ -8,6 +8,7 @@
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
   "factorial.ml": 3628800,
+  "frequency.ml": 5,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "lambda_calc.ml": 7,

lib/ocaml/baseline/frequency.ml

@@ -0,0 +1,18 @@
+let count_chars s =
+  let t = Hashtbl.create 8 in
+  for i = 0 to String.length s - 1 do
+    let c = s.[i] in
+    let n = match Hashtbl.find_opt t c with
+            | Some v -> v + 1
+            | None -> 1
+    in
+    Hashtbl.replace t c n
+  done;
+  t
+
+let max_count t =
+  Hashtbl.fold (fun _ v acc -> if v > acc then v else acc) t 0
+
+;;
+
+max_count (count_chars "abracadabra")

lib/ocaml/runtime.sx

@@ -455,6 +455,15 @@
      let printf fmt = sprintf fmt
    end ;;
 
+   module Format = struct
+     (* Thin alias of Printf for pretty-printing parity. The dynamic
+        runtime doesn't distinguish boxes/breaks — fmt strings work
+        the same as in Printf. *)
+     let sprintf fmt = Printf.sprintf fmt
+     let printf fmt = Printf.sprintf fmt
+     let asprintf fmt = Printf.sprintf fmt
+   end ;;
+
    module Lazy = struct
      let force lz = _lazy_force lz
    end ;;

lib/ocaml/test.sh

@@ -1234,6 +1234,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4961)
 (eval "(ocaml-run \"let counter = ref 0 in let lz = lazy (counter := !counter + 1; 42) in let a = Lazy.force lz in let b = Lazy.force lz in (a + b) * 100 + !counter\")")
 
+;; ── Format alias of Printf ────────────────────────────────────
+(epoch 4970)
+(eval "(ocaml-run \"Format.sprintf \\\"%d\\\" 99\")")
+(epoch 4971)
+(eval "(ocaml-run \"Format.asprintf \\\"%s=%d\\\" \\\"n\\\" 7\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1958,6 +1964,10 @@ check 4951 "Hashtbl.iter ref accum 10+20" '30'
 check 4960 "lazy 1+2 force"               '3'
 check 4961 "lazy memoization counter=1"   '8401'
 
+# ── Format alias ─────────────────────────────────────────────────
+check 4970 "Format.sprintf %d"            '"99"'
+check 4971 "Format.asprintf %s=%d"        '"n=7"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

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.
+- 2026-05-09 Phase 5.1 — frequency.ml baseline + Format module alias
+  (+2 tests, 498 total). frequency.ml builds a Hashtbl of char→count
+  via `Hashtbl.find_opt` + `Hashtbl.replace` inside a `for` loop, then
+  uses `Hashtbl.fold` to find the maximum count. `count_chars
+  "abracadabra"` → max is 5 (a×5). Format module added as a thin
+  alias of Printf — sprintf / printf / asprintf all delegate.
 - 2026-05-09 Phase 4 — `lazy EXPR` + `Lazy.force` (+2 tests, 496
   total). Tokenizer already had `lazy` as a keyword. parse-prefix now
   emits `(:lazy EXPR)`; eval creates a one-element cell with state