ocaml: phase 5.1 shuffle.ml baseline (Fisher-Yates with deterministic Random)

In-place Fisher-Yates shuffle using:

  Random.init 42                  deterministic seed
  let a = Array.of_list xs
  for i = n - 1 downto 1 do       reverse iteration
    let j = Random.int (i + 1)
    let tmp = a.(i) in
    a.(i) <- a.(j);
    a.(j) <- tmp
  done

Sum is invariant under permutation, so the test value (55 for
[1..10] = 1+2+...+10) verifies the shuffle is a valid permutation
regardless of which permutation the seed yields.

Exercises Random.init / Random.int + Array.of_list / to_list /
length / arr.(i) / arr.(i) <- v + downto loop + multi-statement
sequencing within for-body.

33 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -29,6 +29,7 @@
   "quicksort.ml": 44,
   "roman.ml": 44,
   "safe_div.ml": 20,
+  "shuffle.ml": 55,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "unique_set.ml": 9,

lib/ocaml/baseline/shuffle.ml

@@ -0,0 +1,15 @@
+let shuffle xs =
+  Random.init 42;
+  let a = Array.of_list xs in
+  let n = Array.length a in
+  for i = n - 1 downto 1 do
+    let j = Random.int (i + 1) in
+    let tmp = a.(i) in
+    a.(i) <- a.(j);
+    a.(j) <- tmp
+  done;
+  Array.to_list a
+
+;;
+
+List.fold_left (+) 0 (shuffle [1;2;3;4;5;6;7;8;9;10])

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 — shuffle.ml baseline (Fisher-Yates with
+  deterministic Random.init seed). In-place swap loop using `for i =
+  n - 1 downto 1` and `a.(i) <- a.(j)`. Sum is invariant under
+  permutation, so the test value (55 for [1..10]) verifies that the
+  shuffle is a valid permutation regardless of which one. Exercises
+  Random.init / Random.int + Array.of_list / to_list / length /
+  arr.(i) / arr.(i) <- v + downto loop. 33 baseline programs total.
 - 2026-05-09 Phase 5.1 — pi_leibniz.ml baseline (Leibniz formula,
   1000 terms × 100 → 314). Side-quest: `int_of_float` was wrong —
   defined as identity in iteration 94 instead of truncation. Fixed