From 554ef48c6346a04fcdeb78be77b902ade75816ca Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 16:47:56 +0000
Subject: [PATCH] ocaml: phase 5.1 run_decode.ml baseline (RLE decode,
 expansion sum = 21)

Inverse of run_length.ml from iteration 130. Takes a list of
(value, count) tuples and expands:

  let rec rle_decode pairs =
    match pairs with
    | [] -> []
    | (x, n) :: rest ->
      let rec rep k = if k = 0 then [] else x :: rep (k - 1) in
      rep n @ rle_decode rest

  rle_decode [(1,3); (2,2); (3,4); (1,2)]
  = [1;1;1; 2;2; 3;3;3;3; 1;1]
  sum = 3 + 4 + 12 + 2 = 21.

Tests tuple-cons pattern, inner-let recursion, list concat (@), and
the 'List.fold_left (+) 0' invariant on encoding round-trips.

81 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/run_decode.ml | 10 ++++++++++
 plans/ocaml-on-sx.md             |  6 ++++++
 3 files changed, 17 insertions(+)
 create mode 100644 lib/ocaml/baseline/run_decode.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 5bf23786..f21e85c2 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -66,6 +66,7 @@
   "quicksort.ml": 44,
   "roman.ml": 44,
   "rpn.ml": 9,
+  "run_decode.ml": 21,
   "run_length.ml": 11,
   "safe_div.ml": 20,
   "shuffle.ml": 55,
diff --git a/lib/ocaml/baseline/run_decode.ml b/lib/ocaml/baseline/run_decode.ml
new file mode 100644
index 00000000..4a47e92e
--- /dev/null
+++ b/lib/ocaml/baseline/run_decode.ml
@@ -0,0 +1,10 @@
+let rec rle_decode pairs =
+  match pairs with
+  | [] -> []
+  | (x, n) :: rest ->
+    let rec rep k = if k = 0 then [] else x :: rep (k - 1) in
+    rep n @ rle_decode rest
+
+;;
+
+List.fold_left (+) 0 (rle_decode [(1, 3); (2, 2); (3, 4); (1, 2)])
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index cb7d3e01..8a5be0a8 100644
--- a/plans/ocaml-on-sx.md
+++ b/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 — run_decode.ml baseline (RLE decode, sum of
+  expansion = 21). Inverse of run_length.ml: takes a list of
+  `(value, count)` tuples, expands each pair via inner `rep` helper,
+  and concatenates with `@`. Companion to run_length encoding from
+  iteration 130. `[(1,3);(2,2);(3,4);(1,2)]` expands to
+  [1;1;1;2;2;3;3;3;3;1;1] (sum = 21). 81 baseline programs total.
 - 2026-05-09 Phase 5.1 — peano.ml baseline (Peano arithmetic, 5*6 =
   30). Defines `type peano = Zero | Succ of peano` and four
   recursive functions: to_int, from_int, plus, mul. Multiplication