From ce013fa138019943a4740c58b9217ed0b2657d9f Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 15:53:47 +0000
Subject: [PATCH] ocaml: phase 5.1 fib_mod.ml baseline (Fibonacci mod prime,
 fib(100) mod 1000003 = 391360)

Iterative two-ref Fibonacci with modular reduction every step:

  let fib_mod n m =
    let a = ref 0 in
    let b = ref 1 in
    for _ = 1 to n do
      let c = (!a + !b) mod m in
      a := !b;
      b := c
    done;
    !a

The 100th Fibonacci is 354_224_848_179_261_915_075, well past JS
safe-int (2^53). Modular reduction every step keeps intermediate
values within int53 precision so the answer is exact in our
runtime. fib(100) mod 1000003 = 391360.

76 baseline programs total.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/fib_mod.ml    | 13 +++++++++++++
 plans/ocaml-on-sx.md             |  6 ++++++
 3 files changed, 20 insertions(+)
 create mode 100644 lib/ocaml/baseline/fib_mod.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index b7f41169..8c6e86e7 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -24,6 +24,7 @@
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,
   "factorial.ml": 3628800,
+  "fib_mod.ml": 391360,
   "fraction.ml": 7,
   "frequency.ml": 5,
   "gcd_lcm.ml": 60,
diff --git a/lib/ocaml/baseline/fib_mod.ml b/lib/ocaml/baseline/fib_mod.ml
new file mode 100644
index 00000000..9dccf4f7
--- /dev/null
+++ b/lib/ocaml/baseline/fib_mod.ml
@@ -0,0 +1,13 @@
+let fib_mod n m =
+  let a = ref 0 in
+  let b = ref 1 in
+  for _ = 1 to n do
+    let c = (!a + !b) mod m in
+    a := !b;
+    b := c
+  done;
+  !a
+
+;;
+
+fib_mod 100 1000003
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index f29b437d..93229db5 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 — fib_mod.ml baseline (Fibonacci mod prime,
+  fib(100) mod 1000003 = 391360). Iterative two-ref Fibonacci with
+  modular reduction at every step to keep intermediate values
+  bounded. The 100th Fibonacci is 354224848179261915075, which
+  exceeds JS safe-int range; modular arithmetic on each step keeps
+  computations within int53 precision. 76 baseline programs total.
 - 2026-05-09 Phase 5.1 — luhn.ml baseline (Luhn check digit, 2/4
   inputs valid). Walks digits right-to-left, doubles every other
   starting from the second-from-right; if doubled value > 9