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

@@ -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,

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