ocaml: phase 5.1 euler7.ml baseline (100th prime = 541)

Scaled-down PE7 (real version asks for the 10001st prime = 104743).
Trial-division within an outer while loop searching forward from 2,
short-circuited via bool ref:

  let nth_prime n =
    let count = ref 0 in
    let i = ref 1 in
    let result = ref 0 in
    while !count < n do
      i := !i + 1;
      let p = ref true in
      let j = ref 2 in
      while !j * !j <= !i && !p do
        if !i mod !j = 0 then p := false;
        j := !j + 1
      done;
      if !p then begin
        count := !count + 1;
        if !count = n then result := !i
      end
    done;
    !result

  nth_prime 100 = 541

100 keeps the run under our 3-minute budget while exercising the
same algorithm.

112 baseline programs total.

lib/ocaml/baseline/euler7.ml

@@ -0,0 +1,22 @@
+let nth_prime n =
+  let count = ref 0 in
+  let i = ref 1 in
+  let result = ref 0 in
+  while !count < n do
+    i := !i + 1;
+    let p = ref true in
+    let j = ref 2 in
+    while !j * !j <= !i && !p do
+      if !i mod !j = 0 then p := false;
+      j := !j + 1
+    done;
+    if !p then begin
+      count := !count + 1;
+      if !count = n then result := !i
+    end
+  done;
+  !result
+
+;;
+
+nth_prime 100

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "euler4_small.ml": 9009,
   "euler5.ml": 232792560,
   "euler6.ml": 25164150,
+  "euler7.ml": 541,
   "euler9.ml": 31875000,
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,