feat(p19): add lazy collection implementation

p12/lib/dune

@@ -1,3 +1,3 @@
 (library
  (name p12_lib)
- (modules iterative map module rec tailrec))
+ (modules iterative map module rec tailrec lazy))

p12/lib/lazy.ml

@@ -0,0 +1,24 @@
+let triangular_numbers =
+  Seq.unfold
+    (fun n ->
+      let t = n * (n + 1) / 2 in
+      Some (t, n + 1))
+    1
+;;
+
+let count_factors_tail n =
+  let rec aux i count =
+    if i * i > n
+    then count
+    else if n mod i = 0
+    then aux (i + 1) (count + if i * i = n then 1 else 2)
+    else aux (i + 1) count
+  in
+  if n <= 0 then 0 else aux 1 0
+;;
+
+let find_first_triangular_with_factors n =
+  Seq.find (fun x -> count_factors_tail x > n) triangular_numbers
+;;
+
+let result = find_first_triangular_with_factors 500

p12/test/test_p12.ml

@@ -51,6 +51,13 @@ let test_map_find_first _ =
   | None -> assert_failure "Expected a result, got None"
 ;;
 
+(* Lazy collections tests *)
+let test_lazy_find_first _ =
+  match Map.result with
+  | Some x -> assert_equal 76576500 x
+  | None -> assert_failure "Expected a result, got None"
+;;
+
 let suite =
   "Project Euler Problem 12 Tests"
   >::: [ "Tailrec - count_factors" >:: test_tailrec_count_factors
@@ -61,6 +68,7 @@ let suite =
        ; "Iterative - find_triangular" >:: test_iterative_find_triangle
        ; "Module - result" >:: test_module_result
        ; "Map - result" >:: test_map_find_first
+       ; "Lazy - result" >:: test_lazy_find_first
        ]
 ;;