Add iterate() impl to cartesian_base

The final boss...

In theory, we could make `cartesian_product` and `cartesian_product_map` iterable when the first argument is a non-sequence iterable, but that doesn't seem worth the effort.

Author: tcbrindle

include/flux/adaptor/cartesian_base.hpp

@@ -204,11 +204,37 @@ struct cartesian_traits_base_impl : default_sequence_traits {
         }
     }
 
+    template <std::size_t I, typename Self, typename Predicate,
+              typename... PartialElements>
+    static constexpr auto iterate_impl(Self& self, Predicate& pred,
+                                       PartialElements&&... partial_elements) -> bool
+    {
+        if constexpr (I == Arity - 1) {
+            return flux::iterate(get_base<I>(self), [&](auto&& elem) {
+                if constexpr (ReadKind == read_kind::tuple) {
+                    return std::invoke(pred, element_t<Self>(FLUX_FWD(partial_elements)..., FLUX_FWD(elem)));
+                } else {
+                    return std::invoke(pred, std::invoke(self.func_, FLUX_FWD(partial_elements)..., FLUX_FWD(elem)));
+                }
+            });
+        } else {
+            return flux::iterate(get_base<I>(self), [&](auto&& elem) {
+                return iterate_impl<I+1>(self, pred, FLUX_FWD(partial_elements)..., FLUX_FWD(elem));
+            });
+        }
+    }
+
 protected:
     using types = cartesian_traits_types<Arity, CartesianKind, ReadKind, Bases...>;
 
 public:
 
+    template <typename Self, typename Pred>
+    static constexpr auto iterate(Self& self, Pred&& pred) -> bool
+    {
+        return iterate_impl<0>(self, pred);
+    }
+
     template <typename Self>
     static constexpr auto first(Self& self)
         requires (CartesianKind == cartesian_kind::product)

test/test_cartesian_product.cpp

@@ -451,12 +451,12 @@ constexpr bool test_cartesian_product()
         STATIC_CHECK(count_j == 4);
     }
 
-    // `cartesian_product` `for_each_while` short circuits.
+    // `cartesian_product` `iterate` short circuits.
     {
         auto cart = flux::cartesian_product(std::array{100, 200}, std::array{300, 0});
 
         int count = 0;
-        cart.for_each_while(flux::unpack([&] (auto, auto j) {
+        flux::iterate(cart, flux::unpack([&] (auto, auto j) {
                 ++count;
                 return j != 0;
             }));