Feature: fast_io::list::is_single, fast_io::list::has_multiple

include/fast_io_dsal/impl/list.h

@@ -616,6 +616,24 @@ class list
 		return imp.next == __builtin_addressof(imp);
 	}
 
+	/**
+	 * @brief Checks if the list contains only one element.
+	 * @return true if the list contains only one element, false otherwise.
+	 */
+	[[nodiscard]] inline constexpr bool is_single() const noexcept
+	{
+		return imp.next == imp.prev && imp.next != __builtin_addressof(imp);
+	}
+
+	/**
+	 * @brief Checks if the list contains multiple elements (>= 2).
+	 * @return true if the list contains multiple elements, false otherwise.
+	 */
+	[[nodiscard]] inline constexpr bool has_multiple() const noexcept
+	{
+		return imp.next != imp.prev;
+	}
+
 	[[nodiscard]] static inline constexpr size_type max_size() noexcept
 	{
 		constexpr size_type mxvl{SIZE_MAX / sizeof(node_type)};

tests/0026.container/0002.list/list_is_single.cc

@@ -0,0 +1,49 @@
+#include <fast_io_dsal/list.h>
+
+int main() {
+    ::fast_io::list<int> const l{};
+    if (!l.is_empty()) {
+        ::fast_io::fast_terminate();
+    }
+    if (l.is_single()) {
+        ::fast_io::fast_terminate();
+    }
+    if (l.has_multiple()) {
+        ::fast_io::fast_terminate();
+    }
+
+    ::fast_io::list<int> const l2{1};
+    if (l2.is_empty()) {
+        ::fast_io::fast_terminate();
+    }
+    if (!l2.is_single()) {
+        ::fast_io::fast_terminate();
+    }
+    if (l2.has_multiple()) {
+        ::fast_io::fast_terminate();
+    }
+
+    ::fast_io::list<int> const l3{1, 2};
+    if (l3.is_empty()) {
+        ::fast_io::fast_terminate();
+    }
+    if (l3.is_single()) {
+        ::fast_io::fast_terminate();
+    }
+    if (!l3.has_multiple()) {
+        ::fast_io::fast_terminate();
+    }
+
+    ::fast_io::list<int> l4{1, 2, 3};
+    if (l4.is_empty()) {
+        ::fast_io::fast_terminate();
+    }
+    if (l4.is_single()) {
+        ::fast_io::fast_terminate();
+    }
+    if (!l4.has_multiple()) {
+        ::fast_io::fast_terminate();
+    }
+
+    return 0;
+}