From 98dbc62a858e8eac4253eb0763aca57525967a9f Mon Sep 17 00:00:00 2001
From: Arendelle <Arendelle_@outlook.com>
Date: Mon, 17 Nov 2025 13:32:34 +0800
Subject: [PATCH] Feature: fast_io::list::is_single,
 fast_io::list::has_multiple

---
 include/fast_io_dsal/impl/list.h              | 18 +++++++
 .../0002.list/list_is_single.cc               | 49 +++++++++++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 tests/0026.container/0002.list/list_is_single.cc

diff --git a/include/fast_io_dsal/impl/list.h b/include/fast_io_dsal/impl/list.h
index e58c566b..87ac8cf6 100644
--- a/include/fast_io_dsal/impl/list.h
+++ b/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)};
diff --git a/tests/0026.container/0002.list/list_is_single.cc b/tests/0026.container/0002.list/list_is_single.cc
new file mode 100644
index 00000000..4247b2cc
--- /dev/null
+++ b/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;
+}