Add/test pop_front().

include/bitcoin/system/data/collection.hpp

@@ -69,11 +69,16 @@ constexpr insert_sorted(Collection& list,
     const typename Collection::value_type& element,
     Predicate predicate) NOEXCEPT;
 
-/// Pop an element from the stack and return its value.
+/// Pop an element from the stack (back) and return its value.
 template <typename Collection>
 typename Collection::value_type
 inline pop(Collection& stack) NOEXCEPT;
 
+/// Pop an element from the list (front) and return its value.
+template <typename Collection>
+typename Collection::value_type
+inline pop_front(Collection& stack) NOEXCEPT;
+
 /// Determine if a collection contains only distinct members.
 template <typename Collection>
 constexpr bool is_distinct(Collection&& list) NOEXCEPT;

include/bitcoin/system/impl/data/collection.ipp

@@ -159,6 +159,19 @@ inline pop(Collection& stack) NOEXCEPT
     return element;
 }
 
+// Collection requires front and pop_front methods (list).
+template <typename Collection>
+typename Collection::value_type
+inline pop_front(Collection& stack) NOEXCEPT
+{
+    if (std::empty(stack))
+        return {};
+
+    typename Collection::value_type element{ std::move(stack.front()) };
+    stack.pop_front();
+    return element;
+}
+
 // C++17: Parallel policy for std::sort, std::unique.
 template <typename Collection>
 constexpr bool is_distinct(Collection&& list) NOEXCEPT

test/data/collection.cpp

@@ -18,6 +18,7 @@
  */
 #include "../test.hpp"
 #include <array>
+#include <list>
 #include <vector>
 
 BOOST_AUTO_TEST_SUITE(collection_tests)
@@ -189,6 +190,36 @@ BOOST_AUTO_TEST_CASE(collection__pop__multiple__popped_expected)
     BOOST_REQUIRE_EQUAL(stack, expected_stack);
 }
 
+// pop_front
+using data_queue = std::list<uint8_t>;
+
+BOOST_AUTO_TEST_CASE(collection__pop_front__empty__empty_default)
+{
+    data_queue queue{};
+    const auto value = pop_front(queue);
+    BOOST_REQUIRE(queue.empty());
+    BOOST_REQUIRE_EQUAL(value, 0u);
+}
+
+BOOST_AUTO_TEST_CASE(collection__pop_front__single__empty_expected)
+{
+    const uint8_t expected = 42u;
+    data_queue queue{ expected };
+    const auto value = pop_front(queue);
+    BOOST_REQUIRE(queue.empty());
+    BOOST_REQUIRE_EQUAL(value, expected);
+}
+
+BOOST_AUTO_TEST_CASE(collection__pop_front__multiple__popped_expected)
+{
+    const uint8_t expected_value = 42u;
+    data_queue queue{ expected_value, 0, 1, 2, 3 };
+    const data_queue expected_queue{ 0, 1, 2, 3 };
+    const auto value = pop_front(queue);
+    BOOST_REQUIRE_EQUAL(value, expected_value);
+    BOOST_REQUIRE(queue == expected_queue);
+}
+
 // is_distinct
 
 BOOST_AUTO_TEST_CASE(collection__is_distinct__empty__true)