a/util: Add pop_back to HistoryBuffer

src/xrt/auxiliary/util/u_id_ringbuffer.cpp

@@ -165,6 +165,15 @@ u_id_ringbuffer_pop_front(struct u_id_ringbuffer *uirb)
 	DEFAULT_CATCH()
 }
 
+void
+u_id_ringbuffer_pop_back(struct u_id_ringbuffer *uirb)
+{
+	try {
+		uirb->helper.pop_back();
+	}
+	DEFAULT_CATCH()
+}
+
 
 int32_t
 u_id_ringbuffer_get_back(struct u_id_ringbuffer *uirb, uint64_t *out_id)

src/xrt/auxiliary/util/u_id_ringbuffer.h

@@ -65,6 +65,16 @@ u_id_ringbuffer_push_back(struct u_id_ringbuffer *uirb, uint64_t id);
 void
 u_id_ringbuffer_pop_front(struct u_id_ringbuffer *uirb);
 
+/**
+ * Pop an element from the back, if any
+ *
+ * @param uirb self pointer.
+ *
+ * @public @memberof u_id_ringbuffer
+ */
+void
+u_id_ringbuffer_pop_back(struct u_id_ringbuffer *uirb);
+
 /**
  * Get the back (most recent) of the buffer
  *

src/xrt/auxiliary/util/u_template_historybuf.hpp

@@ -52,6 +52,20 @@ public:
 	void
 	push_back(const T &element);
 
+	/*!
+	 * @brief Logically remove the newest element from the buffer.
+	 *
+	 * This is permitted to invalidate iterators. They won't be poisoned,
+	 * but they will return something you don't expect.
+	 *
+	 * @return true if there was something to pop.
+	 */
+	bool
+	pop_back() noexcept
+	{
+		return helper_.pop_back();
+	}
+
 	/*!
 	 * @brief Logically remove the oldest element from the buffer.
 	 *

src/xrt/auxiliary/util/u_template_historybuf_impl_helpers.hpp

@@ -96,6 +96,15 @@ public:
 	void
 	pop_front() noexcept;
 
+	/*!
+	 * @brief Record the logical removal of the back element, if any.
+	 *
+	 * Returns false if the buffer is empty. Does not actually modify the
+	 * value stored in the backing array.
+	 */
+	bool
+	pop_back() noexcept;
+
 	//! Get the inner index of the front (oldest) value, or capacity_ if empty.
 	size_t
 	front_inner_index() const noexcept;
@@ -192,7 +201,17 @@ RingBufferHelper::pop_front() noexcept
 		length_--;
 	}
 }
-
+inline bool
+RingBufferHelper::pop_back() noexcept
+{
+	if (empty()) {
+		return false;
+	}
+	// adding capacity before -1 to avoid overflow
+	latest_inner_idx_ = (latest_inner_idx_ + capacity_ - 1) % capacity_;
+	length_--;
+	return true;
+}
 inline size_t
 RingBufferHelper::front_inner_index() const noexcept
 {

tests/tests_history_buf.cpp

@@ -217,6 +217,11 @@ TEST_CASE("u_template_historybuf")
 		CHECK_FALSE(buffer.begin().valid());
 		CHECK_FALSE(buffer.end().valid());
 		CHECK(buffer.begin() == buffer.end());
+		{
+			INFO("Check after pop_back");
+			REQUIRE_FALSE(buffer.pop_back());
+			CHECK(buffer.empty());
+		}
 	}
 	SECTION("behavior with one")
 	{
@@ -277,6 +282,14 @@ TEST_CASE("u_template_historybuf")
 		CHECK(buffer.back() == 0);
 
 		CHECK(*buffer.begin() == buffer.front());
+
+		{
+			INFO("Check after pop_back");
+			REQUIRE(buffer.pop_back());
+			CHECK(buffer.size() == 0);
+
+			REQUIRE_FALSE(buffer.pop_back());
+		}
 	}
 
 	SECTION("behavior with two")
@@ -352,6 +365,15 @@ TEST_CASE("u_template_historybuf")
 			CHECK(*buffer.begin() == buffer.front());
 			CHECK(buffer.back() == *(--buffer.end()));
 		}
+		SECTION("Check after pop_back")
+		{
+			REQUIRE(buffer.pop_back());
+			CHECK(buffer.size() == 1);
+			CHECK(buffer.front() == 0);
+
+			REQUIRE(buffer.pop_back());
+			CHECK(buffer.size() == 0);
+		}
 	}
 
 	SECTION("algorithm behavior with 3")

tests/tests_id_ringbuffer.cpp

@@ -23,6 +23,12 @@ TEST_CASE("u_template_historybuf")
 		uint64_t out_id = 0;
 		CHECK(u_id_ringbuffer_get_front(buffer, &out_id) < 0);
 		CHECK(u_id_ringbuffer_get_back(buffer, &out_id) < 0);
+
+		{
+			INFO("Check after pop_back");
+			u_id_ringbuffer_pop_back(buffer);
+			CHECK(u_id_ringbuffer_is_empty(buffer));
+		}
 	}
 	SECTION("behavior with one")
 	{
@@ -74,6 +80,16 @@ TEST_CASE("u_template_historybuf")
 			CHECK(u_id_ringbuffer_get_at_clamped_age(buffer, 2, &out_id) == zero_inner_index);
 			CHECK(out_id == 0);
 		}
+
+		{
+			INFO("Check after pop_back");
+			u_id_ringbuffer_pop_back(buffer);
+
+			CHECK(u_id_ringbuffer_is_empty(buffer));
+
+			u_id_ringbuffer_pop_back(buffer);
+			CHECK(u_id_ringbuffer_is_empty(buffer));
+		}
 	}
 
 	SECTION("behavior with two")
@@ -154,6 +170,19 @@ TEST_CASE("u_template_historybuf")
 				CHECK(out_id == 0);
 			}
 		}
+		SECTION("Check after pop_back")
+		{
+			u_id_ringbuffer_pop_back(buffer);
+			CHECK(1 == u_id_ringbuffer_get_size(buffer));
+			uint64_t out_id_front = 55;
+			CHECK(u_id_ringbuffer_get_front(buffer, &out_id_front) == zero_inner_index);
+			CHECK(out_id_front == 0);
+
+			u_id_ringbuffer_pop_back(buffer);
+			CHECK(0 == u_id_ringbuffer_get_size(buffer));
+			uint64_t out_id = 0;
+			CHECK(u_id_ringbuffer_get_front(buffer, &out_id) < 0);
+		}
 	}
 
 	SECTION("algorithm behavior with 3")