Add a SimpleBufferPool for uv::Buffer (#1215)

Buffers are reused on a regular basis in many use cases.

wpiutil/src/main/native/include/wpi/uv/Buffer.h

@@ -15,6 +15,7 @@
 #include <utility>
 
 #include "wpi/ArrayRef.h"
+#include "wpi/SmallVector.h"
 #include "wpi/StringRef.h"
 
 namespace wpi {
@@ -86,6 +87,65 @@ class Buffer : public uv_buf_t {
   }
 };
 
+/**
+ * A simple pool allocator for Buffers.
+ * Buffers are allocated individually but are reused rather than returned
+ * to the heap.
+ * @tparam DEPTH depth of pool
+ */
+template <size_t DEPTH = 4>
+class SimpleBufferPool {
+ public:
+  /**
+   * Constructor.
+   * @param size Size of each buffer to allocate.
+   */
+  explicit SimpleBufferPool(size_t size = 4096) : m_size{size} {}
+
+  /**
+   * Allocate a buffer.
+   */
+  Buffer Allocate() {
+    if (m_pool.empty()) return Buffer::Allocate(m_size);
+    auto buf = m_pool.back();
+    m_pool.pop_back();
+    buf.len = m_size;
+    return buf;
+  }
+
+  /**
+   * Allocate a buffer.
+   */
+  Buffer operator()() { return Allocate(); }
+
+  /**
+   * Release allocated buffers back into the pool.
+   * This is NOT safe to use with arbitrary buffers unless they were
+   * allocated with the same size as the buffer pool allocation size.
+   */
+  void Release(MutableArrayRef<Buffer> bufs) {
+    for (auto& buf : bufs) m_pool.emplace_back(buf.Move());
+  }
+
+  /**
+   * Clear the pool, releasing all buffers.
+   */
+  void Clear() {
+    for (auto& buf : m_pool) buf.Deallocate();
+    m_pool.clear();
+  }
+
+  /**
+   * Get number of buffers left in the pool before a new buffer will be
+   * allocated from the heap.
+   */
+  size_t Remaining() const { return m_pool.size(); }
+
+ private:
+  SmallVector<Buffer, DEPTH> m_pool;
+  size_t m_size;
+};
+
 }  // namespace uv
 }  // namespace wpi
 

wpiutil/src/test/native/cpp/uv/UvBufferTest.cpp

@@ -0,0 +1,50 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "wpi/uv/Buffer.h"  // NOLINT(build/include_order)
+
+#include "gtest/gtest.h"  // NOLINT(build/include_order)
+
+namespace wpi {
+namespace uv {
+
+TEST(UvSimpleBufferPool, ConstructDefault) {
+  SimpleBufferPool<> pool;
+  auto buf1 = pool.Allocate();
+  ASSERT_EQ(buf1.len, 4096u);
+}
+
+TEST(UvSimpleBufferPool, ConstructSize) {
+  SimpleBufferPool<4> pool{8192};
+  auto buf1 = pool.Allocate();
+  ASSERT_EQ(buf1.len, 8192u);
+}
+
+TEST(UvSimpleBufferPool, ReleaseReuse) {
+  SimpleBufferPool<4> pool;
+  auto buf1 = pool.Allocate();
+  auto buf1copy = buf1;
+  auto origSize = buf1.len;
+  buf1.len = 8;
+  pool.Release(buf1);
+  ASSERT_EQ(buf1.base, nullptr);
+  auto buf2 = pool.Allocate();
+  ASSERT_EQ(buf1copy.base, buf2.base);
+  ASSERT_EQ(buf2.len, origSize);
+}
+
+TEST(UvSimpleBufferPool, ClearRemaining) {
+  SimpleBufferPool<4> pool;
+  auto buf1 = pool.Allocate();
+  pool.Release(buf1);
+  ASSERT_EQ(pool.Remaining(), 1u);
+  pool.Clear();
+  ASSERT_EQ(pool.Remaining(), 0u);
+}
+
+}  // namespace uv
+}  // namespace wpi