feat: add small_fifo container

Author: mhx

CMakeLists.txt

@@ -692,6 +692,7 @@ if(WITH_TESTS)
       test/pcm_sample_transformer_test.cpp
       test/pinned_byte_span_store_test.cpp
       test/scoped_output_capture_test.cpp
+      test/small_fifo_test.cpp
       test/sorted_array_map_test.cpp
       test/sparse_chunk_codec_test.cpp
       test/sparse_file_seeker_test.cpp

include/dwarfs/container/small_fifo.h

@@ -0,0 +1,114 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/**
+ * \author     Marcus Holland-Moritz (github@mhxnet.de)
+ * \copyright  Copyright (c) Marcus Holland-Moritz
+ *
+ * This file is part of dwarfs.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the “Software”), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <cstddef>
+#include <iterator>
+#include <utility>
+
+#include <dwarfs/small_vector.h>
+
+namespace dwarfs::container {
+
+/**
+ * A minimal FIFO over small_vector, for short-lived, usually tiny queues
+ *
+ * std::deque allocates a whole block up front (512 bytes under libstdc++,
+ * 4096 under libc++), even for a single element. That's often wasteful.
+ *
+ * Storage is reclaimed to the inline buffer whenever the queue drains, which
+ * for a produce-a-few / consume-to-empty pattern means no allocation at all
+ * after the first growth beyond N.
+ */
+template <typename T, std::size_t N>
+class small_fifo {
+ public:
+  using value_type = T;
+
+  static constexpr size_t inline_capacity = N;
+
+  small_fifo() = default;
+
+  template <std::input_iterator InputIt>
+  small_fifo(InputIt first, InputIt last)
+      : v_{first, last} {}
+
+  bool empty() const noexcept { return head_ == v_.size(); }
+
+  std::size_t size() const noexcept { return v_.size() - head_; }
+
+  std::size_t capacity() const noexcept { return v_.capacity(); }
+
+  T& front() noexcept { return v_[head_]; }
+  T const& front() const noexcept { return v_[head_]; }
+
+  void pop_front() {
+    ++head_;
+
+    if (head_ == v_.size()) {
+      // drained: return to the inline buffer and reuse it next time round
+      v_.clear();
+      head_ = 0;
+    } else if (head_ >= N && 2 * head_ >= v_.size()) {
+      // amortized O(1): compact once dead slots outnumber live ones
+      v_.erase(v_.begin(), v_.begin() + head_);
+      head_ = 0;
+    }
+  }
+
+  void push_back(T value) { v_.push_back(std::move(value)); }
+
+  template <typename... Args>
+  T& emplace_back(Args&&... args) {
+    return v_.emplace_back(std::forward<Args>(args)...);
+  }
+
+  // bulk append: the common case is an empty queue, where the incoming
+  // buffer can simply be adopted.
+  template <typename Range>
+  void append(Range&& r) {
+    if (empty()) {
+      clear();
+    }
+    for (auto&& e : r) {
+      v_.push_back(std::move(e));
+    }
+  }
+
+  void clear() noexcept {
+    v_.clear();
+    head_ = 0;
+  }
+
+ private:
+  small_vector<T, N> v_;
+  std::size_t head_{0};
+};
+
+} // namespace dwarfs::container