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third/spdlog/details/circular_q.h

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+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
+// circular q view of std::vector.
+#pragma once
+
+#include <cassert>
+#include <vector>
+
+#include "spdlog/common.h"
+
+namespace spdlog {
+namespace details {
+template <typename T>
+class circular_q {
+    size_t max_items_ = 0;
+    typename std::vector<T>::size_type head_ = 0;
+    typename std::vector<T>::size_type tail_ = 0;
+    size_t overrun_counter_ = 0;
+    std::vector<T> v_;
+
+public:
+    using value_type = T;
+
+    // empty ctor - create a disabled queue with no elements allocated at all
+    circular_q() = default;
+
+    explicit circular_q(size_t max_items)
+        : max_items_(max_items + 1)  // one item is reserved as marker for full q
+          ,
+          v_(max_items_) {}
+
+    circular_q(const circular_q &) = default;
+    circular_q &operator=(const circular_q &) = default;
+
+    // move cannot be default,
+    // since we need to reset head_, tail_, etc to zero in the moved object
+    circular_q(circular_q &&other) SPDLOG_NOEXCEPT { copy_moveable(std::move(other)); }
+
+    circular_q &operator=(circular_q &&other) SPDLOG_NOEXCEPT {
+        copy_moveable(std::move(other));
+        return *this;
+    }
+
+    // push back, overrun (oldest) item if no room left
+    void push_back(T &&item) {
+        if (max_items_ > 0) {
+            v_[tail_] = std::move(item);
+            tail_ = (tail_ + 1) % max_items_;
+
+            if (tail_ == head_)  // overrun last item if full
+            {
+                head_ = (head_ + 1) % max_items_;
+                ++overrun_counter_;
+            }
+        }
+    }
+
+    // Return reference to the front item.
+    // If there are no elements in the container, the behavior is undefined.
+    const T &front() const { return v_[head_]; }
+
+    T &front() { return v_[head_]; }
+
+    // Return number of elements actually stored
+    size_t size() const {
+        if (tail_ >= head_) {
+            return tail_ - head_;
+        } else {
+            return max_items_ - (head_ - tail_);
+        }
+    }
+
+    // Return const reference to item by index.
+    // If index is out of range 0…size()-1, the behavior is undefined.
+    const T &at(size_t i) const {
+        assert(i < size());
+        return v_[(head_ + i) % max_items_];
+    }
+
+    // Pop item from front.
+    // If there are no elements in the container, the behavior is undefined.
+    void pop_front() { head_ = (head_ + 1) % max_items_; }
+
+    bool empty() const { return tail_ == head_; }
+
+    bool full() const {
+        // head is ahead of the tail by 1
+        if (max_items_ > 0) {
+            return ((tail_ + 1) % max_items_) == head_;
+        }
+        return false;
+    }
+
+    size_t overrun_counter() const { return overrun_counter_; }
+
+    void reset_overrun_counter() { overrun_counter_ = 0; }
+
+private:
+    // copy from other&& and reset it to disabled state
+    void copy_moveable(circular_q &&other) SPDLOG_NOEXCEPT {
+        max_items_ = other.max_items_;
+        head_ = other.head_;
+        tail_ = other.tail_;
+        overrun_counter_ = other.overrun_counter_;
+        v_ = std::move(other.v_);
+
+        // put &&other in disabled, but valid state
+        other.max_items_ = 0;
+        other.head_ = other.tail_ = 0;
+        other.overrun_counter_ = 0;
+    }
+};
+}  // namespace details
+}  // namespace spdlog