PX4-Autopilot/src/modules/mavlink/timestamped_list.h

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/**
 * @file timestamped list.h
 * Fixed size list with timestamps.
 *
 * The list has a fixed size that is set at instantiation and is based
 * on timestamps. If a new value is put into a full list, the oldest value
 * is overwritten.
 *
 * @author Julian Oes <julian@oes.ch>
 */

#pragma once

#include <drivers/drv_hrt.h>

/**
 * @class TimestampedList
 */
template <class T, int NUM_ITEMS>
class TimestampedList
{
public:
	TimestampedList() = default;
	~TimestampedList() = default;

	/**
	 * Insert a value into the list, overwrite the oldest entry if full.
	 */
	void put(const T &new_value)
	{
		hrt_abstime now = hrt_absolute_time();

		// Insert it wherever there is a free space.
		for (int i = 0; i < NUM_ITEMS; ++i) {
			if (_list[i].timestamp_us == 0) {
				_list[i].timestamp_us = now;
				_list[i].value = new_value;
				return;
			}
		}

		// Find oldest entry.
		int oldest_i = 0;

		for (int i = 1; i < NUM_ITEMS; ++i) {
			if (_list[i].timestamp_us < _list[oldest_i].timestamp_us) {
				oldest_i = i;
			}
		}

		// And overwrite oldest.
		_list[oldest_i].timestamp_us = now;
		_list[oldest_i].value = new_value;
	}

	/**
	 * Before iterating using get_next(), reset to start.
	 */
	void reset_to_start()
	{
		_current_i = -1;
	}

	/**
	 * Iterate through all active values (not sorted).
	 * Return nullptr if at end of list.
	 *
	 * This is basically a poor man's iterator.
	 */
	T *get_next()
	{
		// Increment first, then leave it until called again.
		++_current_i;

		for (int i = _current_i; i < NUM_ITEMS; ++i) {
			if (_list[i].timestamp_us != 0) {
				_current_i = i;
				return &_list[i].value;
			}
		}

		return nullptr;
	}

	/**
	 * Disable the last item that we have gotten.
	 */
	void drop_current()
	{
		if (_current_i < NUM_ITEMS) {
			_list[_current_i].timestamp_us = 0;
		}
	}

	/**
	 * Update the timestamp of the item we have gotten.
	 */
	void update_current()
	{
		if (_current_i < NUM_ITEMS) {
			_list[_current_i].timestamp = hrt_absolute_time();
		}
	}

private:
	struct item_s {
		hrt_abstime timestamp_us = 0; // 0 signals inactive.
		T value{};
	};

	item_s _list[NUM_ITEMS] {};

	int _current_i{-1};
};