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>
class TimestampedList
{
public:
    TimestampedList(int num_items)
    {
        _list = new item_t[num_items];
        _list_len = num_items;
    }

    ~TimestampedList()
    {
        delete[] _list;
    }

    /**
     * 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 < _list_len; ++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 < _list_len; ++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 < _list_len; ++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 < _list_len) {
            _list[_current_i].timestamp_us = 0;
        }
    }

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

    /* do not allow copying or assigning this class */
    TimestampedList(const TimestampedList &) = delete;
    TimestampedList operator=(const TimestampedList &) = delete;

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

    item_t *_list = nullptr;
    int _list_len = 0;
    int _current_i = -1;
};