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Efficient way to implement 'events since x' in Java

I want to be-able to ask an object 'how many events have occurred in the last x seconds' where the x is an argument.

e.g. how many events have occurred in the last 120 seconds..

How I approached is linear based on the number of events occurring but was wanting to see what the most efficient way (space & time) to achieve this requirement?;

public class TimeSinceStat {
private List<DateTime> eventTimes = new ArrayList<>();

public void apply() {
            eventTimes.add(DateTime.now());
           }


public int eventsSince(int seconds) {
    DateTime startTime = DateTime.now().minus(Seconds.seconds(seconds));
    for (int i = 0; i < orderTimes.size(); i++) {
        DateTime dateTime = eventTimes.get(i);
        if (dateTime.compareTo(startTime) > 0)
            return eventTimes.subList(i, eventTimes.size()).size();
    }
    return 0;
}

(PS - i'm using JodaTime for the date/time representation)

Edit:

The key of this algorithm to find all events that have happened in the last x seconds; the exact start time (e.g. now - 30 seconds) is may or maynot be in the collection

Answer*

Since `DateTime` already implements `Comparable` interface, I would recommend storing the data in a `TreeMap` instead, and you could use [`TreeMap#tailMap`](http://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html#tailMap-K-boolean-) to get a subtree of the `DateTime`'s that occurs in the desired time.

Based on your code:

    public class TimeSinceStat {
        //just in case two or more events start at the "same time"
        private NavigableMap<DateTime, Integer> eventTimes = new TreeMap<>();
        //if this class needs to be used in multiple threads, use ConcurrentSkipListMap instead of TreeMap

        public void apply() {
            DateTime dateTime = DateTime.now();
            Integer times = eventTimes.contains(dateTime) != null ? 0 : (eventTimes.get(dateTime) + 1);
            eventTimes.put(dateTime, times);
        }

        public int eventsSince(int seconds) {
            DateTime startTime = DateTime.now().minus(Seconds.seconds(seconds));
            NavigableMap<DateTime, Integer> eventsInRange = eventTimes.tailMap(startTime, true);
            int counter = 0;
            for (Integer time : eventsInRange.values()) {
                counter += time;
            }
            return counter;
        }
    }

Edit Summary*

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@goat so? TreeMap also has the data sorted and handles the algorithm in O(log n), while using the List approach takes O(n), and its usage would be of 1 line.

Luiggi Mendoza
– Luiggi Mendoza

2014-06-12 14:41:02 +00:00
Commented Jun 12, 2014 at 14:41
@goat but it's not sorted by event time - OP is recommended to store it in a structure sorted by event time

eis
– eis

2014-06-12 14:42:13 +00:00
Commented Jun 12, 2014 at 14:42
OP it's storing only the time, not the event, so I think a Set suits better than a Map

Pablo Lozano
– Pablo Lozano

2014-06-12 14:48:04 +00:00
Commented Jun 12, 2014 at 14:48

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Efficient way to implement 'events since x' in Java

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