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From the website: http://symbo1ics.com/blog/?p=2055 I implemented an interview questions:
Given a timer time() with nanosecond accuracy and given the interface
interface RealTimeCounter: void increment() int getCountInLastSecond() int getCountInLastMinute() int getCountInLastHour() int getCountInLastDay()
implement the interface. The getCountInL…
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| import java.lang.System; | |
| import java.util.Collections; | |
| import java.util.Comparator; | |
| import java.util.ArrayList; | |
| public class RealTimeCounterImpl implements RealTimeCounter { | |
| private static final int MAX_COUNTS = Integer.MAX_VALUE >> 8; | |
| private static final Long SECOND = 1000L; | |
| private static final Long MINUTE = 1000L*60L; | |
| private static final Long HOUR = 1000L*60L*60L; | |
| private static final Long DAY = 1000L*60L*60L*24L; | |
| private ArrayList<Long> listCounter; | |
| public int size() { | |
| return listCounter.size(); | |
| } | |
| public void increment() { | |
| if (listCounter.size() == MAX_COUNTS) { | |
| cleanup(DAY); | |
| } | |
| listCounter.add(System.currentTimeMillis()); | |
| } | |
| public int getCountInLastSecond() { | |
| return timeSince(System.currentTimeMillis(),SECOND); | |
| } | |
| public int getCountInLastMinute() { | |
| return timeSince(System.currentTimeMillis(),MINUTE); | |
| } | |
| public int getCountInLastHour() { | |
| return timeSince(System.currentTimeMillis(),HOUR); | |
| } | |
| public int getCountInLastDay() { | |
| return timeSince(System.currentTimeMillis(), DAY); | |
| } | |
| /** | |
| * This takes the current time and a timeframe | |
| * and returns the number of values in listCounter | |
| * that is larger than the now - timeframe. | |
| */ | |
| private int timeSince(Long now, Long timeFrame) { | |
| if (listCounter.isEmpty()) return 0; | |
| Long currentTime = now - timeFrame; | |
| // Do binary search to find the insertion place | |
| // This is an O(log n) opertation | |
| int insertionPoint = Collections.binarySearch(listCounter, | |
| currentTime); | |
| // If a value is not found with binary search | |
| // the insertion point is defined as (-(insertion point) - 1). | |
| if (insertionPoint >= 0) { | |
| // Exact match | |
| return listCounter.size() - insertionPoint; | |
| } | |
| else { | |
| return listCounter.size() + insertionPoint + 1; | |
| } | |
| } | |
| /** | |
| * Remove all items older than this timeframe. | |
| */ | |
| private void cleanup(Long timeFrame) { | |
| // Find all items older than a day an remove them | |
| Long currentTime = System.currentTimeMillis() - timeFrame ; | |
| int index = Collections.binarySearch(listCounter, currentTime); // O (log n) | |
| index = (index >= 0)?listCounter.size()-index:listCounter.size() + index + 1; | |
| // ArrayList doesnt have range remove | |
| // http://stackoverflow.com/questions/2289183/why-is-javas-abstractlists-removerange-method-protected/2289191#2289191 | |
| //listCounter.removeRange(0, index); | |
| listCounter.subList(0, listCounter.size()-index).clear(); // Possibly O(n) | |
| } | |
| public RealTimeCounterImpl () { | |
| listCounter = new ArrayList<Long>(MAX_COUNTS+1); | |
| } | |
| public static void main(String[] args) throws InterruptedException { | |
| RealTimeCounterImpl rtc = new RealTimeCounterImpl(); | |
| System.err.println("rtc.getCountInLastSecond(): " + rtc.getCountInLastSecond() + " [0]"); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| System.err.println("rtc.getCountInLastSecond(): " + rtc.getCountInLastSecond() + " [4]"); | |
| java.lang.Thread.sleep(5000); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| java.lang.Thread.sleep(500); | |
| System.err.println("rtc.getCountInLastSecond(): " + rtc.getCountInLastSecond() + " [7]"); | |
| System.err.println("rtc.getCountInLastMinute(): " + rtc.getCountInLastMinute() + " [11]"); | |
| java.lang.Thread.sleep(1000); | |
| rtc.increment(); | |
| rtc.increment(); | |
| rtc.increment(); | |
| System.err.println("rtc.getCountInLastSecond(): " + rtc.getCountInLastSecond() + " [3]"); | |
| System.err.println("rtc.size(): " + rtc.size() + " [14]"); | |
| //rtc.cleanup(MINUTE); | |
| rtc.cleanup(SECOND); | |
| System.err.println("cleanup"); | |
| System.err.println("rtc.size(): " + rtc.size() + " [3]"); | |
| System.err.println("rtc.getCountInLastSecond(): " + rtc.getCountInLastSecond() + " [3]"); | |
| } | |
| } | |
| interface RealTimeCounter { | |
| public void increment(); | |
| public int getCountInLastSecond(); | |
| public int getCountInLastMinute(); | |
| public int getCountInLastHour(); | |
| public int getCountInLastDay(); | |
| } |
Author
@cwasher I updated it to complete it. I could implement the ring buffer version if you wouldn't want to.
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@cegme If I'm not mistaken, trying to manage the size of your buffer is one of the biggest challenges with the problem. For example using a circular buffer with a set size of 100 runs fine so long as you don't have 101+ increments within the period, etc. As OP mentioned you could expect this to eventually fail (if you enforce buffer size) or run into the unbounded memory issue (for unbound buffers).
FWIW I think an optimal solution to this problem would run your increment() in O(1) to match the speed at which you're able to call counter.increment(). emphasis on the phrase "realtime counter"