从DateTime列表中找到最接近的上一个DateTime

Question

Is there a faster way of obtaining the closest previous (past) DateTime from a list of DateTimes when compared to a specific time? (the list comes from a SQL database)

public DateTime GetClosestPreviousDateTime(List<DateTime> dateTimes, DateTime specificTime)
{
    DateTime ret = null;
    var lowestDifference = TimeSpan.MaxValue;

    foreach (var date in dateTimes)
    {
        if (date >= specificTime)
            continue;

        var difference = specificTime- date;

        if (difference < lowestDifference)
        {
            lowestDifference = difference;
            ret = date;
        }
    }

    return ret;
}

The source list will be sorted since the dates in the list come from a SQL database where they are written consecutively.

Answer 1

It depends what you mean by "faster". The algorithm you show is O(N) so no you won't get faster than that - if by faster you mean is there a way to not have to iterate over all dates.

But if you mean can you shave off a few microseconds with some code that doesn't emit quite as many op codes, then yes of course. But is that really the issue here?

The answer will also change based on the size of the list, how accurate you need the answer to be, whether we can make any assumptions on the data (eg is it already sorted).

Answer 2

dateTimes.Where(x => x < specificTime).Max()

or if you want to handle the case where none exist:

dateTimes.Where(x => x < specificTime).DefaultIfEmpty().Max()

---

Later edit: Now you introduce new information that the List<> is already sorted. That was not in the question before.

With a sorted List<> , your algorithm is silly since it foreach es on and on, even after having reached the point where the entries "pass" the threshold specificTime . You can use instead BinarySearch (assuming List<> is sorted in ascending order and contains no duplicates ):

static DateTime GetClosestPreviousDateTime(List<DateTime> dateTimes, DateTime specificTime)
{
  var search = dateTimes.BinarySearch(specificTime);
  var index = (search < 0 ? ~search : search) - 1;
  if (index == -1)
    throw new InvalidOperationException("Not found");
  return dateTimes[index];
}

If you want to do it faster, just ask the database for the value, it will know how to find the answer fast; do not fetch the entire List<> to memory first. Use SQL or LINQ to the database.

Answer 3

Your problem is a classic search algorithm and binary search might suit you.

1. Sort list: dateTimes.Sort();
2. Apply Binary Search algo with similar logic as in your for statement.

Answer 4

dateTimes.Sort();
var closest = dateTimes[dateTimes.IndexOf(search) - 1];