Question

I was looking for some fast and efficient method do merge items in array. This is my scenario. The collection is sorted by From. Adjacent element not necessarily differ by 1, that is there can be gaps between the last To and the next From, but they never overlap.

var list = new List<Range>();
list.Add(new Range() { From = 0, To = 1, Category = "AB" });
list.Add(new Range() { From = 2, To = 3, Category = "AB" });
list.Add(new Range() { From = 4, To = 5, Category = "AB" });
list.Add(new Range() { From = 6, To = 8, Category = "CD" });
list.Add(new Range() { From = 9, To = 11, Category = "AB" }); // 12 is missing, this is ok
list.Add(new Range() { From = 13, To = 15, Category = "AB" });

I would like the above collection to be merged in such way that the first three (this number can vary, from at least 2 elements to as many as the condition is satisfied) elements become one element. Cannot merge elements with different category.

new Range() { From = 0, To = 5, Category = "AB" };

So that the resulting collection would have 4 elements total.

0 - 5    AB
6 - 8    CD
9 - 11   AB // no merging here, 12 is missing
13 - 15  AB

I have a very large collection with over 2.000.000 items and I would like to this as efficiently as possible.

Answer 1

Here’s a generic, reusable solution rather than an ad hoc, specific solution.
(Updated based on comments)

IEnumerable<T> Merge<T>(this IEnumerable<T> coll, 
                      Func<T,T,bool> canBeMerged, Func<T,T,T>mergeItems)
{
    using(IEnumerator<T> iter = col.GetEnumerator())
    {
      if (iter.MoveNext())
      {
          T lhs = iter.Current;
          while(iter.MoveNext())
          {
              T rhs = iter.Current;
              if (canBeMerged(lhs, rhs)
                 lhs=mergeItems(lhs, rhs);
              else
              {
                 yield return lhs;
                 lhs= rhs;
              }
          }
          yield return lhs;
      }
    }
}

You will have to provide method to determine if the item can be merged, and to merge them.
These really should be part of your Range class, so it would be called like them:

list.Merge((l,r)=> l.IsFollowedBy(r), (l,r)=> l.CombineWith(r));

If you don’t have these method, then you would have to call it like:

list.Merge((l,r)=> l.Category==r.Category && l.To +1 == r.From,
           (l,r)=> new Range(){From = l.From, To=r.To, Category = l.Category});