// O(nrlogr) instead of O(nr^2) naive algorithm (n = max length of lists, r = number of lists)
public static IEnumerable <T> MergeMany <T>(this IEnumerable <IEnumerable <T> > lists, Func <T, T, T> mergeFunc) where T : IComparable <T>
{
var enumerators = lists.Select(list => list.GetEnumerator()).ToArray();
bool[] hasCur = new bool[enumerators.Length];
int numRunning = enumerators.Length;
var BST = new SortedDictionary <ComparableWrapper <T>, KeyValuePair <Wrapper <T>, List <int> > >();
bool first = true;
while (numRunning > 0)
{
IEnumerable <int> indices;
if (first)
{
first = false;
indices = Enumerable.Range(0, enumerators.Length);
for (int i = 0; i < hasCur.Length; i++)
{
hasCur[i] = true;
}
}
else
{
var top = BST.First();
BST.Remove(top.Key);
indices = top.Value.Value;
yield return(top.Key.Value);
}
foreach (int i in indices)
{
if (hasCur[i])
{
if (enumerators[i].MoveNext())
{
var val = enumerators[i].Current;
KeyValuePair <Wrapper <T>, List <int> > res;
if (BST.TryGetValue(new ComparableWrapper <T>(val), out res))
{
res.Key.Value = mergeFunc(res.Key.Value, val);
res.Value.Add(i);
}
else
{
var wrapper = new ComparableWrapper <T>(val);
res = new KeyValuePair <Wrapper <T>, List <int> >(wrapper, new List <int>());
res.Value.Add(i);
BST.Add(wrapper, res);
}
}
else
{
hasCur[i] = false;
numRunning--;
}
}
}
}
}
public int CompareTo(ComparableWrapper <T> other)
{
return(_comparer.Compare(Value, other.Value));
}
