public GroupedItems <T, K, I> GroupItems(IEnumerable <T> items)
{
List <I> nullTo = null;
List <Pair <I, T> > nonKeyTo = null;
IDictionary <K, IList <Pair <I, T> > > groupItems = null;
// Perform grouping
if (CollectionUtil.IsNotNullOrEmpty(items))
{
int i = 0;
foreach (var item in items)
{
I index = indexCreator(item, i++);
// register any nulls
if (item == null)
{
if (nullTo == null)
{
nullTo = new List <I>();
}
nullTo.Add(index);
}
else
{
K key = keyCreator(item);
if (key == null)
{
// register null key
if (nonKeyTo == null)
{
nonKeyTo = new List <Pair <I, T> >();
}
nonKeyTo.Add(new Pair <I, T>(index, item));
}
else
{
// register keyed item
if (groupItems == null)
{
groupItems = new Dictionary <K, IList <Pair <I, T> > >();
}
CollectionUtil.AddToMappedList(groupItems, key, new Pair <I, T>(index, item));
}
}
}
}
return(new GroupedItems <T, K, I>()
{
NullItems = nullTo ?? Immutable.ReadOnlyList <I>(),
NullKeyItems = nonKeyTo ?? Immutable.ReadOnlyList <Pair <I, T> >(),
GroupItems = groupItems ?? Immutable.Dictionary <K, IList <Pair <I, T> > >()
});
}