IndexSet<E> class abstract final Collections

A hash set where element iteration order is independent of their hash codes.

Iteration Order

The keys, E, have a consistent order that is determined by the sequence of add and remove calls (or similar operations):

final set = IndexSet<int>();
set.add(1);
set.add(2);
set.add(3);
set.remove(2);
print(set); // {1, 3}

Unlike LinkedHashSet, remove calls (and similar operations) shift the iteration order; a removed element is swapped with the last element in the set, and then removed:

final set = IndexSet<int>();
set.add(1);
set.add(2);
set.add(3);
set.remove(1);
print(set); // {3, 2}

Index Access

IndexSet provides an additional set of operators, operator[] and operator[]=, to access elements by their order in the iteration sequence; the index is zero-based and must be within bounds (0 <= index < length):

final set = IndexSet<int>();
set.add(1);
set.add(2);
print(set[0]); // 1
print(set[1]); // 2

Performance

If you want the properties of IndexSet, or its strongest performance characteristics fit your workload, it might be the fastest pure Dart Set implementation available; for example package:sector uses it internally for its graph search algorithms.

IndexSet derives some performance facts from how it is constructed:

1. Iteration is as fast as a List (~2-3x faster than dart:collection).
2. Removals are nearly as fast as HashSet (~300-400x faster).
3. Creation is ~2x slower than HashSet, 5x faster than LinkedHashSet.