ImmutableSet

A faster, immutable replacement for Ruby's Set.

On Ruby >= 2.4, all operations are faster, some by several orders of magnitude (see benchmarks).

Useful for ...

• creating and working with large sorted sets
• intersecting, merging, diffing, checking for subsets etc.
• the advantages of immutability

Not useful for ...

• small sets and other cases where performance is negligible
• sets with mixed members or any members that are not mutually comparable
• doing a lot of adding, removing, and checking of single items

Usage

require 'immutable_set'

class MySet < ImmutableSet; end

Mutating methods of Set (e.g. #add, #delete) are removed. They can be substituted like this if needed:

set1 = MySet[1, 2, 3]
set1 += MySet[4] # => MySet[1, 2, 3, 4]
set1 -= MySet[3] # => MySet[1, 2, 4]

Immutability is required for most of the performance optimizations.

All other methods behave as in Set/SortedSet, so see the official documentation for details about what they do.

New methods

#distinct_bounds?

Returns true iff the passed set is beyond the #minmax boundaries of self.

MySet[2, 4].distinct_bounds?(MySet[3]) # => false
MySet[2, 4].distinct_bounds?(MySet[5]) # => true

::from_ranges

Returns a set built from all passed Ranges.

MySet.from_ranges(2..4, 6..8) # => MySet[2, 3, 4, 6, 7, 8]

#inversion

Returns a new set of the same class, containing all members from..upto that are not in self. Faster than Set.new(from..upto) - self.

MySet[3, 5].inversion(from: 1, upto: 4) # => MySet[1, 2, 4]`
MySet['c'].inversion(from: 'a', upto: 'd') # => MySet['a', 'b', 'd']

Performance optimizations

The cost of many methods is reduced from O(m*n) to O(m+n) or better. The underlying ideas are:

• never needing to sort, because the internal @hash is built in order and then frozen
• remembering #max cheaply whenever possible
• this allows skipping unneeded checks for members outside the own #minmax boundaries
• avoiding unneeded lookups during comparisons by iterating over both sets in parallel in C
• parallel iteration can skip over gaps in either set since both hashes are ordered
• when using Ruby, preferring #while over slower, scope-building iteration methods

Benchmarks

Run rake benchmark or see BENCHMARK.md.