class MoreMath::Subset

def self.for(collection, rank = 0)

Returns the subset of the collection +collection+ for the rank +rank+. 
def self.for(collection, rank = 0)
  subset = new(collection.size, rank)
  subset.instance_variable_set(:@collection, collection)
  subset
end

def self.power_set(collection)

Returns the power set of the collection +collection+. 
def self.power_set(collection)
  self.for(collection).map(&:value)
end

def initialize(size, rank = 0)

+rank+.
Returns a Subset instance for a collection of size +size+ with the rank 
def initialize(size, rank = 0)
  @size, self.rank = size, rank
  @last = (1 << size) - 1
end

def project(data = nil)

ensuring the input data size matches the subset's size.
subset's indices determined by its rank and returns the result, while
This method maps elements from a given dataset based on the 
def project(data = nil)
  data ||= @collection || (0...size).to_a
  raise ArgumentError, "data size is != #{size}!" if data.size != size
  value.map { |i| data[i] }
end

def rank=(m)

Assigns m to the rank attribute of this object. 
def rank=(m)
  @rank = m % (1 << size)
end

def value

set it applies to the array [ 0, 1, ..., size - 1 ] instead.)
Returns the subset for rank #rank and #collection. (If no collection was 
def value
  result = []
  c = @collection || (0...size).to_a
  r = @rank
  0.upto(size) do |i|
    r[i] == 1 and result << c[i]
  end
  result
end