Class: Gecode::Constraints::IntEnum::Channel::ChannelConstraint

Inherits:
Constraint
  • Object
show all
Defined in:
lib/gecoder/interface/constraints/int_enum/channel.rb

Overview

Describes a channel constraint which “channels” two enumerations of integer variables or one enumeration of integer variables and one enumeration of set variables. Channel constraints are used to give access to multiple viewpoints when modelling.

The channel constraint can be thought of as constraining the arrays to be each other’s inverses. I.e. if the i:th value in the first enumeration is j, then the j:th value in the second enumeration is constrained to be i.

Neither reification nor negation is supported.

Example

Lets say that we’re modelling a sequence of numbers that must be distinct and that we want access to the following two view simultaneously.

First view

The sequence is modelled as an array of integer variables where the first variable holds the value of the first position in the sequence, the second the value of the second position and so on.

# n variables with values from 0 to n-1.
elements = int_var_array(n, 0...n)
elements.must_be.distinct

That way elements will contain the actual sequence when the problem has been solved.

Second view

The sequence is modelled as the positions of each value in 0..(n-1) in the sequence. That way the first variable would hold the positions of 0 in the sequence, the second variable would hold the positions of 1 in the sequence and so on.

positions = int_var_array(n, 0...n)
positions.must_be.distinct

Connecting the views

In essence the relationship between the two arrays elements and positions is that

elements.map{ |e| e.val }[i] == positions.map{ |p| p.val }.index(i)

for all i in 0..(n-1). This relationship is enforced by the channel constraint as follows. 

elements.must.channel positions

Instance Method Summary collapse

Methods inherited from Constraint

#initialize

Constructor Details

This class inherits a constructor from Gecode::Constraints::Constraint

Instance Method Details

#post ⇒ Object 



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# File 'lib/gecoder/interface/constraints/int_enum/channel.rb', line 81

def post
  lhs, rhs = @params.values_at(:lhs, :rhs)
  Gecode::Raw::channel(@model.active_space, lhs.to_int_var_array, 
    rhs.to_int_var_array, *propagation_options)
end