Class: Sequitur::Production

Inherits:
Object
  • Object
show all
Defined in:
lib/sequitur/production.rb

Overview

In a context-free grammar, a production is a rule in which its left-hand side (LHS) consists solely of a non-terminal symbol and the right-hand side (RHS) consists of a sequence of symbols. The symbols in RHS can be either terminal or non-terminal symbols. The rule stipulates that the LHS is equivalent to the RHS, in other words every occurrence of the LHS can be substituted to corresponding RHS. Implementation note: the object id of the production is taken as its LHS.

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize ⇒ Production

Constructor. Build a production with an empty RHS.



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# File 'lib/sequitur/production.rb', line 26

def initialize()
  @rhs = SymbolSequence.new
  @refcount = 0
  @digrams = []
end

Instance Attribute Details

#digrams ⇒ Object (readonly)

The sequence of digrams appearing in the RHS



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# File 'lib/sequitur/production.rb', line 22

def digrams
  @digrams
end

#refcount ⇒ Object (readonly)

The reference count (= how times other productions reference this one)



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# File 'lib/sequitur/production.rb', line 19

def refcount
  @refcount
end

#rhs ⇒ Object (readonly)

The right-hand side (rhs) consists of a sequence of grammar symbols



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# File 'lib/sequitur/production.rb', line 16

def rhs
  @rhs
end

Instance Method Details

#==(other) ⇒ Object

Identity testing.



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# File 'lib/sequitur/production.rb', line 35

def ==(other)
  return true if object_id == other.object_id

  result = if other.is_a?(ProductionRef)
             (other == self)
           else
             false
           end

  return result
end

#accept(aVisitor) ⇒ Object

Part of the 'visitee' role in Visitor design pattern.



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# File 'lib/sequitur/production.rb', line 223

def accept(aVisitor)
  aVisitor.start_visit_production(self)

  rhs.accept(aVisitor)

  aVisitor.end_visit_production(self)
end

#append_symbol(aSymbol) ⇒ Object

Add a (grammar) symbol at the end of the RHS.



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# File 'lib/sequitur/production.rb', line 132

def append_symbol(aSymbol)
  case aSymbol
    when Production
      new_symb = ProductionRef.new(aSymbol)
    when ProductionRef
      if aSymbol.unbound?
        msg = 'Fail to append reference to nil production in '
        msg << to_string
        raise StandardError, msg
      end
      new_symb = aSymbol.dup
    else
      new_symb = aSymbol
  end

  rhs << new_symb
  digrams << Digram.new(rhs[-2], rhs[-1], self) if rhs.size >= 2
end

#clear_rhs ⇒ Object

Clear the right-hand side. Any referenced production has its reference counter decremented.



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# File 'lib/sequitur/production.rb', line 153

def clear_rhs()
  rhs.clear
end

#decr_refcount ⇒ Object

Decrement the reference count by one.

Raises:

  • (StandardError) 


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# File 'lib/sequitur/production.rb', line 60

def decr_refcount()
  raise StandardError, 'Internal error' if @refcount.zero?
  @refcount -= 1
end

#derive_step(another) ⇒ Object

Replace every occurrence of 'another' production in self.rhs by the symbols in the rhs of 'another'.

Given the production p_A : a p_B b p_B c

And the production p_B : x y

Then...

p_A.derive_step(p_B)

Modifies p_A as into: p_A -> a x y b x y c

Examples:

Synopsis





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# File 'lib/sequitur/production.rb', line 208

def derive_step(another)
  (0...rhs.size).to_a.reverse_each do |index|
    next unless rhs[index] == another

    rhs.insert_at(index + 1, another.rhs)
    another.decr_refcount
    rhs.delete_at(index)
  end

  recalc_digrams
end

#empty? ⇒ Boolean

Is the rhs empty? @ return true if the rhs has no members.



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# File 'lib/sequitur/production.rb', line 50

def empty?
  return rhs.empty?
end

#incr_refcount ⇒ Object

Increment the reference count by one.



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# File 'lib/sequitur/production.rb', line 55

def incr_refcount()
  @refcount += 1
end

#last_digram ⇒ Digram

Retrieve the last digram appearing in the RHS (if any).



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# File 'lib/sequitur/production.rb', line 116

def last_digram()
  result = digrams.empty? ? nil : digrams.last
  return result
end

#positions_of(symb1, symb2) ⇒ Array

Find all the positions where the digram occurs in the rhs

Examples:

# Given the production p : a b c a b a b d
#Then ...
p.positions_of(a, b) # => [0, 3, 5]
# Caution: "overlapping" digrams shouldn't be counted
# Given the production p : a a b a a a c d
# Then ...
p.positions_of(a, a) # => [0, 3]



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# File 'lib/sequitur/production.rb', line 169

def positions_of(symb1, symb2)
  # Find the positions where the digram occur in rhs
  indices = [ -2 ] # Dummy index!
  (0...rhs.size).each do |i|
    next if i == indices.last + 1
    indices << i if (rhs[i] == symb1) && (rhs[i + 1] == symb2)
  end

  indices.shift

  return indices
end

#recalc_digrams ⇒ Array

Enumerate the digrams appearing in the right-hand side (rhs)



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# File 'lib/sequitur/production.rb', line 83

def recalc_digrams()
  return [] if rhs.size < 2

  result = []
  rhs.symbols.each_cons(2) { |couple| result << Digram.new(*couple, self) }
  @digrams = result
end

#reduce_step(another) ⇒ Object

Given that the production P passed as argument has exactly 2 symbols in its rhs s1 s2, substitute in the rhs of self all occurrences of s1 s2 by a reference to P.



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# File 'lib/sequitur/production.rb', line 188

def reduce_step(another)
  (symb1, symb2) = another.rhs.symbols
  pos = positions_of(symb1, symb2).reverse

  # Replace the two symbol sequence by the production
  pos.each { |index| rhs.reduce_step(index, another) }

  recalc_digrams
end

#references ⇒ Array of ProductionRef

Select the references to production appearing in the rhs.



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# File 'lib/sequitur/production.rb', line 68

def references()
  return rhs.references
end

#references_of(a_prod) ⇒ Array

Look in the rhs all the references to a production passed a argument. aProduction [aProduction or ProductionRef] The production to search for.



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# File 'lib/sequitur/production.rb', line 75

def references_of(a_prod)
  real_prod = a_prod.is_a?(ProductionRef) ? a_prod.production : a_prod
  return rhs.references_of(real_prod)
end

#repeated_digram? ⇒ true/false

Detect whether the last digram occurs twice Assumption: when a digram occurs twice in a production then it must occur at the end of the rhs



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# File 'lib/sequitur/production.rb', line 104

def repeated_digram?()
  return false if rhs.size < 3

  my_digrams = digrams
  all_keys = my_digrams.map(&:key)
  last_key = all_keys.pop
  same_key_found = all_keys.index(last_key)
  return !same_key_found.nil?
end

#single_digram? ⇒ true/false

Does the rhs have exactly one digram only (= 2 symbols)?



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# File 'lib/sequitur/production.rb', line 95

def single_digram?
  return rhs.size == 2
end

#to_string ⇒ String

Emit a text representation of the production rule. Text is of the form: object id of production : rhs as space-separated sequence of symbols.



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# File 'lib/sequitur/production.rb', line 126

def to_string()
  return "#{object_id} : #{rhs.to_string}."
end