Class: MiniKraken::Composite::ConsCellVisitor

Inherits:
Object
  • Object
show all
Defined in:
lib/mini_kraken/composite/cons_cell_visitor.rb

Overview

Factory class. Purpose: to create Fiber specialized in the visit of cons cells.

Class Method Summary collapse

Class Method Details

.df_visitor(aCell) ⇒ Fiber

Build a depth-first in-order expression tree visitor. The visitor is implemented as a Fiber. The Fiber yields couples of the form: [member, visitee] where member is one of :car, :cdr The end of visit is signalled with the couple [:stop, nil]

Parameters:

Returns:

  • (Fiber) —

    A Fiber that yields couples



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# File 'lib/mini_kraken/composite/cons_cell_visitor.rb', line 18

def self.df_visitor(aCell)
  first = aCell	# The visit will start from the provided cons cell
  # puts "#{__callee__} called with #{aCell.object_id.to_s(16)}"
  visitor = Fiber.new do |skipping|
    # Initialization part: will run once
    visitees = Set.new # Keep track of the conscell already visited
    visit_stack = first.nil? ? [] : [[:car, first]] # The LIFO queue of cells to visit

    until visit_stack.empty?	# Traversal part (as a loop)
      side, cell = visit_stack.pop
      next if visitees.include?(cell) && side == :car

      visitees << cell if cell.kind_of?(ConsCell)

      skip_children = Fiber.yield [side, cell]
      next if skip_children || skipping

      skipping = false
      if cell.is_a?(ConsCell) && !cell.null?
        visit_stack.push([:cdr, cell.cdr])
        visit_stack.push([:car, cell.car])
      end
    end

    # Send stop mark
    Fiber.yield [:stop, nil]
  end

  return visitor
end