From: Joel VanderWerf <vjoel@...>
Date: 2001-09-01T07:50:01+09:00
Subject: [ruby-talk:20646] Iterating by links


This is part of an Enumerable extension package I'm going to submit to RAA 
eventually, but it's useful, so maybe the ruby-list will forgive the size...

Suppose you want to iterate over a linked list, or more generally a 
sequence of objects that can be generated by iteratively calling some 
function. You can do this by calling the function in a while loop. Or you 
can use "by", as in these examples (source follows):

class Foo
  attr_reader :value, :next_foo
  def initialize value, next_foo = nil
    @value = value
    @next_foo = next_foo
  end
end

list = Foo.new(0, Foo.new(1, Foo.new(2, Foo.new(3))))
for foo in list.by :next_foo
  p foo.value
end
puts

for foo in list.by { |foo| foo.next_foo.next_foo }
  p foo.value
end
puts

tree = [[0, [1, 2]], 3, [4, 5, [6]]]
for node in tree.by(:at, 2) 
  p node
end
puts

p 0.by { |x| x<10 ? x+1 : nil }.to_a
puts

f = proc { |x, incr, limit| y = x + incr; y <= limit ? y : nil }
p 0.by(f, 10, 50).to_a
puts

class A; end
class B < A; end
class C < B; end

for cl in C.by :superclass
 p cl
end
puts

# Popping down a stack:
p [0,1,2,3,4].by { |y| y.pop; y != [] && y.dup }.entries

########################
module Enumerable

  class LinkedListDelegator
    include Enumerable
    
    attr_reader :first, :next_name, :next_proc, :args

    def initialize first, next_name_or_proc = nil, *args, &next_proc
      @first = first
      @args = args
      case next_name_or_proc
      when Symbol
        @next_name = next_name_or_proc
      when String
        @next_name = next_name_or_proc.intern
      when Proc
        @next_proc = next_name_or_proc
      when nil
        @next_proc = next_proc
      end
    end

    def each
      cur = @first
      if @next_name
        loop do
          break unless cur
          yield cur
          cur = cur.send @next_name, *@args
        end
      elsif @next_proc
        loop do
          break unless cur
          yield cur
          cur = @next_proc[cur, *@args]
        end
      else
        raise ArgumentError,
          "No method or procedure specified for getting next in LinkedList."
      end

      self
    end
    
  end
  
end

class Object
  def by next_name = nil, *args, &next_getter
    Enumerable::LinkedListDelegator.new self, next_name, *args, &next_getter
  end
end