words_path.rb

#!/usr/bin/env ruby

from, to = ARGV
a = Time.now

$words = {}
File.readlines('/usr/share/dict/words').each do |line|
  $words[line.chomp.downcase] = true if line.length == from.length+1
end

def siblings from
  Enumerator.new do |enum|
    from.length.times do |i|
      w = from.dup
      ('a'..'z').each do |l|
        w[i] = l
        enum.yield w.dup if $words.delete(w)
      end
    end
  end
end

def bfs from, to
  parent = {}
  queue = [from]

  while !parent[to] and n = queue.shift
    siblings(n).each do |child|
      parent[child] = n
      queue.push(child)
    end
  end

  path = [to]
  while parent[to]
    path.unshift(parent[to])
    to = parent[to]
  end
  path
end

b = Time.now
puts bfs(from, to).join(' -> ')
puts "load: #{b - a}; search: #{Time.now - b}"