Generate permutations of a string using the rank/unrank method (Skiena)

string-permute.rb

def rank(s)
  if s.length == 1
    return 0
  else
    multiplier = s.split("").sort.index(s[0].chr)
    subcombos = (1..(s.length-1)).reduce { |a,b| a * b }
    return (multiplier * subcombos) + rank(s.slice(1..-1))
  end
end

def unrank(n, s)
  sorted = s.split("").sort
  if n == 0
    return sorted.join("")
  end
  subcombos = (1..(s.length-1)).reduce { |a,b| a * b }
  index = n/subcombos
  char = sorted[index]
  sorted.delete_at(index)
  return char + unrank(n-(index*subcombos), sorted.join(""))
end

# Sort of an ad hoc test framework.
rank_tests = [["abf", 0],
              ["afb", 1],
              ["baf", 2],
              ["bfa", 3],
              ["fab", 4],
              ["fba", 5]]

unrank_tests = [[0, "abf"],
                [1, "afb"],
                [2, "baf"],
                [3, "bfa"],
                [4, "fab"],
                [5, "fba"]]

passed = 0
rank_tests.each do |test, expected|
  if rank(test) != expected
    puts "Failed on input: #{test}"
  else
    passed += 1
  end
end
unrank_tests.each do |test, expected|
  if unrank(test, "abf") != expected
    puts "Failed on input: #{test}"
  else
    passed += 1
  end
end
puts "Passed #{passed} tests."