Permutations in Ruby from scratch

permutations

def get_perm str
  return [str] if str.size == 1
  perms = []

  first_char = str.slice!(0)
  words = get_perm(str)
  words.each do |word|
    perms += insert_each(first_char, word)
  end
  perms
end

def insert_each char, str
  results = []
  size = str.size-1
  results << char + str

  for i in (0..size)
    results << str[0..i] + char + str[i+1..size]
  end
  results
end