Rule 4 for the MIU System.

miu_system4.rb

# Returns true if we can apply Rule 4 to the given string.
# (Rule 4: If "UU" appears anywhere in our string, we can drop it.)
def can_apply_rule4?(str)
  !(str.match(/UU/).nil?)
end

# Returns an array of indices for the given string where Rule 4 can be applied.
def rule4_indices(str)
  # We find all of the places where we have 2 or more U's.
  matches_for_U = str.to_enum(:scan, /U{2,}/).map { Regexp.last_match }

  # Again, we can have matches like "UUUUU" where there are multiple "UU"s that
  # we can choose from (eg. 4 of them here, starting from indices 0, 1, 2, 3.)
  # To get the full set of indices, we find the number of ways to get "UU" in 
  # the larger "UUU..." string and return that many indices, starting from 0.
  indices = matches_for_U.map do |match|
    match_length = match[0].length
    number_of_UUs = match_length - "UU".length + 1
    (0...number_of_UUs).map { |i| i + match.begin(0) }.to_a
  end

  indices.flatten
end

# Applies Rule 4: Given the string and an index (that can be found
# using the rule4_indices() method), we drop the 2 characters at that index.
def apply_rule4(str, index)
  changed_str = str.dup
  changed_str[index...(index + "UU".length)] = ""
  changed_str
end