imcat

def caesar_cipher(string, shift, decode = false) 
  alphabet = ('A'..'Z').to_a   # create the range of uppercase letter and convert it to an array: ['A', 'B', … 'Z']
  shift = -shift if decode     # if decoding, negate the shift. Negative value changes the dirrection of Array.rotate
  key = alphabet.rotate(shift) # produce encode or decode array by rotating the alphabet. ['A', 'B', 'C', 'D'].rotate(2) => ['C', 'D', 'A', 'B']  
  
  string.chars.inject("") do |result, char| # string.chars gives the enumerator of the chars in the string. Each char will be processed in the block assigned to 'char'  
    case_changed = char.upcase!   # upcase will return nil if no changes were made       
    result_char = (alphabet.include?(char) ? key[alphabet.index(char)] : char) #if char is in the alphabet, get the corresponding char from key array
    result << (case_changed ? result_char.downcase : result_char) #if original char was lowercase then lowercase the result char and append to result string
  end