Caesar Cipher

# title: Caesar Cipher
# source: https://rosettacode.org/wiki/Caesar_cipher
# category: Rosetta Code Popular
# difficulty: Easy
# summary: Encode and decode text by rotating alphabetic characters through a fixed shift.
# tags: popular, strings, crypto, substitution
# vibe: 0.2

def rotate_index(index, shift)
  rotated = index + shift

  while rotated >= 26
    rotated = rotated - 26
  end

  while rotated < 0
    rotated = rotated + 26
  end

  rotated
end

def rotate_char(char, shift)
  upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
  lower = "abcdefghijklmnopqrstuvwxyz"

  upper_index = upper.index(char)
  if upper_index != nil
    return upper.slice(rotate_index(upper_index, shift))
  end

  lower_index = lower.index(char)
  if lower_index != nil
    return lower.slice(rotate_index(lower_index, shift))
  end

  char
end

def caesar_cipher(text, shift)
  output = ""
  index = 0

  while index < text.length
    output = output + rotate_char(text.slice(index), shift)
    index = index + 1
  end

  output
end

def run
  plain = "The quick brown fox jumps over the lazy dog."
  encoded = caesar_cipher(plain, 3)
  {
    plain: plain,
    encoded: encoded,
    decoded: caesar_cipher(encoded, 23)
  }
end