Palindrome detection

# title: Palindrome detection
# source: https://rosettacode.org/wiki/Palindrome_detection
# category: Rosetta Code
# difficulty: Intro
# summary: Detect palindromes after normalizing letters and digits to lowercase.
# tags: popular, strings, scanning, predicates
# vibe: 0.2

def normalized(text)
  output = ""
  index = 0

  while index < text.length
    char = text.slice(index).downcase
    if (char >= "a" && char <= "z") || (char >= "0" && char <= "9")
      output = output + char
    end
    index = index + 1
  end

  output
end

def palindrome?(text)
  value = normalized(text)
  left = 0
  right = value.length - 1

  while left < right
    if value.slice(left) != value.slice(right)
      return false
    end
    left = left + 1
    right = right - 1
  end

  true
end

def run
  rows = []
  rows = rows.push(["text", "palindrome"])
  rows = rows.push(["racecar", palindrome?("racecar")])
  rows = rows.push(["Able was I ere I saw Elba", palindrome?("Able was I ere I saw Elba")])
  rows = rows.push(["12321", palindrome?("12321")])
  rows = rows.push(["Vibescript", palindrome?("Vibescript")])
  rows
end