Letter frequency

# title: Letter frequency
# source: https://rosettacode.org/wiki/Letter_frequency
# category: Rosetta Code
# difficulty: Intro
# summary: Count alphabetic character frequencies in a sample phrase.
# tags: popular, strings, hashes, counting
# vibe: 0.2

def letter_frequency(text)
  counts = {}
  normalized = text.downcase
  index = 0

  while index < normalized.length
    char = normalized.slice(index)
    if char >= "a" && char <= "z"
      counts[char] = counts.fetch(char, 0) + 1
    end
    index = index + 1
  end

  counts
end

def run
  letter_frequency("Vibescript makes Rosetta Code tasks feel concrete.")
end