yalign

#!/usr/bin/env ruby
# frozen_string_literal: true

class Aligner
  LINE = /
    (?:^|\A)           # beginning of line

    (?<lhs>\s*[^#:]+:) # lhs
    (?:\s*)            # optional space
    (?<rhs>\S.*)       # rhs

    (?:$|\z)           # end of line
  /x.freeze

  def self.call(*args)
    new(*args).()
  end

  def initialize(content, npad = nil)
    @lines          = content.split "\n"
    @npad           = npad || 1
    @longest_length = calculate_longest_length
  end

  def call
    lines.map do |line|
      next line unless (m = line.match(LINE))

      pad = ' ' * (longest_length - m[:lhs].length)
      m[:lhs] + pad + m[:rhs]
    end.join("\n") + "\n"
  end

  private

  attr_reader :lines, :npad, :longest_length

  def calculate_longest_length
    lines.map do |line|
      next 0 unless (m = line.match(LINE))

      (m[:lhs] || '').length
    end.max + npad
  end
end

def main
  puts Aligner.(ARGF.read)
end

main