Bump a gem's version, tag it and commit it to git

bump.rb

#!/usr/bin/env ruby
# bump.rb - Perform a project version bump
#   * If a .gemspec file is present, use it to determine the current project version
#     * Scans for the first line that contains .version and a 1.2.3-ish part
#   * If no gemspec is present, use the highest tagged version
#     * The expected tag format is '1.2.3'
#   * Depending on the command line options 'major', 'minor', 'build' it will
#     bump the 1st, 2nd, 4th digit of the version. No option implies a 'release',
#     which bumps the 3rd digit. Example: 1.2.3 -> 1.2.4 is a release.
#   * Overwrites the current gemspec with the new one if a gemspec was found
#   * Bumps the gemspec date to today's date

require 'date'

def stop(msg,code=1)
  puts msg
  exit code
end

class Array
  def map_with_index(&block)
    ary=[]
    each_with_index do |item, index|
      ary << block.call(item, index)
    end
    ary
  end
end

bump = [nil, nil, nil, nil]
case ARGV.first
when 'major'
  bump = [1,0,0,0]
when 'minor'
  bump = [nil,1,0,0]
when 'build'
  bump = [nil, nil, nil, 1]
else # when 'release'
  bump = [nil, nil, 1, 0]
end

gemspec_file = Dir.glob('*.gemspec').first
if gemspec_file
  gemspec = File.read(gemspec_file)
  version_line = gemspec.grep(/\.version.*\d+(\.\d)+/).first
  version_ary = version_line.scan(/\d+/).map{|digit_str| digit_str.to_i}
  old_version_str = version_ary.join(".")
else # Only commit + bump tag
  tags = `git tag -l`.split("\n").map{|line| line.strip}
  version_tags = tags.select {|tag| tag.match /\A\d(\.\d)+\Z/}
  old_version_str = version_tags.max
  version_ary = old_version_str.split(".").map{|digit_str| digit_str.to_i}
end

new_version = version_ary.map_with_index do |digit, index|
  case bump[index]
  when 0
    0
  when nil
    digit
  else
    digit + bump[index]
  end
end
new_version_str = new_version.join(".")

puts "Bump from %s to %s" % [old_version_str, new_version_str]

if gemspec_file
  # Bump version
  new_version_line = version_line.gsub(old_version_str, new_version_str)
  new_gemspec = gemspec.gsub(version_line, new_version_line)

  # Bump date
  old_date_line = gemspec.grep(/date.*?["'][0-9\-]+["']/).first
  old_date = old_date_line.match(/["']([0-9\-]+)["']/)[1]
  date = Date.today
  new_date = "%04i-%02i-%02i" % [date.year, date.month, date.day]
  new_date_line = old_date_line.gsub(old_date,new_date)
  new_gemspec = new_gemspec.gsub!(old_date_line,new_date_line)

  # Write gemspec
  File.open(gemspec_file,'w') {|f| f.puts(new_gemspec)}
end
system("git commit -va -m \"Version #{new_version_str}\" && git tag -fa #{new_version_str} -m \"Version #{new_version_str}\"")