diff --git a/tool/extlibs.rb b/tool/extlibs.rb
index ee4fd72a16..d59a139b0f 100755
--- a/tool/extlibs.rb
+++ b/tool/extlibs.rb
@@ -7,151 +7,169 @@ require 'fileutils'
 require 'digest'
 require_relative 'downloader'
 
-def do_download(url, base, cache_dir)
-  Downloader.download(url, base, cache_dir, nil)
-end
+class ExtLibs
+  def do_download(url, base, cache_dir)
+    Downloader.download(url, base, cache_dir, nil)
+  end
 
-def do_checksum(cache, chksums)
-  chksums.each do |sum|
-    name, sum = sum.split(/:/)
-    if $VERBOSE
-      $stdout.print "checking #{name} of #{cache} ..."
-      $stdout.flush
-    end
-    hd = Digest(name.upcase).file(cache).hexdigest
-    if hd == sum
+  def do_checksum(cache, chksums)
+    chksums.each do |sum|
+      name, sum = sum.split(/:/)
       if $VERBOSE
-        $stdout.puts " OK"
+        $stdout.print "checking #{name} of #{cache} ..."
         $stdout.flush
       end
+      hd = Digest(name.upcase).file(cache).hexdigest
+      if hd == sum
+        if $VERBOSE
+          $stdout.puts " OK"
+          $stdout.flush
+        end
+      else
+        if $VERBOSE
+          $stdout.puts " NG"
+          $stdout.flush
+        end
+        raise "checksum mismatch: #{cache}, #{name}:#{hd}, expected #{sum}"
+      end
+    end
+  end
+
+  def do_extract(cache, dir)
+    if $VERBOSE
+      $stdout.puts "extracting #{cache} into #{dir}"
+      $stdout.flush
+    end
+    ext = File.extname(cache)
+    case ext
+    when '.gz', '.tgz'
+      f = IO.popen(["gzip", "-dc", cache])
+      cache = cache.chomp('.gz')
+    when '.bz2', '.tbz'
+      f = IO.popen(["bzip2", "-dc", cache])
+      cache = cache.chomp('.bz2')
+    when '.xz', '.txz'
+      f = IO.popen(["xz", "-dc", cache])
+      cache = cache.chomp('.xz')
     else
-      if $VERBOSE
-        $stdout.puts " NG"
-        $stdout.flush
-      end
-      raise "checksum mismatch: #{cache}, #{name}:#{hd}, expected #{sum}"
+      inp = cache
     end
+    inp ||= f.binmode
+    ext = File.extname(cache)
+    case ext
+    when '.tar', /\A\.t[gbx]z\z/
+      pid = Process.spawn("tar", "xpf", "-", in: inp, chdir: dir)
+    when '.zip'
+      pid = Process.spawn("unzip", inp, "-d", dir)
+    end
+    f.close if f
+    Process.wait(pid)
+    $?.success? or raise "failed to extract #{cache}"
   end
-end
 
-def do_extract(cache, dir)
-  if $VERBOSE
-    $stdout.puts "extracting #{cache} into #{dir}"
-    $stdout.flush
-  end
-  ext = File.extname(cache)
-  case ext
-  when '.gz', '.tgz'
-    f = IO.popen(["gzip", "-dc", cache])
-    cache = cache.chomp('.gz')
-  when '.bz2', '.tbz'
-    f = IO.popen(["bzip2", "-dc", cache])
-    cache = cache.chomp('.bz2')
-  when '.xz', '.txz'
-    f = IO.popen(["xz", "-dc", cache])
-    cache = cache.chomp('.xz')
-  else
-    inp = cache
-  end
-  inp ||= f.binmode
-  ext = File.extname(cache)
-  case ext
-  when '.tar', /\A\.t[gbx]z\z/
-    pid = Process.spawn("tar", "xpf", "-", in: inp, chdir: dir)
-  when '.zip'
-    pid = Process.spawn("unzip", inp, "-d", dir)
-  end
-  f.close if f
-  Process.wait(pid)
-  $?.success? or raise "failed to extract #{cache}"
-end
-
-def do_patch(dest, patch, args)
-  if $VERBOSE
-    $stdout.puts "applying #{patch} under #{dest}"
-    $stdout.flush
-  end
-  Process.wait(Process.spawn("patch", "-d", dest, "-i", patch, *args))
-  $?.success? or raise "failed to patch #{patch}"
-end
-
-cache_dir = ENV['CACHE_DIR'] || ".downloaded-cache"
-mode = :all
-until ARGV.empty?
-  case ARGV[0]
-  when '--download'
-    mode = :download
-  when '--extract'
-    mode = :extract
-  when '--patch'
-    mode = :patch
-  when '--all'
-    mode = :all
-  when '--cache'
-    ARGV.shift
-    cache_dir = ARGV[0]
-  when /\A--cache=/
-    cache_dir = $'
-  when '--'
-    ARGV.shift
-    break
-  when /\A-/
-    abort "unknown option: #{ARGV[0]}"
-  else
-    break
-  end
-  ARGV.shift
-end
-
-FileUtils.mkdir_p(cache_dir)
-
-success = true
-ARGV.each do |dir|
-  Dir.glob("#{dir}/**/extlibs") do |list|
+  def do_patch(dest, patch, args)
     if $VERBOSE
-      $stdout.puts "downloading for #{list}"
+      $stdout.puts "applying #{patch} under #{dest}"
       $stdout.flush
     end
+    Process.wait(Process.spawn("patch", "-d", dest, "-i", patch, *args))
+    $?.success? or raise "failed to patch #{patch}"
+  end
+
+  def do_command(mode, dest, url, cache_dir, chksums)
+    base = File.basename(url)
+    cache = File.join(cache_dir, base)
+    target = File.join(dest, base[/.*(?=\.tar(?:\.\w+)?\z)/])
+
     extracted = false
-    dest = File.dirname(list)
-    IO.foreach(list) do |line|
-      line.sub!(/\s*#.*/, '')
-      if /^\t/ =~ line
-        if extracted and (mode == :all or mode == :patch)
-          patch, *args = line.split
-          do_patch(dest, patch, args)
-        end
-        next
+    case mode
+    when :download
+      do_download(url, base, cache_dir)
+      do_checksum(cache, chksums)
+    when :extract
+      unless File.directory?(target)
+        do_checksum(cache, chksums)
+        extracted = do_extract(cache, dest)
       end
-      url, *chksums = line.split(' ')
-      next unless url
-      extracted = false
-      base = File.basename(url)
-      cache = File.join(cache_dir, base)
-      target = File.join(dest, base[/.*(?=\.tar(?:\.\w+)?\z)/])
-      begin
-        case mode
-        when :download
-          do_download(url, base, cache_dir)
-          do_checksum(cache, chksums)
-        when :extract
-          unless File.directory?(target)
-            do_checksum(cache, chksums)
-            extracted = do_extract(cache, dest)
-          end
-        when :all
-          do_download(url, base, cache_dir)
-          unless File.directory?(target)
-            do_checksum(cache, chksums)
-            extracted = do_extract(cache, dest)
-          end
-        end
-      rescue => e
-        warn e.inspect
-        success = false
+    when :all
+      do_download(url, base, cache_dir)
+      unless File.directory?(target)
+        do_checksum(cache, chksums)
+        extracted = do_extract(cache, dest)
       end
     end
+    extracted
+  end
+
+  def run(argv)
+    cache_dir = ENV['CACHE_DIR'] || ".downloaded-cache"
+    mode = :all
+    until argv.empty?
+      case argv[0]
+      when '--download'
+        mode = :download
+      when '--extract'
+        mode = :extract
+      when '--patch'
+        mode = :patch
+      when '--all'
+        mode = :all
+      when '--cache'
+        argv.shift
+        cache_dir = argv[0]
+      when /\A--cache=/
+        cache_dir = $'
+      when '--'
+        argv.shift
+        break
+      when /\A-/
+        warn "unknown option: #{argv[0]}"
+        return false
+      else
+        break
+      end
+      argv.shift
+    end
+
+    FileUtils.mkdir_p(cache_dir)
+
+    success = true
+    argv.each do |dir|
+      Dir.glob("#{dir}/**/extlibs") do |list|
+        if $VERBOSE
+          $stdout.puts "downloading for #{list}"
+          $stdout.flush
+        end
+        extracted = false
+        dest = File.dirname(list)
+        IO.foreach(list) do |line|
+          line.sub!(/\s*#.*/, '')
+          if /^\t/ =~ line
+            if extracted and (mode == :all or mode == :patch)
+              patch, *args = line.split
+              do_patch(dest, patch, args)
+            end
+            next
+          end
+          url, *chksums = line.split(' ')
+          next unless url
+          begin
+            extracted = do_command(mode, dest, url, cache_dir, chksums)
+          rescue => e
+            warn e.inspect
+            success = false
+          end
+        end
+      end
+    end
+    success
+  end
+
+  def self.run(argv)
+    self.new.run(argv)
   end
 end
 
-exit(success)
+if $0 == __FILE__
+  exit ExtLibs.run(ARGV)
+end