```ruby
require 'securerandom'

module Celluloid
  # Clearly Ruby doesn't have enough UUID libraries
  # This one aims to be fast and simple with good support for multiple threads
  # If there's a better UUID library I can use with similar multithreaded
  # performance, I certainly wouldn't mind using a gem for this!
  module UUID
    values = SecureRandom.hex(9).match(/(.{8})(.{4})(.{3})(.{3})/)
    PREFIX = "#{values[1]}-#{values[2]}-4#{values[3]}-8#{values[4]}".freeze
    BLOCK_SIZE = 10_000

    @counter = 0
    @counter_mutex = Mutex.new

    def self.generate
      thread = Thread.current

      unless thread.uuid_limit
        @counter_mutex.synchronize do
          block_base = @counter
          @counter += BLOCK_SIZE
          thread.uuid_counter = block_base
          thread.uuid_limit   = @counter
        end
      end

      counter = thread.uuid_counter
      if thread.uuid_counter >= thread.uuid_limit
        thread.uuid_counter = thread.uuid_limit = nil
      else
        thread.uuid_counter += 1
      end

      "#{PREFIX}-#{sprintf("%012x", counter)}".freeze
    end
  end
end
```

Use a block size that aligns with hexadecimal UUIDs
user
```ruby
require 'securerandom'

module Celluloid
  # Clearly Ruby doesn't have enough UUID libraries
  # This one aims to be fast and simple with good support for multiple threads
  # If there's a better UUID library I can use with similar multithreaded
  # performance, I certainly wouldn't mind using a gem for this!
  module UUID
    values = SecureRandom.hex(9).match(/(.{8})(.{4})(.{3})(.{3})/)
    PREFIX = "#{values[1]}-#{values[2]}-4#{values[3]}-8#{values[4]}".freeze
    BLOCK_SIZE = 0x10000

    @counter = 0
    @counter_mutex = Mutex.new

    def self.generate
      thread = Thread.current

      unless thread.uuid_limit
        @counter_mutex.synchronize do
          block_base = @counter
          @counter += BLOCK_SIZE
          thread.uuid_counter = block_base
          thread.uuid_limit   = @counter
        end
      end

      counter = thread.uuid_counter
      if thread.uuid_counter >= thread.uuid_limit
        thread.uuid_counter = thread.uuid_limit = nil
      else
        thread.uuid_counter += 1
      end

      "#{PREFIX}-#{sprintf("%012x", counter)}".freeze
    end
  end
end
```
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