require 'digest.so'

module Digest
  # A mutex for Digest().
  REQUIRE_MUTEX = Mutex.new

  def self.const_missing(name) # :nodoc:
    case name
    when :SHA256, :SHA384, :SHA512
      lib = 'digest/sha2.so'
    else
      lib = File.join('digest', name.to_s.downcase)
    end

    begin
      require lib
    rescue LoadError
      raise LoadError, "library not found for class Digest::#{name} -- #{lib}", caller(1)
    end
    unless Digest.const_defined?(name)
      raise NameError, "uninitialized constant Digest::#{name}", caller(1)
    end
    Digest.const_get(name)
  end

  class ::Digest::Class
    # Creates a digest object and reads a given file, _name_.
    # Optional arguments are passed to the constructor of the digest
    # class.
    #
    #   p Digest::SHA256.file("X11R6.8.2-src.tar.bz2").hexdigest
    #   # => "f02e3c85572dc9ad7cb77c2a638e3be24cc1b5bea9fdbb0b0299c9668475c534"
    def self.file(name, *args)
      new(*args).file(name)
    end

    # Returns the base64 encoded hash value of a given _string_.  The
    # return value is properly padded with '=' and contains no line
    # feeds.
    def self.base64digest(str, *args)
      [digest(str, *args)].pack('m0')
    end
  end

  module Instance
    # Updates the digest with the contents of a given file _name_ and
    # returns self.
    def file(name)
      File.open(name, "rb") {|f|
        buf = ""
        while f.read(16384, buf)
          update buf
        end
      }
      self
    end

    # If none is given, returns the resulting hash value of the digest
    # in a base64 encoded form, keeping the digest's state.
    #
    # If a +string+ is given, returns the hash value for the given
    # +string+ in a base64 encoded form, resetting the digest to the
    # initial state before and after the process.
    #
    # In either case, the return value is properly padded with '=' and
    # contains no line feeds.
    def base64digest(str = nil)
      [str ? digest(str) : digest].pack('m0')
    end

    # Returns the resulting hash value and resets the digest to the
    # initial state.
    def base64digest!
      [digest!].pack('m0')
    end
  end
end

# call-seq:
#   Digest(name) -> digest_subclass
#
# Returns a Digest subclass by +name+ in a thread-safe manner even
# when on-demand loading is involved.
#
#   require 'digest'
#
#   Digest("MD5")
#   # => Digest::MD5
#
#   Digest(:SHA256)
#   # => Digest::SHA256
#
#   Digest(:Foo)
#   # => LoadError: library not found for class Digest::Foo -- digest/foo
def Digest(name)
  const = name.to_sym
  Digest::REQUIRE_MUTEX.synchronize {
    # Ignore autoload's because it is void when we have #const_missing
    Digest.const_missing(const)
  }
rescue LoadError
  # Constants do not necessarily rely on digest/*.
  if Digest.const_defined?(const)
    Digest.const_get(const)
  else
    raise
  end
end
