# frozen_string_literal: true

# A simple stateful class for keeping track of a CRC32 value through multiple writes
class ZipKit::StreamCRC32
  include ZipKit::WriteShovel

  STRINGS_HAVE_CAPACITY_SUPPORT = begin
    String.new("", capacity: 1)
    true
  rescue ArgumentError
    false
  end
  CRC_BUF_SIZE = 1024 * 512
  private_constant :STRINGS_HAVE_CAPACITY_SUPPORT, :CRC_BUF_SIZE

  # Compute a CRC32 value from an IO object. The object should respond to `read` and `eof?`
  #
  # @param io[IO] the IO to read the data from
  # @return [Integer] the computed CRC32 value
  def self.from_io(io)
    # If we can specify the string capacity upfront we will not have to resize
    # the string during operation. This saves time but is only available on
    # recent Ruby 2.x versions.
    blob = STRINGS_HAVE_CAPACITY_SUPPORT ? String.new("", capacity: CRC_BUF_SIZE) : +""
    crc = new
    crc << io.read(CRC_BUF_SIZE, blob) until io.eof?
    crc.to_i
  end

  # Creates a new streaming CRC32 calculator
  def initialize
    @crc = Zlib.crc32
  end

  # Append data to the CRC32. Updates the contained CRC32 value in place.
  #
  # @param blob[String] the string to compute the CRC32 from
  # @return [self]
  def <<(blob)
    @crc = Zlib.crc32(blob, @crc)
    self
  end

  # Returns the CRC32 value computed so far
  #
  # @return [Integer] the updated CRC32 value for all the blobs so far
  def to_i
    @crc
  end

  # Appends a known CRC32 value to the current one, and combines the
  # contained CRC32 value in-place.
  #
  # @param crc32[Integer] the CRC32 value to append
  # @param blob_size[Integer] the size of the daata the `crc32` is computed from
  # @return [Integer] the updated CRC32 value for all the blobs so far
  def append(crc32, blob_size)
    @crc = Zlib.crc32_combine(@crc, crc32, blob_size)
  end
end
