Module: Omnizip::Algorithms::Zstandard::FSE::Interleaved

Defined in:
lib/omnizip/algorithms/zstandard/fse/interleaved.rb

Overview

2-state interleaved FSE stream decoder (RFC 8878 ยง4.2.1.2, "FSE compression of Huffman weights"; mirrors the C reference FSE_decompress_usingDTable).

Symbols are produced until the bitstream overflows; after an overflow following one state's transition, one final symbol is decoded from the other state without transitioning.

Class Method Summary collapse

Class Method Details

.check_length!(out, max_output) ⇒ Object

Raises:



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# File 'lib/omnizip/algorithms/zstandard/fse/interleaved.rb', line 82

def check_length!(out, max_output)
  return unless out.length >= max_output

  raise Omnizip::DecompressionError,
        "FSE decode exceeded max output #{max_output}"
end

.decode_stream(table, bitstream_bytes, max_output) ⇒ Array<Integer>

Decode a 2-state interleaved FSE stream.

Parameters:

  • table (Table)
  • bitstream_bytes (String)
  • max_output (Integer) —

    cap on decoded length (corrupt input protection)

Returns:

  • (Array<Integer>) —

    decoded symbols



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# File 'lib/omnizip/algorithms/zstandard/fse/interleaved.rb', line 44

def decode_stream(table, bitstream_bytes, max_output)
  bs = BitStream.new(bitstream_bytes)
  accuracy_log = table.accuracy_log

  s1 = bs.read_bits(accuracy_log)
  bs.reload
  s2 = bs.read_bits(accuracy_log)
  bs.reload

  out = []

  loop do
    check_length!(out, max_output)
    e1 = table[s1]
    out << e1.symbol
    s1 = e1.baseline + bs.read_bits(e1.num_bits)

    if bs.reload_status == BitStream::OVERFLOW
      check_length!(out, max_output)
      out << table[s2].symbol
      break
    end

    check_length!(out, max_output)
    e2 = table[s2]
    out << e2.symbol
    s2 = e2.baseline + bs.read_bits(e2.num_bits)

    if bs.reload_status == BitStream::OVERFLOW
      check_length!(out, max_output)
      out << table[s1].symbol
      break
    end
  end

  out
end