Class: Cabriolet::Huffman::Encoder

Inherits:
Object
  • Object
show all
Defined in:
lib/cabriolet/huffman/encoder.rb

Overview

Encoder encodes symbols using Huffman codes for compression

Constant Summary collapse

MAX_BITS =

Maximum code length supported

16

Class Method Summary collapse

Class Method Details

.build_codes(lengths, num_symbols) ⇒ Hash

Build Huffman codes from code lengths (RFC 1951 algorithm)

This generates the actual Huffman code values from code lengths. The algorithm ensures canonical Huffman codes where codes of the same length are assigned sequentially.

Parameters:

  • lengths (Array<Integer>) —

    Code lengths for each symbol

  • num_symbols (Integer) —

    Number of symbols

Returns:

  • (Hash) —

    Hash mapping symbol to value, bits: length



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# File 'lib/cabriolet/huffman/encoder.rb', line 19

def self.build_codes(lengths, num_symbols)
  # Count the number of codes for each length
  bl_count = Array.new(MAX_BITS + 1, 0)
  lengths[0, num_symbols].each do |len|
    bl_count[len] += 1 if len.positive?
  end

  # Find the numerical value of the smallest code for each length
  code = 0
  bl_count[0] = 0
  next_code = Array.new(MAX_BITS + 1, 0)

  (1..MAX_BITS).each do |bits|
    code = (code + bl_count[bits - 1]) << 1
    next_code[bits] = code
  end

  # Assign codes to symbols
  codes = {}
  num_symbols.times do |symbol|
    len = lengths[symbol]
    next unless len.positive?

    codes[symbol] = {
      code: next_code[len],
      bits: len,
    }
    next_code[len] += 1
  end

  codes
end

.build_fixed_codes ⇒ Hash

Build fixed Huffman codes for DEFLATE (RFC 1951)

Returns:

  • (Hash) —

    Hash with :literal and :distance code tables



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# File 'lib/cabriolet/huffman/encoder.rb', line 55

def self.build_fixed_codes
  # Fixed literal/length code lengths
  literal_lengths = Array.new(288, 0)
  (0...144).each { |i| literal_lengths[i] = 8 }
  (144...256).each { |i| literal_lengths[i] = 9 }
  (256...280).each { |i| literal_lengths[i] = 7 }
  (280...288).each { |i| literal_lengths[i] = 8 }

  # Fixed distance code lengths (all 5 bits)
  distance_lengths = Array.new(32, 5)

  {
    literal: build_codes(literal_lengths, 288),
    distance: build_codes(distance_lengths, 32),
  }
end

.encode_symbol(symbol, codes, bitstream) ⇒ void

This method returns an undefined value.

Encode a symbol using Huffman codes and write to bitstream

Per RFC 1951 Section 3.1.1, Huffman codes are written LSB-first, so we must reverse the bits before writing to the bitstream.

Parameters:

  • symbol (Integer) —

    Symbol to encode

  • codes (Hash) —

    Code table mapping symbols to bits:

  • bitstream (Binary::BitstreamWriter) —

    Output bitstream



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# File 'lib/cabriolet/huffman/encoder.rb', line 81

def self.encode_symbol(symbol, codes, bitstream)
  entry = codes[symbol]
  unless entry
    raise Cabriolet::CompressionError,
          "No code for symbol #{symbol}"
  end

  # Reverse bits for LSB-first writing per RFC 1951
  reversed_code = reverse_bits(entry[:code], entry[:bits])
  bitstream.write_bits(reversed_code, entry[:bits])
end

.reverse_bits(value, num_bits) ⇒ Integer

Reverse bits for writing (some formats need reversed bit order)

Parameters:

  • value (Integer) —

    Value to reverse

  • num_bits (Integer) —

    Number of bits

Returns:

  • (Integer) —

    Reversed value



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# File 'lib/cabriolet/huffman/encoder.rb', line 98

def self.reverse_bits(value, num_bits)
  result = 0
  num_bits.times do
    result = (result << 1) | (value & 1)
    value >>= 1
  end
  result
end