Class: Pura::Webp::BoolDecoder

Inherits:
Object
  • Object
show all
Defined in:
lib/pura/webp/bool_decoder.rb

Overview

VP8 arithmetic bit decoder. Direct port of golang.org/x/image/vp8/partition.go. Follows libwebp’s optimized formulation with precomputed shift/range lookup tables so normalization is O(1) per bit.

Constant Summary collapse

LUT_SHIFT = 
[
  7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
  3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
  1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
].freeze
LUT_RANGE_M1 = 
[
  127,
  127, 191,
  127, 159, 191, 223,
  127, 143, 159, 175, 191, 207, 223, 239,
  127, 135, 143, 151, 159, 167, 175, 183, 191, 199, 207, 215, 223, 231, 239, 247,
  127, 131, 135, 139, 143, 147, 151, 155, 159, 163, 167, 171, 175, 179, 183, 187,
  191, 195, 199, 203, 207, 211, 215, 219, 223, 227, 231, 235, 239, 243, 247, 251,
  127, 129, 131, 133, 135, 137, 139, 141, 143, 145, 147, 149, 151, 153, 155, 157,
  159, 161, 163, 165, 167, 169, 171, 173, 175, 177, 179, 181, 183, 185, 187, 189,
  191, 193, 195, 197, 199, 201, 203, 205, 207, 209, 211, 213, 215, 217, 219, 221,
  223, 225, 227, 229, 231, 233, 235, 237, 239, 241, 243, 245, 247, 249, 251, 253
].freeze
UNIFORM_PROB = 
128

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(buf = nil) ⇒ BoolDecoder

Returns a new instance of BoolDecoder.



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# File 'lib/pura/webp/bool_decoder.rb', line 54

def initialize(buf = nil)
  init(buf) if buf
end

Instance Attribute Details

#unexpected_eof ⇒ Object (readonly)

Returns the value of attribute unexpected_eof.



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# File 'lib/pura/webp/bool_decoder.rb', line 43

def unexpected_eof
  @unexpected_eof
end

Instance Method Details

#init(buf) ⇒ Object 



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# File 'lib/pura/webp/bool_decoder.rb', line 45

def init(buf)
  @buf = buf
  @r = 0
  @range_m1 = 254
  @bits = 0
  @n_bits = 0
  @unexpected_eof = false
end

#read_bit(prob) ⇒ Object

Reads a single arithmetic-coded bit. Returns true or false (not 0/1).



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# File 'lib/pura/webp/bool_decoder.rb', line 59

def read_bit(prob)
  if @n_bits < 8
    if @r >= @buf.bytesize
      @unexpected_eof = true
      return false
    end
    x = @buf.getbyte(@r)
    @bits |= x << (8 - @n_bits)
    @r += 1
    @n_bits += 8
  end
  split = ((@range_m1 * prob) >> 8) + 1
  bit = @bits >= (split << 8)
  if bit
    @range_m1 -= split
    @bits -= split << 8
  else
    @range_m1 = split - 1
  end
  if @range_m1 < 127
    shift = LUT_SHIFT[@range_m1]
    @range_m1 = LUT_RANGE_M1[@range_m1]
    @bits = (@bits << shift) & 0xFFFFFFFF
    @n_bits -= shift
  end
  bit
end

#read_int(prob, n) ⇒ Object

Reads an n-bit signed integer: magnitude bits followed by sign bit.



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# File 'lib/pura/webp/bool_decoder.rb', line 98

def read_int(prob, n)
  u = read_uint(prob, n)
  read_bit(prob) ? -u : u
end

#read_optional_int(prob, n) ⇒ Object

Reads an “optional int”: 1 prob-bit flag; if set, read n-bit signed int; otherwise return 0.



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# File 'lib/pura/webp/bool_decoder.rb', line 105

def read_optional_int(prob, n)
  return 0 unless read_bit(prob)

  read_int(prob, n)
end

#read_uint(prob, n) ⇒ Object

Reads an n-bit unsigned integer, MSB first.



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# File 'lib/pura/webp/bool_decoder.rb', line 88

def read_uint(prob, n)
  u = 0
  while n.positive?
    n -= 1
    u |= (1 << n) if read_bit(prob)
  end
  u
end