byteio - bigendianwriter_varint.go

  package byteio
  
  // maxBENineByte is the largest number that would be stored in 63 bits with
  // this varint format without using a special case.
  // This is needed to we can shift the bits to store the last bit within the
  // carry bit of the ninth byte
  // The byte structure would be as follows: -
  // 	[0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f]
  // With the high bit of each of the first 8 bytes being the carry bit, this
  // would lead to a number of 0xffffffffffffff7f, except that the carry bit also
  // adds one to the value of each byte.
  // Thus, we end up with a value of 0x80 for the first eight bytes, and 0x7f for
  // the last.
  // Numbers greater than this value use the high bit of the ninth byte to store
  // the eighth bit of the number, shifting the remaining bits down by one.
  const maxBENineByte = 0x80<<56 | 0x80<<49 | 0x80<<42 | 0x80<<35 | 0x80<<28 | 0x80<<21 | 0x80<<14 | 0x80<<7 | 0x7f
  
  // WriteUintX writes the unsigned integer using a variable number of bytes
  func (e *BigEndianWriter) WriteUintX(d uint64) (int, error) {
  	pos := 8
  	if d > maxBENineByte {
  		e.buffer[8] = byte(d)
  		d >>= 8
  	} else {
  		e.buffer[8] = byte(d & 0x7f)
  		d >>= 7
  	}
  	for ; d > 0; d >>= 7 {
  		pos--
  		d--
  		e.buffer[pos] = byte(d&0x7f) | 0x80
  	}
  	return e.Write(e.buffer[pos:])
  }
  
  // WriteIntX writes the integer using a variable number of bytes
  func (e *BigEndianWriter) WriteIntX(d int64) (int, error) {
  	return e.WriteUintX(zigzag(d))
  }