github.com/andybalholm/brotli@v1.0.6/write_bits.go (about)

     1  package brotli
     2  
     3  import "encoding/binary"
     4  
     5  /* Copyright 2010 Google Inc. All Rights Reserved.
     6  
     7     Distributed under MIT license.
     8     See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
     9  */
    10  
    11  /* Write bits into a byte array. */
    12  
    13  /* This function writes bits into bytes in increasing addresses, and within
    14     a byte least-significant-bit first.
    15  
    16     The function can write up to 56 bits in one go with WriteBits
    17     Example: let's assume that 3 bits (Rs below) have been written already:
    18  
    19     BYTE-0     BYTE+1       BYTE+2
    20  
    21     0000 0RRR    0000 0000    0000 0000
    22  
    23     Now, we could write 5 or less bits in MSB by just sifting by 3
    24     and OR'ing to BYTE-0.
    25  
    26     For n bits, we take the last 5 bits, OR that with high bits in BYTE-0,
    27     and locate the rest in BYTE+1, BYTE+2, etc. */
    28  func writeBits(n_bits uint, bits uint64, pos *uint, array []byte) {
    29  	/* This branch of the code can write up to 56 bits at a time,
    30  	   7 bits are lost by being perhaps already in *p and at least
    31  	   1 bit is needed to initialize the bit-stream ahead (i.e. if 7
    32  	   bits are in *p and we write 57 bits, then the next write will
    33  	   access a byte that was never initialized). */
    34  	p := array[*pos>>3:]
    35  	v := uint64(p[0])
    36  	v |= bits << (*pos & 7)
    37  	binary.LittleEndian.PutUint64(p, v)
    38  	*pos += n_bits
    39  }
    40  
    41  func writeSingleBit(bit bool, pos *uint, array []byte) {
    42  	if bit {
    43  		writeBits(1, 1, pos, array)
    44  	} else {
    45  		writeBits(1, 0, pos, array)
    46  	}
    47  }
    48  
    49  func writeBitsPrepareStorage(pos uint, array []byte) {
    50  	assert(pos&7 == 0)
    51  	array[pos>>3] = 0
    52  }