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Source file src/compress/bzip2/move_to_front.go

Documentation: compress/bzip2

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package bzip2
     6  
     7  // moveToFrontDecoder implements a move-to-front list. Such a list is an
     8  // efficient way to transform a string with repeating elements into one with
     9  // many small valued numbers, which is suitable for entropy encoding. It works
    10  // by starting with an initial list of symbols and references symbols by their
    11  // index into that list. When a symbol is referenced, it's moved to the front
    12  // of the list. Thus, a repeated symbol ends up being encoded with many zeros,
    13  // as the symbol will be at the front of the list after the first access.
    14  type moveToFrontDecoder []byte
    15  
    16  // newMTFDecoder creates a move-to-front decoder with an explicit initial list
    17  // of symbols.
    18  func newMTFDecoder(symbols []byte) moveToFrontDecoder {
    19  	if len(symbols) > 256 {
    20  		panic("too many symbols")
    21  	}
    22  	return moveToFrontDecoder(symbols)
    23  }
    24  
    25  // newMTFDecoderWithRange creates a move-to-front decoder with an initial
    26  // symbol list of 0...n-1.
    27  func newMTFDecoderWithRange(n int) moveToFrontDecoder {
    28  	if n > 256 {
    29  		panic("newMTFDecoderWithRange: cannot have > 256 symbols")
    30  	}
    31  
    32  	m := make([]byte, n)
    33  	for i := 0; i < n; i++ {
    34  		m[i] = byte(i)
    35  	}
    36  	return moveToFrontDecoder(m)
    37  }
    38  
    39  func (m moveToFrontDecoder) Decode(n int) (b byte) {
    40  	// Implement move-to-front with a simple copy. This approach
    41  	// beats more sophisticated approaches in benchmarking, probably
    42  	// because it has high locality of reference inside of a
    43  	// single cache line (most move-to-front operations have n < 64).
    44  	b = m[n]
    45  	copy(m[1:], m[:n])
    46  	m[0] = b
    47  	return
    48  }
    49  
    50  // First returns the symbol at the front of the list.
    51  func (m moveToFrontDecoder) First() byte {
    52  	return m[0]
    53  }
    54  

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