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Source file src/compress/flate/token.go

Documentation: compress/flate

     1  // Copyright 2009 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 flate
     6  
     7  const (
     8  	// 2 bits:   type   0 = literal  1=EOF  2=Match   3=Unused
     9  	// 8 bits:   xlength = length - MIN_MATCH_LENGTH
    10  	// 22 bits   xoffset = offset - MIN_OFFSET_SIZE, or literal
    11  	lengthShift = 22
    12  	offsetMask  = 1<<lengthShift - 1
    13  	typeMask    = 3 << 30
    14  	literalType = 0 << 30
    15  	matchType   = 1 << 30
    16  )
    17  
    18  // The length code for length X (MIN_MATCH_LENGTH <= X <= MAX_MATCH_LENGTH)
    19  // is lengthCodes[length - MIN_MATCH_LENGTH]
    20  var lengthCodes = [...]uint32{
    21  	0, 1, 2, 3, 4, 5, 6, 7, 8, 8,
    22  	9, 9, 10, 10, 11, 11, 12, 12, 12, 12,
    23  	13, 13, 13, 13, 14, 14, 14, 14, 15, 15,
    24  	15, 15, 16, 16, 16, 16, 16, 16, 16, 16,
    25  	17, 17, 17, 17, 17, 17, 17, 17, 18, 18,
    26  	18, 18, 18, 18, 18, 18, 19, 19, 19, 19,
    27  	19, 19, 19, 19, 20, 20, 20, 20, 20, 20,
    28  	20, 20, 20, 20, 20, 20, 20, 20, 20, 20,
    29  	21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
    30  	21, 21, 21, 21, 21, 21, 22, 22, 22, 22,
    31  	22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
    32  	22, 22, 23, 23, 23, 23, 23, 23, 23, 23,
    33  	23, 23, 23, 23, 23, 23, 23, 23, 24, 24,
    34  	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    35  	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    36  	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    37  	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
    38  	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
    39  	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
    40  	25, 25, 26, 26, 26, 26, 26, 26, 26, 26,
    41  	26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
    42  	26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
    43  	26, 26, 26, 26, 27, 27, 27, 27, 27, 27,
    44  	27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
    45  	27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
    46  	27, 27, 27, 27, 27, 28,
    47  }
    48  
    49  var offsetCodes = [...]uint32{
    50  	0, 1, 2, 3, 4, 4, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
    51  	8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9,
    52  	10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
    53  	11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11,
    54  	12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
    55  	12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
    56  	13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
    57  	13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
    58  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
    59  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
    60  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
    61  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
    62  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
    63  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
    64  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
    65  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
    66  }
    67  
    68  type token uint32
    69  
    70  // Convert a literal into a literal token.
    71  func literalToken(literal uint32) token { return token(literalType + literal) }
    72  
    73  // Convert a < xlength, xoffset > pair into a match token.
    74  func matchToken(xlength uint32, xoffset uint32) token {
    75  	return token(matchType + xlength<<lengthShift + xoffset)
    76  }
    77  
    78  // Returns the literal of a literal token
    79  func (t token) literal() uint32 { return uint32(t - literalType) }
    80  
    81  // Returns the extra offset of a match token
    82  func (t token) offset() uint32 { return uint32(t) & offsetMask }
    83  
    84  func (t token) length() uint32 { return uint32((t - matchType) >> lengthShift) }
    85  
    86  func lengthCode(len uint32) uint32 { return lengthCodes[len] }
    87  
    88  // Returns the offset code corresponding to a specific offset
    89  func offsetCode(off uint32) uint32 {
    90  	if off < uint32(len(offsetCodes)) {
    91  		return offsetCodes[off]
    92  	}
    93  	if off>>7 < uint32(len(offsetCodes)) {
    94  		return offsetCodes[off>>7] + 14
    95  	}
    96  	return offsetCodes[off>>14] + 28
    97  }
    98  

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