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Source file src/compress/flate/token.go

Documentation: compress/flate

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package flate
  
  const (
  	// 2 bits:   type   0 = literal  1=EOF  2=Match   3=Unused
  	// 8 bits:   xlength = length - MIN_MATCH_LENGTH
  	// 22 bits   xoffset = offset - MIN_OFFSET_SIZE, or literal
  	lengthShift = 22
  	offsetMask  = 1<<lengthShift - 1
  	typeMask    = 3 << 30
  	literalType = 0 << 30
  	matchType   = 1 << 30
  )
  
  // The length code for length X (MIN_MATCH_LENGTH <= X <= MAX_MATCH_LENGTH)
  // is lengthCodes[length - MIN_MATCH_LENGTH]
  var lengthCodes = [...]uint32{
  	0, 1, 2, 3, 4, 5, 6, 7, 8, 8,
  	9, 9, 10, 10, 11, 11, 12, 12, 12, 12,
  	13, 13, 13, 13, 14, 14, 14, 14, 15, 15,
  	15, 15, 16, 16, 16, 16, 16, 16, 16, 16,
  	17, 17, 17, 17, 17, 17, 17, 17, 18, 18,
  	18, 18, 18, 18, 18, 18, 19, 19, 19, 19,
  	19, 19, 19, 19, 20, 20, 20, 20, 20, 20,
  	20, 20, 20, 20, 20, 20, 20, 20, 20, 20,
  	21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
  	21, 21, 21, 21, 21, 21, 22, 22, 22, 22,
  	22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
  	22, 22, 23, 23, 23, 23, 23, 23, 23, 23,
  	23, 23, 23, 23, 23, 23, 23, 23, 24, 24,
  	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
  	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
  	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
  	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
  	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
  	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
  	25, 25, 26, 26, 26, 26, 26, 26, 26, 26,
  	26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
  	26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
  	26, 26, 26, 26, 27, 27, 27, 27, 27, 27,
  	27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
  	27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
  	27, 27, 27, 27, 27, 28,
  }
  
  var offsetCodes = [...]uint32{
  	0, 1, 2, 3, 4, 4, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
  	8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9,
  	10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
  	11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11,
  	12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
  	12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
  	13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  	13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  }
  
  type token uint32
  
  // Convert a literal into a literal token.
  func literalToken(literal uint32) token { return token(literalType + literal) }
  
  // Convert a < xlength, xoffset > pair into a match token.
  func matchToken(xlength uint32, xoffset uint32) token {
  	return token(matchType + xlength<<lengthShift + xoffset)
  }
  
  // Returns the literal of a literal token
  func (t token) literal() uint32 { return uint32(t - literalType) }
  
  // Returns the extra offset of a match token
  func (t token) offset() uint32 { return uint32(t) & offsetMask }
  
  func (t token) length() uint32 { return uint32((t - matchType) >> lengthShift) }
  
  func lengthCode(len uint32) uint32 { return lengthCodes[len] }
  
  // Returns the offset code corresponding to a specific offset
  func offsetCode(off uint32) uint32 {
  	if off < uint32(len(offsetCodes)) {
  		return offsetCodes[off]
  	}
  	if off>>7 < uint32(len(offsetCodes)) {
  		return offsetCodes[off>>7] + 14
  	}
  	return offsetCodes[off>>14] + 28
  }
  

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