Source file src/pkg/compress/flate/token.go

     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 type of a token
    79	func (t token) typ() uint32 { return uint32(t) & typeMask }
    80	
    81	// Returns the literal of a literal token
    82	func (t token) literal() uint32 { return uint32(t - literalType) }
    83	
    84	// Returns the extra offset of a match token
    85	func (t token) offset() uint32 { return uint32(t) & offsetMask }
    86	
    87	func (t token) length() uint32 { return uint32((t - matchType) >> lengthShift) }
    88	
    89	func lengthCode(len uint32) uint32 { return lengthCodes[len] }
    90	
    91	// Returns the offset code corresponding to a specific offset
    92	func offsetCode(off uint32) uint32 {
    93		const n = uint32(len(offsetCodes))
    94		switch {
    95		case off < n:
    96			return offsetCodes[off]
    97		case off>>7 < n:
    98			return offsetCodes[off>>7] + 14
    99		default:
   100			return offsetCodes[off>>14] + 28
   101		}
   102		panic("unreachable")
   103	}