1. 1
2. 2
3. 3
4. 4
5. 5
6. 6
7. 7
8. 8
9. 9
10. 10
11. 11
12. 12
13. 13
14. 14
15. 15
16. 16
17. 17
18. 18
19. 19
20. 20
21. 21
22. 22
23. 23
24. 24
25. 25
26. 26
27. 27
28. 28
29. 29
30. 30
31. 31
32. 32
33. 33
34. 34
35. 35
36. 36
37. 37
38. 38
39. 39
40. 40
41. 41
42. 42
43. 43
44. 44
45. 45
46. 46
47. 47
48. 48
49. 49
50. 50
51. 51
52. 52
53. 53
54. 54
55. 55
56. 56
57. 57
58. 58
59. 59
60. 60
61. 61
62. 62
63. 63
64. 64
65. 65
66. 66
67. 67
68. 68
69. 69
70. 70
71. 71
72. 72
73. 73
74. 74
75. 75
76. 76
77. 77
78. 78
79. 79
80. 80
81. 81
82. 82
83. 83
84. 84
85. 85
86. 86
87. 87
88. 88
89. 89
90. 90
91. 91
92. 92
93. 93
94. 94
95. 95
96. 96
97. 97
98. 98
99. 99
100. 100
101. 101
102. 102
103. 103
104. 104
105. 105
106. 106
107. 107
108. 108
109. 109
110. 110
111. 111
112. 112
113. 113
114. 114
115. 115
116. 116
117. 117
118. 118
119. 119
120. 120
121. 121
122. 122
123. 123
124. 124
125. 125
126. 126
127. 127
128. 128
129. 129
130. 130
131. 131

// Copyright 2015, Joe Tsai. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.md file.

package bzip2

import "github.com/dsnet/compress/internal/errors"

// moveToFront implements both the MTF and RLE stages of bzip2 at the same time.
// Any runs of zeros in the encoded output will be replaced by a sequence of
// RUNA and RUNB symbols are encode the length of the run.
//
// The RLE encoding used can actually be encoded to and decoded from using
// normal two's complement arithmetic. The methodology for doing so is below.
//
// Assuming the following:
//	num: The value being encoded by RLE encoding.
//	run: A sequence of RUNA and RUNB symbols represented as a binary integer,
//	where RUNA is the 0 bit, RUNB is the 1 bit, and least-significant RUN
//	symbols are at the least-significant bit positions.
//	cnt: The number of RUNA and RUNB symbols.
//
// Then the RLE encoding used by bzip2 has this mathematical property:
//	num+1 == (1<<cnt) | run
type moveToFront struct {
	dictBuf [256]uint8
	dictLen int

	vals    []byte
	syms    []uint16
	blkSize int
}

func (mtf *moveToFront) Init(dict []uint8, blkSize int) {
	if len(dict) > len(mtf.dictBuf) {
		panicf(errors.Internal, "alphabet too large")
	}
	copy(mtf.dictBuf[:], dict)
	mtf.dictLen = len(dict)
	mtf.blkSize = blkSize
}

func (mtf *moveToFront) Encode(vals []byte) (syms []uint16) {
	dict := mtf.dictBuf[:mtf.dictLen]
	syms = mtf.syms[:0]

	if len(vals) > mtf.blkSize {
		panicf(errors.Internal, "exceeded block size")
	}

	var lastNum uint32
	for _, val := range vals {
		// Normal move-to-front transform.
		var idx uint8 // Reverse lookup idx in dict
		for di, dv := range dict {
			if dv == val {
				idx = uint8(di)
				break
			}
		}
		copy(dict[1:], dict[:idx])
		dict[0] = val

		// Run-length encoding augmentation.
		if idx == 0 {
			lastNum++
			continue
		}
		if lastNum > 0 {
			for rc := lastNum + 1; rc != 1; rc >>= 1 {
				syms = append(syms, uint16(rc&1))
			}
			lastNum = 0
		}
		syms = append(syms, uint16(idx)+1)
	}
	if lastNum > 0 {
		for rc := lastNum + 1; rc != 1; rc >>= 1 {
			syms = append(syms, uint16(rc&1))
		}
	}
	mtf.syms = syms
	return syms
}

func (mtf *moveToFront) Decode(syms []uint16) (vals []byte) {
	dict := mtf.dictBuf[:mtf.dictLen]
	vals = mtf.vals[:0]

	var lastCnt uint
	var lastRun uint32
	for _, sym := range syms {
		// Run-length encoding augmentation.
		if sym < 2 {
			lastRun |= uint32(sym) << lastCnt
			lastCnt++
			continue
		}
		if lastCnt > 0 {
			cnt := int((1<<lastCnt)|lastRun) - 1
			if len(vals)+cnt > mtf.blkSize || lastCnt > 24 {
				panicf(errors.Corrupted, "run-length decoding exceeded block size")
			}
			for i := cnt; i > 0; i-- {
				vals = append(vals, dict[0])
			}
			lastCnt, lastRun = 0, 0
		}

		// Normal move-to-front transform.
		val := dict[sym-1] // Forward lookup val in dict
		copy(dict[1:], dict[:sym-1])
		dict[0] = val

		if len(vals) >= mtf.blkSize {
			panicf(errors.Corrupted, "run-length decoding exceeded block size")
		}
		vals = append(vals, val)
	}
	if lastCnt > 0 {
		cnt := int((1<<lastCnt)|lastRun) - 1
		if len(vals)+cnt > mtf.blkSize || lastCnt > 24 {
			panicf(errors.Corrupted, "run-length decoding exceeded block size")
		}
		for i := cnt; i > 0; i-- {
			vals = append(vals, dict[0])
		}
	}
	mtf.vals = vals
	return vals
}