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Pragya Arya324337e2020-02-20 14:35:08 +05301// Copyright 2019+ Klaus Post. All rights reserved.
2// License information can be found in the LICENSE file.
3// Based on work by Yann Collet, released under BSD License.
4
5package zstd
6
7import "math/bits"
8
9type seqCoders struct {
10 llEnc, ofEnc, mlEnc *fseEncoder
11 llPrev, ofPrev, mlPrev *fseEncoder
12}
13
14// swap coders with another (block).
15func (s *seqCoders) swap(other *seqCoders) {
16 *s, *other = *other, *s
17}
18
19// setPrev will update the previous encoders to the actually used ones
20// and make sure a fresh one is in the main slot.
21func (s *seqCoders) setPrev(ll, ml, of *fseEncoder) {
22 compareSwap := func(used *fseEncoder, current, prev **fseEncoder) {
23 // We used the new one, more current to history and reuse the previous history
24 if *current == used {
25 *prev, *current = *current, *prev
26 c := *current
27 p := *prev
28 c.reUsed = false
29 p.reUsed = true
30 return
31 }
32 if used == *prev {
33 return
34 }
35 // Ensure we cannot reuse by accident
36 prevEnc := *prev
37 prevEnc.symbolLen = 0
38 return
39 }
40 compareSwap(ll, &s.llEnc, &s.llPrev)
41 compareSwap(ml, &s.mlEnc, &s.mlPrev)
42 compareSwap(of, &s.ofEnc, &s.ofPrev)
43}
44
45func highBit(val uint32) (n uint32) {
46 return uint32(bits.Len32(val) - 1)
47}
48
49var llCodeTable = [64]byte{0, 1, 2, 3, 4, 5, 6, 7,
50 8, 9, 10, 11, 12, 13, 14, 15,
51 16, 16, 17, 17, 18, 18, 19, 19,
52 20, 20, 20, 20, 21, 21, 21, 21,
53 22, 22, 22, 22, 22, 22, 22, 22,
54 23, 23, 23, 23, 23, 23, 23, 23,
55 24, 24, 24, 24, 24, 24, 24, 24,
56 24, 24, 24, 24, 24, 24, 24, 24}
57
58// Up to 6 bits
59const maxLLCode = 35
60
61// llBitsTable translates from ll code to number of bits.
62var llBitsTable = [maxLLCode + 1]byte{
63 0, 0, 0, 0, 0, 0, 0, 0,
64 0, 0, 0, 0, 0, 0, 0, 0,
65 1, 1, 1, 1, 2, 2, 3, 3,
66 4, 6, 7, 8, 9, 10, 11, 12,
67 13, 14, 15, 16}
68
69// llCode returns the code that represents the literal length requested.
70func llCode(litLength uint32) uint8 {
71 const llDeltaCode = 19
72 if litLength <= 63 {
73 // Compiler insists on bounds check (Go 1.12)
74 return llCodeTable[litLength&63]
75 }
76 return uint8(highBit(litLength)) + llDeltaCode
77}
78
79var mlCodeTable = [128]byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
80 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
81 32, 32, 33, 33, 34, 34, 35, 35, 36, 36, 36, 36, 37, 37, 37, 37,
82 38, 38, 38, 38, 38, 38, 38, 38, 39, 39, 39, 39, 39, 39, 39, 39,
83 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40,
84 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41,
85 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
86 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42}
87
88// Up to 6 bits
89const maxMLCode = 52
90
91// mlBitsTable translates from ml code to number of bits.
92var mlBitsTable = [maxMLCode + 1]byte{
93 0, 0, 0, 0, 0, 0, 0, 0,
94 0, 0, 0, 0, 0, 0, 0, 0,
95 0, 0, 0, 0, 0, 0, 0, 0,
96 0, 0, 0, 0, 0, 0, 0, 0,
97 1, 1, 1, 1, 2, 2, 3, 3,
98 4, 4, 5, 7, 8, 9, 10, 11,
99 12, 13, 14, 15, 16}
100
101// note : mlBase = matchLength - MINMATCH;
102// because it's the format it's stored in seqStore->sequences
103func mlCode(mlBase uint32) uint8 {
104 const mlDeltaCode = 36
105 if mlBase <= 127 {
106 // Compiler insists on bounds check (Go 1.12)
107 return mlCodeTable[mlBase&127]
108 }
109 return uint8(highBit(mlBase)) + mlDeltaCode
110}
111
112func ofCode(offset uint32) uint8 {
113 // A valid offset will always be > 0.
114 return uint8(bits.Len32(offset) - 1)
115}