1// 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	}
39	compareSwap(ll, &s.llEnc, &s.llPrev)
40	compareSwap(ml, &s.mlEnc, &s.mlPrev)
41	compareSwap(of, &s.ofEnc, &s.ofPrev)
42}
43
44func highBit(val uint32) (n uint32) {
45	return uint32(bits.Len32(val) - 1)
46}
47
48var llCodeTable = [64]byte{0, 1, 2, 3, 4, 5, 6, 7,
49	8, 9, 10, 11, 12, 13, 14, 15,
50	16, 16, 17, 17, 18, 18, 19, 19,
51	20, 20, 20, 20, 21, 21, 21, 21,
52	22, 22, 22, 22, 22, 22, 22, 22,
53	23, 23, 23, 23, 23, 23, 23, 23,
54	24, 24, 24, 24, 24, 24, 24, 24,
55	24, 24, 24, 24, 24, 24, 24, 24}
56
57// Up to 6 bits
58const maxLLCode = 35
59
60// llBitsTable translates from ll code to number of bits.
61var llBitsTable = [maxLLCode + 1]byte{
62	0, 0, 0, 0, 0, 0, 0, 0,
63	0, 0, 0, 0, 0, 0, 0, 0,
64	1, 1, 1, 1, 2, 2, 3, 3,
65	4, 6, 7, 8, 9, 10, 11, 12,
66	13, 14, 15, 16}
67
68// llCode returns the code that represents the literal length requested.
69func llCode(litLength uint32) uint8 {
70	const llDeltaCode = 19
71	if litLength <= 63 {
72		// Compiler insists on bounds check (Go 1.12)
73		return llCodeTable[litLength&63]
74	}
75	return uint8(highBit(litLength)) + llDeltaCode
76}
77
78var mlCodeTable = [128]byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
79	16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
80	32, 32, 33, 33, 34, 34, 35, 35, 36, 36, 36, 36, 37, 37, 37, 37,
81	38, 38, 38, 38, 38, 38, 38, 38, 39, 39, 39, 39, 39, 39, 39, 39,
82	40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40,
83	41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41,
84	42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
85	42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42}
86
87// Up to 6 bits
88const maxMLCode = 52
89
90// mlBitsTable translates from ml code to number of bits.
91var mlBitsTable = [maxMLCode + 1]byte{
92	0, 0, 0, 0, 0, 0, 0, 0,
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	1, 1, 1, 1, 2, 2, 3, 3,
97	4, 4, 5, 7, 8, 9, 10, 11,
98	12, 13, 14, 15, 16}
99
100// note : mlBase = matchLength - MINMATCH;
101// because it's the format it's stored in seqStore->sequences
102func mlCode(mlBase uint32) uint8 {
103	const mlDeltaCode = 36
104	if mlBase <= 127 {
105		// Compiler insists on bounds check (Go 1.12)
106		return mlCodeTable[mlBase&127]
107	}
108	return uint8(highBit(mlBase)) + mlDeltaCode
109}
110
111func ofCode(offset uint32) uint8 {
112	// A valid offset will always be > 0.
113	return uint8(bits.Len32(offset) - 1)
114}
115