1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
// SPDX-License-Identifier: BSD-3-Clause
// Copyright (c) 2026 Fernando Sahmkow
#include "bit_io.h"
namespace whiteout {
// ============================================================================
// MsbBitReader
// ============================================================================
void MsbBitReader::init(const u8* d, size_t s, size_t startOffset) {
data = d;
size = s;
bytePos = startOffset;
bitBuf = 0;
bitsAvail = 0;
pendingMarker = 0;
}
void MsbBitReader::refill() {
while (bitsAvail <= 24) {
u8 byte = 0;
if (pendingMarker || bytePos >= size) {
// Pad with zero bits when no more data is available.
} else {
byte = data[bytePos++];
if (byte == 0xFF) {
if (bytePos < size) {
u8 const nextByte = data[bytePos];
if (nextByte == 0x00) {
bytePos++; // Byte-stuffing: literal 0xFF value
} else {
pendingMarker = nextByte; // Real marker encountered
bytePos++;
byte = 0;
}
} else {
byte = 0;
}
}
}
bitBuf |= static_cast<u32>(byte) << (24 - bitsAvail);
bitsAvail += 8;
}
}
u32 MsbBitReader::peekBits(i32 count) {
if (bitsAvail < count) {
refill();
}
return (bitBuf >> (32 - count)) & BIT_MASK[count];
}
void MsbBitReader::consumeBits(i32 count) {
bitBuf <<= count;
bitsAvail -= count;
}
u32 MsbBitReader::readBits(i32 count) {
if (count == 0) {
return 0;
}
u32 const value = peekBits(count);
consumeBits(count);
return value;
}
void MsbBitReader::handleRestartMarker() {
bitBuf = 0;
bitsAvail = 0;
// JPEG restart markers: 0xD0\u20130xD7.
if (pendingMarker >= 0xD0 && pendingMarker <= 0xD7) {
// refill() already consumed the marker bytes — just clear the flag.
pendingMarker = 0;
} else if (pendingMarker == 0 && bytePos + 1 < size && data[bytePos] == 0xFF &&
data[bytePos + 1] >= 0xD0 && data[bytePos + 1] <= 0xD7) {
// refill() didn't reach the marker — skip past it manually.
bytePos += 2;
}
}
// ============================================================================
// MsbBitWriter
// ============================================================================
void MsbBitWriter::init(std::vector<u8>* o) {
out = o;
bitBuf = 0;
bitsUsed = 0;
}
void MsbBitWriter::writeBits(u32 value, i32 count) {
// Shift value into the MSB-aligned position, leaving room for existing bits.
bitBuf |= (value & ((1u << count) - 1)) << (32 - bitsUsed - count);
bitsUsed += count;
// Flush complete bytes.
while (bitsUsed >= 8) {
u8 const byte = static_cast<u8>(bitBuf >> 24);
out->push_back(byte);
if (byte == 0xFF) {
out->push_back(0x00); // Byte-stuffing.
}
bitBuf <<= 8;
bitsUsed -= 8;
}
}
void MsbBitWriter::flushWithPadding() {
if (bitsUsed > 0) {
i32 const padBits = 8 - bitsUsed;
writeBits((1u << padBits) - 1, padBits);
}
}
} // namespace whiteout