jcs.org
Serenity Operating System
1/*
2 * Copyright (c) 2021, Hunter Salyer <thefalsehonesty@gmail.com>
3 * Copyright (c) 2022, Gregory Bertilson <zaggy1024@gmail.com>
4 *
5 * SPDX-License-Identifier: BSD-2-Clause
6 */
7
8#include <AK/BuiltinWrappers.h>
9
10#include "BooleanDecoder.h"
11
12namespace Video::VP9 {
13
14/* 9.2.1 */
15ErrorOr<BooleanDecoder> BooleanDecoder::initialize(MaybeOwned<BigEndianInputBitStream> bit_stream, size_t bytes)
16{
17 VERIFY(bit_stream->is_aligned_to_byte_boundary());
18 auto value = TRY(bit_stream->read_value<u8>());
19 u8 range = 255;
20 u64 max_bits = (8 * bytes) - 8;
21 BooleanDecoder decoder { move(bit_stream), value, range, max_bits };
22 if (TRY(decoder.read_bool(128)))
23 return Error::from_string_literal("Range decoder marker was non-zero");
24 return decoder;
25}
26
27/* 9.2.2 */
28ErrorOr<bool> BooleanDecoder::read_bool(u8 probability)
29{
30 auto split = 1u + (((m_range - 1u) * probability) >> 8u);
31 bool return_bool;
32
33 if (m_value < split) {
34 m_range = split;
35 return_bool = false;
36 } else {
37 m_range -= split;
38 m_value -= split;
39 return_bool = true;
40 }
41
42 if (m_range < 128) {
43 u8 bits_to_shift_into_range = count_leading_zeroes(m_range);
44
45 if (bits_to_shift_into_range > m_bits_left)
46 return Error::from_string_literal("Range decoder is out of data");
47
48 m_range <<= bits_to_shift_into_range;
49 m_value = (m_value << bits_to_shift_into_range) | TRY(m_bit_stream->read_bits<u8>(bits_to_shift_into_range));
50 m_bits_left -= bits_to_shift_into_range;
51 }
52
53 return return_bool;
54}
55
56ErrorOr<u8> BooleanDecoder::read_literal(u8 bits)
57{
58 u8 return_value = 0;
59 for (size_t i = 0; i < bits; i++) {
60 return_value = (2 * return_value) + TRY(read_bool(128));
61 }
62 return return_value;
63}
64
65size_t BooleanDecoder::bits_remaining() const
66{
67 return m_bits_left;
68}
69
70/* 9.2.3 */
71ErrorOr<void> BooleanDecoder::finish_decode()
72{
73 while (m_bits_left > 0) {
74 auto padding_read_size = min(m_bits_left, 64);
75 auto padding_bits = TRY(m_bit_stream->read_bits(padding_read_size));
76 m_bits_left -= padding_read_size;
77
78 if (padding_bits != 0)
79 return Error::from_string_literal("Range decoder has non-zero padding element");
80 }
81
82 // FIXME: It is a requirement of bitstream conformance that enough padding bits are inserted to ensure that the final coded byte of a frame is not equal to a superframe marker.
83 // A byte b is equal to a superframe marker if and only if (b & 0xe0)is equal to 0xc0, i.e. if the most significant 3 bits are equal to 0b110.
84 return {};
85}
86
87}