zahirscan 0.3.6

Token-efficient content compression for AI analysis using probabilistic template mining
Documentation 
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
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199

//! MP3-specific parsing utilities, including LAME tag reading

use std::fs::File;
use std::io::{Read, Seek, SeekFrom};

use crate::parsers::FileType;
use crate::parsers::media_helpers::BitrateMode;
use crate::utils::filetypes::is_codec_for_file_type;

/// Check if a codec string represents MP3
/// Verifies against `FILE_EXTENSION_MAP` so we only treat known audio codecs as MP3.
pub fn is_mp3_codec(codec_ref: &str) -> bool {
    is_codec_for_file_type(codec_ref, FileType::Audio) && codec_ref.to_lowercase().contains("mp3")
}

/// MP3 frame header constants
mod mp3_constants {
    /// MPEG-1 version identifier
    pub const MPEG_VERSION_1: u8 = 0x03;

    /// Channel mode: Mono (single channel)
    pub const CHANNEL_MODE_MONO: u8 = 0x03;

    /// Frame sync pattern: first byte must be 0xFF
    pub const FRAME_SYNC_BYTE: u8 = 0xFF;

    /// Frame sync mask: first 3 bits of second byte must be 0xE0 (11100000)
    pub const FRAME_SYNC_MASK: u8 = 0xE0;

    /// Protection bit mask (bit 1 of frame header byte 1)
    pub const PROTECTION_BIT_MASK: u8 = 0x01;

    /// MPEG version mask (bits 3-4 of frame header byte 1)
    pub const MPEG_VERSION_MASK: u8 = 0x03;

    /// Channel mode mask (bits 6-7 of frame header byte 3)
    pub const CHANNEL_MODE_MASK: u8 = 0x03;

    /// VBR method mask (low 4 bits of revision+VBR byte)
    pub const VBR_METHOD_MASK: u8 = 0x0F;

    /// Xing/Info header flags bit masks
    pub mod xing_flags {
        /// Frame count flag (bit 0)
        pub const FRAME_COUNT: u8 = 0x01;
        /// Byte count flag (bit 1)
        pub const BYTE_COUNT: u8 = 0x02;
        /// TOC flag (bit 2)
        pub const TOC: u8 = 0x04;
        /// Quality flag (bit 3)
        pub const QUALITY: u8 = 0x08;
    }

    /// LAME VBR method values
    pub mod vbr_method {
        pub const CBR: u8 = 1;
        pub const ABR: u8 = 2;
        pub const VBR_OLD: u8 = 3;
        pub const VBR_NEW: u8 = 4;
        pub const VBR_MT: u8 = 5;
        pub const VBR_MTRH: u8 = 6;
        pub const VBR_ABR_ALT: u8 = 8;
    }

    /// Side info sizes (in bytes) for different MPEG versions and channel modes
    pub mod side_info_size {
        /// MPEG-1 Layer III mono
        pub const MPEG1_MONO: usize = 17;
        /// MPEG-1 Layer III stereo/dual/joint
        pub const MPEG1_STEREO: usize = 32;
        /// MPEG-2/2.5 Layer III mono
        pub const MPEG2_MONO: usize = 9;
        /// MPEG-2/2.5 Layer III stereo/dual/joint
        pub const MPEG2_STEREO: usize = 17;
    }
}

/// Skip `ID3v2` if present, otherwise rewind to file start. Returns `None` on I/O failure.
fn skip_id3v2_prefix(file_ref: &mut File) -> Option<()> {
    let mut header = [0u8; 10];
    file_ref.read_exact(&mut header).ok()?;
    if &header[0..3] == b"ID3" {
        let size = (u32::from(header[6]) << 21)
            | (u32::from(header[7]) << 14)
            | (u32::from(header[8]) << 7)
            | u32::from(header[9]);
        let id3_size = 10 + u64::from(size);
        file_ref.seek(SeekFrom::Start(id3_size)).ok()?;
    } else {
        file_ref.seek(SeekFrom::Start(0)).ok()?;
    }
    Some(())
}

fn mp3_frame_sync_valid(frame_header: [u8; 4]) -> bool {
    frame_header[0] == mp3_constants::FRAME_SYNC_BYTE
        && (frame_header[1] & mp3_constants::FRAME_SYNC_MASK) == mp3_constants::FRAME_SYNC_MASK
}

fn side_info_byte_count(mpeg_version: u8, channel_mode: u8) -> usize {
    match mpeg_version {
        mp3_constants::MPEG_VERSION_1 => {
            if channel_mode == mp3_constants::CHANNEL_MODE_MONO {
                mp3_constants::side_info_size::MPEG1_MONO
            } else {
                mp3_constants::side_info_size::MPEG1_STEREO
            }
        }
        _ => {
            if channel_mode == mp3_constants::CHANNEL_MODE_MONO {
                mp3_constants::side_info_size::MPEG2_MONO
            } else {
                mp3_constants::side_info_size::MPEG2_STEREO
            }
        }
    }
}

/// After the first frame header: skip CRC (if any), side info, then read Xing/LAME VBR byte.
fn bitrate_mode_after_first_frame(
    file_ref: &mut File,
    frame_header: [u8; 4],
) -> Option<BitrateMode> {
    let has_crc = (frame_header[1] & mp3_constants::PROTECTION_BIT_MASK) == 0;
    if has_crc {
        file_ref.seek(SeekFrom::Current(2)).ok()?;
    }
    let mpeg_version = (frame_header[1] >> 3) & mp3_constants::MPEG_VERSION_MASK;
    let channel_mode = (frame_header[3] >> 6) & mp3_constants::CHANNEL_MODE_MASK;
    let side_n = side_info_byte_count(mpeg_version, channel_mode);
    let side_off = i64::try_from(side_n).unwrap_or(i64::MAX);
    file_ref.seek(SeekFrom::Current(side_off)).ok()?;

    let mut xing_header = [0u8; 4];
    file_ref.read_exact(&mut xing_header).ok()?;
    if &xing_header != b"Xing" && &xing_header != b"Info" {
        return None;
    }

    let mut flags = [0u8; 4];
    file_ref.read_exact(&mut flags).ok()?;

    let mut skip_bytes = 0u64;
    if (flags[3] & mp3_constants::xing_flags::FRAME_COUNT) != 0 {
        skip_bytes += 4;
    }
    if (flags[3] & mp3_constants::xing_flags::BYTE_COUNT) != 0 {
        skip_bytes += 4;
    }
    if (flags[3] & mp3_constants::xing_flags::TOC) != 0 {
        skip_bytes += 100;
    }
    if (flags[3] & mp3_constants::xing_flags::QUALITY) != 0 {
        skip_bytes += 4;
    }
    file_ref
        .seek(SeekFrom::Current(skip_bytes.cast_signed()))
        .ok()?;

    let mut lame_version = [0u8; 9];
    file_ref.read_exact(&mut lame_version).ok()?;
    if &lame_version[0..4] != b"LAME" {
        return None;
    }

    let mut revision_vbr_byte = [0u8; 1];
    file_ref.read_exact(&mut revision_vbr_byte).ok()?;
    let vbr_method = revision_vbr_byte[0] & mp3_constants::VBR_METHOD_MASK;

    match vbr_method {
        mp3_constants::vbr_method::CBR => Some(BitrateMode::Cbr),
        mp3_constants::vbr_method::ABR => Some(BitrateMode::Abr),
        mp3_constants::vbr_method::VBR_OLD
        | mp3_constants::vbr_method::VBR_NEW
        | mp3_constants::vbr_method::VBR_MT
        | mp3_constants::vbr_method::VBR_MTRH
        | mp3_constants::vbr_method::VBR_ABR_ALT => Some(BitrateMode::Vbr),
        _ => None,
    }
}

/// Read LAME tag from MP3 file to determine bitrate mode (CBR/VBR/ABR)
///
/// The LAME tag is embedded in the first MP3 frame's Xing/Info header.
/// Returns the bitrate mode if successfully parsed, None otherwise.
///
/// Note: Many MP3 files (especially CBR) don't have a Xing/Info header,
/// so this will return None for those files.
pub fn read_lame_tag_bitrate_mode(file_path_ref: &str) -> Option<BitrateMode> {
    let mut file = File::open(file_path_ref).ok()?;
    skip_id3v2_prefix(&mut file)?;

    let mut frame_header = [0u8; 4];
    file.read_exact(&mut frame_header).ok()?;
    if !mp3_frame_sync_valid(frame_header) {
        return None;
    }
    bitrate_mode_after_first_frame(&mut file, frame_header)
}