dvb_si/descriptors/
vbi_teletext.rs

1//! VBI Teletext Descriptor — ETSI EN 300 468 §6.2.48 (tag 0x46).
2//!
3//! Table 108 (PDF p. 111). Identical wire layout to the teletext_descriptor
4//! (Table 101): a loop of 5-byte entries, each a 3-char ISO 639 language code
5//! plus teletext_type (5 bits) / magazine_number (3 bits) / page_number
6//! (8 bits). Signals teletext also carried in the analogue VBI lines.
7
8use super::descriptor_body;
9use super::teletext::TeletextType;
10use crate::error::{Error, Result};
11use crate::text::LangCode;
12use alloc::vec::Vec;
13use dvb_common::{Parse, Serialize};
14
15/// Descriptor tag for VBI_teletext_descriptor.
16pub const TAG: u8 = 0x46;
17const HEADER_LEN: usize = 2;
18const ENTRY_LEN: usize = 5;
19const LANG_LEN: usize = 3;
20/// Maximum body length expressible in the 8-bit `descriptor_length` field.
21const MAX_BODY_LEN: usize = u8::MAX as usize;
22
23/// One VBI teletext component.
24#[derive(Debug, Clone, Copy, PartialEq, Eq)]
25#[cfg_attr(feature = "serde", derive(serde::Serialize))]
26pub struct VbiTeletextEntry {
27    /// ISO 639-2 language code of this teletext service.
28    pub language_code: LangCode,
29    /// 5-bit teletext_type (EN 300 468 Table 102).
30    pub teletext_type: TeletextType,
31    /// 3-bit teletext_magazine_number.
32    pub magazine_number: u8,
33    /// 8-bit BCD teletext_page_number.
34    pub page_number: u8,
35}
36
37/// VBI Teletext Descriptor.
38#[derive(Debug, Clone, PartialEq, Eq)]
39#[cfg_attr(feature = "serde", derive(serde::Serialize))]
40pub struct VbiTeletextDescriptor {
41    /// Teletext components in wire order.
42    pub entries: Vec<VbiTeletextEntry>,
43}
44
45impl<'a> Parse<'a> for VbiTeletextDescriptor {
46    type Error = crate::error::Error;
47    fn parse(bytes: &'a [u8]) -> Result<Self> {
48        let body = descriptor_body(
49            bytes,
50            TAG,
51            "VbiTeletextDescriptor",
52            "unexpected tag for VBI_teletext_descriptor",
53        )?;
54        if body.len() % ENTRY_LEN != 0 {
55            return Err(Error::InvalidDescriptor {
56                tag: TAG,
57                reason: "descriptor_length must be a multiple of 5",
58            });
59        }
60        let mut entries = Vec::with_capacity(body.len() / ENTRY_LEN);
61        for chunk in body.chunks_exact(ENTRY_LEN) {
62            let language_code = LangCode([chunk[0], chunk[1], chunk[2]]);
63            let type_and_mag = chunk[LANG_LEN];
64            entries.push(VbiTeletextEntry {
65                language_code,
66                teletext_type: TeletextType::from_u8((type_and_mag >> 3) & 0x1F),
67                magazine_number: type_and_mag & 0x07,
68                page_number: chunk[LANG_LEN + 1],
69            });
70        }
71        Ok(Self { entries })
72    }
73}
74
75impl Serialize for VbiTeletextDescriptor {
76    type Error = crate::error::Error;
77    fn serialized_len(&self) -> usize {
78        HEADER_LEN + ENTRY_LEN * self.entries.len()
79    }
80
81    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
82        let body_len = ENTRY_LEN * self.entries.len();
83        // 8-bit descriptor_length field: error rather than silently truncate.
84        if body_len > MAX_BODY_LEN {
85            return Err(Error::InvalidDescriptor {
86                tag: TAG,
87                reason: "VBI_teletext_descriptor body exceeds 255 bytes",
88            });
89        }
90        let len = self.serialized_len();
91        if buf.len() < len {
92            return Err(Error::OutputBufferTooSmall {
93                need: len,
94                have: buf.len(),
95            });
96        }
97        buf[0] = TAG;
98        buf[1] = body_len as u8;
99        let mut pos = HEADER_LEN;
100        for e in &self.entries {
101            buf[pos..pos + LANG_LEN].copy_from_slice(&e.language_code.0);
102            buf[pos + LANG_LEN] =
103                ((e.teletext_type.to_u8() & 0x1F) << 3) | (e.magazine_number & 0x07);
104            buf[pos + LANG_LEN + 1] = e.page_number;
105            pos += ENTRY_LEN;
106        }
107        Ok(len)
108    }
109}
110impl<'a> crate::traits::DescriptorDef<'a> for VbiTeletextDescriptor {
111    const TAG: u8 = TAG;
112    const NAME: &'static str = "VBI_TELETEXT";
113}
114
115#[cfg(test)]
116mod tests {
117    use super::*;
118
119    #[test]
120    fn parse_single_entry() {
121        let bytes = [TAG, 5, b'e', b'n', b'g', (1 << 3) | 2, 0x10];
122        let d = VbiTeletextDescriptor::parse(&bytes).unwrap();
123        assert_eq!(d.entries.len(), 1);
124        assert_eq!(d.entries[0].language_code, LangCode(*b"eng"));
125        assert_eq!(d.entries[0].teletext_type, TeletextType::InitialPage);
126        assert_eq!(d.entries[0].magazine_number, 2);
127        assert_eq!(d.entries[0].page_number, 0x10);
128    }
129
130    #[test]
131    fn parse_multiple_entries() {
132        let bytes = [
133            TAG,
134            10,
135            b'e',
136            b'n',
137            b'g',
138            (1 << 3) | 1,
139            0x10,
140            b'f',
141            b'r',
142            b'a',
143            (2 << 3) | 1,
144            0x20,
145        ];
146        let d = VbiTeletextDescriptor::parse(&bytes).unwrap();
147        assert_eq!(d.entries.len(), 2);
148        assert_eq!(d.entries[1].teletext_type, TeletextType::SubtitlePage);
149        assert_eq!(d.entries[1].language_code, LangCode(*b"fra"));
150    }
151
152    #[test]
153    fn parse_rejects_wrong_tag() {
154        assert!(matches!(
155            VbiTeletextDescriptor::parse(&[0x47, 0]).unwrap_err(),
156            Error::InvalidDescriptor { tag: 0x47, .. }
157        ));
158    }
159
160    #[test]
161    fn parse_rejects_short_buffer() {
162        let bytes = [TAG, 5, b'e', b'n'];
163        assert!(matches!(
164            VbiTeletextDescriptor::parse(&bytes).unwrap_err(),
165            Error::BufferTooShort { .. }
166        ));
167    }
168
169    #[test]
170    fn parse_rejects_length_not_multiple_of_5() {
171        let bytes = [TAG, 4, 0, 0, 0, 0];
172        assert!(matches!(
173            VbiTeletextDescriptor::parse(&bytes).unwrap_err(),
174            Error::InvalidDescriptor { tag: TAG, .. }
175        ));
176    }
177
178    #[test]
179    fn empty_descriptor_valid() {
180        let d = VbiTeletextDescriptor::parse(&[TAG, 0]).unwrap();
181        assert!(d.entries.is_empty());
182    }
183
184    #[test]
185    fn serialize_round_trip() {
186        let d = VbiTeletextDescriptor {
187            entries: vec![VbiTeletextEntry {
188                language_code: LangCode(*b"fra"),
189                teletext_type: TeletextType::SubtitlePage,
190                magazine_number: 8 & 0x07,
191                page_number: 0x88,
192            }],
193        };
194        let mut buf = vec![0u8; d.serialized_len()];
195        d.serialize_into(&mut buf).unwrap();
196        assert_eq!(VbiTeletextDescriptor::parse(&buf).unwrap(), d);
197    }
198
199    #[test]
200    fn serialize_rejects_over_range_body() {
201        // 52 entries = 260 body bytes, past the u8 length field.
202        let d = VbiTeletextDescriptor {
203            entries: vec![
204                VbiTeletextEntry {
205                    language_code: LangCode(*b"eng"),
206                    teletext_type: TeletextType::Reserved(1),
207                    magazine_number: 1,
208                    page_number: 0,
209                };
210                52
211            ],
212        };
213        let mut buf = vec![0u8; d.serialized_len()];
214        assert!(matches!(
215            d.serialize_into(&mut buf).unwrap_err(),
216            Error::InvalidDescriptor { tag: TAG, .. }
217        ));
218    }
219
220    #[cfg(feature = "serde")]
221    #[test]
222    fn serde_round_trip() {
223        let d = VbiTeletextDescriptor {
224            entries: vec![VbiTeletextEntry {
225                language_code: LangCode(*b"eng"),
226                teletext_type: TeletextType::SubtitlePage,
227                magazine_number: 1,
228                page_number: 0x10,
229            }],
230        };
231        let json = serde_json::to_string(&d).unwrap();
232        // Serialize-only: assert the emitted JSON re-parses (serialize-stable).
233        let _v: serde_json::Value = serde_json::from_str(&json).unwrap();
234    }
235}
dvb_si/descriptors/vbi_teletext.rs

dvb_si/descriptors/
vbi_teletext.rs
