dvb_si/descriptors/
component.rs

1//! Component Descriptor — ETSI EN 300 468 §6.2.8 (tag 0x50).
2//!
3//! Describes one elementary component (audio/video/data/teletext/subtitle)
4//! with a language code and free-text label. Carried inside EIT event
5//! descriptor loops.
6
7use crate::error::{Error, Result};
8use crate::traits::Descriptor;
9use dvb_common::{Parse, Serialize};
10
11/// Descriptor tag for component_descriptor.
12pub const TAG: u8 = 0x50;
13const HEADER_LEN: usize = 2;
14/// stream_content(1) + component_type(1) + component_tag(1) + language_code(3) = 6
15const PRE_TEXT_LEN: usize = 6;
16const STREAM_CONTENT_MASK: u8 = 0x0F;
17
18/// Component Descriptor.
19#[derive(Debug, Clone, PartialEq, Eq)]
20#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
21pub struct ComponentDescriptor<'a> {
22    /// 4-bit stream_content_ext (high nibble) — combines with `stream_content`
23    /// to identify the component (EN 300 468 §6.2.8 Table 25).
24    pub stream_content_ext: u8,
25    /// 4-bit stream_content (0x01 video, 0x02 audio, 0x03 teletext, …).
26    pub stream_content: u8,
27    /// component_type byte (sub-classification inside the stream_content).
28    pub component_type: u8,
29    /// component_tag for cross-reference with stream_identifier_descriptor.
30    pub component_tag: u8,
31    /// ISO 639-2 language code.
32    pub language_code: [u8; 3],
33    /// Raw DVB-encoded text label for this component.
34    #[cfg_attr(feature = "serde", serde(borrow))]
35    pub text: &'a [u8],
36}
37
38impl<'a> Parse<'a> for ComponentDescriptor<'a> {
39    type Error = crate::error::Error;
40    fn parse(bytes: &'a [u8]) -> Result<Self> {
41        if bytes.len() < HEADER_LEN {
42            return Err(Error::BufferTooShort {
43                need: HEADER_LEN,
44                have: bytes.len(),
45                what: "ComponentDescriptor header",
46            });
47        }
48        if bytes[0] != TAG {
49            return Err(Error::InvalidDescriptor {
50                tag: bytes[0],
51                reason: "unexpected tag for component_descriptor",
52            });
53        }
54        let length = bytes[1] as usize;
55        let end = HEADER_LEN + length;
56        if bytes.len() < end {
57            return Err(Error::BufferTooShort {
58                need: end,
59                have: bytes.len(),
60                what: "ComponentDescriptor body",
61            });
62        }
63        if length < PRE_TEXT_LEN {
64            return Err(Error::InvalidDescriptor {
65                tag: TAG,
66                reason: "component_descriptor body shorter than minimum 6 bytes",
67            });
68        }
69        let stream_content_ext = bytes[HEADER_LEN] >> 4;
70        let stream_content = bytes[HEADER_LEN] & STREAM_CONTENT_MASK;
71        let component_type = bytes[HEADER_LEN + 1];
72        let component_tag = bytes[HEADER_LEN + 2];
73        let language_code = [
74            bytes[HEADER_LEN + 3],
75            bytes[HEADER_LEN + 4],
76            bytes[HEADER_LEN + 5],
77        ];
78        let text = &bytes[HEADER_LEN + PRE_TEXT_LEN..end];
79        Ok(Self {
80            stream_content_ext,
81            stream_content,
82            component_type,
83            component_tag,
84            language_code,
85            text,
86        })
87    }
88}
89
90impl Serialize for ComponentDescriptor<'_> {
91    type Error = crate::error::Error;
92    fn serialized_len(&self) -> usize {
93        HEADER_LEN + PRE_TEXT_LEN + self.text.len()
94    }
95
96    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
97        let len = self.serialized_len();
98        if buf.len() < len {
99            return Err(Error::OutputBufferTooSmall {
100                need: len,
101                have: buf.len(),
102            });
103        }
104        buf[0] = TAG;
105        buf[1] = (len - HEADER_LEN) as u8;
106        // High nibble = stream_content_ext, low nibble = stream_content (§6.2.8).
107        buf[HEADER_LEN] = (self.stream_content_ext << 4) | (self.stream_content & STREAM_CONTENT_MASK);
108        buf[HEADER_LEN + 1] = self.component_type;
109        buf[HEADER_LEN + 2] = self.component_tag;
110        buf[HEADER_LEN + 3..HEADER_LEN + 6].copy_from_slice(&self.language_code);
111        buf[HEADER_LEN + PRE_TEXT_LEN..len].copy_from_slice(self.text);
112        Ok(len)
113    }
114}
115
116impl<'a> Descriptor<'a> for ComponentDescriptor<'a> {
117    const TAG: u8 = TAG;
118    fn descriptor_length(&self) -> u8 {
119        (self.serialized_len() - HEADER_LEN) as u8
120    }
121}
122
123#[cfg(test)]
124mod tests {
125    use super::*;
126
127    #[test]
128    fn parse_extracts_all_fields() {
129        let bytes = [
130            TAG, 12, 0x02, 0x01, 0x42, b'e', b'n', b'g', b'S', b'T', b'E', b'R', b'E', b'O',
131        ];
132        let d = ComponentDescriptor::parse(&bytes).unwrap();
133        assert_eq!(d.stream_content, 2);
134        assert_eq!(d.component_type, 0x01);
135        assert_eq!(d.component_tag, 0x42);
136        assert_eq!(&d.language_code, b"eng");
137        assert_eq!(d.text, b"STEREO");
138    }
139
140    #[test]
141    fn parse_rejects_wrong_tag() {
142        assert!(matches!(
143            ComponentDescriptor::parse(&[0x51, 6, 0, 0, 0, b'e', b'n', b'g']).unwrap_err(),
144            Error::InvalidDescriptor { tag: 0x51, .. }
145        ));
146    }
147
148    #[test]
149    fn parse_rejects_short_body() {
150        let bytes = [TAG, 5, 0x01, 0x00, 0x00, b'e', b'n'];
151        assert!(matches!(
152            ComponentDescriptor::parse(&bytes).unwrap_err(),
153            Error::InvalidDescriptor { .. }
154        ));
155    }
156
157    #[test]
158    fn parse_rejects_truncated_buffer() {
159        let bytes = [TAG, 6, 0x01];
160        assert!(matches!(
161            ComponentDescriptor::parse(&bytes).unwrap_err(),
162            Error::BufferTooShort { .. }
163        ));
164    }
165
166    #[test]
167    fn parse_with_empty_text_valid() {
168        let bytes = [TAG, 6, 0x01, 0x01, 0x01, b'e', b'n', b'g'];
169        let d = ComponentDescriptor::parse(&bytes).unwrap();
170        assert_eq!(d.text, &[] as &[u8]);
171    }
172
173    #[test]
174    fn serialize_round_trip() {
175        let d = ComponentDescriptor {
176            stream_content_ext: 0x0F,
177            stream_content: 0x03,
178            component_type: 0x10,
179            component_tag: 0x05,
180            language_code: *b"fra",
181            text: b"Sous-titres",
182        };
183        let mut buf = vec![0u8; d.serialized_len()];
184        d.serialize_into(&mut buf).unwrap();
185        assert_eq!(ComponentDescriptor::parse(&buf).unwrap(), d);
186    }
187
188    #[test]
189    fn stream_content_masked_to_low_nibble() {
190        // Byte = 0xF2 → high nibble is stream_content_ext (0xF), low nibble is stream_content=2.
191        let bytes = [TAG, 6, 0xF2, 0x00, 0x00, b'e', b'n', b'g'];
192        let d = ComponentDescriptor::parse(&bytes).unwrap();
193        assert_eq!(d.stream_content, 2);
194    }
195}
dvb_si/descriptors/component.rs

dvb_si/descriptors/
component.rs
