dvb_si/descriptors/
system_clock.rs

1//! System Clock Descriptor — ISO/IEC 13818-1 §2.6.20 (tag 0x0B).
2//!
3//! Describes the system clock accuracy.
4
5use super::descriptor_body;
6use crate::error::{Error, Result};
7use dvb_common::{Parse, Serialize};
8
9/// Descriptor tag for system_clock_descriptor.
10pub const TAG: u8 = 0x0B;
11const HEADER_LEN: usize = 2;
12const BODY_LEN: u8 = 2;
13
14/// System Clock Descriptor.
15#[derive(Debug, Clone, PartialEq, Eq)]
16#[cfg_attr(feature = "serde", derive(serde::Serialize))]
17#[cfg_attr(feature = "yoke", derive(yoke::Yokeable))]
18pub struct SystemClockDescriptor {
19    /// External clock reference indicator.
20    pub external_clock_reference_indicator: bool,
21    /// Clock accuracy integer (6 bits).
22    pub clock_accuracy_integer: u8,
23    /// Clock accuracy exponent (3 bits).
24    pub clock_accuracy_exponent: u8,
25}
26
27impl<'a> Parse<'a> for SystemClockDescriptor {
28    type Error = crate::error::Error;
29
30    fn parse(bytes: &'a [u8]) -> Result<Self> {
31        let body = descriptor_body(
32            bytes,
33            TAG,
34            "SystemClockDescriptor",
35            "unexpected tag for system_clock_descriptor",
36        )?;
37        if body.len() != BODY_LEN as usize {
38            return Err(Error::InvalidDescriptor {
39                tag: TAG,
40                reason: "system_clock_descriptor length must equal 2",
41            });
42        }
43        let b0 = body[0];
44        let external_clock_reference_indicator = (b0 & 0x80) != 0;
45        let clock_accuracy_integer = b0 & 0x3F;
46        let b1 = body[1];
47        let clock_accuracy_exponent = b1 >> 5;
48        Ok(Self {
49            external_clock_reference_indicator,
50            clock_accuracy_integer,
51            clock_accuracy_exponent,
52        })
53    }
54}
55
56impl Serialize for SystemClockDescriptor {
57    type Error = crate::error::Error;
58
59    fn serialized_len(&self) -> usize {
60        HEADER_LEN + (BODY_LEN as usize)
61    }
62
63    fn serialize_into(&self, buf: &mut [u8]) -> Result<usize> {
64        let len = self.serialized_len();
65        if buf.len() < len {
66            return Err(Error::OutputBufferTooSmall {
67                need: len,
68                have: buf.len(),
69            });
70        }
71        buf[0] = TAG;
72        buf[1] = BODY_LEN;
73        buf[HEADER_LEN] = ((self.external_clock_reference_indicator as u8) << 7)
74            | (self.clock_accuracy_integer & 0x3F);
75        buf[HEADER_LEN + 1] = (self.clock_accuracy_exponent & 0x07) << 5;
76        Ok(len)
77    }
78}
79impl<'a> crate::traits::DescriptorDef<'a> for SystemClockDescriptor {
80    const TAG: u8 = TAG;
81    const NAME: &'static str = "SYSTEM_CLOCK";
82}
83
84#[cfg(test)]
85mod tests {
86    use super::*;
87
88    #[test]
89    fn parse() {
90        let bytes = [
91            TAG,
92            2,
93            0b1011_1111, // external=1, reserved=0, integer=63
94            0b101_00000, // exponent=5, reserved=0
95        ];
96        let d = SystemClockDescriptor::parse(&bytes).unwrap();
97        assert!(d.external_clock_reference_indicator);
98        assert_eq!(d.clock_accuracy_integer, 63);
99        assert_eq!(d.clock_accuracy_exponent, 5);
100    }
101
102    #[test]
103    fn serialize_round_trip() {
104        let d = SystemClockDescriptor {
105            external_clock_reference_indicator: false,
106            clock_accuracy_integer: 42,
107            clock_accuracy_exponent: 2,
108        };
109        let mut buf = vec![0u8; d.serialized_len()];
110        d.serialize_into(&mut buf).unwrap();
111        let reparsed = SystemClockDescriptor::parse(&buf).unwrap();
112        assert_eq!(d, reparsed);
113    }
114
115    #[test]
116    fn parse_rejects_wrong_tag() {
117        let err = SystemClockDescriptor::parse(&[0x0C, 2, 0, 0]).unwrap_err();
118        assert!(matches!(err, Error::InvalidDescriptor { tag: 0x0C, .. }));
119    }
120
121    #[test]
122    fn parse_rejects_wrong_length() {
123        let err = SystemClockDescriptor::parse(&[TAG, 3, 0, 0, 0]).unwrap_err();
124        assert!(matches!(err, Error::InvalidDescriptor { tag: TAG, .. }));
125    }
126
127    #[test]
128    fn serialize_rejects_small_buffer() {
129        let d = SystemClockDescriptor {
130            external_clock_reference_indicator: false,
131            clock_accuracy_integer: 0,
132            clock_accuracy_exponent: 0,
133        };
134        let mut tiny = vec![0u8; 2];
135        let err = d.serialize_into(&mut tiny).unwrap_err();
136        assert!(matches!(err, Error::OutputBufferTooSmall { .. }));
137    }
138}
dvb_si/descriptors/system_clock.rs

dvb_si/descriptors/
system_clock.rs
