dvb-si 12.1.0

//! Checked big-endian field readers.
//!
//! Replaces the `*bytes[off..].first_chunk::<N>().unwrap()` idiom: a short
//! buffer is reported as [`Error::BufferTooShort`] instead of panicking.

use crate::error::{Error, Result};

/// Read a big-endian `u16` at `off`, or `BufferTooShort` naming `what`.
pub(crate) fn be_u16(bytes: &[u8], off: usize, what: &'static str) -> Result<u16> {
    match bytes.get(off..).and_then(|s| s.first_chunk::<2>()) {
        Some(b) => Ok(u16::from_be_bytes(*b)),
        None => Err(Error::BufferTooShort {
            need: off.saturating_add(2),
            have: bytes.len(),
            what,
        }),
    }
}

/// Read a big-endian `u32` at `off`, or `BufferTooShort` naming `what`.
pub(crate) fn be_u32(bytes: &[u8], off: usize, what: &'static str) -> Result<u32> {
    match bytes.get(off..).and_then(|s| s.first_chunk::<4>()) {
        Some(b) => Ok(u32::from_be_bytes(*b)),
        None => Err(Error::BufferTooShort {
            need: off.saturating_add(4),
            have: bytes.len(),
            what,
        }),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn exact_ok() {
        assert_eq!(be_u16(&[1, 2], 0, "t").unwrap(), 0x0102);
        assert_eq!(be_u16(&[9, 1, 2], 1, "t").unwrap(), 0x0102);
        assert_eq!(be_u32(&[1, 2, 3, 4], 0, "t").unwrap(), 0x0102_0304);
    }

    #[test]
    fn short_is_err_not_panic() {
        assert!(matches!(
            be_u16(&[1], 0, "t"),
            Err(Error::BufferTooShort {
                need: 2,
                have: 1,
                what: "t"
            })
        ));
        assert!(be_u16(&[1, 2], 1, "t").is_err());
        assert!(be_u16(&[1, 2], 9, "t").is_err());
        assert!(be_u16(&[], usize::MAX, "t").is_err());
        assert!(be_u32(&[1, 2, 3], 0, "t").is_err());
    }
}