nmea-kit 0.7.4

Bidirectional NMEA 0183 parser and encoder with AIS decoding
Documentation 
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//! AIS aid-to-navigation report — Type 21.
//!
//! Transmitted by AIS-equipped buoys, lighthouses, and beacons. Provides real-time
//! position and identity for navigational aids. Always single-frame.

use crate::ais::armor::{extract_i32, extract_string, extract_u32};

use super::utils::{decode_latitude, decode_longitude};

/// AIS Type 21 — Aid-to-Navigation Report.
///
/// ITU-R M.1371 bit layout (minimum 272 bits):
/// - bits   0–5:   message type (= 21)
/// - bits   6–7:   repeat indicator
/// - bits   8–37:  MMSI (30 bits)
/// - bits  38–42:  type of AID (5 bits, 1–31)
/// - bits  43–162: name (20 × 6-bit ASCII chars)
/// - bit   163:    position accuracy
/// - bits 164–191: longitude (28 bits, 1/10000 min, same encoding as Type 1)
/// - bits 192–218: latitude  (27 bits, 1/10000 min, same encoding as Type 1)
#[derive(Debug, Clone, PartialEq)]
pub struct AidToNavigation {
    /// MMSI of the aid-to-navigation transponder.
    pub mmsi: u32,
    /// Type of navigational aid (ITU-R M.1371 Table 67).
    /// E.g. 1=default/unspecified, 16=buoy, 20=LANBY, 31=IALA special mark.
    pub aid_type: u8,
    /// Name of the aid (e.g. "PORTLAND BILL LT"), trimmed of padding.
    pub name: String,
    /// Latitude in decimal degrees. `None` if not available (sentinel 91°).
    pub lat: Option<f64>,
    /// Longitude in decimal degrees. `None` if not available (sentinel 181°).
    pub lon: Option<f64>,
}

impl AidToNavigation {
    pub(crate) fn decode(bits: &[u8]) -> Option<Self> {
        if bits.len() < 272 {
            return None;
        }
        let mmsi = extract_u32(bits, 8, 30)?;
        let aid_type = extract_u32(bits, 38, 5)? as u8;
        let name = extract_string(bits, 43, 20)?;
        let lon_raw = extract_i32(bits, 164, 28)?;
        let lat_raw = extract_i32(bits, 192, 27)?;
        Some(Self {
            mmsi,
            aid_type,
            name,
            lat: decode_latitude(lat_raw),
            lon: decode_longitude(lon_raw),
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ais::messages::test_helpers::set_bits;

    #[test]
    fn aton_name_preserves_leading_space() {
        // Type 21 needs >= 272 bits. name @43, 20 chars: char0=32 (' '), char1=1 ('A').
        let mut bits = vec![0u8; 272];
        set_bits(&mut bits, 0, 6, 21); // msg_type 21
        set_bits(&mut bits, 43, 6, 32); // ' '
        set_bits(&mut bits, 49, 6, 1); // 'A'
        let aton = AidToNavigation::decode(&bits).expect("decode");
        assert_eq!(
            aton.name, " A",
            "leading space must be preserved (trailing-only trim)"
        );
    }
}