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Fix data packet sign extension (phases below -65536 wrap positive) #1

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Fix data packet sign extension (phases below -65536 wrap positive) #1

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24 changes: 18 additions & 6 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -617,8 +617,7 @@ pub struct DataPacket {

impl SerializePacket for DataPacket {
fn serialize(&self) -> [u8; 8] {
let mut packet = 0xff00_0000_0000_0000u64;
packet |= (self.x as u64).wrapping_shl(64 - 10);
let mut packet = (self.x as u64).wrapping_shl(64 - 10);
packet |= (self.y as u64).wrapping_shl(64 - 10 - 10);
packet |= (self.timestamp as u64 & 0x1ff).wrapping_shl(64 - 10 - 10 - 9);
packet |= (self.phase as u64 & 0x3ffff).wrapping_shl(64 - 10 - 10 - 9 - 18);
Expand Down Expand Up @@ -685,9 +684,10 @@ fn parse_data(input: &mut Stream<'_>) -> ModalResult<DataPacket> {
take(10usize).verify(|i| *i < 256),
take(10usize).verify(|i| *i < 256),
take(9usize),
// Use an arithmetic right shift to sign extend these
take(18usize).map(|input: i32| input.wrapping_shl(32 - 17).wrapping_shr(32 - 17)),
take(17usize).map(|input: i32| input.wrapping_shl(32 - 16).wrapping_shr(32 - 16)),
// Phase and baseline are 18- and 17-bit two's complement fields;
// shift each field's sign bit into bit 31, then arithmetic-shift back
take(18usize).map(|input: i32| input.wrapping_shl(32 - 18).wrapping_shr(32 - 18)),
take(17usize).map(|input: i32| input.wrapping_shl(32 - 17).wrapping_shr(32 - 17)),
))
.parse_next(input)
.map_err(|_| ErrMode::<ContextError>::Cut(ContextError::new()))?;
Expand Down Expand Up @@ -1226,6 +1226,16 @@ mod tests {
);
}

#[test]
fn test_data_full_width_sign_extension() {

@ld-cd ld-cd Oct 11, 2026

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something a little less verbose like test_sign_extension

// Values outside the 17-bit phase / 16-bit baseline range, including a real
// MEC 850 nm photon (phase -100212), must not wrap to the opposite sign
for (phase, baseline) in [(-100212, -9286), (-131072, -65536), (131071, 65535), (-65537, -32769)] {
let packet = DataPacket { x: 99, y: 44, timestamp: 131, phase, baseline };
assert_eq!(parse_data(&mut stream(&packet.serialize())), Ok(packet));
}
}

#[test]
fn test_header() {
let mut input_header = stream(&[0xff, 0xe8, 0x76, 0x0b, 0x46, 0x72, 0x2c, 0xfb]);
Expand Down Expand Up @@ -1254,7 +1264,9 @@ mod tests {
})
);

assert_eq!(input_data, stream(&[]))
assert_eq!(input_data, stream(&[]));
let packet = DataPacket { x: 99, y: 44, timestamp: 131, phase: -34148, baseline: -4334 };

@ld-cd ld-cd Oct 11, 2026

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This should be a seperate test as serialization and deserialization are very nearly orthogonal as implemented here

assert_eq!(packet.serialize(), [0x18, 0xc2, 0xc4, 0x1e, 0xf5, 0x39, 0xef, 0x12]);
}

#[test]
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