OXYGENHUB
# c2e1d09047db oxygen / crates / oxy-core / src / flat.rs
4.2 KiB · Rust · last changed A state index for the working copy's base 1 month ago source blame history raw
f9575918 Andy McCall ↶ 1 
//! A flat binary layout for local indexes (oxygen#41): fixed-width
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//! little-endian numbers and length-prefixed bytes, read in one pass
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//! without building a generic value tree. Only for caches this repository
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//! derives and can derive again: never exchanged, never trusted beyond the
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//! key they are stored under, and a reader that finds anything amiss fails
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//! so the caller rebuilds.
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use crate::hash::ObjectRef;
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use crate::objects::ObjectError;
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#[derive(Default)]
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pub struct Writer(pub Vec<u8>);
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impl Writer {
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    pub fn u8(&mut self, v: u8) {
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        self.0.push(v);
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    }
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    pub fn u32(&mut self, v: u32) {
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        self.0.extend_from_slice(&v.to_le_bytes());
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    }
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    pub fn u64(&mut self, v: u64) {
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        self.0.extend_from_slice(&v.to_le_bytes());
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    }
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    /// A count, which must fit in 32 bits.
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    pub fn count(&mut self, n: usize) {
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        self.u32(u32::try_from(n).expect("an index count fits in 32 bits"));
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    }
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    pub fn fixed(&mut self, b: &[u8]) {
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        self.0.extend_from_slice(b);
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    }
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    pub fn bytes(&mut self, b: &[u8]) {
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        self.count(b.len());
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        self.0.extend_from_slice(b);
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    }
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    pub fn object(&mut self, r: &ObjectRef) {
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        self.fixed(&r.0);
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    }
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    pub fn opt_object(&mut self, r: &Option<ObjectRef>) {
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        match r {
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            None => self.u8(0),
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            Some(r) => {
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                self.u8(1);
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                self.object(r);
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            }
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        }
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    }
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}
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pub struct Reader<'a> {
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    buf: &'a [u8],
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    pos: usize,
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}
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fn short() -> ObjectError {
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    ObjectError::Schema("index: truncated".into())
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}
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impl<'a> Reader<'a> {
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    pub fn new(buf: &'a [u8]) -> Reader<'a> {
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        Reader { buf, pos: 0 }
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    }
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    pub fn at_end(&self) -> bool {
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        self.pos == self.buf.len()
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    }
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    fn take(&mut self, n: usize) -> Result<&'a [u8], ObjectError> {
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        let end = self.pos.checked_add(n).ok_or_else(short)?;
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        let s = self.buf.get(self.pos..end).ok_or_else(short)?;
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        self.pos = end;
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        Ok(s)
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    }
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    pub fn u8(&mut self) -> Result<u8, ObjectError> {
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        Ok(self.take(1)?[0])
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    }
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    pub fn u32(&mut self) -> Result<u32, ObjectError> {
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        Ok(u32::from_le_bytes(self.fixed::<4>()?))
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    }
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    pub fn u64(&mut self) -> Result<u64, ObjectError> {
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        Ok(u64::from_le_bytes(self.fixed::<8>()?))
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    }
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    /// A count; checked against what is left, so a corrupt one cannot ask
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    /// for more room than the index could hold.
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    pub fn count(&mut self) -> Result<usize, ObjectError> {
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        let n = self.u32()? as usize;
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        if n > self.buf.len() - self.pos {
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            return Err(short());
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        }
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        Ok(n)
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    }
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    pub fn fixed<const N: usize>(&mut self) -> Result<[u8; N], ObjectError> {
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        Ok(self.take(N)?.try_into().expect("N bytes"))
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    }
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    pub fn bytes(&mut self) -> Result<&'a [u8], ObjectError> {
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        let n = self.u32()? as usize;
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        self.take(n)
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    }
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    pub fn text(&mut self) -> Result<String, ObjectError> {
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        String::from_utf8(self.bytes()?.to_vec())
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            .map_err(|_| ObjectError::Schema("index: text is not UTF-8".into()))
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    }
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    pub fn object(&mut self) -> Result<ObjectRef, ObjectError> {
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        Ok(ObjectRef(self.fixed::<32>()?))
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    }
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    pub fn opt_object(&mut self) -> Result<Option<ObjectRef>, ObjectError> {
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        Ok(match self.u8()? {
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            0 => None,
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            _ => Some(self.object()?),
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        })
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    }
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}
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#[cfg(test)]
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mod tests {
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    use super::*;
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    #[test]
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    fn numbers_and_bytes_round_trip_and_truncation_fails() {
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        let mut w = Writer::default();
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        w.u8(7);
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        w.u32(70_000);
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        w.u64(1 << 40);
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        w.bytes(b"path/name");
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        w.opt_object(&Some(ObjectRef([3; 32])));
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        w.opt_object(&None);
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        let mut r = Reader::new(&w.0);
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        assert_eq!(r.u8().unwrap(), 7);
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        assert_eq!(r.u32().unwrap(), 70_000);
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        assert_eq!(r.u64().unwrap(), 1 << 40);
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        assert_eq!(r.text().unwrap(), "path/name");
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        assert_eq!(r.opt_object().unwrap(), Some(ObjectRef([3; 32])));
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        assert_eq!(r.opt_object().unwrap(), None);
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        assert!(r.at_end());
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        let mut r = Reader::new(&w.0[..w.0.len() - 3]);
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        r.fixed::<13>().unwrap();
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        assert!(r.text().is_ok());
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        assert!(r.opt_object().is_err());
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    }
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}