thin-fetch 0.1.0

Low-level Git crate to fetch objects with high granularity
Documentation 
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//! Applying deltas onto raw objects.
//!
//! Code copied from `gix_pack/data/delta.rs` and modified for thin-fetch.
//! Original copyright: MIT or Apache-2.0, Gitoxide contributors and slop machines.

use crate::{Error, WithError as _, bail, error};

/// Given the decompressed pack delta `d`, decode a size in bytes (either the base object size or the result object size)
/// Equivalent to [this canonical git function](https://github.com/git/git/blob/311531c9de557d25ac087c1637818bd2aad6eb3a/delta.h#L89)
pub(crate) fn decode_header_size(d: &[u8]) -> Result<(usize, usize), Error> {
    let mut shift = 0;
    let mut size = 0u64;
    let mut consumed = 0;

    for cmd in d {
        if shift >= u64::BITS {
            bail!("delta header size uses more bits than fit into u64");
        }
        consumed += 1;
        size |= (u64::from(*cmd) & 0x7f) << shift;
        shift += 7;
        if *cmd & 0x80 == 0 {
            let size =
                usize::try_from(size).with_err(|| error!("delta size exceeds usize: {size}"))?;

            return Ok((size, consumed));
        }
    }

    bail!("delta header size is truncated");
}

pub(crate) fn apply(base: &[u8], mut target: &mut [u8], data: &[u8]) -> Result<(), Error> {
    fn next_byte(data: &[u8], i: &mut usize) -> Result<u8, Error> {
        let byte = *data
            .get(*i)
            .with_err_msg("delta copy instruction is truncated")?;
        *i += 1;
        Ok(byte)
    }

    let mut i = 0;
    while let Some(cmd) = data.get(i) {
        i += 1;
        let bytes = match cmd {
            cmd if cmd & 0b1000_0000 != 0 => {
                let (mut ofs, mut size): (u32, u32) = (0, 0);
                if cmd & 0b0000_0001 != 0 {
                    ofs = u32::from(next_byte(data, &mut i)?);
                }
                if cmd & 0b0000_0010 != 0 {
                    ofs |= u32::from(next_byte(data, &mut i)?) << 8;
                }
                if cmd & 0b0000_0100 != 0 {
                    ofs |= u32::from(next_byte(data, &mut i)?) << 16;
                }
                if cmd & 0b0000_1000 != 0 {
                    ofs |= u32::from(next_byte(data, &mut i)?) << 24;
                }
                if cmd & 0b0001_0000 != 0 {
                    size = u32::from(next_byte(data, &mut i)?);
                }
                if cmd & 0b0010_0000 != 0 {
                    size |= u32::from(next_byte(data, &mut i)?) << 8;
                }
                if cmd & 0b0100_0000 != 0 {
                    size |= u32::from(next_byte(data, &mut i)?) << 16;
                }
                if size == 0 {
                    size = 0x10000; // 65536
                }
                let ofs = ofs as usize;
                let end = ofs
                    .checked_add(size as usize)
                    .with_err_msg("delta copy range overflows")?;
                base.get(ofs..end).with_err(|| {
                    error!(
                        "delta copy range ({ofs}..{end}) exceeds base object size {}",
                        base.len()
                    )
                })?
            }
            0 => {
                bail!("delta command 0 is reserved and invalid");
            }
            size => {
                let end = i
                    .checked_add(*size as usize)
                    .with_err_msg("delta insert range overflows")?;
                let bytes = data
                    .get(i..end)
                    .with_err_msg("delta insert data is truncated")?;
                i = end;
                bytes
            }
        };
        let (out, rest) = target
            .split_at_mut_checked(bytes.len())
            .with_err_msg("delta instructions produced more bytes than promised")?;
        out.copy_from_slice(bytes);
        target = rest;
    }
    debug_assert_eq!(
        i,
        data.len(),
        "delta instructions were not consumed completely, should be impossible"
    );
    if !target.is_empty() {
        bail!("delta instructions produced fewer bytes than promised");
    }

    Ok(())
}