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use std::io;
use super::{BLOB, COMMIT, OFS_DELTA, REF_DELTA, TAG, TREE};
use crate::data;
/// The header portion of a pack data entry, identifying the kind of stored object.
#[derive(PartialEq, Eq, Debug, Hash, Ord, PartialOrd, Clone, Copy)]
#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
#[allow(missing_docs)]
pub enum Header {
/// The object is a commit
Commit,
/// The object is a tree
Tree,
/// The object is a blob
Blob,
/// The object is a tag
Tag,
/// Describes a delta-object which needs to be applied to a base. The base object is identified by the `base_id` field
/// which is found within the parent repository.
/// Most commonly used for **thin-packs** when receiving pack files from the server to refer to objects that are not
/// part of the pack but expected to be present in the receivers repository.
///
/// # Note
/// This could also be an object within this pack if the LSB encoded offset would be larger than 20 bytes, which is unlikely to
/// happen.
///
/// **The naming** is exactly the same as the canonical implementation uses, namely **REF_DELTA**.
RefDelta { base_id: git_hash::ObjectId },
/// Describes a delta-object present in this pack which acts as base for this object.
/// The base object is measured as a distance from this objects
/// pack offset, so that `base_pack_offset = this_objects_pack_offset - base_distance`
///
/// # Note
///
/// **The naming** is exactly the same as the canonical implementation uses, namely **OFS_DELTA**.
OfsDelta { base_distance: u64 },
}
impl Header {
/// Subtract `distance` from `pack_offset` safely without the chance for overflow or no-ops if `distance` is 0.
pub fn verified_base_pack_offset(pack_offset: data::Offset, distance: u64) -> Option<data::Offset> {
if distance == 0 {
return None;
}
pack_offset.checked_sub(distance)
}
/// Convert the header's object kind into [`git_object::Kind`] if possible
pub fn as_kind(&self) -> Option<git_object::Kind> {
use git_object::Kind::*;
Some(match self {
Header::Tree => Tree,
Header::Blob => Blob,
Header::Commit => Commit,
Header::Tag => Tag,
Header::RefDelta { .. } | Header::OfsDelta { .. } => return None,
})
}
/// Convert this header's object kind into the packs internal representation
pub fn as_type_id(&self) -> u8 {
use Header::*;
match self {
Blob => BLOB,
Tree => TREE,
Commit => COMMIT,
Tag => TAG,
OfsDelta { .. } => OFS_DELTA,
RefDelta { .. } => REF_DELTA,
}
}
/// Return's true if this is a delta object, i.e. not a full object.
pub fn is_delta(&self) -> bool {
matches!(self, Header::OfsDelta { .. } | Header::RefDelta { .. })
}
/// Return's true if this is a base object, i.e. not a delta object.
pub fn is_base(&self) -> bool {
!self.is_delta()
}
}
impl Header {
/// Encode this header along the given `decompressed_size_in_bytes` into the `out` write stream for use within a data pack.
///
/// Returns the amount of bytes written to `out`.
/// `decompressed_size_in_bytes` is the full size in bytes of the object that this header represents
pub fn write_to(&self, decompressed_size_in_bytes: u64, mut out: impl io::Write) -> io::Result<usize> {
let mut size = decompressed_size_in_bytes;
let mut written = 1;
let mut c: u8 = (self.as_type_id() << 4) | (size as u8 & 0b0000_1111);
size >>= 4;
while size != 0 {
out.write_all(&[c | 0b1000_0000])?;
written += 1;
c = size as u8 & 0b0111_1111;
size >>= 7;
}
out.write_all(&[c])?;
use Header::*;
match self {
RefDelta { base_id: oid } => {
out.write_all(oid.as_slice())?;
written += oid.as_slice().len();
}
OfsDelta { base_distance } => {
let mut buf = [0u8; 10];
let buf = leb64_encode(*base_distance, &mut buf);
out.write_all(buf)?;
written += buf.len();
}
Blob | Tree | Commit | Tag => {}
}
Ok(written)
}
/// The size of the header in bytes when serialized
pub fn size(&self, decompressed_size: u64) -> usize {
self.write_to(decompressed_size, io::sink())
.expect("io::sink() to never fail")
}
}
#[inline]
fn leb64_encode(mut n: u64, buf: &mut [u8; 10]) -> &[u8] {
let mut bytes_written = 1;
buf[buf.len() - 1] = n as u8 & 0b0111_1111;
for out in buf.iter_mut().rev().skip(1) {
n >>= 7;
if n == 0 {
break;
}
n -= 1;
*out = 0b1000_0000 | (n as u8 & 0b0111_1111);
bytes_written += 1;
}
debug_assert_eq!(n, 0, "BUG: buffer must be large enough to hold a 64 bit integer");
&buf[buf.len() - bytes_written..]
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn leb64_encode_max_int() {
let mut buf = [0u8; 10];
let buf = leb64_encode(u64::MAX, &mut buf);
assert_eq!(buf.len(), 10, "10 bytes should be used when 64bits are encoded");
}
}