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A stab at implementing lookup_prefix - to no avail (#298)
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Probably something is missing, what could it be :)?
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Byron committed Feb 10, 2022
1 parent 7f0efa7 commit 850a91f
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Showing 2 changed files with 43 additions and 2 deletions.
17 changes: 17 additions & 0 deletions git-hash/src/owned.rs
Original file line number Diff line number Diff line change
Expand Up @@ -71,6 +71,23 @@ impl Prefix {
pub fn hex_len(&self) -> usize {
self.hex_len
}

/// Provided with candidate id which is a full hash, determine if this prefix fully matches
/// the corresponding part in the candidate id.
pub fn matches_candidate(&self, candidate: &oid) -> bool {
let common_len = self.hex_len / 2;

if !(self.bytes.as_bytes()[..common_len] == candidate.as_bytes()[..common_len]) {
return false;
}

if self.hex_len % 2 == 1 {
let half_byte_idx = self.hex_len / 2;
self.bytes.as_bytes()[half_byte_idx] == (candidate.as_bytes()[half_byte_idx] & 0xf0)
} else {
true
}
}
}

/// An owned hash identifying objects, most commonly Sha1
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28 changes: 26 additions & 2 deletions git-pack/src/multi_index/access.rs
Original file line number Diff line number Diff line change
Expand Up @@ -68,8 +68,32 @@ impl File {
}

/// TODO
pub fn lookup_prefix(&self, _prefix: git_hash::Prefix) -> Option<PrefixLookupResult> {
todo!()
pub fn lookup_prefix(&self, prefix: git_hash::Prefix) -> Option<PrefixLookupResult> {
let prefix_oid = prefix.as_oid();
let first_byte = prefix_oid.first_byte() as usize;
let mut upper_bound = self.fan[first_byte];
let mut lower_bound = if first_byte != 0 { self.fan[first_byte - 1] } else { 0 };

// Bisect using indices
let mut candidate = None;
while lower_bound < upper_bound {
let mid = (lower_bound + upper_bound) / 2;
let mid_sha = self.oid_at_index(mid);
candidate = Some(mid);

use std::cmp::Ordering::*;
match prefix_oid.cmp(mid_sha) {
Less => upper_bound = mid,
Equal if prefix.hex_len() == prefix_oid.kind().len_in_hex() => return Some(Ok(mid)),
Equal => break,
Greater => lower_bound = mid + 1,
}
}

let candidate = candidate?;
prefix
.matches_candidate(self.oid_at_index(candidate))
.then(|| Ok(candidate))
}

/// Find the index ranging from 0 to [File::num_objects()] that belongs to data associated with `id`, or `None` if it wasn't found.
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