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index: adjust binary search function to conform to std behavior

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This removes redundant case from resolve_neighbor_commit_ids(). The returned
position should never be lower than the prefix id.

The implementation is basically a copy of slice::binary_search_by(). We still
use (low + high) / 2 as the size wouldn't exceed 2^31.

https://github.com/rust-lang/rust/blob/1.76.0/library/core/src/slice/mod.rs#L2825
This commit is contained in:
Yuya Nishihara 2024-02-11 20:05:54 +09:00
parent e2c8a8fabd
commit 91a68b950d
1 changed files with 26 additions and 37 deletions
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63
lib/src/default_index/readonly.rs
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@ -367,27 +367,27 @@ impl ReadonlyIndexSegment {
.collect()
}
fn commit_id_byte_prefix_to_lookup_pos(&self, prefix: &CommitId) -> Option<u32> {
if self.num_local_commits == 0 {
// Avoid overflow when subtracting 1 below
return None;
}
/// Binary searches commit id by `prefix`.
///
/// If the `prefix` matches exactly, returns `Ok` with the lookup position.
/// Otherwise, returns `Err` containing the position where the id could be
/// inserted.
fn commit_id_byte_prefix_to_lookup_pos(&self, prefix: &CommitId) -> Result<u32, u32> {
let mut low = 0;
let mut high = self.num_local_commits - 1;
// binary search for the commit id
loop {
let mut high = self.num_local_commits;
while low < high {
let mid = (low + high) / 2;
if high == low {
return Some(mid);
}
let entry = self.commit_lookup_entry(mid);
if entry.commit_id_bytes() < prefix.as_bytes() {
low = mid + 1;
} else {
high = mid;
let cmp = entry.commit_id_bytes().cmp(prefix.as_bytes());
// According to Rust std lib, this produces cmov instructions.
// https://github.com/rust-lang/rust/blob/1.76.0/library/core/src/slice/mod.rs#L2845-L2855
low = if cmp == Ordering::Less { mid + 1 } else { low };
high = if cmp == Ordering::Greater { mid } else { high };
if cmp == Ordering::Equal {
return Ok(mid);
}
}
Err(low)
}
}
@ -409,41 +409,30 @@ impl IndexSegment for ReadonlyIndexSegment {
}
fn commit_id_to_pos(&self, commit_id: &CommitId) -> Option<LocalPosition> {
let lookup_pos = self.commit_id_byte_prefix_to_lookup_pos(commit_id)?;
let lookup_pos = self.commit_id_byte_prefix_to_lookup_pos(commit_id).ok()?;
let entry = self.commit_lookup_entry(lookup_pos);
(&entry.commit_id() == commit_id).then(|| entry.local_pos())
Some(entry.local_pos())
}
fn resolve_neighbor_commit_ids(
&self,
commit_id: &CommitId,
) -> (Option<CommitId>, Option<CommitId>) {
if let Some(lookup_pos) = self.commit_id_byte_prefix_to_lookup_pos(commit_id) {
let entry_commit_id = self.commit_lookup_entry(lookup_pos).commit_id();
let (prev_lookup_pos, next_lookup_pos) = match entry_commit_id.cmp(commit_id) {
Ordering::Less => {
assert_eq!(lookup_pos + 1, self.num_local_commits);
(Some(lookup_pos), None)
}
Ordering::Equal => {
let succ = ((lookup_pos + 1)..self.num_local_commits).next();
(lookup_pos.checked_sub(1), succ)
}
Ordering::Greater => (lookup_pos.checked_sub(1), Some(lookup_pos)),
let (prev_lookup_pos, next_lookup_pos) =
match self.commit_id_byte_prefix_to_lookup_pos(commit_id) {
Ok(pos) => (pos.checked_sub(1), (pos + 1..self.num_local_commits).next()),
Err(pos) => (pos.checked_sub(1), (pos..self.num_local_commits).next()),
};
let prev_id = prev_lookup_pos.map(|p| self.commit_lookup_entry(p).commit_id());
let next_id = next_lookup_pos.map(|p| self.commit_lookup_entry(p).commit_id());
(prev_id, next_id)
} else {
(None, None)
}
let prev_id = prev_lookup_pos.map(|p| self.commit_lookup_entry(p).commit_id());
let next_id = next_lookup_pos.map(|p| self.commit_lookup_entry(p).commit_id());
(prev_id, next_id)
}
fn resolve_commit_id_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<CommitId> {
let min_bytes_prefix = CommitId::from_bytes(prefix.min_prefix_bytes());
let lookup_pos = self
.commit_id_byte_prefix_to_lookup_pos(&min_bytes_prefix)
.unwrap_or(self.num_local_commits);
.unwrap_or_else(|pos| pos);
let mut matches = (lookup_pos..self.num_local_commits)
.map(|pos| self.commit_lookup_entry(pos).commit_id())
.take_while(|id| prefix.matches(id))