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Reduce element shifting in Record::retain_mut (#10915)

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# Description
Replaces the `Vec::remove` in `Record::retain_mut` with some swaps which
should eliminate the `O(n^2)` complexity due to repeated shifting of
elements.
This commit is contained in:
Ian Manske 2023-11-02 19:01:46 +00:00 committed by GitHub
parent 29591c97a7
commit 56e35fc3f9
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1 changed files with 18 additions and 9 deletions
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27
crates/nu-protocol/src/value/record.rs
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@ -172,24 +172,33 @@ impl Record {
where
F: FnMut(&str, &mut Value) -> bool,
{
// `Vec::retain` is able to optimize memcopies internally.
// For maximum benefit, `retain` is used on `vals`,
// as `Value` is a larger struct than `String`.
//
// To do a simultaneous retain on the `cols`, three portions of it are tracked:
// [..retained, ..dropped, ..unvisited]
// number of elements keep so far, start of ..dropped and length of ..retained
let mut retained = 0;
// current index of element being checked, start of ..unvisited
let mut idx = 0;
// `Vec::retain` is able to optimize memcopies internally. For maximum benefit as `Value`
// is a larger struct than `String` use `retain` on `vals`
//
// The calls to `Vec::remove` are suboptimal as they need memcopies to shift each time.
//
// As the operations should remain inplace, we don't allocate a separate index `Vec` which
// could be used to avoid the repeated shifting of `Vec::remove` in cols.
self.vals.retain_mut(|val| {
if keep(self.cols[idx].as_str(), val) {
if keep(&self.cols[idx], val) {
// skip swaps for first consecutive run of kept elements
if idx != retained {
self.cols.swap(idx, retained);
}
retained += 1;
idx += 1;
true
} else {
self.cols.remove(idx);
idx += 1;
false
}
});
self.cols.truncate(retained);
}
pub fn columns(&self) -> Columns {