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Fix last memory use (#7178)

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Currently `last n` memory use is O(input), while it should be O(n). This
patch replaces code collecting all of last's input into a Vec<_> with
collecting into a bounded VecDeque<_>. UI/UX remain are unchanged.
This commit is contained in:
Kamil Koczurek 2022-11-21 18:19:31 +01:00 committed by GitHub
parent 7bcd96fc65
commit 88a0705df1
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1 changed files with 22 additions and 21 deletions
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43
crates/nu-command/src/filters/last.rs
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@ -1,3 +1,5 @@
use std::collections::VecDeque;
use nu_engine::CallExt;
use nu_protocol::ast::Call;
@ -75,18 +77,31 @@ impl Command for Last {
let span = call.head;
let rows: Option<i64> = call.opt(engine_state, stack, 0)?;
let v: Vec<_> = input.into_iter().collect();
let vlen: i64 = v.len() as i64;
let beginning_rows_to_skip = rows_to_skip(vlen, rows);
let to_keep = rows.unwrap_or(1).try_into().unwrap_or(0);
// early exit for `last 0`
if to_keep == 0 {
return Ok(Vec::<Value>::new()
.into_pipeline_data(engine_state.ctrlc.clone())
.set_metadata(metadata));
}
// only keep last `to_keep` rows in memory
let mut buf = VecDeque::<_>::new();
for row in input.into_iter() {
if buf.len() == to_keep {
buf.pop_front();
}
buf.push_back(row);
}
if rows.is_some() {
let iter = v.into_iter().skip(beginning_rows_to_skip as usize);
Ok(iter
Ok(buf
.into_pipeline_data(engine_state.ctrlc.clone())
.set_metadata(metadata))
} else {
let last = v.into_iter().nth(beginning_rows_to_skip as usize);
let last = buf.pop_back();
if let Some(last) = last {
Ok(last.into_pipeline_data().set_metadata(metadata))
@ -97,20 +112,6 @@ impl Command for Last {
}
}
fn rows_to_skip(count: i64, rows: Option<i64>) -> i64 {
let end_rows_desired = if let Some(quantity) = rows {
quantity
} else {
1
};
if end_rows_desired < count {
count - end_rows_desired
} else {
0
}
}
#[cfg(test)]
mod test {
use super::*;