comparison rust/hg-core/src/dirstate_tree/status.rs @ 47119:15395fd8ab28

dirstate-tree: Use HashMap instead of BTreeMap

BTreeMap has the advantage of its "natural" iteration order being the one we need
in the status algorithm. With HashMap however, iteration order is undefined so
we need to allocate a Vec and sort it explicitly.

Unfortunately many BTreeMap operations are slower than in HashMap, and skipping
that extra allocation and sort is not enough to compensate.

Switching to HashMap + sort makes `hg status` 17% faster in one test case,
as measure with hyperfine:

```
Benchmark #1: ../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1
  Time (mean ± σ):     765.0 ms ±   8.8 ms    [User: 1.352 s, System: 0.747 s]
  Range (min … max):   751.8 ms … 778.7 ms    10 runs

Benchmark #2: ./hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1
  Time (mean ± σ):     651.8 ms ±   9.9 ms    [User: 1.251 s, System: 0.799 s]
  Range (min … max):   642.2 ms … 671.8 ms    10 runs

Summary
  './hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1' ran
    1.17 ± 0.02 times faster than '../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1'
```

* ./hg is this revision
* ../hg2/hg is its parent
* $REPO is an old snapshot of mozilla-central

Differential Revision: https://phab.mercurial-scm.org/D10553

47118:c92e63762573

            return;
        };

        // `merge_join_by` requires both its input iterators to be sorted:

        //
        // * `BTreeMap` iterates according to keys’ ordering by definition

        // `sort_unstable_by_key` doesn’t allow keys borrowing from the value:
        // https://github.com/rust-lang/rust/issues/34162
        fs_entries.sort_unstable_by(|e1, e2| e1.base_name.cmp(&e2.base_name));

        itertools::merge_join_by(
            dirstate_nodes,
            &fs_entries,

47119:15395fd8ab28

            return;
        };

        // `merge_join_by` requires both its input iterators to be sorted:

        let mut dirstate_nodes: Vec<_> = dirstate_nodes.iter_mut().collect();
        // `sort_unstable_by_key` doesn’t allow keys borrowing from the value:
        // https://github.com/rust-lang/rust/issues/34162
        dirstate_nodes
            .sort_unstable_by(|(path1, _), (path2, _)| path1.cmp(path2));
        fs_entries.sort_unstable_by(|e1, e2| e1.base_name.cmp(&e2.base_name));

        itertools::merge_join_by(
            dirstate_nodes,
            &fs_entries,