view rust/hg-core/src/dagops.rs @ 49491:c6a1beba27e9

bisect: avoid copying ancestor list for non-merge commits

During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.

To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.

During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:

Setup:
  $ cd mozilla-central
  $ hg bisect --reset
  $ hg bisect --good 0
  $ hg log -r tip -T '{rev}\n'
  628417

Test:
  $ time hg bisect --bad tip --noupdate

Before:
  real    3m35.927s
  user    3m35.553s
  sys     0m0.319s

After:
  real    1m41.142s
  user    1m40.810s
  sys     0m0.285s

// dagops.rs
//
// Copyright 2019 Georges Racinet <[e-mail]>
//
// This software may be used and distributed according to the terms of the
// GNU General Public License version 2 or any later version.

//! Miscellaneous DAG operations
//!
//! # Terminology
//! - By *relative heads* of a collection of revision numbers (`Revision`), we
//!   mean those revisions that have no children among the collection.
//! - Similarly *relative roots* of a collection of `Revision`, we mean those
//!   whose parents, if any, don't belong to the collection.
use super::{Graph, GraphError, Revision, NULL_REVISION};
use crate::ancestors::AncestorsIterator;
use std::collections::{BTreeSet, HashSet};

fn remove_parents<S: std::hash::BuildHasher>(
    graph: &impl Graph,
    rev: Revision,
    set: &mut HashSet<Revision, S>,
) -> Result<(), GraphError> {
    for parent in graph.parents(rev)?.iter() {
        if *parent != NULL_REVISION {
            set.remove(parent);
        }
    }
    Ok(())
}

/// Relative heads out of some revisions, passed as an iterator.
///
/// These heads are defined as those revisions that have no children
/// among those emitted by the iterator.
///
/// # Performance notes
/// Internally, this clones the iterator, and builds a `HashSet` out of it.
///
/// This function takes an `Iterator` instead of `impl IntoIterator` to
/// guarantee that cloning the iterator doesn't result in cloning the full
/// construct it comes from.
pub fn heads<'a>(
    graph: &impl Graph,
    iter_revs: impl Clone + Iterator<Item = &'a Revision>,
) -> Result<HashSet<Revision>, GraphError> {
    let mut heads: HashSet<Revision> = iter_revs.clone().cloned().collect();
    heads.remove(&NULL_REVISION);
    for rev in iter_revs {
        if *rev != NULL_REVISION {
            remove_parents(graph, *rev, &mut heads)?;
        }
    }
    Ok(heads)
}

/// Retain in `revs` only its relative heads.
///
/// This is an in-place operation, so that control of the incoming
/// set is left to the caller.
/// - a direct Python binding would probably need to build its own `HashSet`
///   from an incoming iterable, even if its sole purpose is to extract the
///   heads.
/// - a Rust caller can decide whether cloning beforehand is appropriate
///
/// # Performance notes
/// Internally, this function will store a full copy of `revs` in a `Vec`.
pub fn retain_heads<S: std::hash::BuildHasher>(
    graph: &impl Graph,
    revs: &mut HashSet<Revision, S>,
) -> Result<(), GraphError> {
    revs.remove(&NULL_REVISION);
    // we need to construct an iterable copy of revs to avoid itering while
    // mutating
    let as_vec: Vec<Revision> = revs.iter().cloned().collect();
    for rev in as_vec {
        if rev != NULL_REVISION {
            remove_parents(graph, rev, revs)?;
        }
    }
    Ok(())
}

/// Roots of `revs`, passed as a `HashSet`
///
/// They are returned in arbitrary order
pub fn roots<G: Graph, S: std::hash::BuildHasher>(
    graph: &G,
    revs: &HashSet<Revision, S>,
) -> Result<Vec<Revision>, GraphError> {
    let mut roots: Vec<Revision> = Vec::new();
    for rev in revs {
        if graph
            .parents(*rev)?
            .iter()
            .filter(|p| **p != NULL_REVISION)
            .all(|p| !revs.contains(p))
        {
            roots.push(*rev);
        }
    }
    Ok(roots)
}

/// Compute the topological range between two collections of revisions
///
/// This is equivalent to the revset `<roots>::<heads>`.
///
/// Currently, the given `Graph` has to implement `Clone`, which means
/// actually cloning just a reference-counted Python pointer if
/// it's passed over through `rust-cpython`. This is due to the internal
/// use of `AncestorsIterator`
///
/// # Algorithmic details
///
/// This is a two-pass swipe inspired from what `reachableroots2` from
/// `mercurial.cext.parsers` does to obtain the same results.
///
/// - first, we climb up the DAG from `heads` in topological order, keeping
///   them in the vector `heads_ancestors` vector, and adding any element of
///   `roots` we find among them to the resulting range.
/// - Then, we iterate on that recorded vector so that a revision is always
///   emitted after its parents and add all revisions whose parents are already
///   in the range to the results.
///
/// # Performance notes
///
/// The main difference with the C implementation is that
/// the latter uses a flat array with bit flags, instead of complex structures
/// like `HashSet`, making it faster in most scenarios. In theory, it's
/// possible that the present implementation could be more memory efficient
/// for very large repositories with many branches.
pub fn range(
    graph: &(impl Graph + Clone),
    roots: impl IntoIterator<Item = Revision>,
    heads: impl IntoIterator<Item = Revision>,
) -> Result<BTreeSet<Revision>, GraphError> {
    let mut range = BTreeSet::new();
    let roots: HashSet<Revision> = roots.into_iter().collect();
    let min_root: Revision = match roots.iter().cloned().min() {
        None => {
            return Ok(range);
        }
        Some(r) => r,
    };

    // Internally, AncestorsIterator currently maintains a `HashSet`
    // of all seen revision, which is also what we record, albeit in an ordered
    // way. There's room for improvement on this duplication.
    let ait = AncestorsIterator::new(graph.clone(), heads, min_root, true)?;
    let mut heads_ancestors: Vec<Revision> = Vec::new();
    for revres in ait {
        let rev = revres?;
        if roots.contains(&rev) {
            range.insert(rev);
        }
        heads_ancestors.push(rev);
    }

    for rev in heads_ancestors.into_iter().rev() {
        for parent in graph.parents(rev)?.iter() {
            if *parent != NULL_REVISION && range.contains(parent) {
                range.insert(rev);
            }
        }
    }
    Ok(range)
}

#[cfg(test)]
mod tests {

    use super::*;
    use crate::testing::SampleGraph;

    /// Apply `retain_heads()` to the given slice and return as a sorted `Vec`
    fn retain_heads_sorted(
        graph: &impl Graph,
        revs: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        let mut revs: HashSet<Revision> = revs.iter().cloned().collect();
        retain_heads(graph, &mut revs)?;
        let mut as_vec: Vec<Revision> = revs.iter().cloned().collect();
        as_vec.sort();
        Ok(as_vec)
    }

    #[test]
    fn test_retain_heads() -> Result<(), GraphError> {
        assert_eq!(retain_heads_sorted(&SampleGraph, &[4, 5, 6])?, vec![5, 6]);
        assert_eq!(
            retain_heads_sorted(&SampleGraph, &[4, 1, 6, 12, 0])?,
            vec![1, 6, 12]
        );
        assert_eq!(
            retain_heads_sorted(&SampleGraph, &[1, 2, 3, 4, 5, 6, 7, 8, 9])?,
            vec![3, 5, 8, 9]
        );
        Ok(())
    }

    /// Apply `heads()` to the given slice and return as a sorted `Vec`
    fn heads_sorted(
        graph: &impl Graph,
        revs: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        let heads = heads(graph, revs.iter())?;
        let mut as_vec: Vec<Revision> = heads.iter().cloned().collect();
        as_vec.sort();
        Ok(as_vec)
    }

    #[test]
    fn test_heads() -> Result<(), GraphError> {
        assert_eq!(heads_sorted(&SampleGraph, &[4, 5, 6])?, vec![5, 6]);
        assert_eq!(
            heads_sorted(&SampleGraph, &[4, 1, 6, 12, 0])?,
            vec![1, 6, 12]
        );
        assert_eq!(
            heads_sorted(&SampleGraph, &[1, 2, 3, 4, 5, 6, 7, 8, 9])?,
            vec![3, 5, 8, 9]
        );
        Ok(())
    }

    /// Apply `roots()` and sort the result for easier comparison
    fn roots_sorted(
        graph: &impl Graph,
        revs: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        let set: HashSet<_> = revs.iter().cloned().collect();
        let mut as_vec = roots(graph, &set)?;
        as_vec.sort();
        Ok(as_vec)
    }

    #[test]
    fn test_roots() -> Result<(), GraphError> {
        assert_eq!(roots_sorted(&SampleGraph, &[4, 5, 6])?, vec![4]);
        assert_eq!(
            roots_sorted(&SampleGraph, &[4, 1, 6, 12, 0])?,
            vec![0, 4, 12]
        );
        assert_eq!(
            roots_sorted(&SampleGraph, &[1, 2, 3, 4, 5, 6, 7, 8, 9])?,
            vec![1, 8]
        );
        Ok(())
    }

    /// Apply `range()` and convert the result into a Vec for easier comparison
    fn range_vec(
        graph: impl Graph + Clone,
        roots: &[Revision],
        heads: &[Revision],
    ) -> Result<Vec<Revision>, GraphError> {
        range(&graph, roots.iter().cloned(), heads.iter().cloned())
            .map(|bs| bs.into_iter().collect())
    }

    #[test]
    fn test_range() -> Result<(), GraphError> {
        assert_eq!(range_vec(SampleGraph, &[0], &[4])?, vec![0, 1, 2, 4]);
        assert_eq!(range_vec(SampleGraph, &[0], &[8])?, vec![]);
        assert_eq!(
            range_vec(SampleGraph, &[5, 6], &[10, 11, 13])?,
            vec![5, 10]
        );
        assert_eq!(
            range_vec(SampleGraph, &[5, 6], &[10, 12])?,
            vec![5, 6, 9, 10, 12]
        );
        Ok(())
    }
}