// dagops.rs

//

// Copyright 2019 Georges Racinet <georges.racinet@octobus.net>

//

// 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, BaseRevision};

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(())

}

///

/// # Panics

///

/// Can panic if `not_heads` is shorten than the length of graph.

pub fn retain_heads_fast(

    graph: &impl Graph,

    filtered_revs: &HashSet<Revision>,

) -> Result<(), GraphError> {

    for idx in (0..not_heads.len()).rev() {

        let rev = Revision(idx as BaseRevision);

        if !not_heads[idx] && filtered_revs.contains(&rev) {

            continue;

        }

        for parent in graph.parents(rev)?.iter() {

            if *parent != NULL_REVISION {

            }

        }

    }

    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, BaseRevision};

    /// Apply `retain_heads()` to the given slice and return as a sorted `Vec`

    fn retain_heads_sorted(

        graph: &impl Graph,

        revs: &[BaseRevision],

    ) -> Result<Vec<Revision>, GraphError> {

        let mut revs: HashSet<Revision> =

            revs.iter().cloned().map(Revision).collect();

        retain_heads(graph, &mut revs)?;

        let mut as_vec: Vec<Revision> = revs.iter().cloned().collect();

        as_vec.sort_unstable();

        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: &[BaseRevision],

    ) -> Result<Vec<Revision>, GraphError> {

        let iter_revs: Vec<_> = revs.iter().cloned().map(Revision).collect();

        let heads = heads(graph, iter_revs.iter())?;

        let mut as_vec: Vec<Revision> = heads.iter().cloned().collect();

        as_vec.sort_unstable();

        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: &[BaseRevision],

    ) -> Result<Vec<Revision>, GraphError> {

        let set: HashSet<_> = revs.iter().cloned().map(Revision).collect();

        let mut as_vec = roots(graph, &set)?;

        as_vec.sort_unstable();

        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: &[BaseRevision],

        heads: &[BaseRevision],

    ) -> Result<Vec<Revision>, GraphError> {

        range(

            &graph,

            roots.iter().cloned().map(Revision),

            heads.iter().cloned().map(Revision),

        )

        .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::<Revision>::new()

        );

        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(())

    }

}