rust/hg-core/src/copy_tracing.rs

copies-rust: hide most of the comparison details inside a closure

The function that compares values needs various supporting elements that are
the same for each call. We are about to both make change to these element and
change to call sites in our upcoming work. So abstracting most of the details
will help to avoid conflict while these works happen in parallel.

Differential Revision: https://phab.mercurial-scm.org/D9426

use crate::utils::hg_path::HgPath;
use crate::utils::hg_path::HgPathBuf;
use crate::Revision;

use im_rc::ordmap::DiffItem;
use im_rc::ordmap::OrdMap;

use std::cmp::Ordering;
use std::collections::HashMap;
use std::convert::TryInto;

pub type PathCopies = HashMap<HgPathBuf, HgPathBuf>;

#[derive(Clone, Debug, PartialEq)]
struct TimeStampedPathCopy {
    /// revision at which the copy information was added
    rev: Revision,
    /// the copy source, (Set to None in case of deletion of the associated
    /// key)
    path: Option<HgPathBuf>,
}

/// maps CopyDestination to Copy Source (+ a "timestamp" for the operation)
type TimeStampedPathCopies = OrdMap<HgPathBuf, TimeStampedPathCopy>;

/// hold parent 1, parent 2 and relevant files actions.
pub type RevInfo<'a> = (Revision, Revision, ChangedFiles<'a>);

/// represent the files affected by a changesets
///
/// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
/// all the data categories tracked by it.
/// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
/// all the data categories tracked by it.
pub struct ChangedFiles<'a> {
    nb_items: u32,
    index: &'a [u8],
    data: &'a [u8],
}

/// Represent active changes that affect the copy tracing.
enum Action<'a> {
    /// The parent ? children edge is removing a file
    ///
    /// (actually, this could be the edge from the other parent, but it does
    /// not matters)
    Removed(&'a HgPath),
    /// The parent ? children edge introduce copy information between (dest,
    /// source)
    Copied(&'a HgPath, &'a HgPath),
}

/// This express the possible "special" case we can get in a merge
///
/// See mercurial/metadata.py for details on these values.
#[derive(PartialEq)]
enum MergeCase {
    /// Merged: file had history on both side that needed to be merged
    Merged,
    /// Salvaged: file was candidate for deletion, but survived the merge
    Salvaged,
    /// Normal: Not one of the two cases above
    Normal,
}

type FileChange<'a> = (u8, &'a HgPath, &'a HgPath);

const EMPTY: &[u8] = b"";
const COPY_MASK: u8 = 3;
const P1_COPY: u8 = 2;
const P2_COPY: u8 = 3;
const ACTION_MASK: u8 = 28;
const REMOVED: u8 = 12;
const MERGED: u8 = 8;
const SALVAGED: u8 = 16;

impl<'a> ChangedFiles<'a> {
    const INDEX_START: usize = 4;
    const ENTRY_SIZE: u32 = 9;
    const FILENAME_START: u32 = 1;
    const COPY_SOURCE_START: u32 = 5;

    pub fn new(data: &'a [u8]) -> Self {
        assert!(
            data.len() >= 4,
            "data size ({}) is too small to contain the header (4)",
            data.len()
        );
        let nb_items_raw: [u8; 4] = (&data[0..=3])
            .try_into()
            .expect("failed to turn 4 bytes into 4 bytes");
        let nb_items = u32::from_be_bytes(nb_items_raw);

        let index_size = (nb_items * Self::ENTRY_SIZE) as usize;
        let index_end = Self::INDEX_START + index_size;

        assert!(
            data.len() >= index_end,
            "data size ({}) is too small to fit the index_data ({})",
            data.len(),
            index_end
        );

        let ret = ChangedFiles {
            nb_items,
            index: &data[Self::INDEX_START..index_end],
            data: &data[index_end..],
        };
        let max_data = ret.filename_end(nb_items - 1) as usize;
        assert!(
            ret.data.len() >= max_data,
            "data size ({}) is too small to fit all data ({})",
            data.len(),
            index_end + max_data
        );
        ret
    }

    pub fn new_empty() -> Self {
        ChangedFiles {
            nb_items: 0,
            index: EMPTY,
            data: EMPTY,
        }
    }

    /// internal function to return an individual entry at a given index
    fn entry(&'a self, idx: u32) -> FileChange<'a> {
        if idx >= self.nb_items {
            panic!(
                "index for entry is higher that the number of file {} >= {}",
                idx, self.nb_items
            )
        }
        let flags = self.flags(idx);
        let filename = self.filename(idx);
        let copy_idx = self.copy_idx(idx);
        let copy_source = self.filename(copy_idx);
        (flags, filename, copy_source)
    }

    /// internal function to return the filename of the entry at a given index
    fn filename(&self, idx: u32) -> &HgPath {
        let filename_start;
        if idx == 0 {
            filename_start = 0;
        } else {
            filename_start = self.filename_end(idx - 1)
        }
        let filename_end = self.filename_end(idx);
        let filename_start = filename_start as usize;
        let filename_end = filename_end as usize;
        HgPath::new(&self.data[filename_start..filename_end])
    }

    /// internal function to return the flag field of the entry at a given
    /// index
    fn flags(&self, idx: u32) -> u8 {
        let idx = idx as usize;
        self.index[idx * (Self::ENTRY_SIZE as usize)]
    }

    /// internal function to return the end of a filename part at a given index
    fn filename_end(&self, idx: u32) -> u32 {
        let start = (idx * Self::ENTRY_SIZE) + Self::FILENAME_START;
        let end = (idx * Self::ENTRY_SIZE) + Self::COPY_SOURCE_START;
        let start = start as usize;
        let end = end as usize;
        let raw = (&self.index[start..end])
            .try_into()
            .expect("failed to turn 4 bytes into 4 bytes");
        u32::from_be_bytes(raw)
    }

    /// internal function to return index of the copy source of the entry at a
    /// given index
    fn copy_idx(&self, idx: u32) -> u32 {
        let start = (idx * Self::ENTRY_SIZE) + Self::COPY_SOURCE_START;
        let end = (idx + 1) * Self::ENTRY_SIZE;
        let start = start as usize;
        let end = end as usize;
        let raw = (&self.index[start..end])
            .try_into()
            .expect("failed to turn 4 bytes into 4 bytes");
        u32::from_be_bytes(raw)
    }

    /// Return an iterator over all the `Action` in this instance.
    fn iter_actions(&self, parent: Parent) -> ActionsIterator {
        ActionsIterator {
            changes: &self,
            parent: parent,
            current: 0,
        }
    }

    /// return the MergeCase value associated with a filename
    fn get_merge_case(&self, path: &HgPath) -> MergeCase {
        if self.nb_items == 0 {
            return MergeCase::Normal;
        }
        let mut low_part = 0;
        let mut high_part = self.nb_items;

        while low_part < high_part {
            let cursor = (low_part + high_part - 1) / 2;
            let (flags, filename, _source) = self.entry(cursor);
            match path.cmp(filename) {
                Ordering::Less => low_part = cursor + 1,
                Ordering::Greater => high_part = cursor,
                Ordering::Equal => {
                    return match flags & ACTION_MASK {
                        MERGED => MergeCase::Merged,
                        SALVAGED => MergeCase::Salvaged,
                        _ => MergeCase::Normal,
                    };
                }
            }
        }
        MergeCase::Normal
    }
}

/// A struct responsible for answering "is X ancestors of Y" quickly
///
/// The structure will delegate ancestors call to a callback, and cache the
/// result.
#[derive(Debug)]
struct AncestorOracle<'a, A: Fn(Revision, Revision) -> bool> {
    inner: &'a A,
    pairs: HashMap<(Revision, Revision), bool>,
}

impl<'a, A: Fn(Revision, Revision) -> bool> AncestorOracle<'a, A> {
    fn new(func: &'a A) -> Self {
        Self {
            inner: func,
            pairs: HashMap::default(),
        }
    }

    /// returns `true` if `anc` is an ancestors of `desc`, `false` otherwise
    fn is_ancestor(&mut self, anc: Revision, desc: Revision) -> bool {
        if anc > desc {
            false
        } else if anc == desc {
            true
        } else {
            if let Some(b) = self.pairs.get(&(anc, desc)) {
                *b
            } else {
                let b = (self.inner)(anc, desc);
                self.pairs.insert((anc, desc), b);
                b
            }
        }
    }
}

struct ActionsIterator<'a> {
    changes: &'a ChangedFiles<'a>,
    parent: Parent,
    current: u32,
}

impl<'a> Iterator for ActionsIterator<'a> {
    type Item = Action<'a>;

    fn next(&mut self) -> Option<Action<'a>> {
        let copy_flag = match self.parent {
            Parent::FirstParent => P1_COPY,
            Parent::SecondParent => P2_COPY,
        };
        while self.current < self.changes.nb_items {
            let (flags, file, source) = self.changes.entry(self.current);
            self.current += 1;
            if (flags & ACTION_MASK) == REMOVED {
                return Some(Action::Removed(file));
            }
            let copy = flags & COPY_MASK;
            if copy == copy_flag {
                return Some(Action::Copied(file, source));
            }
        }
        return None;
    }
}

/// A small struct whose purpose is to ensure lifetime of bytes referenced in
/// ChangedFiles
///
/// It is passed to the RevInfoMaker callback who can assign any necessary
/// content to the `data` attribute. The copy tracing code is responsible for
/// keeping the DataHolder alive at least as long as the ChangedFiles object.
pub struct DataHolder<D> {
    /// RevInfoMaker callback should assign data referenced by the
    /// ChangedFiles struct it return to this attribute. The DataHolder
    /// lifetime will be at least as long as the ChangedFiles one.
    pub data: Option<D>,
}

pub type RevInfoMaker<'a, D> =
    Box<dyn for<'r> Fn(Revision, &'r mut DataHolder<D>) -> RevInfo<'r> + 'a>;

/// enum used to carry information about the parent → child currently processed
#[derive(Copy, Clone, Debug)]
enum Parent {
    /// The `p1(x) → x` edge
    FirstParent,
    /// The `p2(x) → x` edge
    SecondParent,
}

/// Same as mercurial.copies._combine_changeset_copies, but in Rust.
///
/// Arguments are:
///
/// revs: all revisions to be considered
/// children: a {parent ? [childrens]} mapping
/// target_rev: the final revision we are combining copies to
/// rev_info(rev): callback to get revision information:
///   * first parent
///   * second parent
///   * ChangedFiles
/// isancestors(low_rev, high_rev): callback to check if a revision is an
///                                 ancestor of another
pub fn combine_changeset_copies<A: Fn(Revision, Revision) -> bool, D>(
    revs: Vec<Revision>,
    children: HashMap<Revision, Vec<Revision>>,
    target_rev: Revision,
    rev_info: RevInfoMaker<D>,
    is_ancestor: &A,
) -> PathCopies {
    let mut all_copies = HashMap::new();
    let mut oracle = AncestorOracle::new(is_ancestor);

    for rev in revs {
        // Retrieve data computed in a previous iteration
        let copies = all_copies.remove(&rev);
        let copies = match copies {
            Some(c) => c,
            None => TimeStampedPathCopies::default(), // root of the walked set
        };

        let current_children = match children.get(&rev) {
            Some(c) => c,
            None => panic!("inconsistent `revs` and `children`"),
        };

        for child in current_children {
            // We will chain the copies information accumulated for `rev` with
            // the individual copies information for each of its children.
            // Creating a new PathCopies for each `rev` → `children` vertex.
            let mut d: DataHolder<D> = DataHolder { data: None };
            let (p1, p2, changes) = rev_info(*child, &mut d);

            let parent = if rev == p1 {
                Parent::FirstParent
            } else {
                assert_eq!(rev, p2);
                Parent::SecondParent
            };
            let new_copies =
                add_from_changes(&copies, &changes, parent, *child);

            // Merge has two parents needs to combines their copy information.
            //
            // If the vertex from the other parent was already processed, we
            // will have a value for the child ready to be used. We need to
            // grab it and combine it with the one we already
            // computed. If not we can simply store the newly
            // computed data. The processing happening at
            // the time of the second parent will take care of combining the
            // two TimeStampedPathCopies instance.
            match all_copies.remove(child) {
                None => {
                    all_copies.insert(child, new_copies);
                }
                Some(other_copies) => {
                    let (minor, major) = match parent {
                        Parent::FirstParent => (other_copies, new_copies),
                        Parent::SecondParent => (new_copies, other_copies),
                    };
                    let merged_copies =
                        merge_copies_dict(minor, major, &changes, &mut oracle);
                    all_copies.insert(child, merged_copies);
                }
            };
        }
    }

    // Drop internal information (like the timestamp) and return the final
    // mapping.
    let tt_result = all_copies
        .remove(&target_rev)
        .expect("target revision was not processed");
    let mut result = PathCopies::default();
    for (dest, tt_source) in tt_result {
        if let Some(path) = tt_source.path {
            result.insert(dest, path);
        }
    }
    result
}

/// Combine ChangedFiles with some existing PathCopies information and return
/// the result
fn add_from_changes(
    base_copies: &TimeStampedPathCopies,
    changes: &ChangedFiles,
    parent: Parent,
    current_rev: Revision,
) -> TimeStampedPathCopies {
    let mut copies = base_copies.clone();
    for action in changes.iter_actions(parent) {
        match action {
            Action::Copied(dest, source) => {
                let entry;
                if let Some(v) = base_copies.get(source) {
                    entry = match &v.path {
                        Some(path) => Some((*(path)).to_owned()),
                        None => Some(source.to_owned()),
                    }
                } else {
                    entry = Some(source.to_owned());
                }
                // Each new entry is introduced by the children, we
                // record this information as we will need it to take
                // the right decision when merging conflicting copy
                // information. See merge_copies_dict for details.
                let ttpc = TimeStampedPathCopy {
                    rev: current_rev,
                    path: entry,
                };
                copies.insert(dest.to_owned(), ttpc);
            }
            Action::Removed(f) => {
                // We must drop copy information for removed file.
                //
                // We need to explicitly record them as dropped to
                // propagate this information when merging two
                // TimeStampedPathCopies object.
                if copies.contains_key(f.as_ref()) {
                    let ttpc = TimeStampedPathCopy {
                        rev: current_rev,
                        path: None,
                    };
                    copies.insert(f.to_owned(), ttpc);
                }
            }
        }
    }
    copies
}

/// merge two copies-mapping together, minor and major
///
/// In case of conflict, value from "major" will be picked, unless in some
/// cases. See inline documentation for details.
fn merge_copies_dict<A: Fn(Revision, Revision) -> bool>(
    minor: TimeStampedPathCopies,
    major: TimeStampedPathCopies,
    changes: &ChangedFiles,
    oracle: &mut AncestorOracle<A>,
) -> TimeStampedPathCopies {
    // This closure exist as temporary help while multiple developper are
    // actively working on this code. Feel free to re-inline it once this
    // code is more settled.
    let mut cmp_value =
        |dest: &HgPathBuf,
         src_minor: &TimeStampedPathCopy,
         src_major: &TimeStampedPathCopy| {
            compare_value(changes, oracle, dest, src_minor, src_major)
        };
    if minor.is_empty() {
        major
    } else if major.is_empty() {
        minor
    } else {
        let mut override_minor = Vec::new();
        let mut override_major = Vec::new();

        let mut to_major = |k: &HgPathBuf, v: &TimeStampedPathCopy| {
            override_major.push((k.clone(), v.clone()))
        };
        let mut to_minor = |k: &HgPathBuf, v: &TimeStampedPathCopy| {
            override_minor.push((k.clone(), v.clone()))
        };

        // The diff function leverage detection of the identical subpart if
        // minor and major has some common ancestors. This make it very
        // fast is most case.
        //
        // In case where the two map are vastly different in size, the current
        // approach is still slowish because the iteration will iterate over
        // all the "exclusive" content of the larger on. This situation can be
        // frequent when the subgraph of revision we are processing has a lot
        // of roots. Each roots adding they own fully new map to the mix (and
        // likely a small map, if the path from the root to the "main path" is
        // small.
        //
        // We could do better by detecting such situation and processing them
        // differently.
        for d in minor.diff(&major) {
            match d {
                DiffItem::Add(k, v) => to_minor(k, v),
                DiffItem::Remove(k, v) => to_major(k, v),
                DiffItem::Update { old, new } => {
                    let (dest, src_major) = new;
                    let (_, src_minor) = old;
                    match cmp_value(dest, src_minor, src_major) {
                        MergePick::Major => to_minor(dest, src_major),
                        MergePick::Minor => to_major(dest, src_minor),
                        // If the two entry are identical, no need to do
                        // anything (but diff should not have yield them)
                        MergePick::Any => unreachable!(),
                    }
                }
            };
        }

        let updates;
        let mut result;
        if override_major.is_empty() {
            result = major
        } else if override_minor.is_empty() {
            result = minor
        } else {
            if override_minor.len() < override_major.len() {
                updates = override_minor;
                result = minor;
            } else {
                updates = override_major;
                result = major;
            }
            for (k, v) in updates {
                result.insert(k, v);
            }
        }
        result
    }
}

/// represent the side that should prevail when merging two
/// TimeStampedPathCopies
enum MergePick {
    /// The "major" (p1) side prevails
    Major,
    /// The "minor" (p2) side prevails
    Minor,
    /// Any side could be used (because they are the same)
    Any,
}

/// decide which side prevails in case of conflicting values
#[allow(clippy::if_same_then_else)]
fn compare_value<A: Fn(Revision, Revision) -> bool>(
    changes: &ChangedFiles,
    oracle: &mut AncestorOracle<A>,
    dest: &HgPathBuf,
    src_minor: &TimeStampedPathCopy,
    src_major: &TimeStampedPathCopy,
) -> MergePick {
    if src_major.path == src_minor.path {
        // we have the same value, but from other source;
        if src_major.rev == src_minor.rev {
            // If the two entry are identical, they are both valid
            MergePick::Any
        } else if oracle.is_ancestor(src_major.rev, src_minor.rev) {
            MergePick::Minor
        } else {
            MergePick::Major
        }
    } else if src_major.rev == src_minor.rev {
        // We cannot get copy information for both p1 and p2 in the
        // same rev. So this is the same value.
        unreachable!(
            "conflict information from p1 and p2 in the same revision"
        );
    } else {
        let action = changes.get_merge_case(&dest);
        if src_major.path.is_none() && action == MergeCase::Salvaged {
            // If the file is "deleted" in the major side but was
            // salvaged by the merge, we keep the minor side alive
            MergePick::Minor
        } else if src_minor.path.is_none() && action == MergeCase::Salvaged {
            // If the file is "deleted" in the minor side but was
            // salvaged by the merge, unconditionnaly preserve the
            // major side.
            MergePick::Major
        } else if action == MergeCase::Merged {
            // If the file was actively merged, copy information
            // from each side might conflict.  The major side will
            // win such conflict.
            MergePick::Major
        } else if oracle.is_ancestor(src_major.rev, src_minor.rev) {
            // If the minor side is strictly newer than the major
            // side, it should be kept.
            MergePick::Minor
        } else if src_major.path.is_some() {
            // without any special case, the "major" value win
            // other the "minor" one.
            MergePick::Major
        } else if oracle.is_ancestor(src_minor.rev, src_major.rev) {
            // the "major" rev is a direct ancestors of "minor",
            // any different value should
            // overwrite
            MergePick::Major
        } else {
            // major version is None (so the file was deleted on
            // that branch) and that branch is independant (neither
            // minor nor major is an ancestors of the other one.)
            // We preserve the new
            // information about the new file.
            MergePick::Minor
        }
    }
}