copies-rust: move the parent token to an enum

We carry around information about which parent of a revision is been dealt
with. So far this was a `usize` but as we are about to pass it around to more
function it seems like a good idea to start cleaning this up and use a proper
enum.

Differential Revision: https://phab.mercurial-scm.org/D9420

rust/hg-core/src/copy_tracing.rs

@@ -1,567 +1,576 b''
 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: usize) -> ActionsIterator {
+    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: usize,
+    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 self.parent == 1 && copy == P1_COPY {
-                return Some(Action::Copied(file, source));
-            }
-            if self.parent == 2 && copy == P2_COPY {
+            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 {
-                1
+                Parent::FirstParent
             } else {
                 assert_eq!(rev, p2);
-                2
+                Parent::SecondParent
             };
             let mut new_copies = copies.clone();
 
             for action in changes.iter_actions(parent) {
                 match action {
                     Action::Copied(dest, source) => {
                         let entry;
                         if let Some(v) = 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: *child,
                             path: entry,
                         };
                         new_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 new_copies.contains_key(f.as_ref()) {
                             let ttpc = TimeStampedPathCopy {
                                 rev: *child,
                                 path: None,
                             };
                             new_copies.insert(f.to_owned(), ttpc);
                         }
                     }
                 }
             }
 
             // 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 {
-                        1 => (other_copies, new_copies),
-                        2 => (new_copies, other_copies),
-                        _ => unreachable!(),
+                        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
 }
 
 /// 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.
 #[allow(clippy::if_same_then_else)]
 fn merge_copies_dict<A: Fn(Revision, Revision) -> bool>(
     minor: TimeStampedPathCopies,
     major: TimeStampedPathCopies,
     changes: &ChangedFiles,
     oracle: &mut AncestorOracle<A>,
 ) -> TimeStampedPathCopies {
     if minor.is_empty() {
         return major;
     } else if major.is_empty() {
         return minor;
     }
     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;
                 let mut pick_minor = || (to_major(dest, src_minor));
                 let mut pick_major = || (to_minor(dest, src_major));
                 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, no need to do
                         // anything (but diff should not have yield them)
                         unreachable!();
                     } else if oracle.is_ancestor(src_major.rev, src_minor.rev)
                     {
                         pick_minor();
                     } else {
                         pick_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!();
                 } 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
                         pick_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.
                         pick_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.
                         pick_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.
                         pick_minor();
                     } else if src_major.path.is_some() {
                         // without any special case, the "major" value win
                         // other the "minor" one.
                         pick_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
                         pick_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.
                         pick_minor();
                     }
                 }
             }
         };
     }
 
     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
 }