rust-nodemap: abstracting the indexing

In the forthcoming mutable implementation, we'll have to visit
node trees that are more complex than a single slice, although
the algorithm will still be expressed in simple indexing terms.

We still refrain using `#[inline]` indications as being
premature optimizations, but we strongly hope the compiler will
indeed inline most of the glue.

Differential Revision: https://phab.mercurial-scm.org/D7792

rust/hg-core/src/revlog/nodemap.rs

@@ -1,388 +1,404 b''
 // Copyright 2018-2020 Georges Racinet <[e-mail]>
 //           and Mercurial contributors
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 //! Indexing facilities for fast retrieval of `Revision` from `Node`
 //!
 //! This provides a variation on the 16-ary radix tree that is
 //! provided as "nodetree" in revlog.c, ready for append-only persistence
 //! on disk.
 //!
 //! Following existing implicit conventions, the "nodemap" terminology
 //! is used in a more abstract context.
 
 use super::{
     Node, NodeError, NodePrefix, NodePrefixRef, Revision, RevlogIndex,
 };
 use std::fmt;
 use std::ops::Deref;
+use std::ops::Index;
 
 #[derive(Debug, PartialEq)]
 pub enum NodeMapError {
     MultipleResults,
     InvalidNodePrefix(NodeError),
     /// A `Revision` stored in the nodemap could not be found in the index
     RevisionNotInIndex(Revision),
 }
 
 impl From<NodeError> for NodeMapError {
     fn from(err: NodeError) -> Self {
         NodeMapError::InvalidNodePrefix(err)
     }
 }
 
 /// Mapping system from Mercurial nodes to revision numbers.
 ///
 /// ## `RevlogIndex` and `NodeMap`
 ///
 /// One way to think about their relationship is that
 /// the `NodeMap` is a prefix-oriented reverse index of the `Node` information
 /// carried by a [`RevlogIndex`].
 ///
 /// Many of the methods in this trait take a `RevlogIndex` argument
 /// which is used for validation of their results. This index must naturally
 /// be the one the `NodeMap` is about, and it must be consistent.
 ///
 /// Notably, the `NodeMap` must not store
 /// information about more `Revision` values than there are in the index.
 /// In these methods, an encountered `Revision` is not in the index, a
 /// [`RevisionNotInIndex`] error is returned.
 ///
 /// In insert operations, the rule is thus that the `NodeMap` must always
 /// be updated after the `RevlogIndex`
 /// be updated first, and the `NodeMap` second.
 ///
 /// [`RevisionNotInIndex`]: enum.NodeMapError.html#variant.RevisionNotInIndex
 /// [`RevlogIndex`]: ../trait.RevlogIndex.html
 pub trait NodeMap {
     /// Find the unique `Revision` having the given `Node`
     ///
     /// If no Revision matches the given `Node`, `Ok(None)` is returned.
     fn find_node(
         &self,
         index: &impl RevlogIndex,
         node: &Node,
     ) -> Result<Option<Revision>, NodeMapError> {
         self.find_bin(index, node.into())
     }
 
     /// Find the unique Revision whose `Node` starts with a given binary prefix
     ///
     /// If no Revision matches the given prefix, `Ok(None)` is returned.
     ///
     /// If several Revisions match the given prefix, a [`MultipleResults`]
     /// error is returned.
     fn find_bin<'a>(
         &self,
         idx: &impl RevlogIndex,
         prefix: NodePrefixRef<'a>,
     ) -> Result<Option<Revision>, NodeMapError>;
 
     /// Find the unique Revision whose `Node` hexadecimal string representation
     /// starts with a given prefix
     ///
     /// If no Revision matches the given prefix, `Ok(None)` is returned.
     ///
     /// If several Revisions match the given prefix, a [`MultipleResults`]
     /// error is returned.
     fn find_hex(
         &self,
         idx: &impl RevlogIndex,
         prefix: &str,
     ) -> Result<Option<Revision>, NodeMapError> {
         self.find_bin(idx, NodePrefix::from_hex(prefix)?.borrow())
     }
 }
 
 /// Low level NodeTree [`Blocks`] elements
 ///
 /// These are exactly as for instance on persistent storage.
 type RawElement = i32;
 
 /// High level representation of values in NodeTree
 /// [`Blocks`](struct.Block.html)
 ///
 /// This is the high level representation that most algorithms should
 /// use.
 #[derive(Clone, Debug, Eq, PartialEq)]
 enum Element {
     Rev(Revision),
     Block(usize),
     None,
 }
 
 impl From<RawElement> for Element {
     /// Conversion from low level representation, after endianness conversion.
     ///
     /// See [`Block`](struct.Block.html) for explanation about the encoding.
     fn from(raw: RawElement) -> Element {
         if raw >= 0 {
             Element::Block(raw as usize)
         } else if raw == -1 {
             Element::None
         } else {
             Element::Rev(-raw - 2)
         }
     }
 }
 
 impl From<Element> for RawElement {
     fn from(element: Element) -> RawElement {
         match element {
             Element::None => 0,
             Element::Block(i) => i as RawElement,
             Element::Rev(rev) => -rev - 2,
         }
     }
 }
 
 /// A logical block of the `NodeTree`, packed with a fixed size.
 ///
 /// These are always used in container types implementing `Index<Block>`,
 /// such as `&Block`
 ///
 /// As an array of integers, its ith element encodes that the
 /// ith potential edge from the block, representing the ith hexadecimal digit
 /// (nybble) `i` is either:
 ///
 /// - absent (value -1)
 /// - another `Block` in the same indexable container (value ≥ 0)
 ///  - a `Revision` leaf (value ≤ -2)
 ///
 /// Endianness has to be fixed for consistency on shared storage across
 /// different architectures.
 ///
 /// A key difference with the C `nodetree` is that we need to be
 /// able to represent the [`Block`] at index 0, hence -1 is the empty marker
 /// rather than 0 and the `Revision` range upper limit of -2 instead of -1.
 ///
 /// Another related difference is that `NULL_REVISION` (-1) is not
 /// represented at all, because we want an immutable empty nodetree
 /// to be valid.
 
 #[derive(Clone, PartialEq)]
 pub struct Block([RawElement; 16]);
 
 impl Block {
     fn new() -> Self {
         Block([-1; 16])
     }
 
     fn get(&self, nybble: u8) -> Element {
         Element::from(RawElement::from_be(self.0[nybble as usize]))
     }
 
     fn set(&mut self, nybble: u8, element: Element) {
         self.0[nybble as usize] = RawElement::to_be(element.into())
     }
 }
 
 impl fmt::Debug for Block {
     /// sparse representation for testing and debugging purposes
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_map()
             .entries((0..16).filter_map(|i| match self.get(i) {
                 Element::None => None,
                 element => Some((i, element)),
             }))
             .finish()
     }
 }
 
 /// A 16-radix tree with the root block at the end
 pub struct NodeTree {
     readonly: Box<dyn Deref<Target = [Block]> + Send>,
 }
 
+impl Index<usize> for NodeTree {
+    type Output = Block;
+
+    fn index(&self, i: usize) -> &Block {
+        &self.readonly[i]
+    }
+}
+
 /// Return `None` unless the `Node` for `rev` has given prefix in `index`.
 fn has_prefix_or_none<'p>(
     idx: &impl RevlogIndex,
     prefix: NodePrefixRef<'p>,
     rev: Revision,
 ) -> Result<Option<Revision>, NodeMapError> {
     idx.node(rev)
         .ok_or_else(|| NodeMapError::RevisionNotInIndex(rev))
         .map(|node| {
             if prefix.is_prefix_of(node) {
                 Some(rev)
             } else {
                 None
             }
         })
 }
 
 impl NodeTree {
+    fn len(&self) -> usize {
+        self.readonly.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Main working method for `NodeTree` searches
     ///
     /// This partial implementation lacks special cases for NULL_REVISION
     fn lookup<'p>(
         &self,
         prefix: NodePrefixRef<'p>,
     ) -> Result<Option<Revision>, NodeMapError> {
-        let blocks: &[Block] = &*self.readonly;
-        if blocks.is_empty() {
+        if self.is_empty() {
             return Ok(None);
         }
-        let mut visit = blocks.len() - 1;
+        let mut visit = self.len() - 1;
         for i in 0..prefix.len() {
             let nybble = prefix.get_nybble(i);
-            match blocks[visit].get(nybble) {
+            match self[visit].get(nybble) {
                 Element::None => return Ok(None),
                 Element::Rev(r) => return Ok(Some(r)),
                 Element::Block(idx) => visit = idx,
             }
         }
         Err(NodeMapError::MultipleResults)
     }
 }
 
 impl From<Vec<Block>> for NodeTree {
     fn from(vec: Vec<Block>) -> Self {
         NodeTree {
             readonly: Box::new(vec),
         }
     }
 }
 
 impl fmt::Debug for NodeTree {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let blocks: &[Block] = &*self.readonly;
         write!(f, "readonly: {:?}", blocks)
     }
 }
 
 impl NodeMap for NodeTree {
     fn find_bin<'a>(
         &self,
         idx: &impl RevlogIndex,
         prefix: NodePrefixRef<'a>,
     ) -> Result<Option<Revision>, NodeMapError> {
         self.lookup(prefix.clone()).and_then(|opt| {
             opt.map_or(Ok(None), |rev| has_prefix_or_none(idx, prefix, rev))
         })
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::NodeMapError::*;
     use super::*;
     use crate::revlog::node::{hex_pad_right, Node};
     use std::collections::HashMap;
 
     /// Creates a `Block` using a syntax close to the `Debug` output
     macro_rules! block {
         {$($nybble:tt : $variant:ident($val:tt)),*} => (
             {
                 let mut block = Block::new();
                 $(block.set($nybble, Element::$variant($val)));*;
                 block
             }
         )
     }
 
     #[test]
     fn test_block_debug() {
         let mut block = Block::new();
         block.set(1, Element::Rev(3));
         block.set(10, Element::Block(0));
         assert_eq!(format!("{:?}", block), "{1: Rev(3), 10: Block(0)}");
     }
 
     #[test]
     fn test_block_macro() {
         let block = block! {5: Block(2)};
         assert_eq!(format!("{:?}", block), "{5: Block(2)}");
 
         let block = block! {13: Rev(15), 5: Block(2)};
         assert_eq!(format!("{:?}", block), "{5: Block(2), 13: Rev(15)}");
     }
 
     #[test]
     fn test_raw_block() {
         let mut raw = [-1; 16];
         raw[0] = 0;
         raw[1] = RawElement::to_be(15);
         raw[2] = RawElement::to_be(-2);
         raw[3] = RawElement::to_be(-1);
         raw[4] = RawElement::to_be(-3);
         let block = Block(raw);
         assert_eq!(block.get(0), Element::Block(0));
         assert_eq!(block.get(1), Element::Block(15));
         assert_eq!(block.get(3), Element::None);
         assert_eq!(block.get(2), Element::Rev(0));
         assert_eq!(block.get(4), Element::Rev(1));
     }
 
     type TestIndex = HashMap<Revision, Node>;
 
     impl RevlogIndex for TestIndex {
         fn node(&self, rev: Revision) -> Option<&Node> {
             self.get(&rev)
         }
 
         fn len(&self) -> usize {
             self.len()
         }
     }
 
     /// Pad hexadecimal Node prefix with zeros on the right, then insert
     ///
     /// This avoids having to repeatedly write very long hexadecimal
     /// strings for test data, and brings actual hash size independency.
     fn pad_insert(idx: &mut TestIndex, rev: Revision, hex: &str) {
         idx.insert(rev, Node::from_hex(&hex_pad_right(hex)).unwrap());
     }
 
     fn sample_nodetree() -> NodeTree {
         NodeTree::from(vec![
             block![0: Rev(9)],
             block![0: Rev(0), 1: Rev(9)],
             block![0: Block(1), 1:Rev(1)],
         ])
     }
 
     #[test]
     fn test_nt_debug() {
         let nt = sample_nodetree();
         assert_eq!(
             format!("{:?}", nt),
             "readonly: \
              [{0: Rev(9)}, {0: Rev(0), 1: Rev(9)}, {0: Block(1), 1: Rev(1)}]"
         );
     }
 
     #[test]
     fn test_immutable_find_simplest() -> Result<(), NodeMapError> {
         let mut idx: TestIndex = HashMap::new();
         pad_insert(&mut idx, 1, "1234deadcafe");
 
         let nt = NodeTree::from(vec![block![1: Rev(1)]]);
         assert_eq!(nt.find_hex(&idx, "1")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "12")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "1234de")?, Some(1));
         assert_eq!(nt.find_hex(&idx, "1a")?, None);
         assert_eq!(nt.find_hex(&idx, "ab")?, None);
 
         // and with full binary Nodes
         assert_eq!(nt.find_node(&idx, idx.get(&1).unwrap())?, Some(1));
         let unknown = Node::from_hex(&hex_pad_right("3d")).unwrap();
         assert_eq!(nt.find_node(&idx, &unknown)?, None);
         Ok(())
     }
 
     #[test]
     fn test_immutable_find_one_jump() {
         let mut idx = TestIndex::new();
         pad_insert(&mut idx, 9, "012");
         pad_insert(&mut idx, 0, "00a");
 
         let nt = sample_nodetree();
 
         assert_eq!(nt.find_hex(&idx, "0"), Err(MultipleResults));
         assert_eq!(nt.find_hex(&idx, "01"), Ok(Some(9)));
         assert_eq!(nt.find_hex(&idx, "00"), Ok(Some(0)));
         assert_eq!(nt.find_hex(&idx, "00a"), Ok(Some(0)));
     }
 }