hg-core: add `Changlog` a specialized `Revlog`

hg-core: add `Changlog` a specialized `Revlog`

A facade to `Revlog` to provide a `changelog` specific interface.

Differential Revision: https://phab.mercurial-scm.org/D9010

--- a/rust/hg-core/src/revlog.rs	Wed Sep 02 15:23:25 2020 +0200
+++ b/rust/hg-core/src/revlog.rs	Wed Sep 09 16:25:23 2020 +0200
@@ -8,6 +8,7 @@
 pub mod node;
 pub mod nodemap;
 pub use node::{Node, NodeError, NodePrefix, NodePrefixRef};
+pub mod changelog;
 pub mod index;
 pub mod patch;
 pub mod revlog;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/revlog/changelog.rs	Wed Sep 09 16:25:23 2020 +0200
@@ -0,0 +1,58 @@
+use crate::revlog::revlog::{Revlog, RevlogError};
+use crate::revlog::Revision;
+use std::path::PathBuf;
+
+/// A specialized `Revlog` to work with `changelog` data format.
+pub struct Changelog {
+    /// The generic `revlog` format.
+    revlog: Revlog,
+}
+
+impl Changelog {
+    /// Open the `changelog` of a repository given by its root.
+    pub fn open(root: &PathBuf) -> Result<Self, RevlogError> {
+        let index_file = root.join(".hg/store/00changelog.i");
+        let revlog = Revlog::open(&index_file)?;
+        Ok(Self { revlog })
+    }
+
+    /// Return the `ChangelogEntry` a given node id.
+    pub fn get_node(
+        &self,
+        node: &[u8],
+    ) -> Result<ChangelogEntry, RevlogError> {
+        let rev = self.revlog.get_node_rev(node)?;
+        self.get_rev(rev)
+    }
+
+    /// Return the `ChangelogEntry` of a given node revision.
+    pub fn get_rev(
+        &self,
+        rev: Revision,
+    ) -> Result<ChangelogEntry, RevlogError> {
+        let bytes = self.revlog.get_rev_data(rev)?;
+        Ok(ChangelogEntry { bytes })
+    }
+}
+
+/// `Changelog` entry which knows how to interpret the `changelog` data bytes.
+#[derive(Debug)]
+pub struct ChangelogEntry {
+    /// The data bytes of the `changelog` entry.
+    bytes: Vec<u8>,
+}
+
+impl ChangelogEntry {
+    /// Return an iterator over the lines of the entry.
+    pub fn lines(&self) -> impl Iterator<Item = &[u8]> {
+        self.bytes
+            .split(|b| b == &b'\n')
+            .filter(|line| !line.is_empty())
+    }
+
+    /// Return the node id of the `manifest` referenced by this `changelog`
+    /// entry.
+    pub fn manifest_node(&self) -> Result<&[u8], RevlogError> {
+        self.lines().next().ok_or(RevlogError::Corrupted)
+    }
+}