--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/operations/cat.rs	Fri Sep 11 17:32:53 2020 +0200
@@ -0,0 +1,145 @@
+// list_tracked_files.rs
+//
+// Copyright 2020 Antoine Cezar <antoine.cezar@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.
+
+use std::convert::From;
+use std::path::PathBuf;
+
+use crate::revlog::changelog::Changelog;
+use crate::revlog::manifest::{Manifest, ManifestEntry};
+use crate::revlog::path_encode::path_encode;
+use crate::revlog::revlog::Revlog;
+use crate::revlog::revlog::RevlogError;
+use crate::revlog::Revision;
+use crate::utils::hg_path::HgPathBuf;
+
+/// Kind of error encountered by `CatRev`
+#[derive(Debug)]
+pub enum CatRevErrorKind {
+    /// Error when reading a `revlog` file.
+    IoError(std::io::Error),
+    /// The revision has not been found.
+    InvalidRevision,
+    /// A `revlog` file is corrupted.
+    CorruptedRevlog,
+    /// The `revlog` format version is not supported.
+    UnsuportedRevlogVersion(u16),
+    /// The `revlog` data format is not supported.
+    UnknowRevlogDataFormat(u8),
+}
+
+/// A `CatRev` error
+#[derive(Debug)]
+pub struct CatRevError {
+    /// Kind of error encountered by `CatRev`
+    pub kind: CatRevErrorKind,
+}
+
+impl From<CatRevErrorKind> for CatRevError {
+    fn from(kind: CatRevErrorKind) -> Self {
+        CatRevError { kind }
+    }
+}
+
+impl From<RevlogError> for CatRevError {
+    fn from(err: RevlogError) -> Self {
+        match err {
+            RevlogError::IoError(err) => CatRevErrorKind::IoError(err),
+            RevlogError::UnsuportedVersion(version) => {
+                CatRevErrorKind::UnsuportedRevlogVersion(version)
+            }
+            RevlogError::InvalidRevision => CatRevErrorKind::InvalidRevision,
+            RevlogError::Corrupted => CatRevErrorKind::CorruptedRevlog,
+            RevlogError::UnknowDataFormat(format) => {
+                CatRevErrorKind::UnknowRevlogDataFormat(format)
+            }
+        }
+        .into()
+    }
+}
+
+/// List files under Mercurial control at a given revision.
+pub struct CatRev<'a> {
+    root: &'a PathBuf,
+    /// The revision to cat the files from.
+    rev: &'a str,
+    /// The files to output.
+    files: &'a [HgPathBuf],
+    /// The changelog file
+    changelog: Changelog,
+    /// The manifest file
+    manifest: Manifest,
+    /// The manifest entry corresponding to the revision.
+    ///
+    /// Used to hold the owner of the returned references.
+    manifest_entry: Option<ManifestEntry>,
+}
+
+impl<'a> CatRev<'a> {
+    pub fn new(
+        root: &'a PathBuf,
+        rev: &'a str,
+        files: &'a [HgPathBuf],
+    ) -> Result<Self, CatRevError> {
+        let changelog = Changelog::open(&root)?;
+        let manifest = Manifest::open(&root)?;
+        let manifest_entry = None;
+
+        Ok(Self {
+            root,
+            rev,
+            files,
+            changelog,
+            manifest,
+            manifest_entry,
+        })
+    }
+
+    pub fn run(&mut self) -> Result<Vec<u8>, CatRevError> {
+        let changelog_entry = match self.rev.parse::<Revision>() {
+            Ok(rev) => self.changelog.get_rev(rev)?,
+            _ => {
+                let changelog_node = hex::decode(&self.rev)
+                    .map_err(|_| CatRevErrorKind::InvalidRevision)?;
+                self.changelog.get_node(&changelog_node)?
+            }
+        };
+        let manifest_node = hex::decode(&changelog_entry.manifest_node()?)
+            .map_err(|_| CatRevErrorKind::CorruptedRevlog)?;
+
+        self.manifest_entry = Some(self.manifest.get_node(&manifest_node)?);
+        if let Some(ref manifest_entry) = self.manifest_entry {
+            let mut bytes = vec![];
+
+            for (manifest_file, node_bytes) in
+                manifest_entry.files_with_nodes()
+            {
+                for cat_file in self.files.iter() {
+                    if cat_file.as_bytes() == manifest_file.as_bytes() {
+                        let encoded_bytes =
+                            path_encode(manifest_file.as_bytes());
+                        let revlog_index_string = format!(
+                            ".hg/store/data/{}.i",
+                            String::from_utf8_lossy(&encoded_bytes),
+                        );
+                        let revlog_index_path =
+                            self.root.join(&revlog_index_string);
+                        let file_log = Revlog::open(&revlog_index_path)?;
+                        let file_node = hex::decode(&node_bytes)
+                            .map_err(|_| CatRevErrorKind::CorruptedRevlog)?;
+                        let file_rev = file_log.get_node_rev(&file_node)?;
+                        let data = file_log.get_rev_data(file_rev)?;
+                        bytes.extend(data);
+                    }
+                }
+            }
+
+            Ok(bytes)
+        } else {
+            unreachable!("manifest_entry should have been stored");
+        }
+    }
+}

--- a/rust/hg-core/src/operations/mod.rs	Tue Sep 15 16:46:57 2020 +0200
+++ b/rust/hg-core/src/operations/mod.rs	Fri Sep 11 17:32:53 2020 +0200
@@ -2,10 +2,12 @@
 //! An operation is what can be done whereas a command is what is exposed by
 //! the cli. A single command can use several operations to achieve its goal.
 
+mod cat;
 mod debugdata;
 mod dirstate_status;
 mod find_root;
 mod list_tracked_files;
+pub use cat::{CatRev, CatRevError, CatRevErrorKind};
 pub use debugdata::{
     DebugData, DebugDataError, DebugDataErrorKind, DebugDataKind,
 };