commands: Implement next and prev

This is a naive implementation, which cannot deal with multiple children
or parents stemming from merges.

Note: I currently gave each command separate a separate argument struct
for extensibility. 

Fixes martinvonz#878

src/commands/mod.rs

@@ -87,6 +87,8 @@ enum Commands {
     Diffedit(DiffeditArgs),
     Resolve(ResolveArgs),
     Split(SplitArgs),
+    Next(NextArgs),
+    Prev(PrevArgs),
     /// Merge work from multiple branches
     ///
     /// Unlike most other VCSs, `jj merge` does not implicitly include the
@@ -468,6 +470,68 @@ struct NewArgs {
     insert_before: bool,
 }
 
+/// Move the current working copy commit to the next child revision in the
+/// repository. The command moves you to the next child in a linear fashion.
+///
+/// F        F  @
+/// |        | /
+/// C  @ =>  C
+/// | /      |
+/// B        B
+///
+/// If `edit` is passed as an argument, it will move you directly to the child
+/// revision.
+///
+/// F        F
+/// |        |
+/// C        C
+/// |        |
+/// B  @  => @
+/// | /      |
+/// A        A
+// TODO(#NNN): Handle multiple child revisions properly.
+#[derive(clap::Args, Clone, Debug)]
+struct NextArgs {
+    /// How many revisions to move forward. By default advances to the next
+    /// child.
+    #[default = "1"]
+    amount: usize,
+    /// Instead of moving the empty commit from `jj new`, edit the child
+    /// revision directly. This mirrors the behavior of Mercurial and
+    /// Sapling.
+    edit: bool,
+}
+
+/// Move the working copy commit to the parent of the current revision.
+/// The command moves you to the parent in a linear fashion.
+///
+/// F @     F
+/// |/      |  
+/// A   =>  A  @
+/// |       | /
+/// B       B
+///
+/// If `edit` is passed as an argument, it will move the working copy commit
+/// directly to the parent.
+///
+/// F @     F
+/// |/      |
+/// C   =>  C
+/// |       |
+/// B       @
+/// |       |
+/// A       A
+// TODO(#NNN): Handle multiple parents, e.g merges.
+#[derive(clap::Args, Clone, Debug)]
+struct PrevArgs {
+    /// How many revisions to move backward. By default moves to the parent.
+    #[default = "1"]
+    amount: usize,
+    /// Edit the parent directly, instead of moving the empty revision.
+    /// This mirrors the behavior of Mercurial and Sapling.
+    edit: bool,
+}
+
 /// Move changes from one revision into another
 ///
 /// Use `--interactive` to move only part of the source revision into the
@@ -2176,6 +2240,75 @@ fn combine_messages(
     Ok(description)
 }
 
+fn cmd_next(ui: &mut Ui, command: &CommandHelper, args: &NextArgs) -> Result<(), CommandError> {
+    let mut workspace_command = command.workspace_helper(ui)?;
+    let child = resolve_destination_revs(
+        &workspace_command,
+        &["@-"],
+        /* allow_large_revsets= */ false,
+    );
+
+    let edit = args.edit;
+    assert!(args.amount == 1 || args.amount > 1);
+    // Handle the simple `jj next` call.
+    if args.amount == 1 {}
+    assert!(args.amount > 1, "Expected to descend to further children");
+
+    if edit {}
+    // TODO(#NNN) We currently cannot deal with multiple children, which resulted
+    // from branches.
+    Ok(())
+}
+
+fn cmd_prev(ui: &mut Ui, command: &CommandHelper, args: &NextArgs) -> Result<(), CommandError> {
+    let mut workspace_command = command.workspace_helper(ui)?;
+    let parent = workspace_command.resolve_single_rev("@-")?;
+    let edit = args.edit;
+    assert!(args.amount == 1 || args.amount > 1);
+    // Handle the simple case of a basic `prev` call.
+    if args.amount == 1 {
+        // TODO(#NNN): Handle multiple parents.
+        if parent.parents().len() > 1 {
+            return Err(user_error(
+                "Revision has multiple parents,see issue #NNN for more info",
+            ));
+        }
+        workspace_command.check_rewritable(&parent)?;
+        let mut tx = workspace_command.start_transaction(format!("prev: {current} -> {parent}"))?;
+        if edit {
+            tx.edit(&parent).unwrap();
+            tx.finish(ui)?;
+            Ok(())
+        }
+        // Move the workspace commit after the parent.
+        let new_ws_revision = tx
+            .mut_repo()
+            .new_commit(
+                command.settings(),
+                target_id.clone(),
+                merged_tree.id().clone(),
+            )
+            .write()?;
+        tx.edit(&new_ws_revision).unwrap();
+        tx.finish(ui)?;
+        Ok(())
+    }
+    assert!(args.amount > 1, "Expected more parents to traverse");
+    let amount = args.amount;
+    // Collect all parents and their parents.
+    let all_parents = cur_rev.parents().map(|parent| parent.id()).collect_vec();
+    let mut tx = workspace_helper.start_transaction("prev: moved {amount} {current} -> {parent}");
+
+    // TODO(#NNN): We currently cannot deal with multiple parents.
+    if parent.parents().len() > 1 {
+        return Err(user_error(
+            "Revision has multiple parents,see issue #NNN for more info",
+        ));
+    }
+    if edit {}
+    Ok(())
+}
+
 fn cmd_move(ui: &mut Ui, command: &CommandHelper, args: &MoveArgs) -> Result<(), CommandError> {
     let mut workspace_command = command.workspace_helper(ui)?;
     let source = workspace_command.resolve_single_rev(args.from.as_deref().unwrap_or("@"))?;
@@ -3403,6 +3536,8 @@ pub fn run_command(
         Commands::Duplicate(sub_args) => cmd_duplicate(ui, command_helper, sub_args),
         Commands::Abandon(sub_args) => cmd_abandon(ui, command_helper, sub_args),
         Commands::Edit(sub_args) => cmd_edit(ui, command_helper, sub_args),
+        Commands::Next(sub_args) => cmd_next(ui, command_helper, sub_args),
+        Commands::Prev(sub_args) => cmd_prev(ui, command_helper, sub_args),
         Commands::New(sub_args) => cmd_new(ui, command_helper, sub_args),
         Commands::Move(sub_args) => cmd_move(ui, command_helper, sub_args),
         Commands::Squash(sub_args) => cmd_squash(ui, command_helper, sub_args),

tests/test_next_prev_commands.rs

@@ -0,0 +1 @@
+// TODO: Write insta tests for `jj next` and `jj prev`