How to get last n ancestors as revset

[sullyj3]

I'm trying to figure out how to do the equivalent of git log -n 5. I came up with jj log -r @-----::@. Is there a better way? (This method clearly doesn't scale)

[martinvonz]

The revset engine already knows how to do it, so I think it's just a matter of updating the ancestors() function to accept a generation range (@yuja can correct me if I'm wrong). That code is here:

jj/lib/src/revset.rs

Lines 946 to 950 in b19bf37

	map.insert("ancestors", |name, arguments_pair, state| {
	let arg = expect_one_argument(name, arguments_pair)?;
	let expression = parse_expression_rule(arg.into_inner(), state)?;
	Ok(expression.ancestors())
	});

I'm not sure exactly what the syntax should be. Mercurial has ancestors(x, n) to give up to n generations up to the specified distance. I think it makes sense to copy that. We could also accept ancestors(x, m, n) to also allow a lower bound on the number of generation (i.e. making ancestors(x, n) equal to ancestors(x, 0, n)).

There's also support in the revset engine for descendants in a specified range, so it shouldn't be hard to add syntax for that too.

Do you feel like implementing any of that?

[sullyj3]

I'll have a read through of the code and see if I feel capable of tackling it

[yuja]

Yep, it's just a matter of syntax. Abusing range expression ancestors(x, m..n) or optional start=m/end=n parameters ancestors(x, m, n).

[sullyj3]

PR at #2200