pub(crate) fn newest_fifths<'a>(
ids: &[usize],
group: impl Fn(usize) -> (&'a str, bool),
) -> (Vec<usize>, Vec<usize>)Expand description
The oldest 80 % and the newest 20 % of each group of ids (in time
order): an account’s ham, its spam; an outside source’s. Every group is
on both sides, so that a group whose mail all came late (a provider that
keeps spam a month) is learned from too, and none is told apart by when it came.