Function has_balanced_runs 

fn has_balanced_runs(
    members: impl Iterator<Item = usize>,
    files: &[FileHandle],
    extents: &[Extent],
    tolerance: u64,
) -> bool

Whether the largest sorted run among files holds at most tolerance bytes more than all other runs together. Runs are built greedily in time order, which takes O(K²) conflict checks for K files.