revlog: add function to slice chunk down to a given size It is possible to encounter situations where the slicing based on density did not achieve chunk smaller than the 4*textlength limit. To avoid extra memory consumption in those cases, we need to be able to break down chunk to a given size. Actual caller comes in the next changesets.

revlog: extract density based slicing into its own function We are going to introduce another slicing step. We start by extracting the existing one into its own function.

revlog: document and test _slicechunk

revlog: add a doctest for _segmentspan

revlog: add a doctest to _trimchunk

revlog: introduce a tiny mock of a revlog class This will be used in various function doctest added in the next changesets. The class seems small enough to live in the module directly.

revlog: _segmentspan computes the byte span of a segment Refactor out this code to be used somewhere else in a next changeset

revlog: early return in _slicechunk when span is already small enough If the full span is smaller than the minimum gap size we'll consider, we know we won't do any slicing and we can return earlier.

revlog: early return in _slicechunk when density is already good We don't need to do anything if we know we won't enter the second while loop. Save the overhead of the first loop by returning earlier.

sparse-read: discard gap below 65K only Testing on actual data shows that 65K is more efficient in both time and memory than 256K.