forked from Imagelibrary/littlefs
8732904ef6
B-tree remove/merge is the most annoying part of B-trees. The implementation here follows the same ideas implemented in push/split: 1. Defer splits/merges until compaction. 2. Assume our split/merge will succeed and play it out into the rbyd. 3. On the first sign of failure, revert any unnecessary changes by appending deletes. 4. Do all of this in a single commit to avoid issues with single-prog blocks. Mapping this onto B-tree merge, the condition that triggers merge is when our rbyd is <1/4 the block_size after compaction, and the condition that aborts a merge is when our rbyd is >1/2 the block_size, since that would trigger a split on a later compact. Weaving this into lfsr_btree_commit is a bit subtle, but relatively straightforward all things considered. One downside is it's not physically possible to try merging with both siblings, so we have to choose just one to attempt a merge. We handle the corner case of merging the last sibling in a block explicitly, and in theory the other sibling will eventually trigger a merge during its own compaction. Extra annoying are the corner cases with merges in the root rbyd that make the root rbyd degenerate. We really should avoid a compaction in this case, as otherwise we would erase a block that we immediately inline at a significant cost. However determining if our root rbyd is degenerate is tricky. We can determine a degenerate root with children by checking if our rbyd's weight matches the B-tree's weight when we merge. But determining a degenerate root that is a leaf requires manually looking up both children in lfsr_btree_pop to see if they will result in a degenerate root. Ugh. On the bright side, this does all seem to be working now. Which completes the last of the core B-tree algorithms.