Github/littlefs
1
Fork 0
Code Actions Activity

rbyd-rr: Fixed issue where red alts were just not being pruned

Browse Source 
Not sure how I missed this earlier, but we aren't pruning unreachable/
unavoidable red alts.

There are two cases where we can use red alts to prune. Both cases
effectively collapse a 3-node into a 2-node, while converting isolated
black alts into altns effectively collase a 2-node into a 1-node:

   .---> a rm me
   | .-> b        red prune         .-> b    <-- we weren't handling
  -r-b-> c           =>          ---b-> c        this case correctly

   .---> a                        .---> a
   | .-> b rm me  red prune       |
  -r-b-> c           =>          -b---> c

     .-> a rm me                    v------ altn
   .-b-> b        black flatten   .-b-> b
   | .-> c           =>           | .-> c
  -b-b-> d                       -b-b-> d

Humorously, we were handling the arguably more difficult case of pruning
a black alt following a red alt correctly. But we weren't handling the
case when a red alt itself needs to be pruned.

Fortunately this code is identical to pruning root alts (also arguably a
more tricky case!), so we can just extend the relevant if statement to
cover the case of an unreachable/unavoidable red alt.

And small code change means small code change:

           code          stack
  before: 34288           2864
  after:  34304 (+0.0%)   2864 (+0.0%)
This commit is contained in:
Christopher Haster
2024-04-03 21:11:28 -05:00
parent 37c45e1afc
commit 4f14f3cef4
1 changed files with 7 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lfs.c
+7 -5
View File
@@ -3048,20 +3048,22 @@ again:;
lfs_swap32(&jump, &branch_); lfs_swap32(&jump, &branch_);
} }
// collapse unreachable red alts // prune unreachable red-black alts
if (lfsr_tag_isred(p_alts[0])) { if (lfsr_tag_isred(p_alts[0])) {
alt = p_alts[0] & ~LFSR_TAG_R; alt = p_alts[0] & ~LFSR_TAG_R;
weight = p_weights[0]; weight = p_weights[0];
jump = p_jumps[0]; jump = p_jumps[0];
lfsr_rbyd_p_pop(p_alts, p_weights, p_jumps); lfsr_rbyd_p_pop(p_alts, p_weights, p_jumps);
// collapse unreachable root alts // prune unreachable red alts
} else if (!p_alts[0] && !lfsr_d_isdiverged(d_state)) { } else if (lfsr_tag_isred(alt)
// prune unreachable black alts if root
|| (!p_alts[0] && !lfsr_d_isdiverged(d_state))) {
branch = branch_; branch = branch_;
continue; continue;
// make unreachable non-root black alts alt-nevers, if we // convert unreachable non-root black alts into alt-nevers,
// prune these it would break the coloring of our tree // if we prune these it would break the coloring of our tree
} else { } else {
alt = LFSR_TAG_ALT(LFSR_TAG_LE, LFSR_TAG_B, 0); alt = LFSR_TAG_ALT(LFSR_TAG_LE, LFSR_TAG_B, 0);
weight = 0; weight = 0;