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rbyd: Fixed eager red pruning ruining balance

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In our pruning logic we eagerly prune root alts and red alts as soon as
we notice they're unreachable.

On paper this is fine, but it's hiding post-split yellow alts from being
recolored to maintain the color balance of the tree...

This is a work in progress...

Test changes:

  test_rbyd+balance before: 273/385878 failed
  test_rbyd+balance after:  306/385878 failed (+12.1%)

Code changes:

           code          stack          ctx
  before: 38536           2624          640
  after:  38528 (-0.0%)   2624 (+0.0%)  640 (+0.0%)
This commit is contained in:
Christopher Haster
2025-01-24 18:41:41 -06:00
parent ae27cdf56b
commit 192779ae84
1 changed files with 46 additions and 9 deletions
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lfs.c
+46 -9
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@@ -3087,6 +3087,9 @@ static int lfsr_rbyd_lookupnext_(lfs_t *lfs, const lfsr_rbyd_t *rbyd,
if (!tag__
|| rid__ < rid
|| (rid__ == rid && tag__ < tag)) {
if (height_) {
LFS_DEBUG("not found height: %d\n", *height_);
}
return LFS_ERR_NOENT;
}
@@ -3516,6 +3519,7 @@ static int lfsr_rbyd_appendrat(lfs_t *lfs, lfsr_rbyd_t *rbyd,
lfsr_srid_t d_upper_rid = rbyd->weight;
lfsr_srid_t d_weight = 0;
lfsr_tag_t d_tag = 0;
lfs_size_t d_branch = 0;
// follow the current trunk
lfs_size_t branch = lfsr_rbyd_trunk(rbyd);
@@ -3645,7 +3649,14 @@ trunk:;
lfsr_rbyd_p_pop(p);
}
// // TODO need this?
// // only diverging red? swap
// diverging = lfsr_tag_diverging2(
// alt, weight,
// p[0].alt, p[0].weight,
// lower_rid, upper_rid,
// a_rid, a_tag,
// b_rid, b_tag);
// diverging_red = lfsr_tag_isred(p[0].alt)
// && lfsr_tag_diverging(
// p[0].alt, p[0].weight,
@@ -3690,7 +3701,7 @@ trunk:;
// && lfsr_tag_isblack(alt)
&& (lfsr_tag_isblack(alt)
// give up if we find a yellow alt
|| lfsr_tag_isred(p[0].alt))
|| (lfsr_tag_isred(p[0].alt)))
&& (diverging || diverging_red)) {
LFS_DEBUG("%04x->%04x: diverging",
branch, lfsr_rbyd_eoff(rbyd));
@@ -3717,6 +3728,14 @@ trunk:;
// p[0].alt = alt | LFSR_TAG_R;
// p[0].weight += weight;
// weight = 0;
// }
// // propagate a red edge?
// if (lfsr_tag_isred(alt) && lfsr_tag_isred(p[0].alt)) {
// LFS_DEBUG("%04x->%04x: recolor",
// branch, lfsr_rbyd_eoff(rbyd));
// lfsr_rbyd_p_recolor(p);
// alt &= ~LFSR_TAG_R;
// }
// diverging upper? stitch together both trunks
@@ -3736,6 +3755,9 @@ trunk:;
lower_rid, upper_rid,
alt, d_tag);
// TODO should we assert we're only diverging here?
// not diverging_red?
// TODO is this uh, how much of this is already in
// the diverging alt?
@@ -3820,9 +3842,9 @@ trunk:;
// 1 2 3 4 4 1 2 3 4 4 1
if (lfsr_tag_isred(p[0].alt)
&& lfsr_tag_unreachable(
p[0].alt, p[0].weight,
lower_rid, upper_rid,
lower_tag, upper_tag)) {
p[0].alt, p[0].weight,
lower_rid, upper_rid,
lower_tag, upper_tag)) {
LFS_DEBUG("%04x->%04x: yprune",
branch, lfsr_rbyd_eoff(rbyd));
alt &= ~LFSR_TAG_R;
@@ -3840,10 +3862,10 @@ trunk:;
// | | .----'| | .----'| |
// 1 2 3 4 4 1 2 3 4 4 2
if (lfsr_tag_unreachable2(
alt, weight,
p[0].alt, p[0].weight,
lower_rid, upper_rid,
lower_tag, upper_tag)) {
alt, weight,
p[0].alt, p[0].weight,
lower_rid, upper_rid,
lower_tag, upper_tag)) {
// prune unreachable recolorable alts
// : :
// <r => <b
@@ -3857,6 +3879,7 @@ trunk:;
jump = p[0].jump;
lfsr_rbyd_p_pop(p);
// TODO redoc
// prune unreachable root alts and red alts
// : :
// <r => <b
@@ -3864,7 +3887,7 @@ trunk:;
// | <b | |
// | .-'| | .--'
// 3 4 5 3 4 5 x
} else if (!p[0].alt || lfsr_tag_isred(alt)) {
} else if (!p[0].alt) { //|| lfsr_tag_isred(alt)) {
branch = branch_;
continue;
@@ -4010,6 +4033,10 @@ trunk:;
return err;
}
// TODO need this? can we instead make the trunk
// unreachable?
d_branch = lfsr_rbyd_eoff(rbyd);
// terminate diverged trunk with an unreachable tag
err = lfsr_rbyd_appendrat_(lfs, rbyd, LFSR_RAT(
(lfsr_rbyd_isshrub(rbyd) ? LFSR_TAG_SHRUB : 0)
@@ -4032,6 +4059,16 @@ trunk:;
// calculation
lower_rid = d_rid;
lower_tag = d_tag;
// // TODO ???
// // needed for balance reasons if we end up reachable
// err = lfsr_rbyd_p_push(lfs, rbyd, p,
// LFSR_TAG_ALT(LFSR_TAG_B, LFSR_TAG_LE, lower_tag),
// 0,
// 0);
// if (err) {
// return err;
// }
}
}