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Added some more tree transformation comments to lfsr_rbyd_appendattr

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Hopefully having something to help visualize these tree operations will
help make lfsr_rbyd_appendattr easier to understand.

This one function is probably the most complicated function in littlefs,
but for good reason.
This commit is contained in:
Christopher Haster
2024-04-18 17:12:14 -05:00
parent 77c45827e5
commit 8bfb1be926
1 changed files with 63 additions and 9 deletions
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lfs.c
+63 -9
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@@ -2935,6 +2935,13 @@ trunk:;
}
// do bounds want to take different paths? begin diverging
// >b <b
// .-'| .-'|
// <b => | nb => nb |
// .----'| .--------|--' .-----------' |
// <b <b | <b | nb
// .-'| .-'| | .-'| | .-----'
// 1 2 3 4 1 2 3 4 x 1 2 3 4 x x
bool diverging = lfsr_tag_diverging2(
alt, weight,
p[0].alt, p[0].weight,
@@ -2956,6 +2963,11 @@ trunk:;
diverged = true;
// both diverging? collapse
// <r >b
// .----'| .-'|
// | <b => | |
// | .-'| .-----|--'
// 1 2 3 1 2 3 x
if (diverging && diverging_red) {
LFS_ASSERT(a_rid < b_rid || a_tag < b_tag);
LFS_ASSERT(lfsr_tag_isparallel(alt, p[0].alt));
@@ -2966,6 +2978,13 @@ trunk:;
}
// diverging upper? stitch together both trunks
// >b <b
// .-'| .-'|
// | nb => nb |
// .--------|--' .-----------' |
// | <b | nb
// | .-'| | .-----'
// 1 2 3 4 x 1 2 3 4 x x
if (a_rid > b_rid || a_tag > b_tag) {
lfsr_tag_trim2(
alt, weight,
@@ -2986,9 +3005,13 @@ trunk:;
branch = branch_;
continue;
}
// trim diverging alts so they can be pruned
// diverged?
// : :
// <b => nb
// .-'| .--'
// 3 4 3 4 x
} else if (diverged && diverging) {
// trim so alt is pruned
lfsr_tag_trim(
alt, weight,
&lower_rid, &upper_rid,
@@ -2997,15 +3020,19 @@ trunk:;
}
// prune?
//
// note if only yellow pruning this could be much simpler
// prune unreachable red alts
// <b >b
// .-'| .-'|
// <y | | |
// .-------'| | | |
// | <r | => | <b
// | .----' | .-----------|-'|
// | | <b | <b |
// | | .----'| | .----'| |
// 1 2 3 4 4 1 2 3 4 4 2
// | <r | => | >b
// | .----' | .--------|-'|
// | | <b | <b |
// | | .----'| | .----'| |
// 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,
@@ -3015,12 +3042,28 @@ trunk:;
lfsr_p_pop(p);
}
// prune other unreachable alts
// <b >b
// .-'| .-'|
// <y | | <b
// .-------'| | .-----------|-'|
// | <r | => | | |
// | .----' | | | |
// | | <b | <b |
// | | .----'| | .----'| |
// 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)) {
// prune unreachable red-black alts
// prune unreachable recolorable alts
// : :
// <r => <b
// .----'| .-------'|
// | <b | |
// | .-'| | .-----'
// 1 2 3 1 2 3 x
if (lfsr_tag_isred(p[0].alt)) {
alt = p[0].alt & ~LFSR_TAG_R;
weight = p[0].weight;
@@ -3028,12 +3071,23 @@ trunk:;
lfsr_p_pop(p);
// prune unreachable root alts and red alts
// : :
// <r => <b
// .----'| .----'|
// | <b | |
// | .-'| | .--'
// 3 4 5 3 4 5 x
} else if (!p[0].alt || lfsr_tag_isred(alt)) {
branch = branch_;
continue;
// convert unreachable non-root black alts into alt-nevers,
// if we prune these it would break the coloring of our tree
// if we prune these it would break the color balance of
// our tree
// : :
// <b => nb
// .-'| .--'
// 3 4 3 4 x
} else {
alt = LFSR_TAG_ALT(LFSR_TAG_LE, LFSR_TAG_B, 0);
weight = 0;