From 4f14f3cef412011e3ff8d6c8401127a7c2a59cd4 Mon Sep 17 00:00:00 2001
From: Christopher Haster <geky@geky.net>
Date: Wed, 3 Apr 2024 21:11:28 -0500
Subject: [PATCH] rbyd-rr: Fixed issue where red alts were just not being
 pruned

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%)
---
 lfs.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/lfs.c b/lfs.c
index 53281bd4..c1a6e54d 100644
--- a/lfs.c
+++ b/lfs.c
@@ -3048,20 +3048,22 @@ again:;
                     lfs_swap32(&jump, &branch_);
                 }
 
-                // collapse unreachable red alts
+                // prune unreachable red-black alts
                 if (lfsr_tag_isred(p_alts[0])) {
                     alt = p_alts[0] & ~LFSR_TAG_R;
                     weight = p_weights[0];
                     jump = p_jumps[0];
                     lfsr_rbyd_p_pop(p_alts, p_weights, p_jumps);
 
-                // collapse unreachable root alts
-                } else if (!p_alts[0] && !lfsr_d_isdiverged(d_state)) {
+                // prune unreachable red alts
+                } else if (lfsr_tag_isred(alt)
+                        // prune unreachable black alts if root
+                        || (!p_alts[0] && !lfsr_d_isdiverged(d_state))) {
                     branch = branch_;
                     continue;
 
-                // make unreachable non-root black alts alt-nevers, if we
-                // prune these it would break the coloring of our tree
+                // convert unreachable non-root black alts into alt-nevers,
+                // if we prune these it would break the coloring of our tree
                 } else {
                     alt = LFSR_TAG_ALT(LFSR_TAG_LE, LFSR_TAG_B, 0);
                     weight = 0;