From a7005af6ba6a6506c048ff8e11acf274fdd305d4 Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Tue, 14 May 2024 18:04:35 -0700
Subject: [PATCH] Rotate all the way to the bottom for simplicity

---
 VersionedMap.cpp | 32 +++++++++-----------------------
 1 file changed, 9 insertions(+), 23 deletions(-)

diff --git a/VersionedMap.cpp b/VersionedMap.cpp
index e009279..7481717 100644
--- a/VersionedMap.cpp
+++ b/VersionedMap.cpp
@@ -822,7 +822,6 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
 
     // True if finger is pointing to an entry > than the entry we're removing
     // after we rotate it down
-    bool greaterThan;
 
     // Rotate down until we can remove the entry
     for (;;) {
@@ -831,17 +830,13 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
           child<std::memory_order_relaxed>(node, false, latestVersion);
       const auto r =
           child<std::memory_order_relaxed>(node, true, latestVersion);
-      if (l == 0) {
-        node = r;
-        greaterThan = true;
-        break;
-      } else if (r == 0) {
-        node = l;
-        greaterThan = false;
+      if (l == 0 && r == 0) {
+        // TODO we can avoid some rotations if we stop when l or r == 0
+        node = 0;
         break;
       } else {
-        const bool direction =
-            mm.base[l].entry->priority > mm.base[r].entry->priority;
+        const bool direction = (l == 0 ? 0 : mm.base[l].entry->priority) >
+                               (r == 0 ? 0 : mm.base[r].entry->priority);
         rotate(node, latestVersion, direction);
         assert(node != 0);
         finger.push(
@@ -853,7 +848,8 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
     // propagate up the search path, all the way to the root since we may have
     // more rotations to do even if an update doesn't change a node pointer
     auto node = finger.backNode();
-    const auto oldSize = finger.searchPathSize() - (node == 0);
+    assert(node == 0);
+    const auto oldSize = finger.searchPathSize();
     for (;;) {
       if (finger.searchPathSize() == 1) {
         // Made it to the root
@@ -869,18 +865,8 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
     }
     finger.setSearchPathSizeUnsafe(oldSize);
 
-    if (greaterThan && finger.backNode() != 0) {
-      uint32_t c;
-      while ((c = child<std::memory_order_relaxed>(finger.backNode(), false,
-                                                   latestVersion)) != 0) {
-        finger.push(c, false);
-      }
-    } else {
-      while (finger.searchPathSize() > 1 && finger.backDirection() == true) {
-        finger.pop();
-      }
-      finger.pop();
-    }
+    // finger now points to the insertion point of the node we're removing
+    move<std::memory_order_relaxed>(finger, latestVersion, true);
 
 #ifndef NDEBUG
     if (finger.searchPathSize() > 0) {