From 7e49888becd9e93dab5e5cb59f2ce414ad73e213 Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Wed, 14 Aug 2024 16:40:29 -0700
Subject: [PATCH] More eraseBetween optimizations

---
 ConflictSet.cpp | 44 ++++++++++++++++++++------------------------
 1 file changed, 20 insertions(+), 24 deletions(-)

diff --git a/ConflictSet.cpp b/ConflictSet.cpp
index 5d11e62..19b4793 100644
--- a/ConflictSet.cpp
+++ b/ConflictSet.cpp
@@ -1825,32 +1825,28 @@ void eraseBetween(Node **inTree, Node16 *n, int begin, int end,
 
 void eraseBetween(Node **inTree, Node48 *n, int begin, int end,
                   WriteContext *tls) {
-  const unsigned shiftUpperBound = end - begin;
-  const unsigned shiftAmount = begin;
-  auto inBounds = [&](unsigned c) { return c - shiftAmount < shiftUpperBound; };
-  Node **nodeOut = n->children;
-  uint8_t *indexOut = n->reverseIndex;
-  InternalVersionT *maxVOut = n->childMaxVersion;
-  for (auto &v : n->maxOfMax) {
-    v = tls->zero;
-  }
-  n->bitSet = {};
-  memset(n->index, -1, sizeof(n->index));
-  n->nextFree = 0;
-  for (int i = 0; i < n->numChildren; ++i) {
-    if (inBounds(n->reverseIndex[i])) {
-      eraseTree(n->children[i], tls);
-    } else {
-      *nodeOut++ = n->children[i];
-      *indexOut++ = n->reverseIndex[i];
-      *maxVOut++ = n->childMaxVersion[i];
-      n->maxOfMax[i >> Node48::kMaxOfMaxShift] = std::max(
-          n->maxOfMax[i >> Node48::kMaxOfMaxShift], n->childMaxVersion[i]);
-      n->bitSet.set(n->reverseIndex[i]);
-      n->index[n->reverseIndex[i]] = n->nextFree++;
+  for (int i = n->bitSet.firstSetGeq(begin); i >= 0 && i < end;
+       i = n->bitSet.firstSetGeq(i)) {
+    n->bitSet.reset(i);
+    int8_t toRemoveChildrenIndex = std::exchange(n->index[i], -1);
+    int8_t lastChildrenIndex = --n->nextFree;
+    assert(toRemoveChildrenIndex >= 0);
+    assert(lastChildrenIndex >= 0);
+    eraseTree(n->children[toRemoveChildrenIndex], tls);
+    if (toRemoveChildrenIndex != lastChildrenIndex) {
+      n->children[toRemoveChildrenIndex] = n->children[lastChildrenIndex];
+      n->childMaxVersion[toRemoveChildrenIndex] =
+          n->childMaxVersion[lastChildrenIndex];
+      n->maxOfMax[toRemoveChildrenIndex >> Node48::kMaxOfMaxShift] =
+          std::max(n->maxOfMax[toRemoveChildrenIndex >> Node48::kMaxOfMaxShift],
+                   n->childMaxVersion[toRemoveChildrenIndex]);
+      auto parentIndex = n->children[toRemoveChildrenIndex]->parentsIndex;
+      n->index[parentIndex] = toRemoveChildrenIndex;
+      n->reverseIndex[toRemoveChildrenIndex] = parentIndex;
     }
+    n->childMaxVersion[lastChildrenIndex] = tls->zero;
+    --n->numChildren;
   }
-  n->numChildren = n->nextFree;
 
   if (n->numChildren > Node16::kMaxNodes) {
     // nop