From 7261c914928a38c06b1ae38e0f45d13276de4c70 Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Tue, 20 Aug 2024 09:51:29 -0700
Subject: [PATCH] Remove Node48::nextFree, and improve padding to save 8 bytes

---
 ConflictSet.cpp | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/ConflictSet.cpp b/ConflictSet.cpp
index ee09ed7..1604587 100644
--- a/ConflictSet.cpp
+++ b/ConflictSet.cpp
@@ -264,17 +264,17 @@ struct Node16 : Node {
 struct Node48 : Node {
   constexpr static auto kType = Type_Node48;
   constexpr static auto kMaxNodes = 48;
-  BitSet bitSet;
-  int8_t nextFree;
-  int8_t index[256];
-  Node *children[kMaxNodes];
-  InternalVersionT childMaxVersion[kMaxNodes];
-  uint8_t reverseIndex[kMaxNodes];
   constexpr static int kMaxOfMaxPageSize = 16;
   constexpr static int kMaxOfMaxShift =
       std::countr_zero(uint32_t(kMaxOfMaxPageSize));
   constexpr static int kMaxOfMaxTotalPages = kMaxNodes / kMaxOfMaxPageSize;
+
+  BitSet bitSet;
+  Node *children[kMaxNodes];
+  InternalVersionT childMaxVersion[kMaxNodes];
   InternalVersionT maxOfMax[kMaxOfMaxTotalPages];
+  uint8_t reverseIndex[kMaxNodes];
+  int8_t index[256];
 
   uint8_t *partialKey() { return (uint8_t *)(this + 1); }
 
@@ -404,7 +404,6 @@ inline void Node48::copyChildrenAndKeyFrom(const Node16 &other) {
   }
   memcpy(partialKey(), &other + 1, partialKeyLen);
   bitSet.init();
-  nextFree = Node16::kMaxNodes;
   int i = 0;
   for (auto x : other.index) {
     bitSet.set(x);
@@ -424,7 +423,6 @@ inline void Node48::copyChildrenAndKeyFrom(const Node48 &other) {
   memcpy((char *)this + kNodeCopyBegin, (char *)&other + kNodeCopyBegin,
          kNodeCopySize);
   bitSet = other.bitSet;
-  nextFree = other.nextFree;
   memcpy(index, other.index, sizeof(index));
   memset(children, 0, sizeof(children));
   const auto z = InternalVersionT::zero;
@@ -451,7 +449,6 @@ inline void Node48::copyChildrenAndKeyFrom(const Node256 &other) {
   for (auto &v : childMaxVersion) {
     v = z;
   }
-  nextFree = other.numChildren;
   bitSet = other.bitSet;
   int i = 0;
   bitSet.forEachSet([&](int c) {
@@ -1419,9 +1416,7 @@ Node *&getOrCreateChild(Node *&self, std::span<const uint8_t> &key,
   insert48:
     auto *self48 = static_cast<Node48 *>(self);
     self48->bitSet.set(index);
-    ++self->numChildren;
-    assert(self48->nextFree < 48);
-    int nextFree = self48->nextFree++;
+    auto nextFree = self48->numChildren++;
     self48->index[index] = nextFree;
     self48->reverseIndex[nextFree] = index;
     auto &result = self48->children[nextFree];
@@ -1812,7 +1807,7 @@ Node *erase(Node *self, WriteContext *tls, ConflictSet::Impl *impl,
     parent48->bitSet.reset(parentsIndex);
     int8_t toRemoveChildrenIndex =
         std::exchange(parent48->index[parentsIndex], -1);
-    int8_t lastChildrenIndex = --parent48->nextFree;
+    auto lastChildrenIndex = --parent48->numChildren;
     assert(toRemoveChildrenIndex >= 0);
     assert(lastChildrenIndex >= 0);
     if (toRemoveChildrenIndex != lastChildrenIndex) {
@@ -1831,8 +1826,6 @@ Node *erase(Node *self, WriteContext *tls, ConflictSet::Impl *impl,
     }
     parent48->childMaxVersion[lastChildrenIndex] = tls->zero;
 
-    --parent->numChildren;
-
     if (needsDownsize(parent48)) {
       downsize(parent48, tls, impl, result);
     }