Use sparse scan under a threshold

ConflictSet.cpp

@@ -741,6 +741,7 @@ int64_t maxBetweenExclusive(Node *n, int begin, int end) {
   assert(end <= 256);
   assert(begin < end);
   int64_t result = std::numeric_limits<int64_t>::lowest();
+  constexpr int kSparseThreshold = 32;
   {
     int c = getChildGeq(n, begin + 1);
     if (c >= 0 && c < end) {
@@ -771,19 +772,35 @@ int64_t maxBetweenExclusive(Node *n, int begin, int end) {
   }
   case Type::Node48: {
     auto *self = static_cast<Node48 *>(n);
-    for (int i = begin + 1; i < end; ++i) {
-      if (self->index[i] != -1) {
-        result = std::max(result, self->children[self->index[i]]->maxVersion);
+    if (self->numChildren < kSparseThreshold) {
+      for (int i = self->bitSet.firstSetGeq(begin + 1); i < end && i >= 0;
+           i = self->bitSet.firstSetGeq(i + 1)) {
+        if (self->index[i] != -1) {
+          result = std::max(result, self->children[self->index[i]]->maxVersion);
+        }
+      }
+    } else {
+      for (int i = begin + 1; i < end; ++i) {
+        if (self->index[i] != -1) {
+          result = std::max(result, self->children[self->index[i]]->maxVersion);
+        }
       }
     }
     break;
   }
   case Type::Node256: {
     auto *self = static_cast<Node256 *>(n);
-    for (int i = begin + 1; i < end; ++i) {
-      if (self->children[i] != nullptr) {
+    if (self->numChildren < kSparseThreshold) {
+      for (int i = self->bitSet.firstSetGeq(begin + 1); i < end && i >= 0;
+           i = self->bitSet.firstSetGeq(i + 1)) {
         result = std::max(result, self->children[i]->maxVersion);
       }
+    } else {
+      for (int i = begin + 1; i < end; ++i) {
+        if (self->children[i] != nullptr) {
+          result = std::max(result, self->children[i]->maxVersion);
+        }
+      }
     }
     break;
   }