/*
* Progress condition: lock-free
*
* The dequeue() marks the node that has the item as "logically removed"
* by setting the "marked" bit in node.next
* By default, the "head" is pointing to the first node that has not been
* "logically removed", but if it's the last node (node.next is nullptr),
* then the head will be pointing to the last "logically removed" node.
*/
T* dequeue(const int tid) {
HPGuard hpguard { hp, tid }; // RAII to call hp.clear(tid) when returning
while (true) {
Node* lhead = hp.protectPtr(kHpHead, head.load(), tid);
if (lhead != head.load()) continue;
Node* lcurr = lhead;
for (int i = 0; ;) {
Node* lnext = lcurr->next.load();
if (lnext == getMarked(nullptr)) {
if (lhead != lcurr && casHead(lhead, lcurr)) retireSubList(lhead, lcurr, tid);
return nullptr; // Queue is empty
}
if (isMarked(lnext)) {
hp.protectPtr(kHpNext+(i&0x1), getUnmarked(lnext), tid); // Alternate hps during traversal
if (lhead != head.load()) break;
lcurr = getUnmarked(lnext);
i++;
continue;
}
if (!lcurr->next.compare_exchange_strong(lnext, getMarked(lnext))) continue;
T* item = lcurr->item;
if (lcurr != lhead && casHead(lhead, lcurr)) retireSubList(lhead, lcurr, tid);
return item;
}
}
}
/*
* Progress condition: lock-free
*
* If we don't know maxThreads, we can replace the for() loop with a
* while(true) and it will still be correct.
*/
void enqueue(T* item, const int tid) {
if (item == nullptr) throw std::invalid_argument("item can not be nullptr");
HPGuard hpguard { hp, tid }; // RAII to call hp.clear(tid) when returning
Node* newNode = new Node(item);
while (true) {
Node* ltail = hp.protectPtr(kHpTail, tail.load(), tid);
if (ltail != tail.load()) continue;
Node* lnext = ltail->next.load();
if (getUnmarked(lnext) != nullptr) { // Advance the tail first
casTail(ltail, getUnmarked(lnext)); // "tail" is always unmarked
} else {
for (int i=0; i < 2; i++) {
Node* newNodeMark = isMarked(lnext) ? getMarked(newNode) : newNode; // lnext here is either nullptr or nullptr|0x1
newNode->next.store(nullptr, std::memory_order_relaxed);
if (ltail->next.compare_exchange_strong(lnext, newNodeMark)) {
casTail(ltail, newNode);
return;
}
lnext = ltail->next.load();
if (getUnmarked(lnext) != nullptr) {
casTail(ltail, getUnmarked(lnext)); // "tail" is always unmarked
break;
}
}
}
for (int i = 0; i < maxThreads-1; i++) { // This loop will run at most maxThreads because the CAS can fail at most maxThreads
lnext = ltail->next.load();
if (isMarked(lnext)) break; // This node has been dequeued, must re-read tail. It's ok to be marked as long as it's the first and therefore, nullptr
newNode->next.store(lnext, std::memory_order_relaxed);
if (ltail->next.compare_exchange_strong(lnext, newNode)) return;
}
}
}
BitNextLazyHeadQueue(int maxThreads=MAX_THREADS) : maxThreads{maxThreads} {
Node* sentinelNode = new Node(nullptr);
// The sentinel is already "logically removed"
sentinelNode->next.store(getMarked(nullptr), std::memory_order_relaxed);
head.store(sentinelNode, std::memory_order_relaxed);
tail.store(sentinelNode, std::memory_order_relaxed);
}
/**
* This method is named 'Delete()' in the original paper.
* Taken from Figure 7 of the paper:
* "High Performance Dynamic Lock-Free Hash Tables and List-Based Sets"
*/
bool remove(T* key, const int tid)
{
Node *curr, *next;
std::atomic<Node*> *prev;
while (true) {
/* Try to find the key in the list. */
if (!find(key, &prev, &curr, &next, tid)) {
return false;
}
/* Mark if needed. */
Node *tmp = getUnmarked(next);
if (!curr->next.compare_exchange_strong(tmp, getMarked(next))) {
continue; /* Another thread interfered. */
}
tmp = getUnmarked(curr);
prev->compare_exchange_strong(tmp, getUnmarked(next)); /* Unlink */
/*
* If we want to prevent the possibility of there being an
* unbounded number of unmarked nodes, add "else _find(head,key)."
* This is not necessary for correctness.
*/
return true;
}
}
string Polynomial::toString(bool latex/*, bool mathMode*/)const
{
stringstream s;
/*
if(latex && !mathMode)
s << "$";
*/
bool first=true;
if(terms.empty())
{
s << "0";
return s.str();
}
// If the polynomial has a marked term it is written first
// printString("_");
IntegerVector e=getMarked().m.exponent;
for(TermMap::const_iterator i=terms.begin();i!=terms.end();i++)
if(e==i->first.exponent)
{
s << i->second.toString(i->first.exponent.isZero(),!first,latex);
if((!i->first.exponent.isZero())&&(!i->second.isOne())&&(!(-(i->second)).isOne()))s<<"*";
s << i->first.toString(false,false,latex);
first=false;
}
// printString("_");
for(TermMap::const_iterator i=terms.begin();i!=terms.end();i++)
if(e!=i->first.exponent)
{
s << i->second.toString(i->first.exponent.isZero(),!first,latex);
if((!i->first.exponent.isZero())&&(!i->second.isOne())&&(!(-(i->second)).isOne()))s<<"*";
s << i->first.toString(false,false,latex);
/* printFieldElement(i->second,i->first.exponent.isZero(),!first);
if((!i->first.exponent.isZero())&&(!i->second.isOne())&&(!(-(i->second)).isOne()))printString("*");
printMonomial(i->first,false,false);*/
first=false;
}
/* if(latex && !mathMode)
s << "$";
*/
return s.str();
}
