/*
sort children by converting linked list to vector,
sorting the vector, and reconstructing to linked list again
Requires node mutex to be held.
*/
void UCTNode::sort_children() {
assert(get_mutex().is_held());
std::vector<std::tuple<float, UCTNode*>> tmp;
UCTNode * child = m_firstchild;
while (child != nullptr) {
tmp.emplace_back(child->get_score(), child);
child = child->m_nextsibling;
}
std::sort(begin(tmp), end(tmp));
m_firstchild = nullptr;
for (auto& sortnode : tmp) {
link_child(std::get<1>(sortnode));
}
}
UCTNode* UCTNode::uct_select_child(int color) {
UCTNode * best = nullptr;
float best_value = -1000.0f;
LOCK(get_mutex(), lock);
// Progressive widening
// int childbound = std::max(2, (int)(((log((double)get_visits()) - 3.0) * 3.0) + 2.0));
int childbound = 362;
int childcount = 0;
UCTNode * child = m_firstchild;
// Count parentvisits.
// We do this manually to avoid issues with transpositions.
int parentvisits = 0;
// Make sure we are at a valid successor.
while (child != nullptr && !child->valid()) {
child = child->m_nextsibling;
}
while (child != nullptr && childcount < childbound) {
parentvisits += child->get_visits();
child = child->m_nextsibling;
// Make sure we are at a valid successor.
while (child != nullptr && !child->valid()) {
child = child->m_nextsibling;
}
childcount++;
}
float numerator = std::sqrt((double)parentvisits);
childcount = 0;
child = m_firstchild;
// Make sure we are at a valid successor.
while (child != nullptr && !child->valid()) {
child = child->m_nextsibling;
}
if (child == nullptr) {
return nullptr;
}
// Prune bad probabilities
// auto parent_log = std::log((float)parentvisits);
// auto cutoff_ratio = cfg_cutoff_offset + cfg_cutoff_ratio * parent_log;
// auto best_probability = child->get_score();
// assert(best_probability > 0.001f);
while (child != nullptr && childcount < childbound) {
// Prune bad probabilities
// if (child->get_score() * cutoff_ratio < best_probability) {
// break;
// }
// get_eval() will automatically set first-play-urgency
float winrate = child->get_eval(color);
float psa = child->get_score();
float denom = 1.0f + child->get_visits();
float puct = cfg_puct * psa * (numerator / denom);
float value = winrate + puct;
assert(value > -1000.0f);
if (value > best_value) {
best_value = value;
best = child;
}
child = child->m_nextsibling;
// Make sure we are at a valid successor.
while (child != nullptr && !child->valid()) {
child = child->m_nextsibling;
}
childcount++;
}
assert(best != nullptr);
return best;
}