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PR925 + PR956 update #37

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a90137e
Add logit_q to GetQ
AlexisOlson Sep 21, 2019
7db0ea8
Define FastLogit
AlexisOlson Sep 21, 2019
c1cab44
Add logit_Q to GetFpu
AlexisOlson Sep 21, 2019
aeccdcb
Simplification
AlexisOlson Sep 21, 2019
8fd6aaf
Simplification
AlexisOlson Sep 21, 2019
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9 changes: 6 additions & 3 deletions src/mcts/node.h
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,7 @@
#include "chess/position.h"
#include "neural/encoder.h"
#include "neural/writer.h"
#include "utils/fastmath.h"
#include "utils/mutex.h"

namespace lczero {
Expand Down Expand Up @@ -477,8 +478,10 @@ class EdgeAndNode {
Node* node() const { return node_; }

// Proxy functions for easier access to node/edge.
float GetQ(float default_q) const {
return (node_ && node_->GetN() > 0) ? node_->GetQ() : default_q;
float GetQ(float default_q, bool logit_q = false) const {
return (node_ && node_->GetN() > 0)
? (logit_q ? FastLogit(node_->GetQ()) : node_->GetQ())
: default_q;
}
float GetD() const {
return (node_ && node_->GetN() > 0) ? node_->GetD() : 0.0f;
Expand Down Expand Up @@ -521,7 +524,7 @@ class EdgeAndNode {

int GetVisitsToReachU(float target_score, float numerator,
float default_q) const {
const auto q = GetQ(default_q);
const auto q = GetQ(default_q, true);
if (q >= target_score) return std::numeric_limits<int>::max();
const auto n1 = GetNStarted() + 1;
return std::max(
Expand Down
22 changes: 12 additions & 10 deletions src/mcts/search.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -202,11 +202,13 @@ int64_t Search::GetTimeToDeadline() const {
}

namespace {
inline float GetFpu(const SearchParams& params, Node* node, bool is_root_node) {
inline float GetFpu(const SearchParams& params, Node* node,
bool is_root_node, bool logit_q = false) {
const auto value = params.GetFpuValue(is_root_node);
const auto Q = logit_q ? FastLogit(node->GetQ()) : node->GetQ();
return params.GetFpuAbsolute(is_root_node)
? value
: -node->GetQ() - value * std::sqrt(node->GetVisitedPolicy());
: -Q - value * std::sqrt(node->GetVisitedPolicy());
}

inline float ComputeCpuct(const SearchParams& params, uint32_t N) {
Expand All @@ -230,8 +232,8 @@ std::vector<std::string> Search::GetVerboseStats(Node* node,
std::sort(
edges.begin(), edges.end(),
[&fpu, &U_coeff](EdgeAndNode a, EdgeAndNode b) {
return std::forward_as_tuple(a.GetN(), a.GetQ(fpu) + a.GetU(U_coeff)) <
std::forward_as_tuple(b.GetN(), b.GetQ(fpu) + b.GetU(U_coeff));
return std::forward_as_tuple(a.GetN(), a.GetQ(fpu, true) + a.GetU(U_coeff)) <
std::forward_as_tuple(b.GetN(), b.GetQ(fpu, true) + b.GetU(U_coeff));
});

std::vector<std::string> infos;
Expand Down Expand Up @@ -260,7 +262,7 @@ std::vector<std::string> Search::GetVerboseStats(Node* node,
<< ") ";

oss << "(Q+U: " << std::setw(8) << std::setprecision(5)
<< edge.GetQ(fpu) + edge.GetU(U_coeff) << ") ";
<< edge.GetQ(fpu, true) + edge.GetU(U_coeff) << ") ";

oss << "(V: ";
optional<float> v;
Expand Down Expand Up @@ -981,7 +983,7 @@ SearchWorker::NodeToProcess SearchWorker::PickNodeToExtend(
float best = std::numeric_limits<float>::lowest();
float second_best = std::numeric_limits<float>::lowest();
int possible_moves = 0;
const float fpu = GetFpu(params_, node, is_root_node);
const float fpu = GetFpu(params_, node, is_root_node, true);
bool parent_upperbounded = node->IsOnlyUBounded();
for (auto child : node->Edges()) {
if (is_root_node) {
Expand Down Expand Up @@ -1015,7 +1017,7 @@ SearchWorker::NodeToProcess SearchWorker::PickNodeToExtend(
}
++possible_moves;
}
float Q = child.GetQ(fpu);
float Q = child.GetQ(fpu, true);

// Certainty Propagation. Avoid suboptimal childs.
if (params_.GetCertaintyPropagation()) {
Expand Down Expand Up @@ -1243,11 +1245,11 @@ int SearchWorker::PrefetchIntoCache(Node* node, int budget) {
const float cpuct = ComputeCpuct(params_, node->GetN());
const float puct_mult =
cpuct * std::sqrt(std::max(node->GetChildrenVisits(), 1u));
const float fpu = GetFpu(params_, node, node == search_->root_node_);
const float fpu = GetFpu(params_, node, node == search_->root_node_, true);
for (auto edge : node->Edges()) {
if (edge.GetP() == 0.0f) continue;
// Flip the sign of a score to be able to easily sort.
scores.emplace_back(-edge.GetU(puct_mult) - edge.GetQ(fpu), edge);
scores.emplace_back(-edge.GetU(puct_mult) - edge.GetQ(fpu, true), edge);
}

size_t first_unsorted_index = 0;
Expand Down Expand Up @@ -1277,7 +1279,7 @@ int SearchWorker::PrefetchIntoCache(Node* node, int budget) {
if (i != scores.size() - 1) {
// Sign of the score was flipped for sorting, so flip it back.
const float next_score = -scores[i + 1].first;
const float q = edge.GetQ(-fpu);
const float q = edge.GetQ(-fpu, true);
if (next_score > q) {
budget_to_spend =
std::min(budget, int(edge.GetP() * puct_mult / (next_score - q) -
Expand Down
8 changes: 8 additions & 0 deletions src/utils/fastmath.h
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Original file line number Diff line number Diff line change
Expand Up @@ -65,4 +65,12 @@ inline float FastLog(const float a) {
return 0.6931471805599453f * FastLog2(a);
}

// Fast logit for more readable code.
// This is actually 0.5 * logit((a+1)/2) = atanh(a).
inline float FastLogit(const float a) {
// Scale Q slightly to avoid logit(1) = infinity.
float b = 0.9999999f * a;
return 0.5f * FastLog((1.0f + b) / (1.0f - b));
}

} // namespace lczero