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// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "cc/mcts_node.h"
#include <algorithm>
#include <cmath>
#include <functional>
#include <tuple>
#include <utility>
#include "absl/strings
8000
/str_format.h"
#include "cc/algorithm.h"
#include "cc/logging.h"
namespace minigo {
namespace {
void InitLegalMoves(MctsNode* node) {
auto& position = node->position;
auto position_hash = position.stone_hash();
auto to_play = position.to_play();
for (int c = 0; c < kN * kN; ++c) {
switch (position.ClassifyMove(c)) {
case Position::MoveType::kIllegal: {
// The move is trivially not legal.
node->legal_moves[c] = false;
break;
}
case Position::MoveType::kNoCapture: {
// The move will not capture any stones: we can calculate the new
// position's stone hash directly.
auto new_hash = position_hash ^ zobrist::MoveHash(c, to_play);
node->legal_moves[c] = !node->HasPositionBeenPlayedBefore(new_hash);
break;
}
case Position::MoveType::kCapture: {
// The move will capture some opponent stones: in order to calculate the
// stone hash, we actually have to play the move.
Position new_position(position);
// It's safe to call AddStoneToBoard instead of PlayMove because:
// - we know the move is not kPass.
// - the move is legal (modulo superko).
// - we only care about new_position's stone_hash and not the rest of
// the bookkeeping that PlayMove updates.
new_position.AddStoneToBoard(c, to_play);
auto new_hash = new_position.stone_hash();
node->legal_moves[c] = !node->HasPositionBeenPlayedBefore(new_hash);
break;
}
}
}
node->legal_moves[Coord::kPass] = true;
}
constexpr int kSuperKoCacheStride = 8;
} // namespace
MctsNode::MctsNode(EdgeStats* stats, const Position& position)
: parent(nullptr), stats(stats), move(Coord::kInvalid), position(position) {
InitLegalMoves(this);
}
MctsNode::MctsNode(MctsNode* parent, Coord move)
: parent(parent),
stats(&parent->edges[move]),
move(move),
position(parent->position) {
position.PlayMove(move);
// Insert a cache of ancestor Zobrist hashes at regular depths in the tree.
// See the comment for superko_cache in the mcts_node.h for more details.
if ((position.n() % kSuperKoCacheStride) == 0) {
superko_cache = absl::make_unique<SuperkoCache>();
superko_cache->reserve(position.n() + 1);
superko_cache->insert(position.stone_hash());
for (auto* node = parent; node != nullptr; node = node->parent) {
if (node->superko_cache != nullptr) {
superko_cache->insert(node->superko_cache->begin(),
node->superko_cache->end());
break;
}
superko_cache->insert(node->position.stone_hash());
}
}
InitLegalMoves(this);
}
Coord MctsNode::GetMostVisitedMove() const {
// Find the set of moves with the largest N.
inline_vector<Coord, kNumMoves> moves;
int best_N = -1;
for (int i = 0; i < kNumMoves; ++i) {
int cn = child_N(i);
if (cn >= best_N) {
if (cn > best_N) {
moves.clear();
best_N = cn;
}
moves.push_back(i);
}
}
MG_CHECK(!moves.empty());
// If there's only one move with the largest N, we're done.
if (moves.size() == 1) {
return moves[0];
}
// Otherwise, break score using the child action score.
float to_play = position.to_play() == Color::kBlack ? 1 : -1;
float U_scale = kPuct * std::sqrt(1.0f + N());
Coord c = moves[0];
float best_cas =
CalculateSingleMoveChildActionScore(to_play, U_scale, moves[0]);
for (int i = 0; i < moves.size(); ++i) {
float cas = CalculateSingleMoveChildActionScore(to_play, U_scale, moves[i]);
if (cas > best_cas) {
best_cas = cas;
c = moves[i];
}
}
return c;
}
std::string MctsNode::Describe() const {
auto child_action_score = CalculateChildActionScore();
using SortInfo = std::tuple<float, float, int>;
std::array<SortInfo, kNumMoves> sort_order;
for (int i = 0; i < kNumMoves; ++i) {
sort_order[i] = SortInfo(child_N(i), child_action_score[i], i);
}
std::sort(sort_order.begin(), sort_order.end(), std::greater<SortInfo>());
auto result = absl::StrFormat(
"%0.4f\n%s\n"
"move : action Q U P P-Dir N soft-N p-delta p-rel",
Q(), MostVisitedPathString());
float child_N_sum = 0;
for (const auto& e : edges) {
child_N_sum += e.N;
}
for (int rank = 0; rank < 15; ++rank) {
Coord i = std::get<2>(sort_order[rank]);
float soft_N = child_N(i) / child_N_sum;
float p_delta = soft_N - child_P(i);
float p_rel = p_delta / child_P(i);
absl::StrAppendFormat(
&result,
"\n%-5s: % 4.3f % 4.3f %0.3f %0.3f %0.3f %5d %0.4f % 6.5f % 3.2f",
i.ToKgs(), child_action_score[i], child_Q(i), child_U(i), child_P(i),
child_original_P(i), static_cast<int>(child_N(i)), soft_N, p_delta,
p_rel);
}
return result;
}
std::vector<Coord> MctsNode::MostVisitedPath() const {
std::vector<Coord> path;
const auto* node = this;
while (!node->children.empty()) {
Coord c = node->GetMostVisitedMove();
if (node->child_N(c) == 0) {
// In cases where nodes have been added to the tree manually (after the
// user has played a move, loading an SGF game), it's possible that no
// children have been visited. Break before adding a spurious node to the
// path.
break;
}
path.push_back(c);
auto it = node->children.find(c);
if (it == node->children.end()) {
// When we reach the move limit, last node will have children with visit
// counts but no children.
break;
}
node = it->second.get();
}
return path;
}
std::string MctsNode::MostVisitedPathString() const {
std::string result;
const auto* node = this;
for (Coord c : MostVisitedPath()) {
auto it = node->children.find(c);
MG_CHECK(it != node->children.end());
node = it->second.get();
absl::StrAppendFormat(&result, "%s (%d) ==> ", node->move.ToKgs(),
static_cast<int>(node->N()));
}
absl::StrAppendFormat(&result, "Q: %0.5f", node->Q());
return result;
}
void MctsNode::GetMoveHistory(
int num_moves, std::vector<const Position::Stones*>* history) const {
history->clear();
history->reserve(num_moves);
const auto* node = this;
for (int j = 0; j < num_moves; ++j) {
history->push_back(&node->position.stones());
node = node->parent;
if (node == nullptr) {
break;
}
}
}
void MctsNode::InjectNoise(const std::array<float, kNumMoves>& noise) {
// NOTE: our interpretation is to only add dirichlet noise to legal moves.
// Because dirichlet entries are independent we can simply zero and rescale.
float scalar = 0;
for (int i = 0; i < kNumMoves; ++i) {
if (legal_moves[i]) {
scalar += noise[i];
}
}
if (scalar > std::numeric_limits<float>::min()) {
scalar = 1.0 / scalar;
}
for (int i = 0; i < kNumMoves; ++i) {
float scaled_noise = scalar * (legal_moves[i] ? noise[i] : 0);
edges[i].P = 0.75f * edges[i].P + 0.25f * scaled_noise;
}
}
MctsNode* MctsNode::SelectLeaf() {
auto* node = this;
for (;;) {
// If a node has never been evaluated, we have no basis to select a child.
if (!node->HasFlag(Flag::kExpanded)) {
return node;
}
// HACK: if last move was a pass, always investigate double-pass first
// to avoid situations where we auto-lose by passing too early.
if (node->position.previous_move() == Coord::kPass &&
node->child_N(Coord::kPass) == 0) {
node = node->MaybeAddChild(Coord::kPass);
continue;
}
auto child_action_score = node->CalculateChildActionScore();
Coord best_move = ArgMax(child_action_score);
node = node->MaybeAddChild(best_move);
}
}
void MctsNode::IncorporateResults(absl::Span<const float> move_probabilities,
float value, MctsNode* up_to) {
MG_DCHECK(move_probabilities.size() == kNumMoves);
// A finished game should not be going through this code path, it should
// directly call BackupValue on the result of the game.
MG_DCHECK(!game_over());
// If the node has already been selected for the next inference batch, we
// shouldn't 'expand' it again.
if (HasFlag(Flag::kExpanded)) {
return;
}
float policy_scalar = 0;
for (int i = 0; i < kNumMoves; ++i) {
if (legal_moves[i]) {
policy_scalar += move_probabilities[i];
}
}
if (policy_scalar > std::numeric_limits<float>::min()) {
policy_scalar = 1 / policy_scalar;
}
SetFlag(Flag::kExpanded);
for (int i = 0; i < kNumMoves; ++i) {
// Zero out illegal moves, and re-normalize move_probabilities.
float move_prob =
legal_moves[i] ? policy_scalar * move_probabilities[i] : 0;
edges[i].original_P = edges[i].P = move_prob;
// Initialize child Q as current node's value, to prevent dynamics where
// if B is winning, then B will only ever explore 1 move, because the Q
// estimation will be so much larger than the 0 of the other moves.
//
// Conversely, if W is winning, then B will explore all 362 moves before
// continuing to explore the most favorable move. This is a waste of
// search.
//
// The value seeded here acts as a prior, and gets averaged into Q
// calculations.
edges[i].W += value;
}
BackupValue(value, up_to);
}
void MctsNode::IncorporateEndGameResult(float value, MctsNode* up_to) {
MG_DCHECK(game_over() || at_move_limit());
MG_DCHECK(!HasFlag(Flag::kExpanded));
BackupValue(value, up_to);
}
void MctsNode::BackupValue(float value, MctsNode* up_to) {
auto* node = this;
for (;;) {
node->stats->W += value;
++node->stats->N;
if (node == up_to) {
return;
}
node = node->parent;
}
}
void MctsNode::AddVirtualLoss(MctsNode* up_to) {
auto* node = this;
for (;;) {
++node->num_virtual_losses_applied;
node->stats->W += node->position.to_play() == Color::kBlack ? 1 : -1;
if (node == up_to) {
return;
}
node = node->parent;
}
}
void MctsNode::RevertVirtualLoss(MctsNode* up_to) {
auto* node = this;
for (;;) {
--node->num_virtual_losses_applied;
node->stats->W -= node->position.to_play() == Color::kBlack ? 1 : -1;
if (node == up_to) {
return;
}
node = node->parent;
}
}
void MctsNode::PruneChildren(Coord c) {
auto child = std::move(children[c]);
children.clear();
children[c] = std::move(child);
}
std::array<float, kNumMoves> MctsNode::CalculateChildActionScore() const {
float to_play = position.to_play() == Color::kBlack ? 1 : -1;
float U_scale = kPuct * std::sqrt(std::max<float>(1, N() - 1));
std::array<float, kNumMoves> result;
for (int i = 0; i < kNumMoves; ++i) {
result[i] = CalculateSingleMoveChildActionScore(to_play, U_scale, i);
}
return result;
}
MctsNode* MctsNode::MaybeAddChild(Coord c) {
auto it = children.find(c);
if (it == children.end()) {
auto child = absl::make_unique<MctsNode>(this, c);
MctsNode* result = child.get();
children[c] = std::move(child);
return result;
} else {
return it->second.get();
}
}
bool MctsNode::HasPositionBeenPlayedBefore(zobrist::Hash stone_hash) const {
for (const auto* node = this; node != nullptr; node = node->parent) {
if (node->superko_cache != nullptr) {
return node->superko_cache->contains(stone_hash);
} else {
if (node->position.stone_hash() == stone_hash) {
return true;
}
}
}
return false;
}
std::string MctsNode::CalculateTreeStats() const {
// TODO(sethtroisi): Make this return a struct instead of string
long num_nodes = 0;
long num_leaf_nodes = 0;
long depth_sum = 0;
long depth_max = 0;
std::function<void(const MctsNode&, int)> traverse =
[&](const MctsNode& node, int depth) {
num_nodes += 1;
num_leaf_nodes += node.N() <= 1;
depth_sum += depth;
if (depth > depth_max) {
depth_max = depth;
}
for (const auto& child : node.children) {
traverse(*child.second.get(), depth + 1);
}
};
traverse(*this, 0);
return absl::StrFormat(
"%d nodes, %d leaf, %.1f average children\n"
"%.1f average depth, %d max depth\n",
num_nodes, num_leaf_nodes,
1.0f * num_nodes / std::max(1L, num_nodes - num_leaf_nodes),
1.0f * depth_sum / num_nodes, depth_max);
}
} // namespace minigo