320 lines
10 KiB
C++
320 lines
10 KiB
C++
/* ----------------------------------------------------------------------------
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* GTSAM Copyright 2010, Georgia Tech Research Corporation,
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* Atlanta, Georgia 30332-0415
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* All Rights Reserved
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* Authors: Frank Dellaert, et al. (see THANKS for the full author list)
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* See LICENSE for the license information
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* -------------------------------------------------------------------------- */
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/**
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* @file BayesTreeMarginalizationHelper.h
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* @brief Helper functions for marginalizing variables from a Bayes Tree.
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*
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* @author Jeffrey (Zhiwei Wang)
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* @date Oct 28, 2024
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*/
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// \callgraph
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#pragma once
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#include <unordered_map>
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#include <deque>
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#include <gtsam/inference/BayesTree.h>
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#include <gtsam/inference/BayesTreeCliqueBase.h>
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#include <gtsam/base/debug.h>
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#include "gtsam_unstable/dllexport.h"
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namespace gtsam {
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/**
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* This class provides helper functions for marginalizing variables from a Bayes Tree.
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*/
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template <typename BayesTree>
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class GTSAM_UNSTABLE_EXPORT BayesTreeMarginalizationHelper {
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public:
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using Clique = typename BayesTree::Clique;
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using sharedClique = typename BayesTree::sharedClique;
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/**
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* This function identifies variables that need to be re-eliminated before
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* performing marginalization.
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*
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* Re-elimination is necessary for a clique containing marginalizable
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* variables if:
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*
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* 1. Some non-marginalizable variables appear before marginalizable ones
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* in that clique;
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* 2. Or it has a child node depending on a marginalizable variable AND the
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* subtree rooted at that child contains non-marginalizables.
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*
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* In addition, for any descendant node depending on a marginalizable
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* variable, if the subtree rooted at that descendant contains
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* non-marginalizable variables (i.e., it lies on a path from one of the
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* aforementioned cliques that require re-elimination to a node containing
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* non-marginalizable variables at the leaf side), then it also needs to
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* be re-eliminated.
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*
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* @param[in] bayesTree The Bayes tree
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* @param[in] marginalizableKeys Keys to be marginalized
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* @return Set of additional keys that need to be re-eliminated
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*/
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static std::set<Key> gatherAdditionalKeysToReEliminate(
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const BayesTree& bayesTree,
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const KeyVector& marginalizableKeys) {
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const bool debug = ISDEBUG("BayesTreeMarginalizationHelper");
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std::set<Key> additionalKeys;
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std::set<Key> marginalizableKeySet(
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marginalizableKeys.begin(), marginalizableKeys.end());
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CachedSearch cachedSearch;
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// Check each clique that contains a marginalizable key
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for (const sharedClique& clique :
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getCliquesContainingKeys(bayesTree, marginalizableKeySet)) {
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if (needsReelimination(clique, marginalizableKeySet, &cachedSearch)) {
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// Add frontal variables from current clique
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addCliqueToKeySet(clique, &additionalKeys);
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// Then add the dependent cliques
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for (const sharedClique& dependent :
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gatherDependentCliques(clique, marginalizableKeySet, &cachedSearch)) {
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addCliqueToKeySet(dependent, &additionalKeys);
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}
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}
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}
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if (debug) {
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std::cout << "BayesTreeMarginalizationHelper: Additional keys to re-eliminate: ";
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for (const Key& key : additionalKeys) {
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std::cout << DefaultKeyFormatter(key) << " ";
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}
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std::cout << std::endl;
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}
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return additionalKeys;
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}
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protected:
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/**
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* Gather the cliques containing any of the given keys.
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*
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* @param[in] bayesTree The Bayes tree
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* @param[in] keysOfInterest Set of keys of interest
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* @return Set of cliques that contain any of the given keys
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*/
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static std::set<sharedClique> getCliquesContainingKeys(
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const BayesTree& bayesTree,
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const std::set<Key>& keysOfInterest) {
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std::set<sharedClique> cliques;
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for (const Key& key : keysOfInterest) {
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cliques.insert(bayesTree[key]);
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}
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return cliques;
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}
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/**
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* A struct to cache the results of the below two functions.
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*/
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struct CachedSearch {
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std::unordered_map<Clique*, bool> wholeMarginalizableCliques;
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std::unordered_map<Clique*, bool> wholeMarginalizableSubtrees;
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};
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/**
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* Check if all variables in the clique are marginalizable.
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*
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* Note we use a cache map to avoid repeated searches.
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*/
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static bool isWholeCliqueMarginalizable(
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const sharedClique& clique,
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const std::set<Key>& marginalizableKeys,
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CachedSearch* cache) {
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auto it = cache->wholeMarginalizableCliques.find(clique.get());
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if (it != cache->wholeMarginalizableCliques.end()) {
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return it->second;
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} else {
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bool ret = true;
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for (Key key : clique->conditional()->frontals()) {
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if (!marginalizableKeys.count(key)) {
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ret = false;
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break;
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}
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}
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cache->wholeMarginalizableCliques.insert({clique.get(), ret});
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return ret;
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}
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}
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/**
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* Check if all variables in the subtree are marginalizable.
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*
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* Note we use a cache map to avoid repeated searches.
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*/
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static bool isWholeSubtreeMarginalizable(
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const sharedClique& subtree,
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const std::set<Key>& marginalizableKeys,
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CachedSearch* cache) {
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auto it = cache->wholeMarginalizableSubtrees.find(subtree.get());
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if (it != cache->wholeMarginalizableSubtrees.end()) {
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return it->second;
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} else {
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bool ret = true;
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if (isWholeCliqueMarginalizable(subtree, marginalizableKeys, cache)) {
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for (const sharedClique& child : subtree->children) {
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if (!isWholeSubtreeMarginalizable(child, marginalizableKeys, cache)) {
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ret = false;
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break;
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}
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}
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} else {
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ret = false;
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}
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cache->wholeMarginalizableSubtrees.insert({subtree.get(), ret});
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return ret;
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}
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}
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/**
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* Check if a clique contains variables that need reelimination due to
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* elimination ordering conflicts.
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*
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* @param[in] clique The clique to check
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* @param[in] marginalizableKeys Set of keys to be marginalized
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* @return true if any variables in the clique need re-elimination
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*/
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static bool needsReelimination(
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const sharedClique& clique,
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const std::set<Key>& marginalizableKeys,
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CachedSearch* cache) {
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bool hasNonMarginalizableAhead = false;
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// Check each frontal variable in order
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for (Key key : clique->conditional()->frontals()) {
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if (marginalizableKeys.count(key)) {
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// If we've seen non-marginalizable variables before this one,
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// we need to reeliminate
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if (hasNonMarginalizableAhead) {
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return true;
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}
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// Check if any child depends on this marginalizable key and the
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// subtree rooted at that child contains non-marginalizables.
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for (const sharedClique& child : clique->children) {
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if (hasDependency(child, key) &&
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!isWholeSubtreeMarginalizable(child, marginalizableKeys, cache)) {
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return true;
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}
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}
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} else {
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hasNonMarginalizableAhead = true;
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}
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}
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return false;
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}
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/**
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* Gather all dependent nodes that lie on a path from the root clique
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* to a clique containing a non-marginalizable variable at the leaf side.
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*
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* @param[in] rootClique The root clique
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* @param[in] marginalizableKeys Set of keys to be marginalized
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*/
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static std::set<sharedClique> gatherDependentCliques(
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const sharedClique& rootClique,
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const std::set<Key>& marginalizableKeys,
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CachedSearch* cache) {
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std::vector<sharedClique> dependentChildren;
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dependentChildren.reserve(rootClique->children.size());
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for (const sharedClique& child : rootClique->children) {
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if (hasDependency(child, marginalizableKeys)) {
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dependentChildren.push_back(child);
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}
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}
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return gatherDependentCliquesFromChildren(dependentChildren, marginalizableKeys, cache);
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}
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/**
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* A helper function for the above gatherDependentCliques().
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*/
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static std::set<sharedClique> gatherDependentCliquesFromChildren(
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const std::vector<sharedClique>& dependentChildren,
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const std::set<Key>& marginalizableKeys,
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CachedSearch* cache) {
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std::deque<sharedClique> descendants(
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dependentChildren.begin(), dependentChildren.end());
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std::set<sharedClique> dependentCliques;
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while (!descendants.empty()) {
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sharedClique descendant = descendants.front();
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descendants.pop_front();
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// If the subtree rooted at this descendant contains non-marginalizables,
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// it must lie on a path from the root clique to a clique containing
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// non-marginalizables at the leaf side.
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if (!isWholeSubtreeMarginalizable(descendant, marginalizableKeys, cache)) {
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dependentCliques.insert(descendant);
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}
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// Add all children of the current descendant to the set descendants.
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for (const sharedClique& child : descendant->children) {
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descendants.push_back(child);
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}
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}
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return dependentCliques;
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}
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/**
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* Add all frontal variables from a clique to a key set.
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*
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* @param[in] clique Clique to add keys from
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* @param[out] additionalKeys Pointer to the output key set
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*/
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static void addCliqueToKeySet(
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const sharedClique& clique,
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std::set<Key>* additionalKeys) {
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for (Key key : clique->conditional()->frontals()) {
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additionalKeys->insert(key);
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}
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}
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/**
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* Check if the clique depends on the given key.
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*
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* @param[in] clique Clique to check
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* @param[in] key Key to check for dependencies
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* @return true if clique depends on the key
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*/
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static bool hasDependency(
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const sharedClique& clique, Key key) {
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auto conditional = clique->conditional();
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if (std::find(conditional->beginParents(),
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conditional->endParents(), key)
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!= conditional->endParents()) {
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return true;
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} else {
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return false;
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}
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}
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/**
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* Check if the clique depends on any of the given keys.
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*/
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static bool hasDependency(
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const sharedClique& clique, const std::set<Key>& keys) {
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for (Key key : keys) {
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if (hasDependency(clique, key)) {
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return true;
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}
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}
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return false;
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}
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};
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// BayesTreeMarginalizationHelper
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}/// namespace gtsam
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