/******************************************************************************

 * Top contributors (to current version):

 *   Andrew Reynolds, Gereon Kremer, Haniel Barbosa

 *

 * This file is part of the cvc5 project.

 *

 * Copyright (c) 2009-2021 by the authors listed in the file AUTHORS

 * in the top-level source directory and their institutional affiliations.

 * All rights reserved.  See the file COPYING in the top-level source

 * directory for licensing information.

 * ****************************************************************************

 *

 * The proof-producing equality engine.

 */

#include "cvc5_private.h"

#ifndef CVC5__THEORY__UF__PROOF_EQUALITY_ENGINE_H

#define CVC5__THEORY__UF__PROOF_EQUALITY_ENGINE_H

#include <vector>

#include "context/cdhashmap.h"

#include "context/cdhashset.h"

#include "expr/buffered_proof_generator.h"

#include "expr/lazy_proof.h"

#include "expr/node.h"

#include "theory/eager_proof_generator.h"

namespace cvc5 {

class ProofNode;

class ProofNodeManager;

namespace theory {

namespace eq {

class EqualityEngine;

/**

 * A layer on top of an EqualityEngine. The goal of this class is manage the

 * use of an EqualityEngine object in such a way that the proper proofs are

 * internally constructed, and can be retrieved from this class when

 * necessary.

 *

 * Notice that this class is intended to be a *partial layer* on top of

 * equality engine. A user of this class should still issue low-level calls

 * (getRepresentative, areEqual, areDisequal, etc.) on the underlying equality

 * engine directly. The methods that should *not* be called directly on the

 * underlying equality engine are:

 * - assertEquality/assertPredicate [*]

 * - explain

 * Instead, the user should use variants of the above methods provided by

 * the public interface of this class.

 *

 * [*] the exception is that assertions from the fact queue (who are their own

 * explanation) should be sent directly to the underlying equality engine. This

 * is for the sake of efficiency.

 *

 * This class tracks the reason for why all facts are added to an EqualityEngine

 * in a SAT-context dependent manner in a context-dependent (CDProof) object.

 * It furthermore maintains an internal FactProofGenerator class for managing

 * proofs of facts whose steps are explicitly provided (those that are given

 * concrete PfRule, children, and args). Call these "simple facts".

 *

 * Overall, this class is an eager proof generator (theory/proof_generator.h),

 * in that it stores (copies) of proofs for lemmas at the moment they are sent

 * out.

 *

 * A theory that is proof producing and uses the equality engine may use this

 * class to manage proofs that are justified by its underlying equality engine.

 * In particular, the following interfaces are available for constructing

 * a TrustNode:

 * - assertConflict, when the user of the equality engine has discovered that

 * false can be derived from the current state,

 * - assertLemma, for lemmas/conflicts that can be (partially) explained in the

 * current state,

 * - explain, for explaining why a literal is true in the current state.

 * Details on these methods can be found below.

 */

class ProofEqEngine : public EagerProofGenerator

{

  typedef context::CDHashSet<Node> NodeSet;

  typedef context::CDHashMap<Node, std::shared_ptr<ProofNode>> NodeProofMap;

 public:

  ProofEqEngine(context::Context* c,

                context::UserContext* u,

                EqualityEngine& ee,

                ProofNodeManager* pnm);

  //-------------------------- assert fact

  /**

   * Assert the literal lit by proof step id, given explanation exp and

   * arguments args. This fact is

   *

   * @param lit The literal to assert to the equality engine

   * @param id The proof rule of the proof step concluding lit

   * @param exp The premises of the proof step concluding lit. These are also

   * the premises that are used when calling explain(lit).

   * @param args The arguments to the proof step concluding lit.

   * @return true if this fact was processed by this method. If lit already

   * holds in the equality engine, this method returns false.

   */

  bool assertFact(Node lit,

                  PfRule id,

                  const std::vector<Node>& exp,

                  const std::vector<Node>& args);

  /** Same as above but where exp is (conjunctive) node */

  bool assertFact(Node lit, PfRule id, Node exp, const std::vector<Node>& args);

  /**

   * Multi-step version of assert fact via a proof step buffer. This method

   * is similar to above, but the justification for lit may have multiple steps.

   * In particular, we assume that psb has a list of proof steps where the

   * proof step concluding lit has free assumptions exp.

   *

   * For example, a legal call to this method is such that:

   *   lit: A

   *   exp: B

   *   psb.d_steps: { A by (step id1 {B,C} {}), C by (step id2 {} {}) )

   * In other words, A holds by a proof step with rule id1 and premises

   * B and C, and C holds by proof step with rule id2 and no premises.

   *

   * @param lit The literal to assert to the equality engine.

   * @param exp The premises of the proof steps concluding lit. These are also

   * the premises that are used when calling explain(lit).

   * @param psb The proof step buffer containing the proof steps.

   * @return true if this fact was processed by this method. If lit already

   * holds in the equality engine, this method returns false.

   */

  bool assertFact(Node lit, Node exp, ProofStepBuffer& psb);

  /**

   * Assert fact via generator pg. This method asserts lit with explanation exp

   * to the equality engine of this class. It must be the case that pg can

   * provide a proof for lit in terms of exp. More precisely, pg should be

   * prepared in the remainder of the SAT context to respond to a call to

   * ProofGenerator::getProofFor(lit), and return a proof whose free

   * assumptions are a subset of the conjuncts of exp.

   *

   * @param lit The literal to assert to the equality engine.

   * @param exp The premises of the proof concluding lit. These are also

   * the premises that are used when calling explain(lit).

   * @param pg The proof generator that can provide a proof concluding lit

   * from free asumptions in exp.

   * @return true if this fact was processed by this method. If lit already

   * holds in the equality engine, this method returns false.

   */

  bool assertFact(Node lit, Node exp, ProofGenerator* pg);

  //-------------------------- assert conflicts

  /**

   * This method is called when the equality engine of this class is

   * inconsistent (false has been proven) by a contradictory literal lit. This

   * returns the trust node corresponding to the current conflict.

   *

   * @param lit The conflicting literal, which must rewrite to false.

   * @return The trust node capturing the fact that this class can provide a

   * proof for this conflict.

   */

  TrustNode assertConflict(Node lit);

  /**

   * Get proven conflict from contradictory facts. This method is called when

   * the proof rule with premises exp and arguments args implies a contradiction

   * by proof rule id.

   *

   * This method returns the TrustNode containing the corresponding conflict

   * resulting from adding this step, and ensures that a proof has been stored

   * internally so that this class may respond to a call to

   * ProofGenerator::getProof(...).

   */

  TrustNode assertConflict(PfRule id,

                           const std::vector<Node>& exp,

                           const std::vector<Node>& args);

  /** Generator version, where pg has a proof of false from assumptions exp */

  TrustNode assertConflict(const std::vector<Node>& exp, ProofGenerator* pg);

  //-------------------------- assert lemma

  /**

   * Called when we have concluded conc, typically via theory specific

   * reasoning. The purpose of this method is to construct a TrustNode of

   * kind TrustNodeKind::LEMMA or TrustNodeKind::CONFLICT corresponding to the

   * lemma or conflict to be sent on the output channel of the Theory.

   *

   * The user provides the explanation of conc in two parts:

   * (1) (exp \ noExplain), which are literals that hold in the equality engine

   * of this class,

   * (2) noExplain, which do not necessarily hold in the equality engine of this

   * class.

   * Notice that noExplain is a subset of exp.

   *

   * The proof for conc follows from exp by proof rule with the given

   * id and arguments.

   *

   * This call corresponds to a conflict if conc is false and noExplain is

   * empty.

   *

   * This returns the TrustNode corresponding to the formula corresonding to

   * the call to this method [*], for which a proof can be provided by this

   * generator in the remainder of the user context.

   *

   * [*]

   * a. If this call does not correspond to a conflict, then this formula is:

   *   ( ^_{e in exp \ noExplain} <explain>(e) ^ noExplain ) => conc

   * where <explain>(e) is a conjunction of literals L1 ^ ... ^ Ln such that

   * L1 ^ ... ^ Ln entail e, and each Li was passed as an explanation to a

   * call to assertFact in the current SAT context. This explanation method

   * always succeeds, provided that e is a literal that currently holds in

   * the equality engine of this class. Notice that if the antecedant is empty,

   * the formula above is assumed to be conc itself. The above formula is

   * intended to be valid in Theory that owns this class.

   * b. If this call is a conflict, then this formula is:

   *   ^_{e in exp} <explain>(e)

   * The formula can be queried via TrustNode::getProven in the standard way.

   */

  TrustNode assertLemma(Node conc,

                        PfRule id,

                        const std::vector<Node>& exp,

                        const std::vector<Node>& noExplain,

                        const std::vector<Node>& args);

  /** Generator version, where pg has a proof of conc */

  TrustNode assertLemma(Node conc,

                        const std::vector<Node>& exp,

                        const std::vector<Node>& noExplain,

                        ProofGenerator* pg);

  //-------------------------- explain

  /**

   * Explain literal conc. This calls the appropriate methods in the underlying

   * equality engine of this class to construct the explanation of why conc

   * currently holds.

   *

   * It returns a trust node of kind TrustNodeKind::PROP_EXP whose node

   * is the explanation of conc (a conjunction of literals that implies it).

   * The proof that can be proven by this generator is then (=> exp conc), see

   * TrustNode::getPropExpProven(conc,exp);

   *

   * @param conc The conclusion to explain

   * @return The trust node indicating the explanation of conc and the generator

   * (this class) that can prove the implication.

   */

  TrustNode explain(Node conc);

 private:

  /** Assert internal */

  bool assertFactInternal(TNode pred, bool polarity, TNode reason);

  /** holds */

  bool holds(TNode pred, bool polarity);

  /**

   * Ensure proof for fact. This is called by the above method after we have

   * determined the final set of assumptions used for showing conc. This

   * method is used for lemmas, conflicts, and explanations for propagations.

   * The argument tnk is the kind of trust node to return.

   */

  TrustNode ensureProofForFact(Node conc,

                               const std::vector<TNode>& assumps,

                               TrustNodeKind tnk,

                               ProofGenerator* curr);

  /**

   * This ensures the proof of the literals that are in exp but not in

   * noExplain have been added to curr. This additionally adds the

   * explanation of exp to assumps. It updates tnk to LEMMA if there

   * are any literals in exp that are not in noExplain.

   */

  void explainVecWithProof(TrustNodeKind& tnk,

                           std::vector<TNode>& assumps,

                           const std::vector<Node>& exp,

                           const std::vector<Node>& noExplain,

                           LazyCDProof* curr);

  /** Explain

   *

   * This adds to assumps the set of facts that were asserted to this

   * class in the current SAT context that are required for showing lit.

   *

   * This additionally registers the equality proof steps required to

   * regress the explanation of lit in curr.

   */

  void explainWithProof(Node lit,

                        std::vector<TNode>& assumps,

                        LazyCDProof* curr);

  /** Reference to the equality engine */

  eq::EqualityEngine& d_ee;

  /** The default proof generator (for simple facts) */

  BufferedProofGenerator d_factPg;

  /** common nodes */

  Node d_true;

  Node d_false;

  /** the proof node manager */

  ProofNodeManager* d_pnm;

  /** The SAT-context-dependent proof object */

  LazyCDProof d_proof;

  /**

   * The keep set of this class. This set is maintained to ensure that

   * facts and their explanations are reference counted. Since facts and their

   * explanations are SAT-context-dependent, this set is also

   * SAT-context-dependent.

   */

  NodeSet d_keep;

};

}  // namespace eq

}  // namespace theory

}  // namespace cvc5

#endif /* CVC5__THEORY__STRINGS__PROOF_MANAGER_H */