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

 * Top contributors (to current version):

 *   Andrew Reynolds, Andres Noetzli, Tianyi Liang

 *

 * 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.

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

 *

 * Core solver for the theory of strings, responsible for reasoning

 * string concatenation plus length constraints.

 */

#include "cvc5_private.h"

#ifndef CVC5__THEORY__STRINGS__CORE_SOLVER_H

#define CVC5__THEORY__STRINGS__CORE_SOLVER_H

#include "context/cdhashset.h"

#include "context/cdlist.h"

#include "theory/strings/base_solver.h"

#include "theory/strings/infer_info.h"

#include "theory/strings/inference_manager.h"

#include "theory/strings/normal_form.h"

#include "theory/strings/solver_state.h"

#include "theory/strings/term_registry.h"

namespace cvc5 {

namespace theory {

namespace strings {

/**

 * This data structure encapsulates an inference for the core solver of the

 * theory of strings. This includes the form of the inference to be processed

 * by the inference manager, the side effects it generates for the core solver,

 * and information used for heuristics and debugging.

 */

{

 public:

  CoreInferInfo(InferenceId id);

  /** The infer info of this class */

  InferInfo d_infer;

  /**

   * The pending phase requirements, see InferenceManager::sendPhaseRequirement.

   */

  std::map<Node, bool> d_pendingPhase;

  /**

   * The index in the normal forms under which this inference is addressing.

   * For example, if the inference is inferring x = y from |x|=|y| and

   *   w ++ x ++ ... = w ++ y ++ ...

   * then d_index is 1, since x and y are at index 1 in these concat terms.

   */

  unsigned d_index;

  /**

   * The normal form pair that is cached as a result of this inference.

   */

  Node d_nfPair[2];

  /** for debugging

   *

   * The base pair of strings d_i/d_j that led to the inference, and whether

   * (d_rev) we were processing the normal forms of these strings in reverse

   * direction.

   */

  Node d_i;

  Node d_j;

  bool d_rev;

};

/** The core solver for the theory of strings

 *

 * This implements techniques for handling (dis)equalities involving

 * string concatenation terms based on the procedure by Liang et al CAV 2014.

 */

class CoreSolver

{

  friend class InferenceManager;

  using NodeIntMap = context::CDHashMap<Node, int>;

 public:

  CoreSolver(SolverState& s,

             InferenceManager& im,

             TermRegistry& tr,

             BaseSolver& bs);

  ~CoreSolver();

  //-----------------------inference steps

  /** check cycles

   *

   * This inference schema ensures that a containment ordering < over the

   * string equivalence classes is acyclic. We define this ordering < such that

   * for equivalence classes e1 = { t1...tn } and e2 = { s1...sm }, e1 < e2

   * if there exists a ti whose flat form (see below) is [w1...sj...wk] for

   * some i,j. If e1 < ... < en < e1 for some chain, we infer that the flat

   * form components that do not constitute this chain, e.g. (w1...wk) \ sj

   * in the flat form above, must be empty.

   *

   * For more details, see the inference S-Cycle in Liang et al CAV 2014.

   */

  void checkCycles();

  /** check flat forms

   *

   * This applies an inference schema based on "flat forms". The flat form of a

   * string term t is a vector of representative terms [r1, ..., rn] such that

   * t is of the form t1 ++ ... ++ tm and r1 ++ ... ++ rn is equivalent to

   * rewrite( [t1] ++ ... ++ [tm] ), where [t1] denotes the representative of

   * the equivalence class containing t1. For example, if t is y ++ z ++ z,

   * E is { y = "", w = z }, and w is the representative of the equivalence

   * class { w, z }, then the flat form of t is [w, w]. Say t1 and t2 are terms

   * in the same equivalence classes with flat forms [r1...rn] and [s1...sm].

   * We may infer various facts based on this pair of terms. For example:

   *   ri = si, if ri != si, rj == sj for each j < i, and len(ri)=len(si),

   *   rn = sn, if n=m and rj == sj for each j < n,

   *   ri = empty, if n=m+1 and ri == rj for each i=1,...,m.

   * We refer to these as "unify", "endpoint-eq" and "endpoint-emp" inferences

   * respectively.

   *

   * Notice that this inference scheme is an optimization and not needed for

   * model-soundness. The motivation for this schema is that it is simpler than

   * checkNormalFormsEq, which can be seen as a recursive version of this

   * schema (see difference of "normal form" vs "flat form" below), and

   * checkNormalFormsEq is complete, in the sense that if it passes with no

   * inferences, we are ensured that all string equalities in the current

   * context are satisfied.

   *

   * Must call checkCycles before this function in a strategy.

   */

  void checkFlatForms();

  /** check normal forms equalities

   *

   * This applies an inference schema based on "normal forms". The normal form

   * of an equivalence class of string terms e = {t1, ..., tn} union {x1....xn},

   * where t1...tn are concatenation terms is a vector of representative terms

   * [r1, ..., rm] such that:

   * (1) if n=0, then m=1 and r1 is the representative of e,

   * (2) if n>0, say

   *   t1 = t^1_1 ++ ... ++ t^1_m_1

   *   ...

   *   tn = t^1_n ++ ... ++ t^_m_n

   * for *each* i=1, ..., n, the result of concenating the normal forms of

   * t^1_1 ++ ... ++ t^1_m_1 is equal to [r1, ..., rm]. If an equivalence class

   * can be assigned a normal form, then all equalities between ti and tj are

   * satisfied by all models that correspond to extensions of the current

   * assignment. For further detail on this terminology, see Liang et al

   * CAV 2014.

   *

   * Notice that all constant words are implicitly considered concatentation

   * of their characters, e.g. "abc" is treated as "a" ++ "b" ++ "c".

   *

   * At a high level, we build normal forms for equivalence classes bottom-up,

   * starting with equivalence classes that are minimal with respect to the

   * containment ordering < computed during checkCycles. While computing a

   * normal form for an equivalence class, we may infer equalities between

   * components of strings that must be equal (e.g. x=y when x++z == y++w when

   * len(x)==len(y) is asserted), derive conflicts if two strings have disequal

   * prefixes/suffixes (e.g. "a" ++ x == "b" ++ y is a conflict), or split

   * string terms into smaller components using fresh skolem variables (see

   * Inference values with names "SPLIT"). We also may introduce regular

   * expression constraints in this method for looping word equations (see

   * the Inference INFER_FLOOP).

   *

   * If this inference schema returns no facts, lemmas, or conflicts, then

   * we have successfully assigned normal forms for all equivalence classes, as

   * stored in d_normal_forms. Otherwise, this method may add a fact, lemma, or

   * conflict based on inferences in the Inference enumeration above.

   */

  void checkNormalFormsEq();

  /** check normal forms disequalities

   *

   * This inference schema can be seen as the converse of the above schema. In

   * particular, it ensures that each pair of distinct equivalence classes

   * e1 and e2 have distinct normal forms.

   *

   * This method considers all pairs of distinct equivalence classes (e1,e2)

   * such that len(x1)==len(x2) is asserted for some x1 in e1 and x2 in e2. It

   * then traverses the normal forms of x1 and x2, say they are [r1, ..., rn]

   * and [s1, ..., sm]. For the minimial i such that ri!=si, if ri and si are

   * disequal and have the same length, then x1 and x2 have distinct normal

   * forms. Otherwise, we may add splitting lemmas on the length of ri and si,

   * or split on an equality between ri and si.

   *

   * If this inference schema returns no facts, lemmas, or conflicts, then all

   * disequalities between string terms are satisfied by all models that are

   * extensions of the current assignment.

   */

  void checkNormalFormsDeq();

  /** check lengths for equivalence classes

   *

   * This inference schema adds lemmas of the form:

   *   E => len( x ) = rewrite( len( r1 ++ ... ++ rn ) )

   * where [r1, ..., rn] is the normal form of the equivalence class containing

   * x. This schema is not required for correctness but experimentally has

   * shown to be helpful.

   */

  void checkLengthsEqc();

  //-----------------------end inference steps

  //--------------------------- query functions

  /**

   * Get normal form for string term n. For details on this data structure,

   * see theory/strings/normal_form.h.

   *

   * This query is valid after a successful call to checkNormalFormsEq, e.g.

   * a call where the inference manager was not given any lemmas or inferences.

   */

  NormalForm& getNormalForm(Node n);

  /** get normal string

   *

   * This method returns the node that is equivalent to the normal form of x,

   * and adds the corresponding explanation to nf_exp.

   *

   * For example, if x = y ++ z is an assertion in the current context, then

   * this method returns the term y ++ z and adds x = y ++ z to nf_exp.

   */

  Node getNormalString(Node x, std::vector<Node>& nf_exp);

  //-------------------------- end query functions

  /**

   * This returns the conclusion of the proof rule corresponding to splitting

   * on the arrangement of terms x and y appearing in an equation of the form

   *   x ++ x' = y ++ y' or x' ++ x = y' ++ y

   * where we are in the second case if isRev is true. This method is called

   * both by the core solver and by the strings proof checker.

   *

   * @param x The first term

   * @param y The second term

   * @param rule The proof rule whose conclusion we are asking for

   * @param isRev Whether the equation is in a reverse direction

   * @param skc The skolem cache (to allocate fresh variables if necessary)

   * @param newSkolems The vector to add new variables to

   * @return The conclusion of the inference.

   */

  static Node getConclusion(Node x,

                            Node y,

                            PfRule rule,

                            bool isRev,

                            SkolemCache* skc,

                            std::vector<Node>& newSkolems);

  /**

   * Get sufficient non-empty overlap of string constants c and d.

   *

   * This is called when handling equations of the form:

   *   x ++ d ++ ... = c ++ ...

   * when x is non-empty and non-constant.

   *

   * This returns the maximal index in c which x must have as a prefix, which

   * notice is an integer >= 1 since x is non-empty.

   *

   * @param c The first constant

   * @param d The second constant

   * @param isRev Whether the equation is in the reverse direction

   * @return The position in c.

   */

  static size_t getSufficientNonEmptyOverlap(Node c, Node d, bool isRev);

  /**

   * This returns the conclusion of the decompose proof rule. This returns

   * a conjunction of splitting string x into pieces based on length l, e.g.:

   *   x = k_1 ++ k_2

   * where k_1 (resp. k_2) is a skolem corresponding to a substring of x of

   * length l if isRev is false (resp. true). The function also adds a

   * length constraint len(k_1) = l (resp. len(k_2) = l). Note that adding this

   * constraint to the conclusion is *not* optional, since the skolems k_1 and

   * k_2 may be shared, hence their length constraint must be guarded by the

   * premises of this inference.

   *

   * @param x The string term

   * @param l The length term

   * @param isRev Whether the equation is in a reverse direction

   * @param skc The skolem cache (to allocate fresh variables if necessary)

   * @param newSkolems The vector to add new variables to

   * @return The conclusion of the inference.

   */

  static Node getDecomposeConclusion(Node x,

                                     Node l,

                                     bool isRev,

                                     SkolemCache* skc,

                                     std::vector<Node>& newSkolems);

 private:

  /**

   * This processes the infer info ii as an inference. In more detail, it calls

   * the inference manager to process the inference, and updates the set of

   * normal form pairs. Returns true if the conclusion of ii was not true

   * after rewriting. If the conclusion is true, this method does nothing.

   */

  bool processInferInfo(CoreInferInfo& ii);

  /** Add that (n1,n2) is a normal form pair in the current context. */

  void addNormalFormPair(Node n1, Node n2);

  /** Is (n1,n2) a normal form pair in the current context? */

  bool isNormalFormPair(Node n1, Node n2);

  //--------------------------for checkFlatForm

  /**

   * This checks whether there are flat form inferences between eqc[start] and

   * eqc[j] for some j>start. If the flag isRev is true, we check for flat form

   * interferences in the reverse direction of the flat forms (note:

   * `d_flat_form` and `d_flat_form_index` must be in reverse order if `isRev`

   * is true). For more details, see checkFlatForms below.

   */

  void checkFlatForm(std::vector<Node>& eqc, size_t start, bool isRev);

  /** debug print current flat forms on trace tc */

  void debugPrintFlatForms(const char* tc);

  //--------------------------end for checkFlatForm

  //--------------------------for checkCycles

  Node checkCycles(Node eqc, std::vector<Node>& curr, std::vector<Node>& exp);

  //--------------------------end for checkCycles

  //--------------------------for checkNormalFormsEq

  /** normalize equivalence class

   *

   * This method attempts to build a "normal form" for the equivalence class

   * of string term n (for more details on normal forms, see normal_form.h

   * or see Liang et al CAV 2014). In particular, this method checks whether the

   * current normal form for each term in this equivalence class is identical.

   * If it is not, then we add an inference via sendInference and abort the

   * call.

   *

   * stype is the string-like type of the equivalence class we are processing.

   */

  void normalizeEquivalenceClass(Node n, TypeNode stype);

  /**

   * For each term in the equivalence class of eqc, this adds data regarding its

   * normal form to normal_forms. The map term_to_nf_index maps terms to the

   * index in normal_forms where their normal form data is located.

   *

   * stype is the string-like type of the equivalence class we are processing.

   */

  void getNormalForms(Node eqc,

                      std::vector<NormalForm>& normal_forms,

                      std::map<Node, unsigned>& term_to_nf_index,

                      TypeNode stype);

  /** process normalize equivalence class

   *

   * This is called when an equivalence class eqc contains a set of terms that

   * have normal forms given by the argument normal_forms. It either

   * verifies that all normal forms in this vector are identical, or otherwise

   * adds a conflict, lemma, or inference via the sendInference method.

   *

   * To prioritize one inference versus another, it builds a set of possible

   * inferences, at most two for each pair of distinct normal forms,

   * corresponding to processing the normal form pair in the (forward, reverse)

   * directions. Once all possible inferences are recorded, it executes the

   * one with highest priority based on the enumeration type Inference.

   *

   * stype is the string-like type of the equivalence class we are processing.

   */

  void processNEqc(Node eqc,

                   std::vector<NormalForm>& normal_forms,

                   TypeNode stype);

  /** process simple normal equality

   *

   * This method is called when two equal terms have normal forms nfi and nfj.

   * It adds (typically at most one) possible inference to the vector pinfer.

   * This inference is in the form of an CoreInferInfo object, which stores the

   * necessary information regarding how to process the inference.

   *

   * index: The index in the normal form vectors (nfi.d_nf and nfj.d_nf) that

   *   we are currently checking. This method will increment this index until

   *   it finds an index where these vectors differ, or until it reaches the

   *   end of these vectors.

   * isRev: Whether we are processing the normal forms in reverse direction.

   *   Notice in this case the normal form vectors have been reversed, hence,

   *   many operations are identical to the forward case, e.g. index is

   *   incremented not decremented, while others require special care, e.g.

   *   constant strings "ABC" in the normal form vectors are not reversed to

   *   "CBA" and hence all operations should assume a flipped semantics for

   *   constants when isRev is true,

   * rproc: the number of string components on the suffix of the normal form of

   *   nfi and nfj that were already processed. This is used when using

   *   fowards/backwards traversals of normal forms to ensure that duplicate

   *   inferences are not processed.

   * pinfer: the set of possible inferences we add to.

   *

   * stype is the string-like type of the equivalence class we are processing.

   */

  void processSimpleNEq(NormalForm& nfi,

                        NormalForm& nfj,

                        unsigned& index,

                        bool isRev,

                        unsigned rproc,

                        std::vector<CoreInferInfo>& pinfer,

                        TypeNode stype);

  //--------------------------end for checkNormalFormsEq

  //--------------------------for checkNormalFormsEq with loops

  bool detectLoop(NormalForm& nfi,

                  NormalForm& nfj,

                  int index,

                  int& loop_in_i,

                  int& loop_in_j,

                  unsigned rproc);

  /**

   * Result of processLoop() below.

   */

  enum class ProcessLoopResult

  {

    /** Loop processing made an inference */

    INFERENCE,

    /** Loop processing detected a conflict */

    CONFLICT,

    /** Loop not processed or no loop detected */

    SKIPPED,

  };

  ProcessLoopResult processLoop(NormalForm& nfi,

                                NormalForm& nfj,

                                int loop_index,

                                int index,

                                CoreInferInfo& info);

  //--------------------------end for checkNormalFormsEq with loops

  //--------------------------for checkNormalFormsDeq

  /**

   * Given a pair of disequal strings with the same length, checks whether the

   * disequality holds. This may result in inferences or conflicts.

   *

   * @param n1 The first string in the disequality

   * @param n2 The second string in the disequality

   */

  void processDeq(Node n1, Node n2);

  /**

   * Given a pair of disequal strings with the same length and their normal

   * forms, checks whether the disequality holds. This may result in

   * inferences.

   *

   * @param nfi The normal form for the first string in the disequality

   * @param nfj The normal form for the second string in the disequality

   * @param ni The first string in the disequality

   * @param nj The second string in the disequality

   * @return true if the disequality is satisfied, false otherwise

   */

  bool processReverseDeq(std::vector<Node>& nfi,

                         std::vector<Node>& nfj,

                         Node ni,

                         Node nj);

  /**

   * Given a pair of disequal strings with the same length and their normal

   * forms, performs some simple checks whether the disequality holds. The

   * check is done starting from a given index and can either be performed on

   * reversed normal forms or the original normal forms. If the function cannot

   * show that a disequality holds, it updates the index to point to the first

   * element in the normal forms for which the relationship is unclear.

   *

   * @param nfi The normal form for the first string in the disequality

   * @param nfj The normal form for the second string in the disequality

   * @param ni The first string in the disequality

   * @param nj The second string in the disequality

   * @param index The index to start at. If this function returns false, the

   *              index points to the first index in the normal forms for which

   *              it is not known whether they are equal or disequal

   * @param isRev This should be true if the normal forms are reversed, false

   *              otherwise

   * @return true if the disequality is satisfied, false otherwise

   */

  bool processSimpleDeq(std::vector<Node>& nfi,

                        std::vector<Node>& nfj,

                        Node ni,

                        Node nj,

                        size_t& index,

                        bool isRev);

  //--------------------------end for checkNormalFormsDeq

  /** The solver state object */

  SolverState& d_state;

  /** The (custom) output channel of the theory of strings */

  InferenceManager& d_im;

  /** Reference to the term registry of theory of strings */

  TermRegistry& d_termReg;

  /** reference to the base solver, used for certain queries */

  BaseSolver& d_bsolver;

  /** Commonly used constants */

  Node d_true;

  Node d_false;

  Node d_zero;

  Node d_one;

  Node d_neg_one;

  /** empty vector (used for trivial explanations) */

  std::vector<Node> d_emptyVec;

  /**

   * The list of equivalence classes of type string. These are ordered based

   * on the ordering described in checkCycles.

   */

  std::vector<Node> d_strings_eqc;

  /** map from terms to their normal forms */

  std::map<Node, NormalForm> d_normal_form;

  /**

   * In certain cases, we know that two terms are equivalent despite

   * not having to verify their normal forms are identical. For example,

   * after applying the R-Loop rule to two terms a and b, we know they

   * are entailed to be equal in the current context without having

   * to look at their normal forms. We call (a,b) a normal form pair.

   *

   * We map representative terms a to a list of nodes b1,...,bn such that

   * (a,b1) ... (a, bn) are normal form pairs. This list is SAT-context

   * dependent. We use a context-dependent integer along with a context

   * indepedent map from nodes to lists of nodes to model this, given by

   * the two data members below.

   */

  NodeIntMap d_nfPairs;

  std::map<Node, std::vector<Node> > d_nf_pairs_data;

  /** list of non-congruent concat terms in each equivalence class */

  std::map<Node, std::vector<Node> > d_eqc;

  /**

   * The flat form for each equivalence class. For a term (str.++ t1 ... tn),

   * this is a list of the form (r_{i1} ... r_{im}) where each empty t1...tn

   * is dropped and the others are replaced in order with their representative.

   */

  std::map<Node, std::vector<Node> > d_flat_form;

  /**

   * For each component r_{i1} ... r_{im} in a flat form, this stores

   * the argument number of the t1 ... tn they were generated from.

   */

  std::map<Node, std::vector<int> > d_flat_form_index;

}; /* class CoreSolver */

}  // namespace strings

}  // namespace theory

}  // namespace cvc5

#endif /* CVC5__THEORY__STRINGS__CORE_SOLVER_H */