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

 * Top contributors (to current version):

 *   Andrew Reynolds, Andres Noetzli, Yoni Zohar

 *

 * 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.

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

 *

 * A cache of skolems for theory of strings.

 */

#include "cvc5_private.h"

#ifndef CVC5__THEORY__STRINGS__SKOLEM_CACHE_H

#define CVC5__THEORY__STRINGS__SKOLEM_CACHE_H

#include <map>

#include <tuple>

#include <unordered_set>

#include "expr/node.h"

#include "expr/skolem_manager.h"

namespace cvc5 {

namespace theory {

class Rewriter;

namespace strings {

/**

 * A cache of all string skolems generated by the TheoryStrings class. This

 * cache is used to ensure that duplicate skolems are not generated when

 * possible, and helps identify what skolems were allocated in the current run.

 */

{

 public:

  /**

   * Constructor.

   *

   * @param rr determines if we aggressively share Skolems based on rewriting or

   * return the constants they are entailed to be equal to. This argument is

   * optional.

   */

  SkolemCache(Rewriter* rr);

  /** Identifiers for skolem types

   *

   * The comments below document the properties of each skolem introduced by

   * inference in the strings solver, where by skolem we mean the fresh

   * string variable that witnesses each of "exists k".

   *

   * The skolems with _REV suffixes are used for the reverse version of the

   * preconditions below, e.g. where we are considering a' ++ a = b' ++ b.

   *

   * All skolems assume a and b are strings unless otherwise stated.

   *

   * Notice that these identifiers are each syntax sugar for constructing a

   * purification skolem. It is required for the purposes of proof checking

   * that this only results in calls to SkolemManager::mkPurifySkolem.

   */

  enum SkolemId

  {

    // exists k. k = a

    SK_PURIFY,

    // a != "" ^ b = "ccccd" ^ a ++ "d" ++ a' = b ++ b' =>

    //    exists k. a = "cccc" ++ k

    SK_ID_C_SPT,

    SK_ID_C_SPT_REV,

    // a != "" ^ b = "c" ^ len(a)!=len(b) ^ a ++ a' = b ++ b' =>

    //    exists k. a = "c" ++ k

    SK_ID_VC_SPT,

    SK_ID_VC_SPT_REV,

    // a != "" ^ b != "" ^ len(a)!=len(b) ^ a ++ a' = b ++ b' =>

    //    exists k1 k2. len( k1 )>0 ^ len( k2 )>0 ^

    //                  ( a ++ k1 = b OR a = b ++ k2 )

    // k1 is the variable for (a,b) and k2 is the skolem for (b,a).

    SK_ID_V_SPT,

    SK_ID_V_SPT_REV,

    // a != "" ^ b != "" ^ len(a)!=len(b) ^ a ++ a' = b ++ b' =>

    //    exists k. len( k )>0 ^ ( a ++ k = b OR a = b ++ k )

    SK_ID_V_UNIFIED_SPT,

    SK_ID_V_UNIFIED_SPT_REV,

    // a != ""  ^ b = "c" ^ a ++ a' != b ++ b' =>

    //    exists k, k_rem.

    //         len( k ) = 1 ^

    //         ( ( a = k ++ k_rem ^ k != "c" ) OR ( a = "c" ++ k_rem ) )

    SK_ID_DC_SPT,

    SK_ID_DC_SPT_REM,

    // a != ""  ^ b != "" ^ len( a ) != len( b ) ^ a ++ a' != b ++ b' =>

    //    exists k_x k_y k_z.

    //         ( len( k_y ) = len( a ) ^ len( k_x ) = len( b ) ^ len( k_z) > 0

    //           ( a = k_x ++ k_z OR b = k_y ++ k_z ) )

    SK_ID_DEQ_X,

    SK_ID_DEQ_Y,

    // contains( a, b ) =>

    //    exists k_pre, k_post. a = k_pre ++ b ++ k_post ^

    //                          ~contains(k_pre ++ substr( b, 0, len(b)-1 ), b)

    //

    // As an optimization, these skolems are reused for positive occurrences of

    // contains, where they have the semantics:

    //

    //   contains( a, b ) =>

    //      exists k_pre, k_post. a = k_pre ++ b ++ k_post

    //

    // We reuse them since it is sound to consider w.l.o.g. the first occurrence

    // of b in a as the witness for contains( a, b ).

    SK_FIRST_CTN_PRE,

    SK_FIRST_CTN_POST,

    // For integer b,

    // len( a ) > b =>

    //    exists k. a = k ++ a' ^ len( k ) = b

    SK_PREFIX,

    // For integer b,

    // b > 0 =>

    //    exists k. a = a' ++ k ^ len( k ) = ite( len(a)>b, len(a)-b, 0 )

    SK_SUFFIX_REM

  };

  /**

   * Returns a skolem of type string that is cached for (a,b,id) and has

   * name c.

   */

  Node mkSkolemCached(Node a, Node b, SkolemId id, const char* c);

  /**

   * Returns a skolem of type string that is cached for (a,[null],id) and has

   * name c.

   */

  Node mkSkolemCached(Node a, SkolemId id, const char* c);

  /** Same as above, but the skolem to construct has a custom type tn */

  Node mkTypedSkolemCached(

      TypeNode tn, Node a, Node b, SkolemId id, const char* c);

  /** Same as mkTypedSkolemCached above for (a,[null],id) */

  Node mkTypedSkolemCached(TypeNode tn, Node a, SkolemId id, const char* c);

  /**

   * Specific version for seq.nth, used for cases where the index is out of

   * range for sequence type seqType.

   */

  static Node mkSkolemSeqNth(TypeNode seqType, const char* c);

  /** Returns a (uncached) skolem of type string with name c */

  Node mkSkolem(const char* c);

  /** Returns true if n is a skolem allocated by this class */

  bool isSkolem(Node n) const;

  /** Make index variable

   *

   * This returns an integer variable of kind BOUND_VARIABLE that is used

   * for axiomatizing the behavior of a term or predicate t. Notice that this

   * index variable does *not* necessarily refer to indices in the term t

   * itself. Instead, it refers to indices in the relevant string in the

   * reduction of t. For example, the index variable for the term str.to_int(s)

   * is used to quantify over the positions in string term s.

   */

  static Node mkIndexVar(Node t);

  /** Make length variable

   *

   * This returns an integer variable of kind BOUND_VARIABLE that is used for

   * axiomatizing the behavior of a term or predicate t. It refers to lengths

   * of strings in the reduction of t. For example, the length variable for the

   * term str.indexof(s, r, n) is used to quantify over the lengths of strings

   * that could be matched by r.

   */

  static Node mkLengthVar(Node t);

  /**

   * Make skolem function, possibly normalizing based on the rewriter of this

   * class. This method should be used whenever it is not possible to define

   * a Skolem identifier that amounts to purification of a term.

   *

   * Notice that this method is not static or constant since it tracks the

   * Skolem we construct (in d_allSkolems), which is used for finite model

   * finding.

   */

  Node mkSkolemFun(SkolemFunId id,

                   TypeNode tn,

                   Node a = Node::null(),

                   Node b = Node::null());

 private:

  /**

   * Simplifies the arguments for a string skolem used for indexing into the

   * cache. In certain cases, we can share skolems with similar arguments e.g.

   * SK_FIRST_CTN(a, c) can be used instead of SK_FIRST_CTN((str.substr a 0 n),

   * c) because the first occurrence of "c" in "(str.substr a 0 n)" is also the

   * first occurrence of "c" in "a" (assuming that "c" appears in both and

   * otherwise the value of SK_FIRST_CTN does not matter).

   *

   * @param id The type of skolem

   * @param a The first argument used for indexing

   * @param b The second argument used for indexing

   * @return A tuple with the new skolem id, the new first, and the new second

   * argument

   */

  std::tuple<SkolemId, Node, Node> normalizeStringSkolem(SkolemId id,

                                                         Node a,

                                                         Node b);

  /** the optional rewriter */

  Rewriter* d_rr;

  /** string type */

  TypeNode d_strType;

  /** Constant node zero */

  Node d_zero;

  /** map from node pairs and identifiers to skolems */

  std::map<Node, std::map<Node, std::map<SkolemId, Node> > > d_skolemCache;

  /** the set of all skolems we have generated */

  std::unordered_set<Node> d_allSkolems;

};

}  // namespace strings

}  // namespace theory

}  // namespace cvc5

#endif /* CVC5__THEORY__STRINGS__SKOLEM_CACHE_H */