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       1                 :            : /******************************************************************************
       2                 :            :  * Top contributors (to current version):
       3                 :            :  *   Andrew Reynolds, Aina Niemetz, Mathias Preiner
       4                 :            :  *
       5                 :            :  * This file is part of the cvc5 project.
       6                 :            :  *
       7                 :            :  * Copyright (c) 2009-2024 by the authors listed in the file AUTHORS
       8                 :            :  * in the top-level source directory and their institutional affiliations.
       9                 :            :  * All rights reserved.  See the file COPYING in the top-level source
      10                 :            :  * directory for licensing information.
      11                 :            :  * ****************************************************************************
      12                 :            :  *
      13                 :            :  * A let binding.
      14                 :            :  */
      15                 :            : 
      16                 :            : #include "cvc5_private.h"
      17                 :            : 
      18                 :            : #ifndef CVC5__PRINTER__LET_BINDING_H
      19                 :            : #define CVC5__PRINTER__LET_BINDING_H
      20                 :            : 
      21                 :            : #include <vector>
      22                 :            : 
      23                 :            : #include "context/cdhashmap.h"
      24                 :            : #include "context/cdlist.h"
      25                 :            : #include "expr/node.h"
      26                 :            : 
      27                 :            : namespace cvc5::internal {
      28                 :            : 
      29                 :            : /**
      30                 :            :  * A flexible let binding class. This class provides functionalities for
      31                 :            :  * printing letified terms. A simple use case is the following for Node n
      32                 :            :  * and LetBinding lbind:
      33                 :            :  * ```
      34                 :            :  *   std::vector<Node> letList;
      35                 :            :  *   lbind.letify(n, letList);
      36                 :            :  * ```
      37                 :            :  * Now, letList contains a list of subterms of n that should be letified based
      38                 :            :  * on the threshold value passed to this class where a value n>0 indicates that
      39                 :            :  * terms with n or more occurrences should be letified.
      40                 :            :  *
      41                 :            :  * The above is equivalent to:
      42                 :            :  * ```
      43                 :            :  *   std::vector<Node> letList;
      44                 :            :  *   lbind.pushScope();
      45                 :            :  *   lbind.process(n);
      46                 :            :  *   lbind.letify(letList);
      47                 :            :  * ```
      48                 :            :  * In fact, multiple terms can be passed to calls to process, in which case the
      49                 :            :  * counting is cumulative.
      50                 :            :  *
      51                 :            :  * All quantified formulas are treated as black boxes. This class can be used
      52                 :            :  * to letify terms with quantifiers, where multiple calls to pushScope /
      53                 :            :  * popScope can be used. In particular, consider:
      54                 :            :  * ```
      55                 :            :  *   std::vector<Node> letList1;
      56                 :            :  *   lbind.letify(n1, letList1);
      57                 :            :  *   std::vector<Node> letList2;
      58                 :            :  *   lbind.letify(n2, letList2);
      59                 :            :  *   ...
      60                 :            :  *   lbind.popScope();
      61                 :            :  *   lbind.popScope();
      62                 :            :  * ```
      63                 :            :  * In a typical use case, n2 is the body of a quantified formula that is a
      64                 :            :  * subterm of n1. We have that letList2 is the list of subterms of n2 that
      65                 :            :  * should be letified, assuming that we have already have let definitions
      66                 :            :  * given by letList1.
      67                 :            :  *
      68                 :            :  * Internally, a let binding is a list and a map that can be printed as a let
      69                 :            :  * expression. In particular, the list d_letList is ordered such that
      70                 :            :  * d_letList[i] does not contain subterm d_letList[j] for j>i.
      71                 :            :  * It is intended that d_letList contains only unique nodes. Each node
      72                 :            :  * in d_letList is mapped to a unique identifier in d_letMap.
      73                 :            :  *
      74                 :            :  * Notice that this class will *not* use introduced let symbols when converting
      75                 :            :  * the bodies of quantified formulas. Consider the formula:
      76                 :            :  * (let ((Q (forall ((x Int)) (= x (+ a a))))) (and (= (+ a a) (+ a a)) Q Q))
      77                 :            :  * where "let" above is from the user. When this is letified by this class,
      78                 :            :  * note that (+ a a) occurs as a subterm of Q, however it is not seen until
      79                 :            :  * after we have seen Q twice, since we traverse in reverse topological order.
      80                 :            :  * Since we do not traverse underneath quantified formulas, this means that Q
      81                 :            :  * may be marked as a term-to-letify before (+ a a), which leads to violation
      82                 :            :  * of the above invariant concerning containment. Thus, when converting, if
      83                 :            :  * a let symbol is introduced for (+ a a), we will not replace the occurrence
      84                 :            :  * of (+ a a) within Q. Instead, the user of this class is responsible for
      85                 :            :  * letifying the bodies of quantified formulas independently.
      86                 :            :  */
      87                 :            : class LetBinding
      88                 :            : {
      89                 :            :   using NodeList = context::CDList<Node>;
      90                 :            :   using NodeIdMap = context::CDHashMap<Node, uint32_t>;
      91                 :            : 
      92                 :            :  public:
      93                 :            :   /**
      94                 :            :    * @param prefix The prefix to use for introduced variables
      95                 :            :    * @param thresh The threshold to use, that is, the number of times a term
      96                 :            :    * must appear before being letified.
      97                 :            :    * @param traverseBinders Whether we should traverse binders, that is, if
      98                 :            :    * this flag is true, we consider terms beneath binders as targets for
      99                 :            :    * letificiation.
     100                 :            :    * @param traverseSkolems Whether we should traverse skolems, that is, if
     101                 :            :    * this flag is true, we consider terms in skolem indices as targets for
     102                 :            :    * letificiation.
     103                 :            :    */
     104                 :            :   LetBinding(const std::string& prefix,
     105                 :            :              uint32_t thresh = 2,
     106                 :            :              bool traverseBinders = false,
     107                 :            :              bool traverseSkolems = false);
     108                 :            :   /** Get threshold */
     109                 :            :   uint32_t getThreshold() const;
     110                 :            :   /**
     111                 :            :    * This updates this let binding to consider the counts for node n.
     112                 :            :    */
     113                 :            :   void process(Node n);
     114                 :            :   /**
     115                 :            :    * This pushes a scope, computes the letification for n, adds the (new) terms
     116                 :            :    * that must be letified in this context to letList.
     117                 :            :    *
     118                 :            :    * Notice that this method does not traverse inside of closures.
     119                 :            :    *
     120                 :            :    * @param n The node to letify
     121                 :            :    * @param letList The list of terms that should be letified within n. This
     122                 :            :    * list is ordered in such a way that letList[i] does not contain subterm
     123                 :            :    * letList[j] for j>i.
     124                 :            :    */
     125                 :            :   void letify(Node n, std::vector<Node>& letList);
     126                 :            :   /**
     127                 :            :    * Same as above, without a node to letify.
     128                 :            :    */
     129                 :            :   void letify(std::vector<Node>& letList);
     130                 :            :   /** Push scope */
     131                 :            :   void pushScope();
     132                 :            :   /** Pop scope for n, reverts the state change of the above method */
     133                 :            :   void popScope();
     134                 :            :   /**
     135                 :            :    * @return the identifier for node n, or 0 if it does not have one.
     136                 :            :    */
     137                 :            :   uint32_t getId(Node n) const;
     138                 :            :   /** Get prefix. */
     139                 :   16386700 :   const std::string& getPrefix() const { return d_prefix; }
     140                 :            :   /**
     141                 :            :    * Convert n based on the state of the let binding. This replaces all
     142                 :            :    * letified subterms of n with a fresh variable whose name prefix is the
     143                 :            :    * given one.
     144                 :            :    *
     145                 :            :    * @param n The node to conver
     146                 :            :    * @param letTop Whether we letify n itself
     147                 :            :    * @return the converted node.
     148                 :            :    */
     149                 :            :   Node convert(Node n, bool letTop = true) const;
     150                 :            : 
     151                 :            :  private:
     152                 :            :   /** The prefix */
     153                 :            :   std::string d_prefix;
     154                 :            :   /**
     155                 :            :    * Compute the count of sub nodes in n, store in d_count. Additionally,
     156                 :            :    * store each node in the domain of d_count in an order in d_visitList
     157                 :            :    * such that d_visitList[i] does not contain sub d_visitList[j] for j>i.
     158                 :            :    */
     159                 :            :   void updateCounts(Node n);
     160                 :            :   /**
     161                 :            :    * Convert a count to a let binding.
     162                 :            :    */
     163                 :            :   void convertCountToLet();
     164                 :            :   /** The dag threshold */
     165                 :            :   uint32_t d_thresh;
     166                 :            :   /** Traverse binders? */
     167                 :            :   bool d_traverseBinders;
     168                 :            :   /** Traverse skolems? */
     169                 :            :   bool d_traverseSkolems;
     170                 :            :   /** An internal context */
     171                 :            :   context::Context d_context;
     172                 :            :   /** Visit list */
     173                 :            :   NodeList d_visitList;
     174                 :            :   /** Count */
     175                 :            :   NodeIdMap d_count;
     176                 :            :   /** The let list */
     177                 :            :   NodeList d_letList;
     178                 :            :  protected:
     179                 :            :   /** The let map */
     180                 :            :   NodeIdMap d_letMap;
     181                 :            : };
     182                 :            : 
     183                 :            : }  // namespace cvc5::internal
     184                 :            : 
     185                 :            : #endif