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/********************* */
/*! \file ackermann.cpp
** \verbatim
** Top contributors (to current version):
** Yoni Zohar, Aina Niemetz, Clark Barrett, Ying Sheng
** This file is part of the CVC4 project.
** Copyright (c) 2009-2019 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.\endverbatim
**
** \brief Ackermannization preprocessing pass.
**
** This implements the Ackermannization preprocessing pass, which enables
** very limited theory combination support for eager bit-blasting via
** Ackermannization. It reduces constraints over the combination of the
** theories of fixed-size bit-vectors and uninterpreted functions as
** described in
** Liana Hadarean, An Efficient and Trustworthy Theory Solver for
** Bit-vectors in Satisfiability Modulo Theories.
** https://cs.nyu.edu/media/publications/hadarean_liana.pdf
**/
#include "preprocessing/passes/ackermann.h"
#include "base/check.h"
#include "options/options.h"
using namespace CVC4;
using namespace CVC4::theory;
namespace CVC4 {
namespace preprocessing {
namespace passes {
/* -------------------------------------------------------------------------- */
namespace {
void addLemmaForPair(TNode args1,
TNode args2,
const TNode func,
AssertionPipeline* assertionsToPreprocess,
NodeManager* nm)
{
Node args_eq;
if (args1.getKind() == kind::APPLY_UF)
{
Assert(args1.getOperator() == func);
Assert(args2.getKind() == kind::APPLY_UF && args2.getOperator() == func);
Assert(args1.getNumChildren() == args2.getNumChildren());
Assert(args1.getNumChildren() >= 1);
std::vector<Node> eqs(args1.getNumChildren());
for (unsigned i = 0, n = args1.getNumChildren(); i < n; ++i)
{
eqs[i] = nm->mkNode(kind::EQUAL, args1[i], args2[i]);
}
if (eqs.size() >= 2)
{
args_eq = nm->mkNode(kind::AND, eqs);
}
else
{
args_eq = eqs[0];
}
}
else
{
Assert(args1.getKind() == kind::SELECT && args1[0] == func);
Assert(args2.getKind() == kind::SELECT && args2[0] == func);
Assert(args1.getNumChildren() == 2);
Assert(args2.getNumChildren() == 2);
args_eq = nm->mkNode(kind::EQUAL, args1[1], args2[1]);
}
Node func_eq = nm->mkNode(kind::EQUAL, args1, args2);
Node lemma = nm->mkNode(kind::IMPLIES, args_eq, func_eq);
assertionsToPreprocess->push_back(lemma);
}
void storeFunctionAndAddLemmas(TNode func,
TNode term,
FunctionToArgsMap& fun_to_args,
SubstitutionMap& fun_to_skolem,
AssertionPipeline* assertions,
NodeManager* nm,
std::vector<TNode>* vec)
{
if (fun_to_args.find(func) == fun_to_args.end())
{
fun_to_args.insert(make_pair(func, TNodeSet()));
}
TNodeSet& set = fun_to_args[func];
if (set.find(term) == set.end())
{
TypeNode tn = term.getType();
Node skolem = nm->mkSkolem("SKOLEM$$",
tn,
"is a variable created by the ackermannization "
"preprocessing pass");
for (const auto& t : set)
{
addLemmaForPair(t, term, func, assertions, nm);
}
fun_to_skolem.addSubstitution(term, skolem);
set.insert(term);
/* Add the arguments of term (newest element in set) to the vector, so that
* collectFunctionsAndLemmas will process them as well.
* This is only needed if the set has at least two elements
* (otherwise, no lemma is generated).
* Therefore, we defer this for term in case it is the first element in the
* set*/
if (set.size() == 2)
{
for (TNode elem : set)
{
vec->insert(vec->end(), elem.begin(), elem.end());
}
}
else if (set.size() > 2)
{
vec->insert(vec->end(), term.begin(), term.end());
}
}
}
/* We only add top-level applications of functions.
* For example: when we see "f(g(x))", we do not add g as a function and x as a
* parameter.
* Instead, we only include f as a function and g(x) as a parameter.
* However, if we see g(x) later on as a top-level application, we will add it
* as well.
* Another example: for the formula f(g(x))=f(g(y)),
* we first only add f as a function and g(x),g(y) as arguments.
* storeFunctionAndAddLemmas will then add the constraint g(x)=g(y) ->
* f(g(x))=f(g(y)).
* Now that we see g(x) and g(y), we explicitly add them as well. */
void collectFunctionsAndLemmas(FunctionToArgsMap& fun_to_args,
SubstitutionMap& fun_to_skolem,
std::vector<TNode>* vec,
AssertionPipeline* assertions)
{
TNodeSet seen;
NodeManager* nm = NodeManager::currentNM();
TNode term;
while (!vec->empty())
{
term = vec->back();
vec->pop_back();
if (seen.find(term) == seen.end())
{
TNode func;
if (term.getKind() == kind::APPLY_UF)
{
storeFunctionAndAddLemmas(term.getOperator(),
term,
fun_to_args,
fun_to_skolem,
assertions,
nm,
vec);
}
else if (term.getKind() == kind::SELECT)
{
storeFunctionAndAddLemmas(
term[0], term, fun_to_args, fun_to_skolem, assertions, nm, vec);
}
else
{
AlwaysAssert(term.getKind() != kind::STORE)
<< "Cannot use Ackermannization on formula with stores to arrays";
/* add children to the vector, so that they are processed later */
for (TNode n : term)
{
vec->push_back(n);
}
}
seen.insert(term);
}
}
}
} // namespace
/* -------------------------------------------------------------------------- */
Ackermann::Ackermann(PreprocessingPassContext* preprocContext)
: PreprocessingPass(preprocContext, "ackermann"),
d_funcToSkolem(preprocContext->getUserContext())
{
}
PreprocessingPassResult Ackermann::applyInternal(
AssertionPipeline* assertionsToPreprocess)
{
AlwaysAssert(!options::incrementalSolving());
/* collect all function applications and generate consistency lemmas
* accordingly */
std::vector<TNode> to_process;
for (const Node& a : assertionsToPreprocess->ref())
{
to_process.push_back(a);
}
collectFunctionsAndLemmas(
d_funcToArgs, d_funcToSkolem, &to_process, assertionsToPreprocess);
/* replace applications of UF by skolems */
// FIXME for model building, github issue #1901
for (unsigned i = 0, size = assertionsToPreprocess->size(); i < size; ++i)
{
assertionsToPreprocess->replace(
i, d_funcToSkolem.apply((*assertionsToPreprocess)[i]));
}
return PreprocessingPassResult::NO_CONFLICT;
}
/* -------------------------------------------------------------------------- */
} // namespace passes
} // namespace preprocessing
} // namespace CVC4
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