234 lines
7.7 KiB
C++
234 lines
7.7 KiB
C++
#include <anthem/Simplification.h>
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#include <experimental/optional>
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#include <anthem/ASTCopy.h>
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#include <anthem/ASTVisitors.h>
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namespace anthem
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{
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////////////////////////////////////////////////////////////////////////////////////////////////////
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//
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// Simplification
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//
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////////////////////////////////////////////////////////////////////////////////////////////////////
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// Determines whether a term is a specific variable
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bool matchesVariableDeclaration(const ast::Term &term, const ast::VariableDeclaration &variableDeclaration)
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{
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if (!term.is<ast::Variable>())
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return false;
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return term.get<ast::Variable>().declaration == &variableDeclaration;
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}
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////////////////////////////////////////////////////////////////////////////////////////////////////
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// Extracts the term t if the given formula is of the form “X = t” and X matches the given variable
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// The input formula is no longer usable after this call if a term is returned
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std::experimental::optional<ast::Term> extractAssignedTerm(ast::Formula &formula, const ast::VariableDeclaration &variableDeclaration)
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{
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if (!formula.is<ast::Comparison>())
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return std::experimental::nullopt;
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auto &comparison = formula.get<ast::Comparison>();
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if (comparison.operator_ != ast::Comparison::Operator::Equal)
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return std::experimental::nullopt;
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if (matchesVariableDeclaration(comparison.left, variableDeclaration))
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return std::move(comparison.right);
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if (matchesVariableDeclaration(comparison.right, variableDeclaration))
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return std::move(comparison.left);
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return std::experimental::nullopt;
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}
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////////////////////////////////////////////////////////////////////////////////////////////////////
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// Replaces all occurrences of a variable in a given term with another term
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struct ReplaceVariableInTermVisitor : public ast::RecursiveTermVisitor<ReplaceVariableInTermVisitor>
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{
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static void accept(ast::Variable &variable, ast::Term &term, const ast::VariableDeclaration &original, const ast::Term &replacement)
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{
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if (variable.declaration == &original)
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// No dangling variables can result from this operation, and hence, fixing them is not necessary
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term = ast::prepareCopy(replacement);
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}
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// Ignore all other types of expressions
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template<class T>
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static void accept(T &, ast::Term &, const ast::VariableDeclaration &, const ast::Term &)
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{
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}
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};
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////////////////////////////////////////////////////////////////////////////////////////////////////
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// Replaces all occurrences of a variable in a given formula with a term
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struct ReplaceVariableInFormulaVisitor : public ast::RecursiveFormulaVisitor<ReplaceVariableInFormulaVisitor>
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{
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static void accept(ast::Comparison &comparison, ast::Formula &, const ast::VariableDeclaration &original, const ast::Term &replacement)
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{
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comparison.left.accept(ReplaceVariableInTermVisitor(), comparison.left, original, replacement);
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comparison.right.accept(ReplaceVariableInTermVisitor(), comparison.right, original, replacement);
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}
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static void accept(ast::In &in, ast::Formula &, const ast::VariableDeclaration &original, const ast::Term &term)
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{
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in.element.accept(ReplaceVariableInTermVisitor(), in.element, original, term);
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in.set.accept(ReplaceVariableInTermVisitor(), in.set, original, term);
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}
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static void accept(ast::Predicate &predicate, ast::Formula &, const ast::VariableDeclaration &original, const ast::Term &replacement)
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{
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for (auto &argument : predicate.arguments)
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argument.accept(ReplaceVariableInTermVisitor(), argument, original, replacement);
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}
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// Ignore all other types of expressions
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template<class T>
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static void accept(T &, ast::Formula &, const ast::VariableDeclaration &, const ast::Term &)
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{
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}
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};
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////////////////////////////////////////////////////////////////////////////////////////////////////
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// Simplifies exists statements by using the equivalence “exists X (X = t and F(X))” == “F(t)”
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// The exists statement has to be of the form “exists <variables> <conjunction>”
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void simplify(ast::Exists &exists, ast::Formula &formula)
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{
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// Simplify formulas like “exists X (X = Y)” to “#true”
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// TODO: check that this covers all cases
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if (exists.argument.is<ast::Comparison>())
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{
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const auto &comparison = exists.argument.get<ast::Comparison>();
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if (comparison.operator_ != ast::Comparison::Operator::Equal)
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return;
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const auto matchingAssignment = std::find_if(exists.variables.cbegin(), exists.variables.cend(),
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[&](const auto &variableDeclaration)
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{
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return matchesVariableDeclaration(comparison.left, *variableDeclaration)
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|| matchesVariableDeclaration(comparison.right, *variableDeclaration);
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});
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if (matchingAssignment != exists.variables.cend())
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formula = ast::Formula::make<ast::Boolean>(true);
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return;
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}
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if (!exists.argument.is<ast::And>())
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return;
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auto &conjunction = exists.argument.get<ast::And>();
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auto &arguments = conjunction.arguments;
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// Simplify formulas of type “exists X (X = t and F(X))” to “F(t)”
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for (auto i = exists.variables.begin(); i != exists.variables.end();)
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{
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const auto &variableDeclaration = **i;
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bool wasVariableReplaced = false;
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// TODO: refactor
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for (auto j = arguments.begin(); j != arguments.end(); j++)
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{
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auto &argument = *j;
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// Find term that is equivalent to the given variable
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auto assignedTerm = extractAssignedTerm(argument, variableDeclaration);
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if (!assignedTerm)
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continue;
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// Replace all occurrences of the variable with the equivalent term
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for (auto k = arguments.begin(); k != arguments.end(); k++)
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{
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if (k == j)
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continue;
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auto &otherArgument = *k;
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otherArgument.accept(ReplaceVariableInFormulaVisitor(), otherArgument, variableDeclaration, assignedTerm.value());
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}
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arguments.erase(j);
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wasVariableReplaced = true;
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break;
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}
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if (wasVariableReplaced)
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{
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i = exists.variables.erase(i);
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continue;
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}
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i++;
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}
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// If there are no arguments left, we had a formula of the form “exists X1, ..., Xn (X1 = Y1 and ... and Xn = Yn)”
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// Such exists statements are useless and can be safely replaced with “#true”
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if (arguments.empty())
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{
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formula = ast::Formula::make<ast::Boolean>(true);
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return;
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}
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// If the argument now is a conjunction with just one element, directly replace the input formula with the argument
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if (arguments.size() == 1)
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exists.argument = std::move(arguments.front());
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// If there are still remaining variables, simplification is over
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if (!exists.variables.empty())
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return;
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assert(!arguments.empty());
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// If there is more than one element in the conjunction, replace the input formula with the conjunction
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formula = std::move(exists.argument);
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}
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////////////////////////////////////////////////////////////////////////////////////////////////////
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// Performs the different simplification techniques
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struct SimplifyFormulaVisitor : public ast::RecursiveFormulaVisitor<SimplifyFormulaVisitor>
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{
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// Forward exists statements to the dedicated simplification function
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static void accept(ast::Exists &exists, ast::Formula &formula)
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{
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simplify(exists, formula);
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}
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// Simplify formulas of type “A in B” to “A = B” if A and B are primitive
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static void accept(ast::In &in, ast::Formula &formula)
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{
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assert(ast::isPrimitive(in.element));
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if (!ast::isPrimitive(in.element) || !ast::isPrimitive(in.set))
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return;
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formula = ast::Comparison(ast::Comparison::Operator::Equal, std::move(in.element), std::move(in.set));
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}
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// Do nothing for all other types of expressions
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template<class T>
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static void accept(T &, ast::Formula &)
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{
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}
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};
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////////////////////////////////////////////////////////////////////////////////////////////////////
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void simplify(ast::Formula &formula)
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{
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formula.accept(SimplifyFormulaVisitor(), formula);
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}
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////////////////////////////////////////////////////////////////////////////////////////////////////
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}
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