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//-----------------------------------------------------------------------------
//
// Copyright (C) Microsoft Corporation. All Rights Reserved.
//
//-----------------------------------------------------------------------------
using System;
using System.Text;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics.Contracts;
using Microsoft.Basetypes;
using Microsoft.Boogie.VCExprAST;
// Ensure that no formulas (expressions of type boolean that are not
// just a variable) occur with terms (expressions of some other
// type). This is done by introducing let-binders for boolean
// variables.
namespace Microsoft.Boogie.VCExprAST
{
public struct FlattenerState {
public readonly int Polarity;
public readonly bool InTerm;
public static FlattenerState INITIAL = new FlattenerState(1, false);
public FlattenerState(int polarity, bool inTerm) {
Polarity = polarity;
InTerm = inTerm;
}
public FlattenerState TogglePolarity { get {
return new FlattenerState(-Polarity, InTerm);
} }
public FlattenerState ZeroPolarity { get {
return new FlattenerState(0, InTerm);
} }
public FlattenerState EnterTerm { get {
return new FlattenerState(Polarity, true);
} }
}
//////////////////////////////////////////////////////////////////////////////
public class TermFormulaFlattener : MutatingVCExprVisitor<FlattenerState> {
public TermFormulaFlattener(VCExpressionGenerator gen):base(gen){
Contract.Requires(gen != null);
}
private readonly IDictionary<VCExpr/*!*/, VCExprVar/*!*/>/*!*/ Bindings =
new Dictionary<VCExpr/*!*/, VCExprVar/*!*/> ();
private int varNameCounter = 0;
public VCExpr Flatten(VCExpr expr){
Contract.Requires(expr != null);
Contract.Ensures(Contract.Result<VCExpr>() != null);
VCExpr/*!*/ res = Mutate(expr, FlattenerState.INITIAL);
Contract.Assert(res != null);
while (Bindings.Count > 0) {
List<VCExprLetBinding/*!*/>/*!*/ letBindings = new List<VCExprLetBinding/*!*/> ();
foreach (KeyValuePair<VCExpr/*!*/, VCExprVar/*!*/> pair in Bindings){Contract.Assert(cce.NonNullElements(pair));
letBindings.Add(Gen.LetBinding(pair.Value, pair.Key));}
Bindings.Clear();
res = AddBindings(letBindings, res, FlattenerState.INITIAL);
}
return res;
}
private VCExprVar GetVarFor(VCExpr expr){
Contract.Requires(expr != null);
Contract.Requires((expr.Type.IsBool));
Contract.Ensures(Contract.Result<VCExprVar>() != null);
VCExprVar res;
if (!Bindings.TryGetValue(expr, out res)) {
string name = "flt" + varNameCounter;
varNameCounter = varNameCounter + 1;
res = Gen.Variable(name, Type.Bool);
Bindings.Add(expr, res);
}
return cce.NonNull(res);
}
// Remove all let-bindings from the field bindings whose rhs
// contains any of the specified variables
private List<VCExprLetBinding/*!*/>/*!*/ RemoveBindingsWithVars(List<VCExprVar/*!*/>/*!*/ boundVars, List<TypeVariable/*!*/>/*!*/ boundTypeVars){
Contract.Requires(cce.NonNullElements(boundTypeVars));
Contract.Requires(cce.NonNullElements(boundVars));
Contract.Ensures(cce.NonNullElements(Contract.Result<List<VCExprLetBinding>>()));
List<VCExprLetBinding/*!*/>/*!*/ res = new List<VCExprLetBinding/*!*/> ();
FreeVariableCollector/*!*/ coll = new FreeVariableCollector ();
foreach (KeyValuePair<VCExpr, VCExprVar> pair in Bindings) {
Contract.Assert(cce.NonNullElements(pair));
coll.Collect(pair.Key);
if (Contract.Exists(boundVars, var => coll.FreeTermVars.ContainsKey(var)) ||
Contract.Exists(boundTypeVars, var => coll.FreeTypeVars.Contains(var)))
res.Add(Gen.LetBinding(pair.Value, pair.Key));
coll.Reset();
}
foreach (VCExprLetBinding b in res){Contract.Assert(b != null);
Bindings.Remove(b.E);}
return res;
}
// Add bindings to a formula using an implication or
// conjunction. The bindings themselves will be flattened as well,
// which might introduce further bindings
private VCExpr AddBindings(List<VCExprLetBinding/*!*/>/*!*/ bindings, VCExpr body, FlattenerState state){
Contract.Requires(body != null);
Contract.Requires(cce.NonNullElements(bindings));
Contract.Requires((body.Type.IsBool));
Contract.Ensures(Contract.Result<VCExpr>() != null);
List<VCExprLetBinding/*!*/>/*!*/ mutatedBindings = FlattenBindings(bindings, state);
Contract.Assert(mutatedBindings != null);
VCExpr/*!*/ bindingEquations = Gen.AsEquations(mutatedBindings);
Contract.Assert(bindingEquations != null);
switch(state.Polarity) {
case 1:
return Gen.Implies(bindingEquations, body);
case -1:
return Gen.And(bindingEquations, body);
case 0:
// also add explicit quantifiers for the bound variables
List<VCExprVar/*!*/>/*!*/ vars = new List<VCExprVar/*!*/> ();
foreach (VCExprLetBinding/*!*/ binding in mutatedBindings){Contract.Assert(binding != null);
vars.Add(binding.V);}
return Gen.Forall(vars, new List<VCTrigger/*!*/>(),
Gen.Implies(bindingEquations, body));
}
Contract.Assert(false); throw new cce.UnreachableException();
}
private List<VCExprLetBinding/*!*/>/*!*/ FlattenBindings(List<VCExprLetBinding/*!*/>/*!*/ bindings, FlattenerState state){
Contract.Requires(cce.NonNullElements(bindings));
Contract.Ensures(cce.NonNullElements(Contract.Result<List<VCExprLetBinding>>()));
FlattenerState stateInBindings = state.ZeroPolarity;
List<VCExprLetBinding/*!*/>/*!*/ mutatedBindings = new List<VCExprLetBinding/*!*/> ();
foreach (VCExprLetBinding/*!*/ b in bindings) {
Contract.Assert(b != null);
mutatedBindings.Add(Gen.LetBinding(b.V, Mutate(b.E, stateInBindings)));
}
return mutatedBindings;
}
////////////////////////////////////////////////////////////////////////////
public override VCExpr Visit(VCExprNAry node, FlattenerState state){
Contract.Requires(node != null);
Contract.Ensures(Contract.Result<VCExpr>() != null);
// track the polarity to know whether implications or conjunctions
// are to be introduced
if (node.Op.Equals(VCExpressionGenerator.NotOp))
return Gen.Not(Mutate(node[0], state.TogglePolarity));
if (node.Op.Equals(VCExpressionGenerator.ImpliesOp)) {
VCExpr newArg0 = Mutate(node[0], state.TogglePolarity);
VCExpr newArg1 = Mutate(node[1], state);
return Gen.Implies(newArg0, newArg1);
}
if (!node.Type.IsBool)
state = state.EnterTerm;
if (!node.Op.Equals(VCExpressionGenerator.AndOp) &&
!node.Op.Equals(VCExpressionGenerator.OrOp) &&
!(node.Op is VCExprLabelOp))
// standard is to set the polarity to 0 (fits most operators)
return base.Visit(node, state.ZeroPolarity);
return base.Visit(node, state);
}
public override VCExpr Visit(VCExprQuantifier node, FlattenerState state){
Contract.Requires(node != null);
Contract.Ensures(Contract.Result<VCExpr>() != null);
if (state.InTerm)
return GetVarFor(node);
// we only flatten within the matrix of the quantified formula,
// not within the triggers (since SMT-solvers do not seem to
// appreciate triggers with let-binders)
VCExpr newBody = Mutate(node.Body, state);
// Check whether any of the extracted terms contain variables
// bound by this quantifier. In this case, we have to add
// let-binders and remove the extracted terms
bool cont = true;
while (cont) {
List<VCExprLetBinding/*!*/>/*!*/ localBindings =
RemoveBindingsWithVars(node.BoundVars, node.TypeParameters);
Contract.Assert(cce.NonNullElements(localBindings));
if (localBindings.Count > 0)
newBody = AddBindings(localBindings, newBody, state);
else
cont = false;
}
return Gen.Quantify(node.Quan, node.TypeParameters, node.BoundVars, node.Triggers, node.Infos, newBody);
}
public override VCExpr Visit(VCExprLet node, FlattenerState state){
Contract.Requires(node != null);
Contract.Ensures(Contract.Result<VCExpr>() != null);
if (state.InTerm)
return GetVarFor(node);
VCExprLet prelimRes = (VCExprLet)cce.NonNull(base.Visit(node, state));
List<VCExprLetBinding/*!*/>/*!*/ allBindings = new List<VCExprLetBinding/*!*/> ();
allBindings.AddRange(prelimRes);
// Check whether any of the extracted terms contain variables
// bound by this binder. In this case, we have to add
// let-binders and remove the extracted terms
bool cont = true;
while (cont) {
List<VCExprLetBinding/*!*/>/*!*/ localBindings =
RemoveBindingsWithVars(prelimRes.BoundVars, new List<TypeVariable/*!*/>());
if (localBindings.Count > 0)
allBindings.AddRange(FlattenBindings(localBindings, state));
else
cont = false;
}
return Gen.Let(allBindings, prelimRes.Body);
}
}
}
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