//===--- CSBindings.cpp - Constraint Solver -------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file implements selection of bindings for type variables.
//
//===----------------------------------------------------------------------===//
#include "ConstraintGraph.h"
#include "ConstraintSystem.h"
#include <tuple>

using namespace swift;
using namespace constraints;

std::pair<ConstraintSystem::PotentialBindings, TypeVariableType *>
ConstraintSystem::determineBestBindings() {
  // Look for potential type variable bindings.
  TypeVariableType *bestTypeVar = nullptr;
  PotentialBindings bestBindings;
  for (auto typeVar : getTypeVariables()) {
    // Skip any type variables that are bound.
    if (typeVar->getImpl().hasRepresentativeOrFixed())
      continue;

    // Get potential bindings.
    auto bindings = getPotentialBindings(typeVar);
    if (!bindings)
      continue;

    if (TC.getLangOpts().DebugConstraintSolver) {
      auto &log = getASTContext().TypeCheckerDebug->getStream();
      bindings.dump(typeVar, log, solverState->depth * 2);
    }

    // If these are the first bindings, or they are better than what
    // we saw before, use them instead.
    if (!bestTypeVar || bindings < bestBindings) {
      bestBindings = std::move(bindings);
      bestTypeVar = typeVar;
    }
  }

  return std::make_pair(bestBindings, bestTypeVar);
}

/// Find the set of type variables that are inferable from the given type.
///
/// \param type The type to search.
/// \param typeVars Collects the type variables that are inferable from the
/// given type. This set is not cleared, so that multiple types can be explored
/// and introduce their results into the same set.
static void
findInferableTypeVars(Type type,
                      SmallPtrSetImpl<TypeVariableType *> &typeVars) {
  type = type->getCanonicalType();
  if (!type->hasTypeVariable())
    return;

  class Walker : public TypeWalker {
    SmallPtrSetImpl<TypeVariableType *> &typeVars;

  public:
    explicit Walker(SmallPtrSetImpl<TypeVariableType *> &typeVars)
        : typeVars(typeVars) {}

    Action walkToTypePre(Type ty) override {
      if (ty->is<DependentMemberType>())
        return Action::SkipChildren;

      if (auto typeVar = ty->getAs<TypeVariableType>())
        typeVars.insert(typeVar);
      return Action::Continue;
    }
  };

  type.walk(Walker(typeVars));
}

/// \brief Return whether a relational constraint between a type variable and a
/// trivial wrapper type (autoclosure, unary tuple) should result in the type
/// variable being potentially bound to the value type, as opposed to the
/// wrapper type.
static bool shouldBindToValueType(Constraint *constraint) {
  switch (constraint->getKind()) {
  case ConstraintKind::OperatorArgumentConversion:
  case ConstraintKind::OperatorArgumentTupleConversion:
  case ConstraintKind::ArgumentConversion:
  case ConstraintKind::ArgumentTupleConversion:
  case ConstraintKind::Conversion:
  case ConstraintKind::BridgingConversion:
  case ConstraintKind::Subtype:
    return true;
  case ConstraintKind::Bind:
  case ConstraintKind::Equal:
  case ConstraintKind::BindParam:
  case ConstraintKind::BindToPointerType:
  case ConstraintKind::ConformsTo:
  case ConstraintKind::LiteralConformsTo:
  case ConstraintKind::CheckedCast:
  case ConstraintKind::SelfObjectOfProtocol:
  case ConstraintKind::ApplicableFunction:
  case ConstraintKind::BindOverload:
  case ConstraintKind::OptionalObject:
    return false;
  case ConstraintKind::DynamicTypeOf:
  case ConstraintKind::EscapableFunctionOf:
  case ConstraintKind::OpenedExistentialOf:
  case ConstraintKind::KeyPath:
  case ConstraintKind::KeyPathApplication:
  case ConstraintKind::ValueMember:
  case ConstraintKind::UnresolvedValueMember:
  case ConstraintKind::Defaultable:
  case ConstraintKind::Disjunction:
    llvm_unreachable("shouldBindToValueType() may only be called on "
                     "relational constraints");
  }

  llvm_unreachable("Unhandled ConstraintKind in switch.");
}

/// \brief Retrieve the set of potential type bindings for the given
/// representative type variable, along with flags indicating whether
/// those types should be opened.
ConstraintSystem::PotentialBindings
ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
  assert(typeVar->getImpl().getRepresentative(nullptr) == typeVar &&
         "not a representative");
  assert(!typeVar->getImpl().getFixedType(nullptr) && "has a fixed type");

  // Gather the constraints associated with this type variable.
  SmallVector<Constraint *, 8> constraints;
  llvm::SmallPtrSet<Constraint *, 4> visitedConstraints;
  getConstraintGraph().gatherConstraints(
      typeVar, constraints, ConstraintGraph::GatheringKind::EquivalenceClass);

  PotentialBindings result;
  Optional<unsigned> lastSupertypeIndex;

  // Local function to add a potential binding to the list of bindings,
  // coalescing supertype bounds when we are able to compute the meet.
  auto addPotentialBinding = [&](PotentialBinding binding,
                                 bool allowJoinMeet = true) {
    assert(!binding.BindingType->is<ErrorType>());
    // If this is a non-defaulted supertype binding, check whether we can
    // combine it with another supertype binding by computing the 'join' of the
    // types.
    if (binding.Kind == AllowedBindingKind::Supertypes &&
        !binding.BindingType->hasTypeVariable() && !binding.DefaultedProtocol &&
        !binding.isDefaultableBinding() && allowJoinMeet) {
      if (lastSupertypeIndex) {
        // Can we compute a join?
        auto &lastBinding = result.Bindings[*lastSupertypeIndex];
        auto lastType = lastBinding.BindingType->getWithoutSpecifierType();
        auto bindingType = binding.BindingType->getWithoutSpecifierType();
        if (auto join = Type::join(lastType, bindingType)) {
          // Replace the last supertype binding with the join. We're done.
          lastBinding.BindingType = join;
          return;
        }
      }

      // Record this as the most recent supertype index.
      lastSupertypeIndex = result.Bindings.size();
    }

    result.Bindings.push_back(std::move(binding));
  };

  // Consider each of the constraints related to this type variable.
  llvm::SmallPtrSet<CanType, 4> exactTypes;
  llvm::SmallPtrSet<ProtocolDecl *, 4> literalProtocols;
  SmallVector<Constraint *, 2> defaultableConstraints;
  bool addOptionalSupertypeBindings = false;
  auto &tc = getTypeChecker();
  bool hasNonDependentMemberRelationalConstraints = false;
  bool hasDependentMemberRelationalConstraints = false;
  for (auto constraint : constraints) {
    // Only visit each constraint once.
    if (!visitedConstraints.insert(constraint).second)
      continue;

    switch (constraint->getKind()) {
    case ConstraintKind::Bind:
    case ConstraintKind::Equal:
    case ConstraintKind::BindParam:
    case ConstraintKind::BindToPointerType:
    case ConstraintKind::Subtype:
    case ConstraintKind::Conversion:
    case ConstraintKind::ArgumentConversion:
    case ConstraintKind::ArgumentTupleConversion:
    case ConstraintKind::OperatorArgumentTupleConversion:
    case ConstraintKind::OperatorArgumentConversion:
    case ConstraintKind::OptionalObject:
      // Relational constraints: break out to look for types above/below.
      break;

    case ConstraintKind::BridgingConversion:
    case ConstraintKind::CheckedCast:
    case ConstraintKind::DynamicTypeOf:
    case ConstraintKind::EscapableFunctionOf:
    case ConstraintKind::OpenedExistentialOf:
    case ConstraintKind::KeyPath:
    case ConstraintKind::KeyPathApplication:
      // Constraints from which we can't do anything.
      continue;

    case ConstraintKind::Defaultable:
      // Do these in a separate pass.
      if (getFixedTypeRecursive(constraint->getFirstType(), true)
              ->getAs<TypeVariableType>() == typeVar) {
        defaultableConstraints.push_back(constraint);
        hasNonDependentMemberRelationalConstraints = true;
      }
      continue;

    case ConstraintKind::Disjunction:
      // FIXME: Recurse into these constraints to see whether this
      // type variable is fully bound by any of them.
      result.InvolvesTypeVariables = true;
      continue;

    case ConstraintKind::ConformsTo:
    case ConstraintKind::SelfObjectOfProtocol:
      // Swift 3 allowed the use of default types for normal conformances
      // to expressible-by-literal protocols.
      if (tc.Context.LangOpts.EffectiveLanguageVersion[0] >= 4)
        continue;

      if (!constraint->getSecondType()->is<ProtocolType>())
        continue;

      LLVM_FALLTHROUGH;

    case ConstraintKind::LiteralConformsTo: {
      // If there is a 'nil' literal constraint, we might need optional
      // supertype bindings.
      if (constraint->getProtocol()->isSpecificProtocol(
              KnownProtocolKind::ExpressibleByNilLiteral))
        addOptionalSupertypeBindings = true;

      // If there is a default literal type for this protocol, it's a
      // potential binding.
      auto defaultType = tc.getDefaultType(constraint->getProtocol(), DC);
      if (!defaultType)
        continue;

      // Note that we have a literal constraint with this protocol.
      literalProtocols.insert(constraint->getProtocol());
      hasNonDependentMemberRelationalConstraints = true;

      // Handle unspecialized types directly.
      if (!defaultType->hasUnboundGenericType()) {
        if (!exactTypes.insert(defaultType->getCanonicalType()).second)
          continue;

        result.foundLiteralBinding(constraint->getProtocol());
        addPotentialBinding({defaultType, AllowedBindingKind::Subtypes,
                             constraint->getProtocol()});
        continue;
      }

      // For generic literal types, check whether we already have a
      // specialization of this generic within our list.
      // FIXME: This assumes that, e.g., the default literal
      // int/float/char/string types are never generic.
      auto nominal = defaultType->getAnyNominal();
      if (!nominal)
        continue;

      bool matched = false;
      for (auto exactType : exactTypes) {
        if (auto exactNominal = exactType->getAnyNominal()) {
          // FIXME: Check parents?
          if (nominal == exactNominal) {
            matched = true;
            break;
          }
        }
      }

      if (!matched) {
        result.foundLiteralBinding(constraint->getProtocol());
        exactTypes.insert(defaultType->getCanonicalType());
        addPotentialBinding({defaultType, AllowedBindingKind::Subtypes,
                             constraint->getProtocol()});
      }

      continue;
    }

    case ConstraintKind::ApplicableFunction:
    case ConstraintKind::BindOverload: {
      if (result.FullyBound && result.InvolvesTypeVariables)
        continue;

      // If this variable is in the left-hand side, it is fully bound.
      SmallPtrSet<TypeVariableType *, 4> typeVars;
      findInferableTypeVars(simplifyType(constraint->getFirstType()), typeVars);
      if (typeVars.count(typeVar))
        result.FullyBound = true;

      if (result.InvolvesTypeVariables)
        continue;

      // If this and another type variable occur, this result involves
      // type variables.
      findInferableTypeVars(simplifyType(constraint->getSecondType()),
                            typeVars);
      if (typeVars.size() > 1 && typeVars.count(typeVar))
        result.InvolvesTypeVariables = true;
      continue;
    }

    case ConstraintKind::ValueMember:
    case ConstraintKind::UnresolvedValueMember:
      // If our type variable shows up in the base type, there's
      // nothing to do.
      // FIXME: Can we avoid simplification here?
      if (ConstraintSystem::typeVarOccursInType(
              typeVar, simplifyType(constraint->getFirstType()),
              &result.InvolvesTypeVariables)) {
        continue;
      }

      // If the type variable is in the list of member type
      // variables, it is fully bound.
      // FIXME: Can we avoid simplification here?
      if (ConstraintSystem::typeVarOccursInType(
              typeVar, simplifyType(constraint->getSecondType()),
              &result.InvolvesTypeVariables)) {
        result.FullyBound = true;
      }
      continue;
    }

    // Handle relational constraints.
    assert(constraint->getClassification() ==
               ConstraintClassification::Relational &&
           "only relational constraints handled here");

    auto first = simplifyType(constraint->getFirstType());
    auto second = simplifyType(constraint->getSecondType());

    if (first->is<TypeVariableType>() && first->isEqual(second))
      continue;

    Type type;
    AllowedBindingKind kind;
    if (first->getAs<TypeVariableType>() == typeVar) {
      // Upper bound for this type variable.
      type = second;
      kind = AllowedBindingKind::Subtypes;
    } else if (second->getAs<TypeVariableType>() == typeVar) {
      // Lower bound for this type variable.
      type = first;
      kind = AllowedBindingKind::Supertypes;
    } else {
      // Can't infer anything.
      if (result.InvolvesTypeVariables)
        continue;

      // Check whether both this type and another type variable are
      // inferable.
      SmallPtrSet<TypeVariableType *, 4> typeVars;
      findInferableTypeVars(first, typeVars);
      findInferableTypeVars(second, typeVars);
      if (typeVars.size() > 1 && typeVars.count(typeVar))
        result.InvolvesTypeVariables = true;
      continue;
    }
    
    // Do not attempt to bind to ErrorType.
    if (type->hasError())
      continue;

    // If the type we'd be binding to is a dependent member, don't try to
    // resolve this type variable yet.
    if (type->is<DependentMemberType>()) {
      if (!ConstraintSystem::typeVarOccursInType(
              typeVar, type, &result.InvolvesTypeVariables)) {
        hasDependentMemberRelationalConstraints = true;
      }
      continue;
    }
    hasNonDependentMemberRelationalConstraints = true;

    // Check whether we can perform this binding.
    // FIXME: this has a super-inefficient extraneous simplifyType() in it.
    bool isNilLiteral = false;
    bool *isNilLiteralPtr = nullptr;
    if (!addOptionalSupertypeBindings && kind == AllowedBindingKind::Supertypes)
      isNilLiteralPtr = &isNilLiteral;
    if (auto boundType = checkTypeOfBinding(typeVar, type, isNilLiteralPtr)) {
      type = *boundType;
      if (type->hasTypeVariable())
        result.InvolvesTypeVariables = true;
    } else {
      // If the bound is a 'nil' literal type, add optional supertype bindings.
      if (isNilLiteral) {
        addOptionalSupertypeBindings = true;
        continue;
      }

      result.InvolvesTypeVariables = true;
      continue;
    }

    // Don't deduce autoclosure types or single-element, non-variadic
    // tuples.
    if (shouldBindToValueType(constraint)) {
      if (auto funcTy = type->getAs<FunctionType>()) {
        if (funcTy->isAutoClosure())
          type = funcTy->getResult();
      }

      type = type->getWithoutImmediateLabel();
    }

    // Don't deduce IUO types.
    Type alternateType;
    bool adjustedIUO = false;
    if (kind == AllowedBindingKind::Supertypes &&
        constraint->getKind() >= ConstraintKind::Conversion &&
        constraint->getKind() <= ConstraintKind::OperatorArgumentConversion) {
      auto innerType = type->getWithoutSpecifierType();
      if (auto objectType =
              lookThroughImplicitlyUnwrappedOptionalType(innerType)) {
        type = OptionalType::get(objectType);
        alternateType = objectType;
        adjustedIUO = true;
      }
    }

    // Make sure we aren't trying to equate type variables with different
    // lvalue-binding rules.
    if (auto otherTypeVar = type->getAs<TypeVariableType>()) {
      if (typeVar->getImpl().canBindToLValue() !=
          otherTypeVar->getImpl().canBindToLValue())
        continue;
    }

    // BindParam constraints are not reflexive and must be treated specially.
    if (constraint->getKind() == ConstraintKind::BindParam) {
      if (kind == AllowedBindingKind::Subtypes) {
        if (auto *lvt = type->getAs<LValueType>()) {
          type = InOutType::get(lvt->getObjectType());
        }
      } else if (kind == AllowedBindingKind::Supertypes) {
        if (auto *iot = type->getAs<InOutType>()) {
          type = LValueType::get(iot->getObjectType());
        }
      }
      kind = AllowedBindingKind::Exact;
    }

    if (exactTypes.insert(type->getCanonicalType()).second)
      addPotentialBinding({type, kind, None}, /*allowJoinMeet=*/!adjustedIUO);
    if (alternateType &&
        exactTypes.insert(alternateType->getCanonicalType()).second)
      addPotentialBinding({alternateType, kind, None}, /*allowJoinMeet=*/false);
  }

  // If we have any literal constraints, check whether there is already a
  // binding that provides a type that conforms to that literal protocol. In
  // such cases, remove the default binding suggestion because the existing
  // suggestion is better.
  if (!literalProtocols.empty()) {
    SmallPtrSet<ProtocolDecl *, 5> coveredLiteralProtocols;
    for (auto &binding : result.Bindings) {
      // Skip defaulted-protocol constraints.
      if (binding.DefaultedProtocol)
        continue;

      Type testType;
      switch (binding.Kind) {
      case AllowedBindingKind::Exact:
        testType = binding.BindingType;
        break;

      case AllowedBindingKind::Subtypes:
      case AllowedBindingKind::Supertypes:
        testType = binding.BindingType->getRValueType();
        break;
      }

      // Check each non-covered literal protocol to determine which ones
      bool updatedBindingType = false;
      for (auto proto : literalProtocols) {
        do {
          // If the type conforms to this protocol, we're covered.
          if (tc.conformsToProtocol(testType, proto, DC,
                                    ConformanceCheckFlags::InExpression)) {
            coveredLiteralProtocols.insert(proto);
            break;
          }

          // If we're allowed to bind to subtypes, look through optionals.
          // FIXME: This is really crappy special case of computing a reasonable
          // result based on the given constraints.
          if (binding.Kind == AllowedBindingKind::Subtypes) {
            if (auto objTy = testType->getAnyOptionalObjectType()) {
              updatedBindingType = true;
              testType = objTy;
              continue;
            }
          }

          updatedBindingType = false;
          break;
        } while (true);
      }

      if (updatedBindingType)
        binding.BindingType = testType;
    }

    // For any literal type that has been covered, remove the default literal
    // type.
    if (!coveredLiteralProtocols.empty()) {
      result.Bindings.erase(
          std::remove_if(result.Bindings.begin(), result.Bindings.end(),
                         [&](PotentialBinding &binding) {
                           return binding.DefaultedProtocol &&
                                  coveredLiteralProtocols.count(
                                      *binding.DefaultedProtocol) > 0;
                         }),
          result.Bindings.end());
    }
  }

  /// Add defaultable constraints last.
  for (auto constraint : defaultableConstraints) {
    Type type = constraint->getSecondType();
    if (!exactTypes.insert(type->getCanonicalType()).second)
      continue;

    ++result.NumDefaultableBindings;
    addPotentialBinding(
        {type, AllowedBindingKind::Exact, None, constraint->getLocator()});
  }

  // Determine if the bindings only constrain the type variable from above with
  // an existential type; such a binding is not very helpful because it's
  // impossible to enumerate the existential type's subtypes.
  result.SubtypeOfExistentialType =
      std::all_of(result.Bindings.begin(), result.Bindings.end(),
                  [](const PotentialBinding &binding) {
                    return binding.BindingType->isExistentialType() &&
                           binding.Kind == AllowedBindingKind::Subtypes;
                  });

  // If we're supposed to add optional supertype bindings, do so now.
  if (addOptionalSupertypeBindings) {
    for (unsigned i : indices(result.Bindings)) {
      auto &binding = result.Bindings[i];
      bool wrapInOptional = false;

      if (binding.Kind == AllowedBindingKind::Supertypes) {
        // If the type doesn't conform to ExpressibleByNilLiteral,
        // produce an optional of that type as a potential binding. We
        // overwrite the binding in place because the non-optional type
        // will fail to type-check against the nil-literal conformance.
        auto nominalBindingDecl =
            binding.BindingType->getRValueType()->getAnyNominal();
        bool conformsToExprByNilLiteral = false;
        if (nominalBindingDecl) {
          SmallVector<ProtocolConformance *, 2> conformances;
          conformsToExprByNilLiteral = nominalBindingDecl->lookupConformance(
              DC->getParentModule(),
              getASTContext().getProtocol(
                  KnownProtocolKind::ExpressibleByNilLiteral),
              conformances);
        }
        wrapInOptional = !conformsToExprByNilLiteral;
      } else if (binding.isDefaultableBinding() &&
                 binding.BindingType->isAny()) {
        wrapInOptional = true;
      }

      if (wrapInOptional) {
        binding.BindingType = OptionalType::get(binding.BindingType);
      }
    }
  }

  // If there were both dependent-member and non-dependent-member relational
  // constraints, consider this "fully bound"; we don't want to touch it.
  if (hasDependentMemberRelationalConstraints) {
    if (hasNonDependentMemberRelationalConstraints)
      result.FullyBound = true;
    else
      result.Bindings.clear();
  }

  return result;
}