using MonoFN.Cecil.Cil;
using System;

namespace MonoFN.Cecil
{
    internal sealed class TypeResolver
    {
        private readonly IGenericInstance _typeDefinitionContext;
        private readonly IGenericInstance _methodDefinitionContext;

        public static TypeResolver For(TypeReference typeReference)
        {
            return typeReference.IsGenericInstance ? new((GenericInstanceType)typeReference) : new TypeResolver();
        }

        public static TypeResolver For(TypeReference typeReference, MethodReference methodReference)
        {
            return new(typeReference as GenericInstanceType, methodReference as GenericInstanceMethod);
        }

        public TypeResolver() { }

        public TypeResolver(GenericInstanceType typeDefinitionContext)
        {
            _typeDefinitionContext = typeDefinitionContext;
        }

        public TypeResolver(GenericInstanceMethod methodDefinitionContext)
        {
            _methodDefinitionContext = methodDefinitionContext;
        }

        public TypeResolver(GenericInstanceType typeDefinitionContext, GenericInstanceMethod methodDefinitionContext)
        {
            _typeDefinitionContext = typeDefinitionContext;
            _methodDefinitionContext = methodDefinitionContext;
        }

        public MethodReference Resolve(MethodReference method)
        {
            MethodReference methodReference = method;
            if (IsDummy())
                return methodReference;

            TypeReference declaringType = Resolve(method.DeclaringType);

            GenericInstanceMethod genericInstanceMethod = method as GenericInstanceMethod;
            if (genericInstanceMethod != null)
            {
                methodReference = new(method.Name, method.ReturnType, declaringType);

                foreach (ParameterDefinition p in method.Parameters)
                    methodReference.Parameters.Add(new(p.Name, p.Attributes, p.ParameterType));

                foreach (GenericParameter gp in genericInstanceMethod.ElementMethod.GenericParameters)
                    methodReference.GenericParameters.Add(new(gp.Name, methodReference));

                methodReference.HasThis = method.HasThis;

                GenericInstanceMethod m = new(methodReference);
                foreach (TypeReference ga in genericInstanceMethod.GenericArguments)
                {
                    m.GenericArguments.Add(Resolve(ga));
                }

                methodReference = m;
            }
            else
            {
                methodReference = new(method.Name, method.ReturnType, declaringType);

                foreach (GenericParameter gp in method.GenericParameters)
                    methodReference.GenericParameters.Add(new(gp.Name, methodReference));

                foreach (ParameterDefinition p in method.Parameters)
                    methodReference.Parameters.Add(new(p.Name, p.Attributes, p.ParameterType));

                methodReference.HasThis = method.HasThis;
            }


            return methodReference;
        }

        public FieldReference Resolve(FieldReference field)
        {
            TypeReference declaringType = Resolve(field.DeclaringType);

            if (declaringType == field.DeclaringType)
                return field;

            return new(field.Name, field.FieldType, declaringType);
        }

        public TypeReference ResolveReturnType(MethodReference method)
        {
            return Resolve(GenericParameterResolver.ResolveReturnTypeIfNeeded(method));
        }

        public TypeReference ResolveParameterType(MethodReference method, ParameterReference parameter)
        {
            return Resolve(GenericParameterResolver.ResolveParameterTypeIfNeeded(method, parameter));
        }

        public TypeReference ResolveVariableType(MethodReference method, VariableReference variable)
        {
            return Resolve(GenericParameterResolver.ResolveVariableTypeIfNeeded(method, variable));
        }

        public TypeReference ResolveFieldType(FieldReference field)
        {
            return Resolve(GenericParameterResolver.ResolveFieldTypeIfNeeded(field));
        }

        public TypeReference Resolve(TypeReference typeReference)
        {
            return Resolve(typeReference, true);
        }

        public TypeReference Resolve(TypeReference typeReference, bool includeTypeDefinitions)
        {
            if (IsDummy())
                return typeReference;

            if (_typeDefinitionContext != null && _typeDefinitionContext.GenericArguments.Contains(typeReference))
                return typeReference;
            if (_methodDefinitionContext != null && _methodDefinitionContext.GenericArguments.Contains(typeReference))
                return typeReference;

            GenericParameter genericParameter = typeReference as GenericParameter;
            if (genericParameter != null)
            {
                if (_typeDefinitionContext != null && _typeDefinitionContext.GenericArguments.Contains(genericParameter))
                    return genericParameter;
                if (_methodDefinitionContext != null && _methodDefinitionContext.GenericArguments.Contains(genericParameter))
                    return genericParameter;
                return ResolveGenericParameter(genericParameter);
            }

            ArrayType arrayType = typeReference as ArrayType;
            if (arrayType != null)
                return new ArrayType(Resolve(arrayType.ElementType), arrayType.Rank);

            PointerType pointerType = typeReference as PointerType;
            if (pointerType != null)
                return new PointerType(Resolve(pointerType.ElementType));

            ByReferenceType byReferenceType = typeReference as ByReferenceType;
            if (byReferenceType != null)
                return new ByReferenceType(Resolve(byReferenceType.ElementType));

            PinnedType pinnedType = typeReference as PinnedType;
            if (pinnedType != null)
                return new PinnedType(Resolve(pinnedType.ElementType));

            GenericInstanceType genericInstanceType = typeReference as GenericInstanceType;
            if (genericInstanceType != null)
            {
                GenericInstanceType newGenericInstanceType = new(genericInstanceType.ElementType);
                foreach (TypeReference genericArgument in genericInstanceType.GenericArguments)
                    newGenericInstanceType.GenericArguments.Add(Resolve(genericArgument));
                return newGenericInstanceType;
            }

            RequiredModifierType requiredModType = typeReference as RequiredModifierType;
            if (requiredModType != null)
                return Resolve(requiredModType.ElementType, includeTypeDefinitions);


            if (includeTypeDefinitions)
            {
                TypeDefinition typeDefinition = typeReference as TypeDefinition;
                if (typeDefinition != null && typeDefinition.HasGenericParameters)
                {
                    GenericInstanceType newGenericInstanceType = new(typeDefinition);
                    foreach (GenericParameter gp in typeDefinition.GenericParameters)
                        newGenericInstanceType.GenericArguments.Add(Resolve(gp));
                    return newGenericInstanceType;
                }
            }

            if (typeReference is TypeSpecification)
                throw new NotSupportedException(string.Format("The type {0} cannot be resolved correctly.", typeReference.FullName));

            return typeReference;
        }

        internal TypeResolver Nested(GenericInstanceMethod genericInstanceMethod)
        {
            return new(_typeDefinitionContext as GenericInstanceType, genericInstanceMethod);
        }

        private TypeReference ResolveGenericParameter(GenericParameter genericParameter)
        {
            if (genericParameter.Owner == null)
                return HandleOwnerlessInvalidILCode(genericParameter);

            MemberReference memberReference = genericParameter.Owner as MemberReference;
            if (memberReference == null)
                throw new NotSupportedException();

            return genericParameter.Type == GenericParameterType.Type ? _typeDefinitionContext.GenericArguments[genericParameter.Position] : _methodDefinitionContext != null ? _methodDefinitionContext.GenericArguments[genericParameter.Position] : genericParameter;
        }

        private TypeReference HandleOwnerlessInvalidILCode(GenericParameter genericParameter)
        {
            // NOTE: If owner is null and we have a method parameter, then we'll assume that the method parameter
            // is actually a type parameter, and we'll use the type parameter from the corresponding position. I think
            // this assumption is valid, but if you're visiting this code then I might have been proven wrong.
            if (genericParameter.Type == GenericParameterType.Method && _typeDefinitionContext != null && genericParameter.Position < _typeDefinitionContext.GenericArguments.Count)
                return _typeDefinitionContext.GenericArguments[genericParameter.Position];

            // NOTE: Owner cannot be null, but sometimes the Mono compiler generates invalid IL and we
            // end up in this situation.
            // When we do, we assume that the runtime doesn't care about the resolved type of the GenericParameter,
            // thus we return a reference to System.Object.
            return genericParameter.Module.TypeSystem.Object;
        }

        private bool IsDummy()
        {
            return _typeDefinitionContext == null && _methodDefinitionContext == null;
        }
    }
}