/// <summary>
/// Superfast deep copier class, which uses Expression trees.
/// </summary>
public static class DeepCopyByExpressionTrees
{
        private static readonly object IsStructTypeToDeepCopyDictionaryLocker = new object();
        private static Dictionary<Type, bool> IsStructTypeToDeepCopyDictionary = new Dictionary<Type, bool>();

        private static readonly object CompiledCopyFunctionsDictionaryLocker = new object();
        private static Dictionary<Type, Func<object, Dictionary<object, object>, object>> CompiledCopyFunctionsDictionary =
            new Dictionary<Type, Func<object, Dictionary<object, object>, object>>();

        private static readonly Type ObjectType = typeof(Object);
        private static readonly Type ObjectDictionaryType = typeof(Dictionary<object, object>);

        /// <summary>
        /// Creates a deep copy of an object.
        /// </summary>
        /// <typeparam name="T">Object type.</typeparam>
        /// <param name="original">Object to copy.</param>
        /// <param name="copiedReferencesDict">Dictionary of already copied objects (Keys: original objects, Values: their copies).</param>
        /// <returns></returns>
        public static T DeepCopyByExpressionTree<T>(this T original, Dictionary<object, object> copiedReferencesDict = null)
        {
            return (T)DeepCopyByExpressionTreeObj(original, false, copiedReferencesDict ?? new Dictionary<object, object>(new ReferenceEqualityComparer()));
        }

        private static object DeepCopyByExpressionTreeObj(object original, bool forceDeepCopy, Dictionary<object, object> copiedReferencesDict)
        {
            if (original == null)
            {
                return null;
            }

            var type = original.GetType();

            if (IsDelegate(type))
            {
                return null;
            }

            if (!forceDeepCopy && !IsTypeToDeepCopy(type))
            {
                return original;
            }

            object alreadyCopiedObject;

            if (copiedReferencesDict.TryGetValue(original, out alreadyCopiedObject))
            {
                return alreadyCopiedObject;
            }

            if (type == ObjectType)
            {
                return new object();
            }

            var compiledCopyFunction = GetOrCreateCompiledLambdaCopyFunction(type);

            object copy = compiledCopyFunction(original, copiedReferencesDict);

            return copy;
        }

        private static Func<object, Dictionary<object, object>, object> GetOrCreateCompiledLambdaCopyFunction(Type type)
        {
            // The following structure ensures that multiple threads can use the dictionary
            // even while dictionary is locked and being updated by other thread.
            // That is why we do not modify the old dictionary instance but
            // we replace it with a new instance everytime.

            Func<object, Dictionary<object, object>, object> compiledCopyFunction;

            if (!CompiledCopyFunctionsDictionary.TryGetValue(type, out compiledCopyFunction))
            {
                lock (CompiledCopyFunctionsDictionaryLocker)
                {
                    if (!CompiledCopyFunctionsDictionary.TryGetValue(type, out compiledCopyFunction))
                    {
                        var uncompiledCopyFunction = CreateCompiledLambdaCopyFunctionForType(type);

                        compiledCopyFunction = uncompiledCopyFunction.Compile();

                        var dictionaryCopy = CompiledCopyFunctionsDictionary.ToDictionary(pair => pair.Key, pair => pair.Value);

                        dictionaryCopy.Add(type, compiledCopyFunction);

                        CompiledCopyFunctionsDictionary = dictionaryCopy;
                    }
                }
            }

            return compiledCopyFunction;
        }

        private static Expression<Func<object, Dictionary<object, object>, object>> CreateCompiledLambdaCopyFunctionForType(Type type)
        {
            ParameterExpression inputParameter;
            ParameterExpression inputDictionary;
            ParameterExpression outputVariable;
            ParameterExpression boxingVariable;
            LabelTarget endLabel;
            List<ParameterExpression> variables;
            List<Expression> expressions;

            ///// INITIALIZATION OF EXPRESSIONS AND VARIABLES

            InitializeExpressions(type,
                                  out inputParameter,
                                  out inputDictionary,
                                  out outputVariable,
                                  out boxingVariable,
                                  out endLabel,
                                  out variables,
                                  out expressions);

            ///// RETURN NULL IF ORIGINAL IS NULL

            IfNullThenReturnNullExpression(inputParameter, endLabel, expressions);

            ///// MEMBERWISE CLONE ORIGINAL OBJECT

            MemberwiseCloneInputToOutputExpression(type, inputParameter, outputVariable, expressions);

            ///// STORE COPIED OBJECT TO REFERENCES DICTIONARY

            if (IsClassOtherThanString(type))
            {
                StoreReferencesIntoDictionaryExpression(inputParameter, inputDictionary, outputVariable, expressions);
            }

            ///// COPY ALL NONVALUE OR NONPRIMITIVE FIELDS

            FieldsCopyExpressions(type,
                                  inputParameter,
                                  inputDictionary,
                                  outputVariable,
                                  boxingVariable,
                                  expressions);

            ///// COPY ELEMENTS OF ARRAY

            if (IsArray(type) && IsTypeToDeepCopy(type.GetElementType()))
            {
                CreateArrayCopyLoopExpression(type,
                                              inputParameter,
                                              inputDictionary,
                                              outputVariable,
                                              variables,
                                              expressions);
            }

            ///// COMBINE ALL EXPRESSIONS INTO LAMBDA FUNCTION

            var lambda = CombineAllIntoLambdaFunctionExpression(inputParameter, inputDictionary, outputVariable, endLabel, variables, expressions);

            return lambda;
        }

        private static void InitializeExpressions(Type type,
                                                  out ParameterExpression inputParameter,
                                                  out ParameterExpression inputDictionary,
                                                  out ParameterExpression outputVariable,
                                                  out ParameterExpression boxingVariable,
                                                  out LabelTarget endLabel,
                                                  out List<ParameterExpression> variables,
                                                  out List<Expression> expressions)
        {

            inputParameter = Expression.Parameter(ObjectType);

            inputDictionary = Expression.Parameter(ObjectDictionaryType);

            outputVariable = Expression.Variable(type);

            boxingVariable = Expression.Variable(ObjectType);

            endLabel = Expression.Label();

            variables = new List<ParameterExpression>();

            expressions = new List<Expression>();

            variables.Add(outputVariable);
            variables.Add(boxingVariable);
        }

        private static void IfNullThenReturnNullExpression(ParameterExpression inputParameter, LabelTarget endLabel, List<Expression> expressions)
        {
            ///// Intended code:
            /////
            ///// if (input == null)
            ///// {
            /////     return null;
            ///// }

            var ifNullThenReturnNullExpression =
                Expression.IfThen(
                    Expression.Equal(
                        inputParameter,
                        Expression.Constant(null, ObjectType)),
                    Expression.Return(endLabel));

            expressions.Add(ifNullThenReturnNullExpression);
        }

        private static void MemberwiseCloneInputToOutputExpression(
            Type type,
            ParameterExpression inputParameter,
            ParameterExpression outputVariable,
            List<Expression> expressions)
        {
            ///// Intended code:
            /////
            ///// var output = (<type>)input.MemberwiseClone();

            var memberwiseCloneMethod = ObjectType.GetMethod("MemberwiseClone", BindingFlags.NonPublic | BindingFlags.Instance);

            var memberwiseCloneInputExpression =
                Expression.Assign(
                    outputVariable,
                    Expression.Convert(
                        Expression.Call(
                            inputParameter,
                            memberwiseCloneMethod),
                        type));

            expressions.Add(memberwiseCloneInputExpression);
        }

        private static void StoreReferencesIntoDictionaryExpression(ParameterExpression inputParameter,
                                                                          ParameterExpression inputDictionary,
                                                                          ParameterExpression outputVariable,
                                                                          List<Expression> expressions)
        {
            ///// Intended code:
            /////
            ///// inputDictionary[(Object)input] = (Object)output;

            var storeReferencesExpression =
                Expression.Assign(
                    Expression.Property(
                        inputDictionary,
                        ObjectDictionaryType.GetProperty("Item"),
                        inputParameter),
                    Expression.Convert(outputVariable, ObjectType));

            expressions.Add(storeReferencesExpression);
        }

        private static Expression<Func<object, Dictionary<object, object>, object>> CombineAllIntoLambdaFunctionExpression(
            ParameterExpression inputParameter,
            ParameterExpression inputDictionary,
            ParameterExpression outputVariable,
            LabelTarget endLabel,
            List<ParameterExpression> variables,
            List<Expression> expressions)
        {
            expressions.Add(Expression.Label(endLabel));

            expressions.Add(Expression.Convert(outputVariable, ObjectType));

            var finalBody = Expression.Block(variables, expressions);

            var lambda = Expression.Lambda<Func<object, Dictionary<object, object>, object>>(finalBody, inputParameter, inputDictionary);

            return lambda;
        }

        private static void CreateArrayCopyLoopExpression(Type type,
                                                          ParameterExpression inputParameter,
                                                          ParameterExpression inputDictionary,
                                                          ParameterExpression outputVariable,
                                                          List<ParameterExpression> variables,
                                                          List<Expression> expressions)
        {
            ///// Intended code:
            /////
            ///// int i1, i2, ..., in;
            /////
            ///// int length1 = inputarray.GetLength(0);
            ///// i1 = 0;
            ///// while (true)
            ///// {
            /////     if (i1 >= length1)
            /////     {
            /////         goto ENDLABELFORLOOP1;
            /////     }
            /////     int length2 = inputarray.GetLength(1);
            /////     i2 = 0;
            /////     while (true)
            /////     {
            /////         if (i2 >= length2)
            /////         {
            /////             goto ENDLABELFORLOOP2;
            /////         }
            /////         ...
            /////         ...
            /////         ...
            /////         int lengthn = inputarray.GetLength(n);
            /////         in = 0;
            /////         while (true)
            /////         {
            /////             if (in >= lengthn)
            /////             {
            /////                 goto ENDLABELFORLOOPn;
            /////             }
            /////             outputarray[i1, i2, ..., in]
            /////                 = (<elementType>)DeepCopyByExpressionTreeObj(
            /////                        (Object)inputarray[i1, i2, ..., in])
            /////             in++;
            /////         }
            /////         ENDLABELFORLOOPn:
            /////         ...
            /////         ...
            /////         ...
            /////         i2++;
            /////     }
            /////     ENDLABELFORLOOP2:
            /////     i1++;
            ///// }
            ///// ENDLABELFORLOOP1:

            var rank = type.GetArrayRank();

            var indices = GenerateIndices(rank);

            variables.AddRange(indices);

            var elementType = type.GetElementType();

            var assignExpression = ArrayFieldToArrayFieldAssignExpression(inputParameter, inputDictionary, outputVariable, elementType, type, indices);

            Expression forExpression = assignExpression;

            for (int dimension = 0; dimension < rank; dimension++)
            {
                var indexVariable = indices[dimension];

                forExpression = LoopIntoLoopExpression(inputParameter, indexVariable, forExpression, dimension);
            }

            expressions.Add(forExpression);
        }

        private static List<ParameterExpression> GenerateIndices(int arrayRank)
        {
            ///// Intended code:
            /////
            ///// int i1, i2, ..., in;

            var indices = new List<ParameterExpression>();

            for (int i = 0; i < arrayRank; i++)
            {
                var indexVariable = Expression.Variable(typeof(Int32));

                indices.Add(indexVariable);
            }

            return indices;
        }

        private static BinaryExpression ArrayFieldToArrayFieldAssignExpression(
            ParameterExpression inputParameter,
            ParameterExpression inputDictionary,
            ParameterExpression outputVariable,
            Type elementType,
            Type arrayType,
            List<ParameterExpression> indices)
        {
            ///// Intended code:
            /////
            ///// outputarray[i1, i2, ..., in]
            /////     = (<elementType>)DeepCopyByExpressionTreeObj(
            /////            (Object)inputarray[i1, i2, ..., in]);

            var indexTo = Expression.ArrayAccess(outputVariable, indices);

            var indexFrom = Expression.ArrayIndex(Expression.Convert(inputParameter, arrayType), indices);

            var forceDeepCopy = elementType != ObjectType;

            var rightSide =
                Expression.Convert(
                    Expression.Call(
                        DeepCopyByExpressionTreeObjMethod,
                        Expression.Convert(indexFrom, ObjectType),
                        Expression.Constant(forceDeepCopy, typeof(Boolean)),
                        inputDictionary),
                    elementType);

            var assignExpression = Expression.Assign(indexTo, rightSide);

            return assignExpression;
        }

        private static BlockExpression LoopIntoLoopExpression(
            ParameterExpression inputParameter,
            ParameterExpression indexVariable,
            Expression loopToEncapsulate,
            int dimension)
        {
            ///// Intended code:
            /////
            ///// int length = inputarray.GetLength(dimension);
            ///// i = 0;
            ///// while (true)
            ///// {
            /////     if (i >= length)
            /////     {
            /////         goto ENDLABELFORLOOP;
            /////     }
            /////     loopToEncapsulate;
            /////     i++;
            ///// }
            ///// ENDLABELFORLOOP:

            var lengthVariable = Expression.Variable(typeof(Int32));

            var endLabelForThisLoop = Expression.Label();

            var newLoop =
                Expression.Loop(
                    Expression.Block(
                        new ParameterExpression[0],
                        Expression.IfThen(
                            Expression.GreaterThanOrEqual(indexVariable, lengthVariable),
                            Expression.Break(endLabelForThisLoop)),
                        loopToEncapsulate,
                        Expression.PostIncrementAssign(indexVariable)),
                    endLabelForThisLoop);

            var lengthAssignment = GetLengthForDimensionExpression(lengthVariable, inputParameter, dimension);

            var indexAssignment = Expression.Assign(indexVariable, Expression.Constant(0));

            return Expression.Block(
                new[] { lengthVariable },
                lengthAssignment,
                indexAssignment,
                newLoop);
        }

        private static BinaryExpression GetLengthForDimensionExpression(
            ParameterExpression lengthVariable,
            ParameterExpression inputParameter,
            int i)
        {
            ///// Intended code:
            /////
            ///// length = ((Array)input).GetLength(i);

            var getLengthMethod = typeof(Array).GetMethod("GetLength", BindingFlags.Public | BindingFlags.Instance);

            var dimensionConstant = Expression.Constant(i);

            return Expression.Assign(
                lengthVariable,
                Expression.Call(
                    Expression.Convert(inputParameter, typeof(Array)),
                    getLengthMethod,
                    new[] { dimensionConstant }));
        }

        private static void FieldsCopyExpressions(Type type,
                                                  ParameterExpression inputParameter,
                                                  ParameterExpression inputDictionary,
                                                  ParameterExpression outputVariable,
                                                  ParameterExpression boxingVariable,
                                                  List<Expression> expressions)
        {
            var fields = GetAllRelevantFields(type);

            var readonlyFields = fields.Where(f => f.IsInitOnly).ToList();
            var writableFields = fields.Where(f => !f.IsInitOnly).ToList();

            ///// READONLY FIELDS COPY (with boxing)

            bool shouldUseBoxing = readonlyFields.Any();

            if (shouldUseBoxing)
            {
                var boxingExpression = Expression.Assign(boxingVariable, Expression.Convert(outputVariable, ObjectType));

                expressions.Add(boxingExpression);
            }

            foreach (var field in readonlyFields)
            {
                if (IsDelegate(field.FieldType))
                {
                    ReadonlyFieldToNullExpression(field, boxingVariable, expressions);
                }
                else
                {
                    ReadonlyFieldCopyExpression(type,
                                                field,
                                                inputParameter,
                                                inputDictionary,
                                                boxingVariable,
                                                expressions);
                }
            }

            if (shouldUseBoxing)
            {
                var unboxingExpression = Expression.Assign(outputVariable, Expression.Convert(boxingVariable, type));

                expressions.Add(unboxingExpression);
            }

            ///// NOT-READONLY FIELDS COPY

            foreach (var field in writableFields)
            {
                if (IsDelegate(field.FieldType))
                {
                    WritableFieldToNullExpression(field, outputVariable, expressions);
                }
                else
                {
                    WritableFieldCopyExpression(type,
                                                field,
                                                inputParameter,
                                                inputDictionary,
                                                outputVariable,
                                                expressions);
                }
            }
        }

        private static FieldInfo[] GetAllRelevantFields(Type type, bool forceAllFields = false)
        {
            var fieldsList = new List<FieldInfo>();

            var typeCache = type;

            while (typeCache != null)
            {
                fieldsList.AddRange(
                    typeCache
                        .GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy)
                        .Where(field => forceAllFields || IsTypeToDeepCopy(field.FieldType)));

                typeCache = typeCache.BaseType;
            }

            return fieldsList.ToArray();
        }

        private static FieldInfo[] GetAllFields(Type type)
        {
            return GetAllRelevantFields(type, forceAllFields: true);
        }

        private static readonly Type FieldInfoType = typeof(FieldInfo);
        private static readonly MethodInfo SetValueMethod = FieldInfoType.GetMethod("SetValue", new[] { ObjectType, ObjectType });

        private static void ReadonlyFieldToNullExpression(FieldInfo field, ParameterExpression boxingVariable, List<Expression> expressions)
        {
            // This option must be implemented by Reflection because of the following:
            // https://visualstudio.uservoice.com/forums/121579-visual-studio-2015/suggestions/2727812-allow-expression-assign-to-set-readonly-struct-f

            ///// Intended code:
            /////
            ///// fieldInfo.SetValue(boxing, <fieldtype>null);

            var fieldToNullExpression =
                    Expression.Call(
                        Expression.Constant(field),
                        SetValueMethod,
                        boxingVariable,
                        Expression.Constant(null, field.FieldType));

            expressions.Add(fieldToNullExpression);
        }

        private static readonly Type ThisType = typeof(DeepCopyByExpressionTrees);
        private static readonly MethodInfo DeepCopyByExpressionTreeObjMethod = ThisType.GetMethod("DeepCopyByExpressionTreeObj", BindingFlags.NonPublic | BindingFlags.Static);

        private static void ReadonlyFieldCopyExpression(Type type,
                                                        FieldInfo field,
                                                        ParameterExpression inputParameter,
                                                        ParameterExpression inputDictionary,
                                                        ParameterExpression boxingVariable,
                                                        List<Expression> expressions)
        {
            // This option must be implemented by Reflection (SetValueMethod) because of the following:
            // https://visualstudio.uservoice.com/forums/121579-visual-studio-2015/suggestions/2727812-allow-expression-assign-to-set-readonly-struct-f

            ///// Intended code:
            /////
            ///// fieldInfo.SetValue(boxing, DeepCopyByExpressionTreeObj((Object)((<type>)input).<field>))

            var fieldFrom = Expression.Field(Expression.Convert(inputParameter, type), field);

            var forceDeepCopy = field.FieldType != ObjectType;

            var fieldDeepCopyExpression =
                Expression.Call(
                    Expression.Constant(field, FieldInfoType),
                    SetValueMethod,
                    boxingVariable,
                    Expression.Call(
                        DeepCopyByExpressionTreeObjMethod,
                        Expression.Convert(fieldFrom, ObjectType),
                        Expression.Constant(forceDeepCopy, typeof(Boolean)),
                        inputDictionary));

            expressions.Add(fieldDeepCopyExpression);
        }

        private static void WritableFieldToNullExpression(FieldInfo field, ParameterExpression outputVariable, List<Expression> expressions)
        {
            ///// Intended code:
            /////
            ///// output.<field> = (<type>)null;

            var fieldTo = Expression.Field(outputVariable, field);

            var fieldToNullExpression =
                Expression.Assign(
                    fieldTo,
                    Expression.Constant(null, field.FieldType));

            expressions.Add(fieldToNullExpression);
        }

        private static void WritableFieldCopyExpression(Type type,
                                                        FieldInfo field,
                                                        ParameterExpression inputParameter,
                                                        ParameterExpression inputDictionary,
                                                        ParameterExpression outputVariable,
                                                        List<Expression> expressions)
        {
            ///// Intended code:
            /////
            ///// output.<field> = (<fieldType>)DeepCopyByExpressionTreeObj((Object)((<type>)input).<field>);

            var fieldFrom = Expression.Field(Expression.Convert(inputParameter, type), field);

            var fieldType = field.FieldType;

            var fieldTo = Expression.Field(outputVariable, field);

            var forceDeepCopy = field.FieldType != ObjectType;

            var fieldDeepCopyExpression =
                Expression.Assign(
                    fieldTo,
                    Expression.Convert(
                        Expression.Call(
                            DeepCopyByExpressionTreeObjMethod,
                            Expression.Convert(fieldFrom, ObjectType),
                            Expression.Constant(forceDeepCopy, typeof(Boolean)),
                            inputDictionary),
                        fieldType));

            expressions.Add(fieldDeepCopyExpression);
        }

        private static bool IsArray(Type type)
        {
            return type.IsArray;
        }

        private static bool IsDelegate(Type type)
        {
            return typeof(Delegate).IsAssignableFrom(type);
        }

        private static bool IsTypeToDeepCopy(Type type)
        {
            return IsClassOtherThanString(type)
                   || IsStructWhichNeedsDeepCopy(type);
        }

        private static bool IsClassOtherThanString(Type type)
        {
            return !type.IsValueType && type != typeof(String);
        }

        private static bool IsStructWhichNeedsDeepCopy(Type type)
        {
            // The following structure ensures that multiple threads can use the dictionary
            // even while dictionary is locked and being updated by other thread.
            // That is why we do not modify the old dictionary instance but
            // we replace it with a new instance everytime.

            bool isStructTypeToDeepCopy;

            if (!IsStructTypeToDeepCopyDictionary.TryGetValue(type, out isStructTypeToDeepCopy))
            {
                lock (IsStructTypeToDeepCopyDictionaryLocker)
                {
                    if (!IsStructTypeToDeepCopyDictionary.TryGetValue(type, out isStructTypeToDeepCopy))
                    {
                        isStructTypeToDeepCopy = IsStructWhichNeedsDeepCopy_NoDictionaryUsed(type);

                        var newDictionary = IsStructTypeToDeepCopyDictionary.ToDictionary(pair => pair.Key, pair => pair.Value);

                        newDictionary[type] = isStructTypeToDeepCopy;

                        IsStructTypeToDeepCopyDictionary = newDictionary;
                    }
                }
            }

            return isStructTypeToDeepCopy;
        }

        private static bool IsStructWhichNeedsDeepCopy_NoDictionaryUsed(Type type)
        {
            return IsStructOtherThanBasicValueTypes(type)
                   && HasInItsHierarchyFieldsWithClasses(type);
        }

        private static bool IsStructOtherThanBasicValueTypes(Type type)
        {
            return type.IsValueType
                   && !type.IsPrimitive
                   && !type.IsEnum
                   && type != typeof(Decimal);
        }

        private static bool HasInItsHierarchyFieldsWithClasses(Type type, HashSet<Type> alreadyCheckedTypes = null)
        {
            alreadyCheckedTypes = alreadyCheckedTypes ?? new HashSet<Type>();

            alreadyCheckedTypes.Add(type);

            var allFields = GetAllFields(type);

            var allFieldTypes = allFields.Select(f => f.FieldType).Distinct().ToList();

            var hasFieldsWithClasses = allFieldTypes.Any(IsClassOtherThanString);

            if (hasFieldsWithClasses)
            {
                return true;
            }

            var notBasicStructsTypes = allFieldTypes.Where(IsStructOtherThanBasicValueTypes).ToList();

            var typesToCheck = notBasicStructsTypes.Where(t => !alreadyCheckedTypes.Contains(t)).ToList();

            foreach (var typeToCheck in typesToCheck)
            {
                if (HasInItsHierarchyFieldsWithClasses(typeToCheck, alreadyCheckedTypes))
                {
                    return true;
                }
            }

            return false;
        }

        public class ReferenceEqualityComparer : EqualityComparer<Object>
        {
            public override bool Equals(object x, object y)
            {
                return ReferenceEquals(x, y);
            }

            public override int GetHashCode(object obj)
            {
                if (obj == null)
                {
                    return 0;
                }

                return obj.GetHashCode();
            }
        }
}