package funcs

import (
	"errors"
	"fmt"
	"sort"

	"github.com/zclconf/go-cty/cty"
	"github.com/zclconf/go-cty/cty/convert"
	"github.com/zclconf/go-cty/cty/function"
	"github.com/zclconf/go-cty/cty/function/stdlib"
	"github.com/zclconf/go-cty/cty/gocty"
)

var ElementFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.DynamicPseudoType,
		},
		{
			Name: "index",
			Type: cty.Number,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		list := args[0]
		listTy := list.Type()
		switch {
		case listTy.IsListType():
			return listTy.ElementType(), nil
		case listTy.IsTupleType():
			if !args[1].IsKnown() {
				// If the index isn't known yet then we can't predict the
				// result type since each tuple element can have its own type.
				return cty.DynamicPseudoType, nil
			}

			etys := listTy.TupleElementTypes()
			var index int
			err := gocty.FromCtyValue(args[1], &index)
			if err != nil {
				// e.g. fractional number where whole number is required
				return cty.DynamicPseudoType, fmt.Errorf("invalid index: %s", err)
			}
			if len(etys) == 0 {
				return cty.DynamicPseudoType, errors.New("cannot use element function with an empty list")
			}
			index = index % len(etys)
			return etys[index], nil
		default:
			return cty.DynamicPseudoType, fmt.Errorf("cannot read elements from %s", listTy.FriendlyName())
		}
	},
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		var index int
		err := gocty.FromCtyValue(args[1], &index)
		if err != nil {
			// can't happen because we checked this in the Type function above
			return cty.DynamicVal, fmt.Errorf("invalid index: %s", err)
		}

		if !args[0].IsKnown() {
			return cty.UnknownVal(retType), nil
		}

		l := args[0].LengthInt()
		if l == 0 {
			return cty.DynamicVal, errors.New("cannot use element function with an empty list")
		}
		index = index % l

		// We did all the necessary type checks in the type function above,
		// so this is guaranteed not to fail.
		return args[0].Index(cty.NumberIntVal(int64(index))), nil
	},
})

var LengthFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name:             "value",
			Type:             cty.DynamicPseudoType,
			AllowDynamicType: true,
			AllowUnknown:     true,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		collTy := args[0].Type()
		switch {
		case collTy == cty.String || collTy.IsTupleType() || collTy.IsObjectType() || collTy.IsListType() || collTy.IsMapType() || collTy.IsSetType() || collTy == cty.DynamicPseudoType:
			return cty.Number, nil
		default:
			return cty.Number, errors.New("argument must be a string, a collection type, or a structural type")
		}
	},
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		coll := args[0]
		collTy := args[0].Type()
		switch {
		case collTy == cty.DynamicPseudoType:
			return cty.UnknownVal(cty.Number), nil
		case collTy.IsTupleType():
			l := len(collTy.TupleElementTypes())
			return cty.NumberIntVal(int64(l)), nil
		case collTy.IsObjectType():
			l := len(collTy.AttributeTypes())
			return cty.NumberIntVal(int64(l)), nil
		case collTy == cty.String:
			// We'll delegate to the cty stdlib strlen function here, because
			// it deals with all of the complexities of tokenizing unicode
			// grapheme clusters.
			return stdlib.Strlen(coll)
		case collTy.IsListType() || collTy.IsSetType() || collTy.IsMapType():
			return coll.Length(), nil
		default:
			// Should never happen, because of the checks in our Type func above
			return cty.UnknownVal(cty.Number), errors.New("impossible value type for length(...)")
		}
	},
})

// CoalesceFunc constructs a function that takes any number of arguments and
// returns the first one that isn't empty. This function was copied from go-cty
// stdlib and modified so that it returns the first *non-empty* non-null element
// from a sequence, instead of merely the first non-null.
var CoalesceFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name:             "vals",
		Type:             cty.DynamicPseudoType,
		AllowUnknown:     true,
		AllowDynamicType: true,
		AllowNull:        true,
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		argTypes := make([]cty.Type, len(args))
		for i, val := range args {
			argTypes[i] = val.Type()
		}
		retType, _ := convert.UnifyUnsafe(argTypes)
		if retType == cty.NilType {
			return cty.NilType, errors.New("all arguments must have the same type")
		}
		return retType, nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		for _, argVal := range args {
			// We already know this will succeed because of the checks in our Type func above
			argVal, _ = convert.Convert(argVal, retType)
			if !argVal.IsKnown() {
				return cty.UnknownVal(retType), nil
			}
			if argVal.IsNull() {
				continue
			}
			if retType == cty.String && argVal.RawEquals(cty.StringVal("")) {
				continue
			}

			return argVal, nil
		}
		return cty.NilVal, errors.New("no non-null, non-empty-string arguments")
	},
})

// CoalesceListFunc constructs a function that takes any number of list arguments
// and returns the first one that isn't empty.
var CoalesceListFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name:             "vals",
		Type:             cty.DynamicPseudoType,
		AllowUnknown:     true,
		AllowDynamicType: true,
		AllowNull:        true,
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		if len(args) == 0 {
			return cty.NilType, errors.New("at least one argument is required")
		}

		argTypes := make([]cty.Type, len(args))

		for i, arg := range args {
			// if any argument is unknown, we can't be certain know which type we will return
			if !arg.IsKnown() {
				return cty.DynamicPseudoType, nil
			}
			ty := arg.Type()

			if !ty.IsListType() && !ty.IsTupleType() {
				return cty.NilType, errors.New("coalescelist arguments must be lists or tuples")
			}

			argTypes[i] = arg.Type()
		}

		last := argTypes[0]
		// If there are mixed types, we have to return a dynamic type.
		for _, next := range argTypes[1:] {
			if !next.Equals(last) {
				return cty.DynamicPseudoType, nil
			}
		}

		return last, nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		for _, arg := range args {
			if !arg.IsKnown() {
				// If we run into an unknown list at some point, we can't
				// predict the final result yet. (If there's a known, non-empty
				// arg before this then we won't get here.)
				return cty.UnknownVal(retType), nil
			}

			if arg.LengthInt() > 0 {
				return arg, nil
			}
		}

		return cty.NilVal, errors.New("no non-null arguments")
	},
})

// CompactFunc constructs a function that takes a list of strings and returns a new list
// with any empty string elements removed.
var CompactFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.List(cty.String),
		},
	},
	Type: function.StaticReturnType(cty.List(cty.String)),
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		listVal := args[0]
		if !listVal.IsWhollyKnown() {
			// If some of the element values aren't known yet then we
			// can't yet return a compacted list
			return cty.UnknownVal(retType), nil
		}

		var outputList []cty.Value

		for it := listVal.ElementIterator(); it.Next(); {
			_, v := it.Element()
			if v.IsNull() || v.AsString() == "" {
				continue
			}
			outputList = append(outputList, v)
		}

		if len(outputList) == 0 {
			return cty.ListValEmpty(cty.String), nil
		}

		return cty.ListVal(outputList), nil
	},
})

// ContainsFunc constructs a function that determines whether a given list or
// set contains a given single value as one of its elements.
var ContainsFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.DynamicPseudoType,
		},
		{
			Name: "value",
			Type: cty.DynamicPseudoType,
		},
	},
	Type: function.StaticReturnType(cty.Bool),
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		arg := args[0]
		ty := arg.Type()

		if !ty.IsListType() && !ty.IsTupleType() && !ty.IsSetType() {
			return cty.NilVal, errors.New("argument must be list, tuple, or set")
		}

		_, err = Index(cty.TupleVal(arg.AsValueSlice()), args[1])
		if err != nil {
			return cty.False, nil
		}

		return cty.True, nil
	},
})

// IndexFunc constructs a function that finds the element index for a given value in a list.
var IndexFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.DynamicPseudoType,
		},
		{
			Name: "value",
			Type: cty.DynamicPseudoType,
		},
	},
	Type: function.StaticReturnType(cty.Number),
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		if !(args[0].Type().IsListType() || args[0].Type().IsTupleType()) {
			return cty.NilVal, errors.New("argument must be a list or tuple")
		}

		if !args[0].IsKnown() {
			return cty.UnknownVal(cty.Number), nil
		}

		if args[0].LengthInt() == 0 { // Easy path
			return cty.NilVal, errors.New("cannot search an empty list")
		}

		for it := args[0].ElementIterator(); it.Next(); {
			i, v := it.Element()
			eq, err := stdlib.Equal(v, args[1])
			if err != nil {
				return cty.NilVal, err
			}
			if !eq.IsKnown() {
				return cty.UnknownVal(cty.Number), nil
			}
			if eq.True() {
				return i, nil
			}
		}
		return cty.NilVal, errors.New("item not found")

	},
})

// DistinctFunc constructs a function that takes a list and returns a new list
// with any duplicate elements removed.
var DistinctFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.List(cty.DynamicPseudoType),
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		return args[0].Type(), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		listVal := args[0]

		if !listVal.IsWhollyKnown() {
			return cty.UnknownVal(retType), nil
		}
		var list []cty.Value

		for it := listVal.ElementIterator(); it.Next(); {
			_, v := it.Element()
			list, err = appendIfMissing(list, v)
			if err != nil {
				return cty.NilVal, err
			}
		}

		if len(list) == 0 {
			return cty.ListValEmpty(retType.ElementType()), nil
		}
		return cty.ListVal(list), nil
	},
})

// ChunklistFunc constructs a function that splits a single list into fixed-size chunks,
// returning a list of lists.
var ChunklistFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.List(cty.DynamicPseudoType),
		},
		{
			Name: "size",
			Type: cty.Number,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		return cty.List(args[0].Type()), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		listVal := args[0]
		if !listVal.IsKnown() {
			return cty.UnknownVal(retType), nil
		}

		if listVal.LengthInt() == 0 {
			return cty.ListValEmpty(listVal.Type()), nil
		}

		var size int
		err = gocty.FromCtyValue(args[1], &size)
		if err != nil {
			return cty.NilVal, fmt.Errorf("invalid index: %s", err)
		}

		if size < 0 {
			return cty.NilVal, errors.New("the size argument must be positive")
		}

		output := make([]cty.Value, 0)

		// if size is 0, returns a list made of the initial list
		if size == 0 {
			output = append(output, listVal)
			return cty.ListVal(output), nil
		}

		chunk := make([]cty.Value, 0)

		l := args[0].LengthInt()
		i := 0

		for it := listVal.ElementIterator(); it.Next(); {
			_, v := it.Element()
			chunk = append(chunk, v)

			// Chunk when index isn't 0, or when reaching the values's length
			if (i+1)%size == 0 || (i+1) == l {
				output = append(output, cty.ListVal(chunk))
				chunk = make([]cty.Value, 0)
			}
			i++
		}

		return cty.ListVal(output), nil
	},
})

// FlattenFunc constructs a function that takes a list and replaces any elements
// that are lists with a flattened sequence of the list contents.
var FlattenFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.DynamicPseudoType,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		if !args[0].IsWhollyKnown() {
			return cty.DynamicPseudoType, nil
		}

		argTy := args[0].Type()
		if !argTy.IsListType() && !argTy.IsSetType() && !argTy.IsTupleType() {
			return cty.NilType, errors.New("can only flatten lists, sets and tuples")
		}

		retVal, known := flattener(args[0])
		if !known {
			return cty.DynamicPseudoType, nil
		}

		tys := make([]cty.Type, len(retVal))
		for i, ty := range retVal {
			tys[i] = ty.Type()
		}
		return cty.Tuple(tys), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		inputList := args[0]
		if inputList.LengthInt() == 0 {
			return cty.EmptyTupleVal, nil
		}

		out, known := flattener(inputList)
		if !known {
			return cty.UnknownVal(retType), nil
		}

		return cty.TupleVal(out), nil
	},
})

// Flatten until it's not a cty.List, and return whether the value is known.
// We can flatten lists with unknown values, as long as they are not
// lists themselves.
func flattener(flattenList cty.Value) ([]cty.Value, bool) {
	out := make([]cty.Value, 0)
	for it := flattenList.ElementIterator(); it.Next(); {
		_, val := it.Element()
		if val.Type().IsListType() || val.Type().IsSetType() || val.Type().IsTupleType() {
			if !val.IsKnown() {
				return out, false
			}

			res, known := flattener(val)
			if !known {
				return res, known
			}
			out = append(out, res...)
		} else {
			out = append(out, val)
		}
	}
	return out, true
}

// KeysFunc constructs a function that takes a map and returns a sorted list of the map keys.
var KeysFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name:         "inputMap",
			Type:         cty.DynamicPseudoType,
			AllowUnknown: true,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		ty := args[0].Type()
		switch {
		case ty.IsMapType():
			return cty.List(cty.String), nil
		case ty.IsObjectType():
			atys := ty.AttributeTypes()
			if len(atys) == 0 {
				return cty.EmptyTuple, nil
			}
			// All of our result elements will be strings, and atys just
			// decides how many there are.
			etys := make([]cty.Type, len(atys))
			for i := range etys {
				etys[i] = cty.String
			}
			return cty.Tuple(etys), nil
		default:
			return cty.DynamicPseudoType, function.NewArgErrorf(0, "must have map or object type")
		}
	},
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		m := args[0]
		var keys []cty.Value

		switch {
		case m.Type().IsObjectType():
			// In this case we allow unknown values so we must work only with
			// the attribute _types_, not with the value itself.
			var names []string
			for name := range m.Type().AttributeTypes() {
				names = append(names, name)
			}
			sort.Strings(names) // same ordering guaranteed by cty's ElementIterator
			if len(names) == 0 {
				return cty.EmptyTupleVal, nil
			}
			keys = make([]cty.Value, len(names))
			for i, name := range names {
				keys[i] = cty.StringVal(name)
			}
			return cty.TupleVal(keys), nil
		default:
			if !m.IsKnown() {
				return cty.UnknownVal(retType), nil
			}

			// cty guarantees that ElementIterator will iterate in lexicographical
			// order by key.
			for it := args[0].ElementIterator(); it.Next(); {
				k, _ := it.Element()
				keys = append(keys, k)
			}
			if len(keys) == 0 {
				return cty.ListValEmpty(cty.String), nil
			}
			return cty.ListVal(keys), nil
		}
	},
})

// ListFunc constructs a function that takes an arbitrary number of arguments
// and returns a list containing those values in the same order.
//
// This function is deprecated in Terraform v0.12
var ListFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name:             "vals",
		Type:             cty.DynamicPseudoType,
		AllowUnknown:     true,
		AllowDynamicType: true,
		AllowNull:        true,
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		if len(args) == 0 {
			return cty.NilType, errors.New("at least one argument is required")
		}

		argTypes := make([]cty.Type, len(args))

		for i, arg := range args {
			argTypes[i] = arg.Type()
		}

		retType, _ := convert.UnifyUnsafe(argTypes)
		if retType == cty.NilType {
			return cty.NilType, errors.New("all arguments must have the same type")
		}

		return cty.List(retType), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		newList := make([]cty.Value, 0, len(args))

		for _, arg := range args {
			// We already know this will succeed because of the checks in our Type func above
			arg, _ = convert.Convert(arg, retType.ElementType())
			newList = append(newList, arg)
		}

		return cty.ListVal(newList), nil
	},
})

// LookupFunc constructs a function that performs dynamic lookups of map types.
var LookupFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "inputMap",
			Type: cty.DynamicPseudoType,
		},
		{
			Name: "key",
			Type: cty.String,
		},
	},
	VarParam: &function.Parameter{
		Name:             "default",
		Type:             cty.DynamicPseudoType,
		AllowUnknown:     true,
		AllowDynamicType: true,
		AllowNull:        true,
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		if len(args) < 1 || len(args) > 3 {
			return cty.NilType, fmt.Errorf("lookup() takes two or three arguments, got %d", len(args))
		}

		ty := args[0].Type()

		switch {
		case ty.IsObjectType():
			if !args[1].IsKnown() {
				return cty.DynamicPseudoType, nil
			}

			key := args[1].AsString()
			if ty.HasAttribute(key) {
				return args[0].GetAttr(key).Type(), nil
			} else if len(args) == 3 {
				// if the key isn't found but a default is provided,
				// return the default type
				return args[2].Type(), nil
			}
			return cty.DynamicPseudoType, function.NewArgErrorf(0, "the given object has no attribute %q", key)
		case ty.IsMapType():
			return ty.ElementType(), nil
		default:
			return cty.NilType, function.NewArgErrorf(0, "lookup() requires a map as the first argument")
		}
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		var defaultVal cty.Value
		defaultValueSet := false

		if len(args) == 3 {
			defaultVal = args[2]
			defaultValueSet = true
		}

		mapVar := args[0]
		lookupKey := args[1].AsString()

		if !mapVar.IsWhollyKnown() {
			return cty.UnknownVal(retType), nil
		}

		if mapVar.Type().IsObjectType() {
			if mapVar.Type().HasAttribute(lookupKey) {
				return mapVar.GetAttr(lookupKey), nil
			}
		} else if mapVar.HasIndex(cty.StringVal(lookupKey)) == cty.True {
			v := mapVar.Index(cty.StringVal(lookupKey))
			if ty := v.Type(); !ty.Equals(cty.NilType) {
				switch {
				case ty.Equals(cty.String):
					return cty.StringVal(v.AsString()), nil
				case ty.Equals(cty.Number):
					return cty.NumberVal(v.AsBigFloat()), nil
				case ty.Equals(cty.Bool):
					return cty.BoolVal(v.True()), nil
				default:
					return cty.NilVal, errors.New("lookup() can only be used with maps of primitive types")
				}
			}
		}

		if defaultValueSet {
			defaultVal, err = convert.Convert(defaultVal, retType)
			if err != nil {
				return cty.NilVal, err
			}
			return defaultVal, nil
		}

		return cty.UnknownVal(cty.DynamicPseudoType), fmt.Errorf(
			"lookup failed to find '%s'", lookupKey)
	},
})

// MapFunc constructs a function that takes an even number of arguments and
// returns a map whose elements are constructed from consecutive pairs of arguments.
//
// This function is deprecated in Terraform v0.12
var MapFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name:             "vals",
		Type:             cty.DynamicPseudoType,
		AllowUnknown:     true,
		AllowDynamicType: true,
		AllowNull:        true,
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		if len(args) < 2 || len(args)%2 != 0 {
			return cty.NilType, fmt.Errorf("map requires an even number of two or more arguments, got %d", len(args))
		}

		argTypes := make([]cty.Type, len(args)/2)
		index := 0

		for i := 0; i < len(args); i += 2 {
			argTypes[index] = args[i+1].Type()
			index++
		}

		valType, _ := convert.UnifyUnsafe(argTypes)
		if valType == cty.NilType {
			return cty.NilType, errors.New("all arguments must have the same type")
		}

		return cty.Map(valType), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		for _, arg := range args {
			if !arg.IsWhollyKnown() {
				return cty.UnknownVal(retType), nil
			}
		}

		outputMap := make(map[string]cty.Value)

		for i := 0; i < len(args); i += 2 {

			key := args[i].AsString()

			err := gocty.FromCtyValue(args[i], &key)
			if err != nil {
				return cty.NilVal, err
			}

			val := args[i+1]

			var variable cty.Value
			err = gocty.FromCtyValue(val, &variable)
			if err != nil {
				return cty.NilVal, err
			}

			// We already know this will succeed because of the checks in our Type func above
			variable, _ = convert.Convert(variable, retType.ElementType())

			// Check for duplicate keys
			if _, ok := outputMap[key]; ok {
				return cty.NilVal, fmt.Errorf("argument %d is a duplicate key: %q", i+1, key)
			}
			outputMap[key] = variable
		}

		return cty.MapVal(outputMap), nil
	},
})

// MatchkeysFunc constructs a function that constructs a new list by taking a
// subset of elements from one list whose indexes match the corresponding
// indexes of values in another list.
var MatchkeysFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "values",
			Type: cty.List(cty.DynamicPseudoType),
		},
		{
			Name: "keys",
			Type: cty.List(cty.DynamicPseudoType),
		},
		{
			Name: "searchset",
			Type: cty.List(cty.DynamicPseudoType),
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		ty, _ := convert.UnifyUnsafe([]cty.Type{args[1].Type(), args[2].Type()})
		if ty == cty.NilType {
			return cty.NilType, errors.New("keys and searchset must be of the same type")
		}

		// the return type is based on args[0] (values)
		return args[0].Type(), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		if !args[0].IsKnown() {
			return cty.UnknownVal(cty.List(retType.ElementType())), nil
		}

		if args[0].LengthInt() != args[1].LengthInt() {
			return cty.ListValEmpty(retType.ElementType()), errors.New("length of keys and values should be equal")
		}

		output := make([]cty.Value, 0)
		values := args[0]

		// Keys and searchset must be the same type.
		// We can skip error checking here because we've already verified that
		// they can be unified in the Type function
		ty, _ := convert.UnifyUnsafe([]cty.Type{args[1].Type(), args[2].Type()})
		keys, _ := convert.Convert(args[1], ty)
		searchset, _ := convert.Convert(args[2], ty)

		// if searchset is empty, return an empty list.
		if searchset.LengthInt() == 0 {
			return cty.ListValEmpty(retType.ElementType()), nil
		}

		if !values.IsWhollyKnown() || !keys.IsWhollyKnown() {
			return cty.UnknownVal(retType), nil
		}

		i := 0
		for it := keys.ElementIterator(); it.Next(); {
			_, key := it.Element()
			for iter := searchset.ElementIterator(); iter.Next(); {
				_, search := iter.Element()
				eq, err := stdlib.Equal(key, search)
				if err != nil {
					return cty.NilVal, err
				}
				if !eq.IsKnown() {
					return cty.ListValEmpty(retType.ElementType()), nil
				}
				if eq.True() {
					v := values.Index(cty.NumberIntVal(int64(i)))
					output = append(output, v)
					break
				}
			}
			i++
		}

		// if we haven't matched any key, then output is an empty list.
		if len(output) == 0 {
			return cty.ListValEmpty(retType.ElementType()), nil
		}
		return cty.ListVal(output), nil
	},
})

// MergeFunc constructs a function that takes an arbitrary number of maps and
// returns a single map that contains a merged set of elements from all of the maps.
//
// If more than one given map defines the same key then the one that is later in
// the argument sequence takes precedence.
var MergeFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name:             "maps",
		Type:             cty.DynamicPseudoType,
		AllowDynamicType: true,
	},
	Type: function.StaticReturnType(cty.DynamicPseudoType),
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		outputMap := make(map[string]cty.Value)

		for _, arg := range args {
			if !arg.IsWhollyKnown() {
				return cty.UnknownVal(retType), nil
			}
			if !arg.Type().IsObjectType() && !arg.Type().IsMapType() {
				return cty.NilVal, fmt.Errorf("arguments must be maps or objects, got %#v", arg.Type().FriendlyName())
			}
			for it := arg.ElementIterator(); it.Next(); {
				k, v := it.Element()
				outputMap[k.AsString()] = v
			}
		}
		return cty.ObjectVal(outputMap), nil
	},
})

// ReverseFunc takes a sequence and produces a new sequence of the same length
// with all of the same elements as the given sequence but in reverse order.
var ReverseFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.DynamicPseudoType,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		argTy := args[0].Type()
		switch {
		case argTy.IsTupleType():
			argTys := argTy.TupleElementTypes()
			retTys := make([]cty.Type, len(argTys))
			for i, ty := range argTys {
				retTys[len(retTys)-i-1] = ty
			}
			return cty.Tuple(retTys), nil
		case argTy.IsListType(), argTy.IsSetType(): // We accept sets here to mimic the usual behavior of auto-converting to list
			return cty.List(argTy.ElementType()), nil
		default:
			return cty.NilType, function.NewArgErrorf(0, "can only reverse list or tuple values, not %s", argTy.FriendlyName())
		}
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		in := args[0].AsValueSlice()
		outVals := make([]cty.Value, len(in))
		for i, v := range in {
			outVals[len(outVals)-i-1] = v
		}
		switch {
		case retType.IsTupleType():
			return cty.TupleVal(outVals), nil
		default:
			if len(outVals) == 0 {
				return cty.ListValEmpty(retType.ElementType()), nil
			}
			return cty.ListVal(outVals), nil
		}
	},
})

// SetProductFunc calculates the cartesian product of two or more sets or
// sequences. If the arguments are all lists then the result is a list of tuples,
// preserving the ordering of all of the input lists. Otherwise the result is a
// set of tuples.
var SetProductFunc = function.New(&function.Spec{
	Params: []function.Parameter{},
	VarParam: &function.Parameter{
		Name: "sets",
		Type: cty.DynamicPseudoType,
	},
	Type: func(args []cty.Value) (retType cty.Type, err error) {
		if len(args) < 2 {
			return cty.NilType, errors.New("at least two arguments are required")
		}

		listCount := 0
		elemTys := make([]cty.Type, len(args))
		for i, arg := range args {
			aty := arg.Type()
			switch {
			case aty.IsSetType():
				elemTys[i] = aty.ElementType()
			case aty.IsListType():
				elemTys[i] = aty.ElementType()
				listCount++
			case aty.IsTupleType():
				// We can accept a tuple type only if there's some common type
				// that all of its elements can be converted to.
				allEtys := aty.TupleElementTypes()
				if len(allEtys) == 0 {
					elemTys[i] = cty.DynamicPseudoType
					listCount++
					break
				}
				ety, _ := convert.UnifyUnsafe(allEtys)
				if ety == cty.NilType {
					return cty.NilType, function.NewArgErrorf(i, "all elements must be of the same type")
				}
				elemTys[i] = ety
				listCount++
			default:
				return cty.NilType, function.NewArgErrorf(i, "a set or a list is required")
			}
		}

		if listCount == len(args) {
			return cty.List(cty.Tuple(elemTys)), nil
		}
		return cty.Set(cty.Tuple(elemTys)), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		ety := retType.ElementType()

		total := 1
		for _, arg := range args {
			// Because of our type checking function, we are guaranteed that
			// all of the arguments are known, non-null values of types that
			// support LengthInt.
			total *= arg.LengthInt()
		}

		if total == 0 {
			// If any of the arguments was an empty collection then our result
			// is also an empty collection, which we'll short-circuit here.
			if retType.IsListType() {
				return cty.ListValEmpty(ety), nil
			}
			return cty.SetValEmpty(ety), nil
		}

		subEtys := ety.TupleElementTypes()
		product := make([][]cty.Value, total)

		b := make([]cty.Value, total*len(args))
		n := make([]int, len(args))
		s := 0
		argVals := make([][]cty.Value, len(args))
		for i, arg := range args {
			argVals[i] = arg.AsValueSlice()
		}

		for i := range product {
			e := s + len(args)
			pi := b[s:e]
			product[i] = pi
			s = e

			for j, n := range n {
				val := argVals[j][n]
				ty := subEtys[j]
				if !val.Type().Equals(ty) {
					var err error
					val, err = convert.Convert(val, ty)
					if err != nil {
						// Should never happen since we checked this in our
						// type-checking function.
						return cty.NilVal, fmt.Errorf("failed to convert argVals[%d][%d] to %s; this is a bug in Terraform", j, n, ty.FriendlyName())
					}
				}
				pi[j] = val
			}

			for j := len(n) - 1; j >= 0; j-- {
				n[j]++
				if n[j] < len(argVals[j]) {
					break
				}
				n[j] = 0
			}
		}

		productVals := make([]cty.Value, total)
		for i, vals := range product {
			productVals[i] = cty.TupleVal(vals)
		}

		if retType.IsListType() {
			return cty.ListVal(productVals), nil
		}
		return cty.SetVal(productVals), nil
	},
})

// SliceFunc constructs a function that extracts some consecutive elements
// from within a list.
var SliceFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "list",
			Type: cty.DynamicPseudoType,
		},
		{
			Name: "start_index",
			Type: cty.Number,
		},
		{
			Name: "end_index",
			Type: cty.Number,
		},
	},
	Type: func(args []cty.Value) (cty.Type, error) {
		arg := args[0]
		argTy := arg.Type()

		if argTy.IsSetType() {
			return cty.NilType, function.NewArgErrorf(0, "cannot slice a set, because its elements do not have indices; use the tolist function to force conversion to list if the ordering of the result is not important")
		}
		if !argTy.IsListType() && !argTy.IsTupleType() {
			return cty.NilType, function.NewArgErrorf(0, "must be a list or tuple value")
		}

		startIndex, endIndex, idxsKnown, err := sliceIndexes(args)
		if err != nil {
			return cty.NilType, err
		}

		if argTy.IsListType() {
			return argTy, nil
		}

		if !idxsKnown {
			// If we don't know our start/end indices then we can't predict
			// the result type if we're planning to return a tuple.
			return cty.DynamicPseudoType, nil
		}
		return cty.Tuple(argTy.TupleElementTypes()[startIndex:endIndex]), nil
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		inputList := args[0]

		if retType == cty.DynamicPseudoType {
			return cty.DynamicVal, nil
		}

		// we ignore idxsKnown return value here because the indices are always
		// known here, or else the call would've short-circuited.
		startIndex, endIndex, _, err := sliceIndexes(args)
		if err != nil {
			return cty.NilVal, err
		}

		if endIndex-startIndex == 0 {
			if retType.IsTupleType() {
				return cty.EmptyTupleVal, nil
			}
			return cty.ListValEmpty(retType.ElementType()), nil
		}

		outputList := inputList.AsValueSlice()[startIndex:endIndex]

		if retType.IsTupleType() {
			return cty.TupleVal(outputList), nil
		}

		return cty.ListVal(outputList), nil
	},
})

func sliceIndexes(args []cty.Value) (int, int, bool, error) {
	var startIndex, endIndex, length int
	var startKnown, endKnown, lengthKnown bool

	if args[0].Type().IsTupleType() || args[0].IsKnown() { // if it's a tuple then we always know the length by the type, but lists must be known
		length = args[0].LengthInt()
		lengthKnown = true
	}

	if args[1].IsKnown() {
		if err := gocty.FromCtyValue(args[1], &startIndex); err != nil {
			return 0, 0, false, function.NewArgErrorf(1, "invalid start index: %s", err)
		}
		if startIndex < 0 {
			return 0, 0, false, function.NewArgErrorf(1, "start index must not be less than zero")
		}
		if lengthKnown && startIndex > length {
			return 0, 0, false, function.NewArgErrorf(1, "start index must not be greater than the length of the list")
		}
		startKnown = true
	}
	if args[2].IsKnown() {
		if err := gocty.FromCtyValue(args[2], &endIndex); err != nil {
			return 0, 0, false, function.NewArgErrorf(2, "invalid end index: %s", err)
		}
		if endIndex < 0 {
			return 0, 0, false, function.NewArgErrorf(2, "end index must not be less than zero")
		}
		if lengthKnown && endIndex > length {
			return 0, 0, false, function.NewArgErrorf(2, "end index must not be greater than the length of the list")
		}
		endKnown = true
	}
	if startKnown && endKnown {
		if startIndex > endIndex {
			return 0, 0, false, function.NewArgErrorf(1, "start index must not be greater than end index")
		}
	}
	return startIndex, endIndex, startKnown && endKnown, nil
}

// TransposeFunc contructs a function that takes a map of lists of strings and
// TransposeFunc constructs a function that takes a map of lists of strings and
// swaps the keys and values to produce a new map of lists of strings.
var TransposeFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "values",
			Type: cty.Map(cty.List(cty.String)),
		},
	},
	Type: function.StaticReturnType(cty.Map(cty.List(cty.String))),
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		inputMap := args[0]
		if !inputMap.IsWhollyKnown() {
			return cty.UnknownVal(retType), nil
		}

		outputMap := make(map[string]cty.Value)
		tmpMap := make(map[string][]string)

		for it := inputMap.ElementIterator(); it.Next(); {
			inKey, inVal := it.Element()
			for iter := inVal.ElementIterator(); iter.Next(); {
				_, val := iter.Element()
				if !val.Type().Equals(cty.String) {
					return cty.MapValEmpty(cty.List(cty.String)), errors.New("input must be a map of lists of strings")
				}

				outKey := val.AsString()
				if _, ok := tmpMap[outKey]; !ok {
					tmpMap[outKey] = make([]string, 0)
				}
				outVal := tmpMap[outKey]
				outVal = append(outVal, inKey.AsString())
				sort.Strings(outVal)
				tmpMap[outKey] = outVal
			}
		}

		for outKey, outVal := range tmpMap {
			values := make([]cty.Value, 0)
			for _, v := range outVal {
				values = append(values, cty.StringVal(v))
			}
			outputMap[outKey] = cty.ListVal(values)
		}

		return cty.MapVal(outputMap), nil
	},
})

// ValuesFunc constructs a function that returns a list of the map values,
// in the order of the sorted keys.
var ValuesFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "values",
			Type: cty.DynamicPseudoType,
		},
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		ty := args[0].Type()
		if ty.IsMapType() {
			return cty.List(ty.ElementType()), nil
		} else if ty.IsObjectType() {
			// The result is a tuple type with all of the same types as our
			// object type's attributes, sorted in lexicographical order by the
			// keys. (This matches the sort order guaranteed by ElementIterator
			// on a cty object value.)
			atys := ty.AttributeTypes()
			if len(atys) == 0 {
				return cty.EmptyTuple, nil
			}
			attrNames := make([]string, 0, len(atys))
			for name := range atys {
				attrNames = append(attrNames, name)
			}
			sort.Strings(attrNames)

			tys := make([]cty.Type, len(attrNames))
			for i, name := range attrNames {
				tys[i] = atys[name]
			}
			return cty.Tuple(tys), nil
		}
		return cty.NilType, errors.New("values() requires a map as the first argument")
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		mapVar := args[0]

		// We can just iterate the map/object value here because cty guarantees
		// that these types always iterate in key lexicographical order.
		var values []cty.Value
		for it := mapVar.ElementIterator(); it.Next(); {
			_, val := it.Element()
			values = append(values, val)
		}

		if retType.IsTupleType() {
			return cty.TupleVal(values), nil
		}
		if len(values) == 0 {
			return cty.ListValEmpty(retType.ElementType()), nil
		}
		return cty.ListVal(values), nil
	},
})

// ZipmapFunc constructs a function that constructs a map from a list of keys
// and a corresponding list of values.
var ZipmapFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "keys",
			Type: cty.List(cty.String),
		},
		{
			Name: "values",
			Type: cty.DynamicPseudoType,
		},
	},
	Type: func(args []cty.Value) (ret cty.Type, err error) {
		keys := args[0]
		values := args[1]
		valuesTy := values.Type()

		switch {
		case valuesTy.IsListType():
			return cty.Map(values.Type().ElementType()), nil
		case valuesTy.IsTupleType():
			if !keys.IsWhollyKnown() {
				// Since zipmap with a tuple produces an object, we need to know
				// all of the key names before we can predict our result type.
				return cty.DynamicPseudoType, nil
			}

			keysRaw := keys.AsValueSlice()
			valueTypesRaw := valuesTy.TupleElementTypes()
			if len(keysRaw) != len(valueTypesRaw) {
				return cty.NilType, fmt.Errorf("number of keys (%d) does not match number of values (%d)", len(keysRaw), len(valueTypesRaw))
			}
			atys := make(map[string]cty.Type, len(valueTypesRaw))
			for i, keyVal := range keysRaw {
				if keyVal.IsNull() {
					return cty.NilType, fmt.Errorf("keys list has null value at index %d", i)
				}
				key := keyVal.AsString()
				atys[key] = valueTypesRaw[i]
			}
			return cty.Object(atys), nil

		default:
			return cty.NilType, errors.New("values argument must be a list or tuple value")
		}
	},
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		keys := args[0]
		values := args[1]

		if !keys.IsWhollyKnown() {
			// Unknown map keys and object attributes are not supported, so
			// our entire result must be unknown in this case.
			return cty.UnknownVal(retType), nil
		}

		// both keys and values are guaranteed to be shallowly-known here,
		// because our declared params above don't allow unknown or null values.
		if keys.LengthInt() != values.LengthInt() {
			return cty.NilVal, fmt.Errorf("number of keys (%d) does not match number of values (%d)", keys.LengthInt(), values.LengthInt())
		}

		output := make(map[string]cty.Value)

		i := 0
		for it := keys.ElementIterator(); it.Next(); {
			_, v := it.Element()
			val := values.Index(cty.NumberIntVal(int64(i)))
			output[v.AsString()] = val
			i++
		}

		switch {
		case retType.IsMapType():
			if len(output) == 0 {
				return cty.MapValEmpty(retType.ElementType()), nil
			}
			return cty.MapVal(output), nil
		case retType.IsObjectType():
			return cty.ObjectVal(output), nil
		default:
			// Should never happen because the type-check function should've
			// caught any other case.
			return cty.NilVal, fmt.Errorf("internally selected incorrect result type %s (this is a bug)", retType.FriendlyName())
		}
	},
})

// helper function to add an element to a list, if it does not already exist
func appendIfMissing(slice []cty.Value, element cty.Value) ([]cty.Value, error) {
	for _, ele := range slice {
		eq, err := stdlib.Equal(ele, element)
		if err != nil {
			return slice, err
		}
		if eq.True() {
			return slice, nil
		}
	}
	return append(slice, element), nil
}

// Element returns a single element from a given list at the given index. If
// index is greater than the length of the list then it is wrapped modulo
// the list length.
func Element(list, index cty.Value) (cty.Value, error) {
	return ElementFunc.Call([]cty.Value{list, index})
}

// Length returns the number of elements in the given collection or number of
// Unicode characters in the given string.
func Length(collection cty.Value) (cty.Value, error) {
	return LengthFunc.Call([]cty.Value{collection})
}

// Coalesce takes any number of arguments and returns the first one that isn't empty.
func Coalesce(args ...cty.Value) (cty.Value, error) {
	return CoalesceFunc.Call(args)
}

// CoalesceList takes any number of list arguments and returns the first one that isn't empty.
func CoalesceList(args ...cty.Value) (cty.Value, error) {
	return CoalesceListFunc.Call(args)
}

// Compact takes a list of strings and returns a new list
// with any empty string elements removed.
func Compact(list cty.Value) (cty.Value, error) {
	return CompactFunc.Call([]cty.Value{list})
}

// Contains determines whether a given list contains a given single value
// as one of its elements.
func Contains(list, value cty.Value) (cty.Value, error) {
	return ContainsFunc.Call([]cty.Value{list, value})
}

// Index finds the element index for a given value in a list.
func Index(list, value cty.Value) (cty.Value, error) {
	return IndexFunc.Call([]cty.Value{list, value})
}

// Distinct takes a list and returns a new list with any duplicate elements removed.
func Distinct(list cty.Value) (cty.Value, error) {
	return DistinctFunc.Call([]cty.Value{list})
}

// Chunklist splits a single list into fixed-size chunks, returning a list of lists.
func Chunklist(list, size cty.Value) (cty.Value, error) {
	return ChunklistFunc.Call([]cty.Value{list, size})
}

// Flatten takes a list and replaces any elements that are lists with a flattened
// sequence of the list contents.
func Flatten(list cty.Value) (cty.Value, error) {
	return FlattenFunc.Call([]cty.Value{list})
}

// Keys takes a map and returns a sorted list of the map keys.
func Keys(inputMap cty.Value) (cty.Value, error) {
	return KeysFunc.Call([]cty.Value{inputMap})
}

// List takes any number of list arguments and returns a list containing those
//  values in the same order.
func List(args ...cty.Value) (cty.Value, error) {
	return ListFunc.Call(args)
}

// Lookup performs a dynamic lookup into a map.
// There are two required arguments, map and key, plus an optional default,
// which is a value to return if no key is found in map.
func Lookup(args ...cty.Value) (cty.Value, error) {
	return LookupFunc.Call(args)
}

// Map takes an even number of arguments and returns a map whose elements are constructed
// from consecutive pairs of arguments.
func Map(args ...cty.Value) (cty.Value, error) {
	return MapFunc.Call(args)
}

// Matchkeys constructs a new list by taking a subset of elements from one list
// whose indexes match the corresponding indexes of values in another list.
func Matchkeys(values, keys, searchset cty.Value) (cty.Value, error) {
	return MatchkeysFunc.Call([]cty.Value{values, keys, searchset})
}

// Merge takes an arbitrary number of maps and returns a single map that contains
// a merged set of elements from all of the maps.
//
// If more than one given map defines the same key then the one that is later in
// the argument sequence takes precedence.
func Merge(maps ...cty.Value) (cty.Value, error) {
	return MergeFunc.Call(maps)
}

// Reverse takes a sequence and produces a new sequence of the same length
// with all of the same elements as the given sequence but in reverse order.
func Reverse(list cty.Value) (cty.Value, error) {
	return ReverseFunc.Call([]cty.Value{list})
}

// SetProduct computes the cartesian product of sets or sequences.
func SetProduct(sets ...cty.Value) (cty.Value, error) {
	return SetProductFunc.Call(sets)
}

// Slice extracts some consecutive elements from within a list.
func Slice(list, start, end cty.Value) (cty.Value, error) {
	return SliceFunc.Call([]cty.Value{list, start, end})
}

// Transpose takes a map of lists of strings and swaps the keys and values to
// produce a new map of lists of strings.
func Transpose(values cty.Value) (cty.Value, error) {
	return TransposeFunc.Call([]cty.Value{values})
}

// Values returns a list of the map values, in the order of the sorted keys.
// This function only works on flat maps.
func Values(values cty.Value) (cty.Value, error) {
	return ValuesFunc.Call([]cty.Value{values})
}

// Zipmap constructs a map from a list of keys and a corresponding list of values.
func Zipmap(keys, values cty.Value) (cty.Value, error) {
	return ZipmapFunc.Call([]cty.Value{keys, values})
}