package querybuilder

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"reflect"
	"strings"

	gqlgen "github.com/99designs/gqlgen/graphql"
	"golang.org/x/exp/slices"
	"golang.org/x/sync/errgroup"
)

// GraphQLMarshaller is an internal interface for marshalling an object into GraphQL.
type GraphQLMarshaller interface {
	// XXX_GraphQLType is an internal function. It returns the native GraphQL type name
	XXX_GraphQLType() string
	// XXX_GraphQLIDType is an internal function. It returns the native GraphQL type name for the ID of this object
	XXX_GraphQLIDType() string
	// XXX_GraphqlID is an internal function. It returns the underlying type ID
	XXX_GraphQLID(ctx context.Context) (string, error)
	json.Marshaler
}

const (
	GraphQLMarshallerType   = "XXX_GraphQLType"
	GraphQLMarshallerIDType = "XXX_GraphQLIDType"
	GraphQLMarshallerID     = "XXX_GraphQLID"
)

type enum interface {
	IsEnum()
}

var (
	gqlMarshaller = reflect.TypeOf((*GraphQLMarshaller)(nil)).Elem()
	enumT         = reflect.TypeOf((*enum)(nil)).Elem()
)

func MarshalGQL(ctx context.Context, v any) (string, error) {
	return marshalValue(ctx, reflect.ValueOf(v))
}

func marshalValue(ctx context.Context, v reflect.Value) (string, error) {
	t := v.Type()

	if t.Implements(gqlMarshaller) {
		return marshalCustom(ctx, v)
	}

	switch t.Kind() {
	case reflect.Bool:
		return fmt.Sprintf("%t", v.Bool()), nil
	case reflect.Int:
		return fmt.Sprintf("%d", v.Int()), nil
	case reflect.String:
		if t.Implements(enumT) {
			// enums render as their literal value
			return v.String(), nil
		}

		// escape strings following graphQL spec
		// https://github.com/graphql/graphql-spec/blob/main/spec/Section%202%20--%20Language.md#string-value
		var buf bytes.Buffer
		gqlgen.MarshalString(v.String()).MarshalGQL(&buf)
		return buf.String(), nil
	case reflect.Pointer, reflect.Interface:
		if v.IsNil() {
			return "null", nil
		}
		return marshalValue(ctx, v.Elem())
	case reflect.Slice:
		n := v.Len()
		elems := make([]string, n)
		eg, gctx := errgroup.WithContext(ctx)
		for i := 0; i < n; i++ {
			i := i
			eg.Go(func() error {
				m, err := marshalValue(gctx, v.Index(i))
				if err != nil {
					return err
				}
				elems[i] = m
				return nil
			})
		}
		if err := eg.Wait(); err != nil {
			return "", err
		}
		return fmt.Sprintf("[%s]", strings.Join(elems, ",")), nil
	case reflect.Struct:
		n := v.NumField()
		elems := make([]string, n)
		eg, gctx := errgroup.WithContext(ctx)
		for i := 0; i < n; i++ {
			i := i
			eg.Go(func() error {
				f := t.Field(i)
				fv := v.Field(i)
				name := f.Name
				jsonTag := strings.Split(f.Tag.Get("json"), ",")
				if jsonTag[0] != "" {
					name = jsonTag[0]
				}
				isOptional := slices.Contains(jsonTag[1:], "omitempty")
				if isOptional && IsZeroValue(fv.Interface()) {
					return nil
				}
				m, err := marshalValue(gctx, fv)
				if err != nil {
					return err
				}
				if m != `""` && m != "null" {
					elems[i] = fmt.Sprintf("%s:%s", name, m)
				}
				return nil
			})
		}
		if err := eg.Wait(); err != nil {
			return "", err
		}
		nonNullElems := make([]string, 0, n)
		for _, elem := range elems {
			if elem != "" {
				nonNullElems = append(nonNullElems, elem)
			}
		}
		return fmt.Sprintf("{%s}", strings.Join(nonNullElems, ",")), nil
	default:
		panic(fmt.Errorf("unsupported argument of kind %s", t.Kind()))
	}
}

func marshalCustom(ctx context.Context, v reflect.Value) (string, error) {
	result := v.MethodByName(GraphQLMarshallerID).Call([]reflect.Value{
		reflect.ValueOf(ctx),
	})
	if len(result) != 2 {
		panic(result)
	}
	err := result[1].Interface()
	if err != nil {
		return "", err.(error)
	}

	return fmt.Sprintf("%q", result[0].String()), nil
}

func IsZeroValue(value any) bool {
	v := reflect.ValueOf(value)
	kind := v.Type().Kind()
	switch kind {
	case reflect.Pointer:
		return v.IsNil()
	case reflect.Slice, reflect.Array:
		return v.Len() == 0
	default:
		return v.IsZero()
	}
}
