package decoder

import (
	"fmt"
	"math"
	"math/bits"
	"sort"
	"strings"
	"unicode"
	"unicode/utf16"
	"unsafe"

	"github.com/goccy/go-json/internal/errors"
)

type structFieldSet struct {
	dec         Decoder
	offset      uintptr
	isTaggedKey bool
	key         string
	keyLen      int64
	err         error
}

type structDecoder struct {
	fieldMap         map[string]*structFieldSet
	stringDecoder    *stringDecoder
	structName       string
	fieldName        string
	isTriedOptimize  bool
	keyBitmapUint8   [][256]uint8
	keyBitmapUint16  [][256]uint16
	sortedFieldSets  []*structFieldSet
	keyDecoder       func(*structDecoder, []byte, int64) (int64, *structFieldSet, error)
	keyStreamDecoder func(*structDecoder, *Stream) (*structFieldSet, string, error)
}

var (
	largeToSmallTable [256]byte
)

func init() {
	for i := 0; i < 256; i++ {
		c := i
		if 'A' <= c && c <= 'Z' {
			c += 'a' - 'A'
		}
		largeToSmallTable[i] = byte(c)
	}
}

func newStructDecoder(structName, fieldName string, fieldMap map[string]*structFieldSet) *structDecoder {
	return &structDecoder{
		fieldMap:         fieldMap,
		stringDecoder:    newStringDecoder(structName, fieldName),
		structName:       structName,
		fieldName:        fieldName,
		keyDecoder:       decodeKey,
		keyStreamDecoder: decodeKeyStream,
	}
}

const (
	allowOptimizeMaxKeyLen   = 64
	allowOptimizeMaxFieldLen = 16
)

func (d *structDecoder) tryOptimize() {
	if d.isTriedOptimize {
		return
	}
	fieldMap := map[string]*structFieldSet{}
	conflicted := map[string]struct{}{}
	for k, v := range d.fieldMap {
		key := strings.ToLower(k)
		if key != k {
			// already exists same key (e.g. Hello and HELLO has same lower case key
			if _, exists := conflicted[key]; exists {
				d.isTriedOptimize = true
				return
			}
			conflicted[key] = struct{}{}
		}
		if field, exists := fieldMap[key]; exists {
			if field != v {
				d.isTriedOptimize = true
				return
			}
		}
		fieldMap[key] = v
	}

	if len(fieldMap) > allowOptimizeMaxFieldLen {
		d.isTriedOptimize = true
		return
	}

	var maxKeyLen int
	sortedKeys := []string{}
	for key := range fieldMap {
		keyLen := len(key)
		if keyLen > allowOptimizeMaxKeyLen {
			d.isTriedOptimize = true
			return
		}
		if maxKeyLen < keyLen {
			maxKeyLen = keyLen
		}
		sortedKeys = append(sortedKeys, key)
	}
	sort.Strings(sortedKeys)

	// By allocating one extra capacity than `maxKeyLen`,
	// it is possible to avoid the process of comparing the index of the key with the length of the bitmap each time.
	bitmapLen := maxKeyLen + 1
	if len(sortedKeys) <= 8 {
		keyBitmap := make([][256]uint8, bitmapLen)
		for i, key := range sortedKeys {
			for j := 0; j < len(key); j++ {
				c := key[j]
				keyBitmap[j][c] |= (1 << uint(i))
			}
			d.sortedFieldSets = append(d.sortedFieldSets, fieldMap[key])
		}
		d.keyBitmapUint8 = keyBitmap
		d.keyDecoder = decodeKeyByBitmapUint8
		d.keyStreamDecoder = decodeKeyByBitmapUint8Stream
	} else {
		keyBitmap := make([][256]uint16, bitmapLen)
		for i, key := range sortedKeys {
			for j := 0; j < len(key); j++ {
				c := key[j]
				keyBitmap[j][c] |= (1 << uint(i))
			}
			d.sortedFieldSets = append(d.sortedFieldSets, fieldMap[key])
		}
		d.keyBitmapUint16 = keyBitmap
		d.keyDecoder = decodeKeyByBitmapUint16
		d.keyStreamDecoder = decodeKeyByBitmapUint16Stream
	}
}

// decode from '\uXXXX'
func decodeKeyCharByUnicodeRune(buf []byte, cursor int64) ([]byte, int64) {
	const defaultOffset = 4
	const surrogateOffset = 6

	r := unicodeToRune(buf[cursor : cursor+defaultOffset])
	if utf16.IsSurrogate(r) {
		cursor += defaultOffset
		if cursor+surrogateOffset >= int64(len(buf)) || buf[cursor] != '\\' || buf[cursor+1] != 'u' {
			return []byte(string(unicode.ReplacementChar)), cursor + defaultOffset - 1
		}
		cursor += 2
		r2 := unicodeToRune(buf[cursor : cursor+defaultOffset])
		if r := utf16.DecodeRune(r, r2); r != unicode.ReplacementChar {
			return []byte(string(r)), cursor + defaultOffset - 1
		}
	}
	return []byte(string(r)), cursor + defaultOffset - 1
}

func decodeKeyCharByEscapedChar(buf []byte, cursor int64) ([]byte, int64) {
	c := buf[cursor]
	cursor++
	switch c {
	case '"':
		return []byte{'"'}, cursor
	case '\\':
		return []byte{'\\'}, cursor
	case '/':
		return []byte{'/'}, cursor
	case 'b':
		return []byte{'\b'}, cursor
	case 'f':
		return []byte{'\f'}, cursor
	case 'n':
		return []byte{'\n'}, cursor
	case 'r':
		return []byte{'\r'}, cursor
	case 't':
		return []byte{'\t'}, cursor
	case 'u':
		return decodeKeyCharByUnicodeRune(buf, cursor)
	}
	return nil, cursor
}

func decodeKeyByBitmapUint8(d *structDecoder, buf []byte, cursor int64) (int64, *structFieldSet, error) {
	var (
		curBit uint8 = math.MaxUint8
	)
	b := (*sliceHeader)(unsafe.Pointer(&buf)).data
	for {
		switch char(b, cursor) {
		case ' ', '\n', '\t', '\r':
			cursor++
		case '"':
			cursor++
			c := char(b, cursor)
			switch c {
			case '"':
				cursor++
				return cursor, nil, nil
			case nul:
				return 0, nil, errors.ErrUnexpectedEndOfJSON("string", cursor)
			}
			keyIdx := 0
			bitmap := d.keyBitmapUint8
			start := cursor
			for {
				c := char(b, cursor)
				switch c {
				case '"':
					fieldSetIndex := bits.TrailingZeros8(curBit)
					field := d.sortedFieldSets[fieldSetIndex]
					keyLen := cursor - start
					cursor++
					if keyLen < field.keyLen {
						// early match
						return cursor, nil, nil
					}
					return cursor, field, nil
				case nul:
					return 0, nil, errors.ErrUnexpectedEndOfJSON("string", cursor)
				case '\\':
					cursor++
					chars, nextCursor := decodeKeyCharByEscapedChar(buf, cursor)
					for _, c := range chars {
						curBit &= bitmap[keyIdx][largeToSmallTable[c]]
						if curBit == 0 {
							return decodeKeyNotFound(b, cursor)
						}
						keyIdx++
					}
					cursor = nextCursor
				default:
					curBit &= bitmap[keyIdx][largeToSmallTable[c]]
					if curBit == 0 {
						return decodeKeyNotFound(b, cursor)
					}
					keyIdx++
				}
				cursor++
			}
		default:
			return cursor, nil, errors.ErrNotAtBeginningOfValue(cursor)
		}
	}
}

func decodeKeyByBitmapUint16(d *structDecoder, buf []byte, cursor int64) (int64, *structFieldSet, error) {
	var (
		curBit uint16 = math.MaxUint16
	)
	b := (*sliceHeader)(unsafe.Pointer(&buf)).data
	for {
		switch char(b, cursor) {
		case ' ', '\n', '\t', '\r':
			cursor++
		case '"':
			cursor++
			c := char(b, cursor)
			switch c {
			case '"':
				cursor++
				return cursor, nil, nil
			case nul:
				return 0, nil, errors.ErrUnexpectedEndOfJSON("string", cursor)
			}
			keyIdx := 0
			bitmap := d.keyBitmapUint16
			start := cursor
			for {
				c := char(b, cursor)
				switch c {
				case '"':
					fieldSetIndex := bits.TrailingZeros16(curBit)
					field := d.sortedFieldSets[fieldSetIndex]
					keyLen := cursor - start
					cursor++
					if keyLen < field.keyLen {
						// early match
						return cursor, nil, nil
					}
					return cursor, field, nil
				case nul:
					return 0, nil, errors.ErrUnexpectedEndOfJSON("string", cursor)
				case '\\':
					cursor++
					chars, nextCursor := decodeKeyCharByEscapedChar(buf, cursor)
					for _, c := range chars {
						curBit &= bitmap[keyIdx][largeToSmallTable[c]]
						if curBit == 0 {
							return decodeKeyNotFound(b, cursor)
						}
						keyIdx++
					}
					cursor = nextCursor
				default:
					curBit &= bitmap[keyIdx][largeToSmallTable[c]]
					if curBit == 0 {
						return decodeKeyNotFound(b, cursor)
					}
					keyIdx++
				}
				cursor++
			}
		default:
			return cursor, nil, errors.ErrNotAtBeginningOfValue(cursor)
		}
	}
}

func decodeKeyNotFound(b unsafe.Pointer, cursor int64) (int64, *structFieldSet, error) {
	for {
		cursor++
		switch char(b, cursor) {
		case '"':
			cursor++
			return cursor, nil, nil
		case '\\':
			cursor++
			if char(b, cursor) == nul {
				return 0, nil, errors.ErrUnexpectedEndOfJSON("string", cursor)
			}
		case nul:
			return 0, nil, errors.ErrUnexpectedEndOfJSON("string", cursor)
		}
	}
}

func decodeKey(d *structDecoder, buf []byte, cursor int64) (int64, *structFieldSet, error) {
	key, c, err := d.stringDecoder.decodeByte(buf, cursor)
	if err != nil {
		return 0, nil, err
	}
	cursor = c
	k := *(*string)(unsafe.Pointer(&key))
	field, exists := d.fieldMap[k]
	if !exists {
		return cursor, nil, nil
	}
	return cursor, field, nil
}

func decodeKeyByBitmapUint8Stream(d *structDecoder, s *Stream) (*structFieldSet, string, error) {
	var (
		curBit uint8 = math.MaxUint8
	)
	_, cursor, p := s.stat()
	for {
		switch char(p, cursor) {
		case ' ', '\n', '\t', '\r':
			cursor++
		case nul:
			s.cursor = cursor
			if s.read() {
				_, cursor, p = s.stat()
				continue
			}
			return nil, "", errors.ErrNotAtBeginningOfValue(s.totalOffset())
		case '"':
			cursor++
		FIRST_CHAR:
			start := cursor
			switch char(p, cursor) {
			case '"':
				cursor++
				s.cursor = cursor
				return nil, "", nil
			case nul:
				s.cursor = cursor
				if s.read() {
					_, cursor, p = s.stat()
					goto FIRST_CHAR
				}
				return nil, "", errors.ErrUnexpectedEndOfJSON("string", s.totalOffset())
			}
			keyIdx := 0
			bitmap := d.keyBitmapUint8
			for {
				c := char(p, cursor)
				switch c {
				case '"':
					fieldSetIndex := bits.TrailingZeros8(curBit)
					field := d.sortedFieldSets[fieldSetIndex]
					keyLen := cursor - start
					cursor++
					s.cursor = cursor
					if keyLen < field.keyLen {
						// early match
						return nil, field.key, nil
					}
					return field, field.key, nil
				case nul:
					s.cursor = cursor
					if s.read() {
						_, cursor, p = s.stat()
						continue
					}
					return nil, "", errors.ErrUnexpectedEndOfJSON("string", s.totalOffset())
				case '\\':
					s.cursor = cursor + 1 // skip '\' char
					chars, err := decodeKeyCharByEscapeCharStream(s)
					if err != nil {
						return nil, "", err
					}
					cursor = s.cursor
					for _, c := range chars {
						curBit &= bitmap[keyIdx][largeToSmallTable[c]]
						if curBit == 0 {
							s.cursor = cursor
							return decodeKeyNotFoundStream(s, start)
						}
						keyIdx++
					}
				default:
					curBit &= bitmap[keyIdx][largeToSmallTable[c]]
					if curBit == 0 {
						s.cursor = cursor
						return decodeKeyNotFoundStream(s, start)
					}
					keyIdx++
				}
				cursor++
			}
		default:
			return nil, "", errors.ErrNotAtBeginningOfValue(s.totalOffset())
		}
	}
}

func decodeKeyByBitmapUint16Stream(d *structDecoder, s *Stream) (*structFieldSet, string, error) {
	var (
		curBit uint16 = math.MaxUint16
	)
	_, cursor, p := s.stat()
	for {
		switch char(p, cursor) {
		case ' ', '\n', '\t', '\r':
			cursor++
		case nul:
			s.cursor = cursor
			if s.read() {
				_, cursor, p = s.stat()
				continue
			}
			return nil, "", errors.ErrNotAtBeginningOfValue(s.totalOffset())
		case '"':
			cursor++
		FIRST_CHAR:
			start := cursor
			switch char(p, cursor) {
			case '"':
				cursor++
				s.cursor = cursor
				return nil, "", nil
			case nul:
				s.cursor = cursor
				if s.read() {
					_, cursor, p = s.stat()
					goto FIRST_CHAR
				}
				return nil, "", errors.ErrUnexpectedEndOfJSON("string", s.totalOffset())
			}
			keyIdx := 0
			bitmap := d.keyBitmapUint16
			for {
				c := char(p, cursor)
				switch c {
				case '"':
					fieldSetIndex := bits.TrailingZeros16(curBit)
					field := d.sortedFieldSets[fieldSetIndex]
					keyLen := cursor - start
					cursor++
					s.cursor = cursor
					if keyLen < field.keyLen {
						// early match
						return nil, field.key, nil
					}
					return field, field.key, nil
				case nul:
					s.cursor = cursor
					if s.read() {
						_, cursor, p = s.stat()
						continue
					}
					return nil, "", errors.ErrUnexpectedEndOfJSON("string", s.totalOffset())
				case '\\':
					s.cursor = cursor + 1 // skip '\' char
					chars, err := decodeKeyCharByEscapeCharStream(s)
					if err != nil {
						return nil, "", err
					}
					cursor = s.cursor
					for _, c := range chars {
						curBit &= bitmap[keyIdx][largeToSmallTable[c]]
						if curBit == 0 {
							s.cursor = cursor
							return decodeKeyNotFoundStream(s, start)
						}
						keyIdx++
					}
				default:
					curBit &= bitmap[keyIdx][largeToSmallTable[c]]
					if curBit == 0 {
						s.cursor = cursor
						return decodeKeyNotFoundStream(s, start)
					}
					keyIdx++
				}
				cursor++
			}
		default:
			return nil, "", errors.ErrNotAtBeginningOfValue(s.totalOffset())
		}
	}
}

// decode from '\uXXXX'
func decodeKeyCharByUnicodeRuneStream(s *Stream) ([]byte, error) {
	const defaultOffset = 4
	const surrogateOffset = 6

	if s.cursor+defaultOffset >= s.length {
		if !s.read() {
			return nil, errors.ErrInvalidCharacter(s.char(), "escaped unicode char", s.totalOffset())
		}
	}

	r := unicodeToRune(s.buf[s.cursor : s.cursor+defaultOffset])
	if utf16.IsSurrogate(r) {
		s.cursor += defaultOffset
		if s.cursor+surrogateOffset >= s.length {
			s.read()
		}
		if s.cursor+surrogateOffset >= s.length || s.buf[s.cursor] != '\\' || s.buf[s.cursor+1] != 'u' {
			s.cursor += defaultOffset - 1
			return []byte(string(unicode.ReplacementChar)), nil
		}
		r2 := unicodeToRune(s.buf[s.cursor+defaultOffset+2 : s.cursor+surrogateOffset])
		if r := utf16.DecodeRune(r, r2); r != unicode.ReplacementChar {
			s.cursor += defaultOffset - 1
			return []byte(string(r)), nil
		}
	}
	s.cursor += defaultOffset - 1
	return []byte(string(r)), nil
}

func decodeKeyCharByEscapeCharStream(s *Stream) ([]byte, error) {
	c := s.buf[s.cursor]
	s.cursor++
RETRY:
	switch c {
	case '"':
		return []byte{'"'}, nil
	case '\\':
		return []byte{'\\'}, nil
	case '/':
		return []byte{'/'}, nil
	case 'b':
		return []byte{'\b'}, nil
	case 'f':
		return []byte{'\f'}, nil
	case 'n':
		return []byte{'\n'}, nil
	case 'r':
		return []byte{'\r'}, nil
	case 't':
		return []byte{'\t'}, nil
	case 'u':
		return decodeKeyCharByUnicodeRuneStream(s)
	case nul:
		if !s.read() {
			return nil, errors.ErrInvalidCharacter(s.char(), "escaped char", s.totalOffset())
		}
		goto RETRY
	default:
		return nil, errors.ErrUnexpectedEndOfJSON("struct field", s.totalOffset())
	}
}

func decodeKeyNotFoundStream(s *Stream, start int64) (*structFieldSet, string, error) {
	buf, cursor, p := s.stat()
	for {
		cursor++
		switch char(p, cursor) {
		case '"':
			b := buf[start:cursor]
			key := *(*string)(unsafe.Pointer(&b))
			cursor++
			s.cursor = cursor
			return nil, key, nil
		case '\\':
			cursor++
			if char(p, cursor) == nul {
				s.cursor = cursor
				if !s.read() {
					return nil, "", errors.ErrUnexpectedEndOfJSON("string", s.totalOffset())
				}
				buf, cursor, p = s.statForRetry()
			}
		case nul:
			s.cursor = cursor
			if !s.read() {
				return nil, "", errors.ErrUnexpectedEndOfJSON("string", s.totalOffset())
			}
			buf, cursor, p = s.statForRetry()
		}
	}
}

func decodeKeyStream(d *structDecoder, s *Stream) (*structFieldSet, string, error) {
	key, err := d.stringDecoder.decodeStreamByte(s)
	if err != nil {
		return nil, "", err
	}
	k := *(*string)(unsafe.Pointer(&key))
	return d.fieldMap[k], k, nil
}

func (d *structDecoder) DecodeStream(s *Stream, depth int64, p unsafe.Pointer) error {
	depth++
	if depth > maxDecodeNestingDepth {
		return errors.ErrExceededMaxDepth(s.char(), s.cursor)
	}

	c := s.skipWhiteSpace()
	switch c {
	case 'n':
		if err := nullBytes(s); err != nil {
			return err
		}
		return nil
	default:
		if s.char() != '{' {
			return errors.ErrNotAtBeginningOfValue(s.totalOffset())
		}
	}
	s.cursor++
	if s.skipWhiteSpace() == '}' {
		s.cursor++
		return nil
	}
	for {
		s.reset()
		field, key, err := d.keyStreamDecoder(d, s)
		if err != nil {
			return err
		}
		if s.skipWhiteSpace() != ':' {
			return errors.ErrExpected("colon after object key", s.totalOffset())
		}
		s.cursor++
		if field != nil {
			if field.err != nil {
				return field.err
			}
			if err := field.dec.DecodeStream(s, depth, unsafe.Pointer(uintptr(p)+field.offset)); err != nil {
				return err
			}
		} else if s.DisallowUnknownFields {
			return fmt.Errorf("json: unknown field %q", key)
		} else {
			if err := s.skipValue(depth); err != nil {
				return err
			}
		}
		c := s.skipWhiteSpace()
		if c == '}' {
			s.cursor++
			return nil
		}
		if c != ',' {
			return errors.ErrExpected("comma after object element", s.totalOffset())
		}
		s.cursor++
	}
}

func (d *structDecoder) Decode(ctx *RuntimeContext, cursor, depth int64, p unsafe.Pointer) (int64, error) {
	buf := ctx.Buf
	depth++
	if depth > maxDecodeNestingDepth {
		return 0, errors.ErrExceededMaxDepth(buf[cursor], cursor)
	}
	buflen := int64(len(buf))
	cursor = skipWhiteSpace(buf, cursor)
	b := (*sliceHeader)(unsafe.Pointer(&buf)).data
	switch char(b, cursor) {
	case 'n':
		if err := validateNull(buf, cursor); err != nil {
			return 0, err
		}
		cursor += 4
		return cursor, nil
	case '{':
	default:
		return 0, errors.ErrNotAtBeginningOfValue(cursor)
	}
	cursor++
	cursor = skipWhiteSpace(buf, cursor)
	if buf[cursor] == '}' {
		cursor++
		return cursor, nil
	}
	for {
		c, field, err := d.keyDecoder(d, buf, cursor)
		if err != nil {
			return 0, err
		}
		cursor = skipWhiteSpace(buf, c)
		if char(b, cursor) != ':' {
			return 0, errors.ErrExpected("colon after object key", cursor)
		}
		cursor++
		if cursor >= buflen {
			return 0, errors.ErrExpected("object value after colon", cursor)
		}
		if field != nil {
			if field.err != nil {
				return 0, field.err
			}
			c, err := field.dec.Decode(ctx, cursor, depth, unsafe.Pointer(uintptr(p)+field.offset))
			if err != nil {
				return 0, err
			}
			cursor = c
		} else {
			c, err := skipValue(buf, cursor, depth)
			if err != nil {
				return 0, err
			}
			cursor = c
		}
		cursor = skipWhiteSpace(buf, cursor)
		if char(b, cursor) == '}' {
			cursor++
			return cursor, nil
		}
		if char(b, cursor) != ',' {
			return 0, errors.ErrExpected("comma after object element", cursor)
		}
		cursor++
	}
}