package json

import (
	"fmt"
	"unsafe"
)

type structFieldSet struct {
	dec    decoder
	offset uintptr
}

type structDecoder struct {
	fieldMap   map[string]*structFieldSet
	keyDecoder *stringDecoder
}

func newStructDecoder(fieldMap map[string]*structFieldSet) *structDecoder {
	return &structDecoder{
		fieldMap:   fieldMap,
		keyDecoder: newStringDecoder(),
	}
}

func (d *structDecoder) decodeStream(s *stream, p unsafe.Pointer) error {
	s.skipWhiteSpace()
	if s.char() == nul {
		s.read()
	}
	if s.char() != '{' {
		return errNotAtBeginningOfValue(s.totalOffset())
	}
	s.cursor++
	for {
		s.reset()
		key, err := d.keyDecoder.decodeStreamByte(s)
		if err != nil {
			return err
		}
		s.skipWhiteSpace()
		if s.char() == nul {
			s.read()
		}
		if s.char() != ':' {
			return errExpected("colon after object key", s.totalOffset())
		}
		s.cursor++
		if s.char() == nul {
			s.read()
		}
		if s.end() {
			return errExpected("object value after colon", s.totalOffset())
		}
		k := *(*string)(unsafe.Pointer(&key))
		field, exists := d.fieldMap[k]
		if exists {
			addr := uintptr(p) + field.offset
			if err := field.dec.decodeStream(s, unsafe.Pointer(addr)); err != nil {
				return err
			}
		} else if s.disallowUnknownFields {
			return fmt.Errorf("json: unknown field %q", k)
		} else {
			if err := s.skipValue(); err != nil {
				return err
			}
		}
		s.skipWhiteSpace()
		if s.char() == nul {
			s.read()
		}
		c := s.char()
		if c == '}' {
			s.cursor++
			return nil
		}
		if c != ',' {
			return errExpected("comma after object element", s.totalOffset())
		}
		s.cursor++
	}
	return nil
}

func (d *structDecoder) decode(buf []byte, cursor int64, p unsafe.Pointer) (int64, error) {
	buflen := int64(len(buf))
	cursor = skipWhiteSpace(buf, cursor)
	if buf[cursor] != '{' {
		return 0, errNotAtBeginningOfValue(cursor)
	}
	if buflen < 2 {
		return 0, errUnexpectedEndOfJSON("object", cursor)
	}
	cursor++
	for ; cursor < buflen; cursor++ {
		key, c, err := d.keyDecoder.decodeByte(buf, cursor)
		if err != nil {
			return 0, err
		}
		cursor = c
		cursor = skipWhiteSpace(buf, cursor)
		if buf[cursor] != ':' {
			return 0, errExpected("colon after object key", cursor)
		}
		cursor++
		if cursor >= buflen {
			return 0, errExpected("object value after colon", cursor)
		}
		k := *(*string)(unsafe.Pointer(&key))
		field, exists := d.fieldMap[k]
		if exists {
			addr := uintptr(p) + field.offset
			c, err := field.dec.decode(buf, cursor, unsafe.Pointer(addr))
			if err != nil {
				return 0, err
			}
			cursor = c
		} else {
			c, err := skipValue(buf, cursor)
			if err != nil {
				return 0, err
			}
			cursor = c
		}
		cursor = skipWhiteSpace(buf, cursor)
		if buf[cursor] == '}' {
			cursor++
			return cursor, nil
		}
		if buf[cursor] != ',' {
			return 0, errExpected("comma after object element", cursor)
		}
	}
	return cursor, nil
}