package ini

import (
	"fmt"
)

type scanState int
type scanFunction func(c byte, e *element) scanState

const (
	scanContinue scanState = 1 << iota
	scanEOL
	scanEOF

	scanBegin
	scanSection
	scanVariable
	scanVariableValue
)

var (
	EOF = fmt.Errorf("END OF FILE")
	EOL = fmt.Errorf("END OF LINE")
)

type SyntaxError struct {
	got byte
	exp byte
	msg string
	off int
}

func (s *SyntaxError) Error() string {
	if s.got == 0 {
		return fmt.Sprintf("Error on position '%d: %s'", s.off, s.msg)
	}

	if s.exp == 0 {
		return fmt.Sprintf("Error on position '%d': %s. Got '%c'.", s.off, s.msg, s.got)
	}

	return fmt.Sprintf("Error on position '%d': %s. Got '%c' while expecting '%c'.", s.off, s.msg, s.got, s.exp)
}

type scanner struct {
	data []byte
	prev byte

	scan scanFunction

	read int
	len  int

	err error
}

func newScanner(d []byte) *scanner {
	s := &scanner{
		data: d,
		prev: 0,
		read: 0,
		len:  len(d),
	}
	s.scan = s.begin
	return s
}

func (s *scanner) loop() (*element, error) {
	e := newElement()
	for _, c := range s.data[s.read:] {
		s.read++
		state := s.scan(c, e)
		s.prev = c

		if s.err != nil {
			return nil, s.err
		}

		switch state {
		case scanEOL:
			return e, EOL
		case scanEOF:
			return e, EOF
		}
	}

	return e, nil
}

func (s *scanner) valid() error {
	for _, err := s.next(); ; _, err = s.next() {
		if err != EOF && err != EOL {
			return err
		}
	}

	return nil
}

func (s *scanner) next() (*element, error) {
	e, err := s.loop()
	return e, err
}

func (s *scanner) ret(state scanState) scanState {
	if state != scanEOF && s.read == s.len {
		s.scan = s.begin
		return scanEOF
	}

	if state == scanEOL {
		s.scan = s.begin
	}

	return state
}

func (s *scanner) begin(c byte, e *element) scanState {

	switch c {
	case ' ', '\n', '\t', '\r':
		return s.ret(scanContinue)
	case '[':
		e.setType(section)
		s.scan = s.section
		return s.ret(scanContinue)
	default:
		e.setType(variable)
		s.scan = s.variable
		return s.scan(c, e)
	}
}

func (s *scanner) section(c byte, e *element) scanState {
	switch c {
	case '\n':
		s.err = &SyntaxError{
			msg: "Missing valid character",
			got: c,
			exp: ']',
			off: s.read,
		}
		fallthrough
	case ']':
		return s.ret(scanEOL)
	default:
		e.setKey(c)
		return s.ret(scanContinue)
	}
}

func (s *scanner) variable(c byte, e *element) scanState {
	switch c {
	case '\n', '\r':
		return s.ret(scanEOL)
	case '=':
		e.removeLastKeyByteMatching(' ')
		s.scan = s.variableValue
		return s.ret(scanContinue)
	default:
		e.setKey(c)
		return s.ret(scanContinue)
	}
}

func (s *scanner) variableValue(c byte, e *element) scanState {
	switch c {
	case '"', '\'':
		return s.ret(scanContinue)
	case '\n', '\r':
		if s.prev == '\\' {
			e.setValue(byte('n'))
			return s.ret(scanContinue)
		}
		return s.ret(scanEOL)
	default:
		if s.prev == '=' && c == ' ' {
			return scanContinue
		}
		e.setValue(c)
		return s.ret(scanContinue)
	}
}