package ini

// Scan a databuffer of INI-formatted data.

import (
	"fmt"
)

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

const (
	scanContinue scanState = 1 << iota
	scanEOL
	scanEOF
	scanError

	scanBegin
	scanSection
	scanVariable
	scanVariableValue
)

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

// SyntaxError is returned when the formatting is wrong.
type SyntaxError struct {
	got byte
	exp byte
	msg string
	off int
}

// Error returns the error as a string
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) reset() {
	s.prev = 0
	s.scan = s.begin
	s.read = 0
	s.err = nil
}

// loop assists next-method
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
}

// Just quickly loops through the INI and returns an error on error
func (s *scanner) valid() error {
	defer s.reset()

	for _, err := s.next(); ; _, err = s.next() {
		switch err {
		case EOL:
			continue
		case EOF:
			return nil
		default:
			if err != nil {
				return err
			}
		}
	}

	return nil
}

// next returns one element and most likely an error (typically EOF/EOL) on every iteration
func (s *scanner) next() (*element, error) {
	e, err := s.loop()

	if e != nil {
		e.trim()
	}

	return e, err
}

// ret ensures that we return the proper error
func (s *scanner) ret(state scanState) scanState {
	if state != scanEOF && s.read == s.len {
		state = scanEOF
	}

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

	return state
}

func (s *scanner) space(c byte) bool {
	return c == ' ' || c == '\n' || c == '\t' || c == '\r'
}

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

	switch {
	case s.space(c):
		return s.ret(scanContinue)
	case c == '[':
		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 {
	case s.space(c):
		s.err = SyntaxError{
			got: c,
			exp: ']',
			off: s.read,
			msg: "Invalid space character.",
		}
		return scanError
	case s.read == s.len:
		s.err = SyntaxError{
			got: 0x3,
			exp: ']',
			off: s.read,
			msg: "Invalid EOF",
		}
		return scanError
	case c == ']':
		return s.ret(scanEOL)
	default:
		e.key.add(c)
		return s.ret(scanContinue)
	}
}

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

func (s *scanner) variableValue(c byte, e *element) scanState {
	switch c {
	case '\n', '\r':
		if e.val.lastByteMatching('\\') >= 0 {
			e.val.removeLastByteMatching('\\')
			e.val.add('\n')
			return s.ret(scanContinue)
		}

		return s.ret(scanEOL)
	default:
		e.val.add(c)
		return s.ret(scanContinue)
	}
}