package argv

import (
	"unicode"
)

// Scanner is a cmdline string scanner.
//
// It split cmdline string to tokens: space, string, pipe, reverse quote string.
type Scanner struct {
	text []rune
	rpos int
}

// NewScanner create a scanner and init it's internal states.
func NewScanner(text string) *Scanner {
	return &Scanner{
		text: []rune(text),
	}
}

const _RuneEOF = 0

func (s *Scanner) nextRune() rune {
	if s.rpos >= len(s.text) {
		return _RuneEOF
	}

	r := s.text[s.rpos]
	s.rpos++
	return r
}

func (s *Scanner) unreadRune(r rune) {
	if r != _RuneEOF {
		s.rpos--
	}
}

func (s *Scanner) isEscapeChars(r rune) (rune, bool) {
	switch r {
	case 'a':
		return '\a', true
	case 'b':
		return '\b', true
	case 'f':
		return '\f', true
	case 'n':
		return '\n', true
	case 'r':
		return '\r', true
	case 't':
		return '\t', true
	case 'v':
		return '\v', true
	case '\\':
		return '\\', true
	case '$':
		return '$', true
	}
	return r, false
}

// TokenType is the type of tokens recognized by the scanner.
type TokenType uint32

// Token is generated by the scanner with a type and value.
type Token struct {
	Type  TokenType
	Value []rune
}

const (
	// TokString for string
	TokString TokenType = iota + 1
	TokStringSingleQuote
	TokStringDoubleQuote
	// TokPipe is the '|' character
	TokPipe
	// TokBackQuote is reverse quoted string
	TokBackQuote
	// TokSpace represent space character sequence
	TokSpace
	// TokEOF means the input end.
	TokEOF
)

// Next return next token, if it reach the end, TOK_EOF will be returned.
//
// Error is returned for invalid syntax such as unpaired quotes.
func (s *Scanner) Next() (Token, error) {
	const (
		Initial = iota + 1
		Space
		BackQuote
		String
		StringSingleQuote
		StringDoubleQuote
	)

	var (
		tok Token

		state uint8 = Initial
	)
	for {
		r := s.nextRune()
		switch state {
		case Initial:
			switch {
			case r == _RuneEOF:
				tok.Type = TokEOF
				return tok, nil
			case r == '|':
				tok.Type = TokPipe
				return tok, nil
			case r == '`':
				state = BackQuote
			case unicode.IsSpace(r):
				state = Space
				s.unreadRune(r)
			case r == '\'':
				state = StringSingleQuote
			case r == '"':
				state = StringDoubleQuote
			default:
				state = String
				s.unreadRune(r)
			}
		case Space:
			if r == _RuneEOF || !unicode.IsSpace(r) {
				s.unreadRune(r)
				tok.Type = TokSpace
				return tok, nil
			}
		case BackQuote:
			switch r {
			case _RuneEOF:
				return tok, ErrInvalidSyntax
			case '`':
				tok.Type = TokBackQuote
				return tok, nil
			default:
				tok.Value = append(tok.Value, r)
			}
		case String:
			switch {
			case r == _RuneEOF, r == '|', r == '`', r == '\'', r == '"', unicode.IsSpace(r):
				tok.Type = TokString
				s.unreadRune(r)
				return tok, nil
			case r == '\\':
				nr := s.nextRune()
				if nr == _RuneEOF {
					return tok, ErrInvalidSyntax
				}
				tok.Value = append(tok.Value, nr)
			default:
				tok.Value = append(tok.Value, r)
			}
		case StringSingleQuote, StringDoubleQuote:
			switch r {
			case _RuneEOF:
				return tok, ErrInvalidSyntax
			case '\'', '"':
				if singleQuote := state == StringSingleQuote; singleQuote == (r == '\'') {
					if singleQuote {
						tok.Type = TokStringSingleQuote
					} else {
						tok.Type = TokStringDoubleQuote
					}
					return tok, nil
				} else {
					tok.Value = append(tok.Value, r)
				}
			case '\\':
				nr := s.nextRune()
				if escape, ok := s.isEscapeChars(nr); ok {
					tok.Value = append(tok.Value, escape)
				} else {
					tok.Value = append(tok.Value, r)
					s.unreadRune(nr)
				}
			default:
				tok.Value = append(tok.Value, r)
			}
		}
	}
}

// Scan is a utility function help split input text as tokens.
func Scan(text string) ([]Token, error) {
	s := NewScanner(text)
	var tokens []Token
	for {
		tok, err := s.Next()
		if err != nil {
			return nil, err
		}
		tokens = append(tokens, tok)
		if tok.Type == TokEOF {
			break
		}
	}
	return tokens, nil
}