package main

import (
	"fmt"
	"strings"
	"unicode/utf8"
)

type stateFunc func(*lexer) stateFunc

type lexer struct {
	input string
	start int
	pos int
	width int
	tokenStream chan Token
}

func (l *lexer) run() {
	for state := lexCommand; state != nil; {
		state = state(l)
	}
	close(l.tokenStream)
}

func (l *lexer) emit(t TokenType) {
	l.tokenStream <- Token{
		typ: t,
		val: l.input[l.start:l.pos],
	}
	l.start = l.pos
}

func (l *lexer) errorf(format string, args ...interface{}) stateFunc {
	l.tokenStream <- Token{
		typ: TokenErr,
		val: fmt.Sprintf(format, args...),
	}
	return nil
}

const eof rune = -1

func (l *lexer) next() rune {
	if l.pos >= len(l.input) {
		l.width = 0
		return eof
	}
	var r rune
	r, l.width = utf8.DecodeRuneInString(l.input[l.pos:])
	l.pos += l.width
	return r
}

func (l *lexer) backup() {
	l.pos -= l.width
}

func (l *lexer) ignore() {
	l.start = l.pos
}

func (l *lexer) reset() {
	l.pos = l.start
}

func (l *lexer) expect(valid string) bool {
	for _, r := range valid {
		if l.next() != r {
			l.backup()
			return false
		}
	}
	return true
}

func (l *lexer) peek() rune {
	w := l.width
	r := l.next()
	l.backup()
	l.width = w
	return r
}

func (l *lexer) accept(valid string) bool {
	if strings.IndexRune(valid, l.next()) >= 0 {
		return true
	}
	l.backup()
	return false
}

func (l *lexer) acceptAll(valid string) {
	for strings.IndexRune(valid, l.next()) >= 0 {}
	l.backup()
}

func (l *lexer) acceptPassing(valid func(rune) bool) bool {
	if valid(l.next()) {
		return true
	}
	l.backup()
	return false
}

func (l *lexer) acceptAllPassing(valid func(rune) bool) {
	for valid(l.next()) {}
	l.backup()
}

type TokenType int

const (
	TokenErr TokenType = iota // Lexing error
	TokenEOF // end of file
	TokenLBrace // {
	TokenRBrace // }
	TokenCommand // A command character
	TokenSubstituteDelimiter // usually / but could be something else
	TokenSubex // A subex
)

type Token struct {
	typ TokenType
	val string
}

func (t Token) String() string {
	switch t.typ {
	case TokenEOF:
		return "EOF"
	case TokenErr:
		return t.val
	}
	if len(t.val) > 10 {
		return fmt.Sprintf("%.10q...", t.val)
	}
	return fmt.Sprintf("%q", t.val)
}

func Lex(input string) chan Token {
	l := &lexer{
		input: input,
		tokenStream: make(chan Token),
	}
	go l.run()
	return l.tokenStream
}

const (
	whitespace string = " \t"
	whitespaceNewlines string = " \t\r\n"
)

func isAlpha(r rune) bool {
	return ('a' <= r && r < 'z') || ('A' <= r && r <= 'Z')
}
func isDigit(r rune) bool {
	return '0' <= r && r <= '9'
}
func isAlphaNumeric(r rune) bool {
	return isAlpha(r) || isDigit(r)
}
func isStringIndexChar(r rune) bool {
	return isAlphaNumeric(r) || r == '_' || r == '-'
}

func lexCommand(l *lexer) stateFunc {
	l.acceptAll(whitespace)
	l.ignore()
	r := l.next()
	switch r {
		case eof:
			l.emit(TokenEOF)
			return nil
		case '{':
			l.emit(TokenLBrace)
			return lexCommand
		case '}':
			l.emit(TokenRBrace)
			return lexCommand
		case 's', 'S', 'f', 'F', 'l', 'L':
			l.emit(TokenCommand)
			return lexSubstitution
	}
	if isAlpha(r) {
		l.emit(TokenCommand)
		return lexCommand
	}
	return l.errorf("Expected command found something else")
}

func lexSubstitution(l *lexer) stateFunc {
	delimiter := l.next()
	if delimiter == eof {
		return l.errorf("Missing subex in substitution command")
	}
	l.emit(TokenSubstituteDelimiter)
	loop: for {
		r := l.next()
		switch r {
			case delimiter:
				l.backup()
				l.emit(TokenSubex)
				l.next()
				l.emit(TokenSubstituteDelimiter)
				break loop
			case eof:
				return l.errorf("Missing closing substitution delimiter")
			default:
		}
	}
	return lexCommand
}