package qcode

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

// Pos represents a byte position in the original input text from which
// this template was parsed.
type Pos int

func (p Pos) Position() Pos {
	return p
}

// item represents a token or text string returned from the scanner.
type item struct {
	typ  itemType // The type of this item.
	pos  Pos      // The starting position, in bytes, of this item in the input string.
	val  string   // The value of this item.
	line int      // The line number at the start of this item.
}

func (i *item) String() string {
	var v string

	switch i.typ {
	case itemEOF:
		v = "EOF"
	case itemError:
		v = "error"
	case itemName:
		v = "name"
	case itemQuery:
		v = "query"
	case itemMutation:
		v = "mutation"
	case itemSub:
		v = "subscription"
	case itemPunctuator:
		v = "punctuator"
	case itemDirective:
		v = "directive"
	case itemVariable:
		v = "variable"
	case itemIntVal:
		v = "int"
	case itemFloatVal:
		v = "float"
	case itemStringVal:
		v = "string"
	}
	return fmt.Sprintf("%s %q", v, i.val)
}

// itemType identifies the type of lex items.
type itemType int

const (
	itemError itemType = iota // error occurred; value is text of error
	itemEOF
	itemName
	itemQuery
	itemMutation
	itemSub
	itemPunctuator
	itemArgsOpen
	itemArgsClose
	itemListOpen
	itemListClose
	itemObjOpen
	itemObjClose
	itemColon
	itemEquals
	itemDirective
	itemVariable
	itemSpread
	itemIntVal
	itemFloatVal
	itemStringVal
	itemBoolVal
)

// !$():=@[]{|}
var punctuators = map[rune]itemType{
	'{': itemObjOpen,
	'}': itemObjClose,
	'[': itemListOpen,
	']': itemListClose,
	'(': itemArgsOpen,
	')': itemArgsClose,
	':': itemColon,
	'=': itemEquals,
}

const eof = -1

// stateFn represents the state of the scanner as a function that returns the next state.
type stateFn func(*lexer) stateFn

// lexer holds the state of the scanner.
type lexer struct {
	name  string // the name of the input; used only for error reports
	input string // the string being scanned
	pos   Pos    // current position in the input
	start Pos    // start position of this item
	width Pos    // width of last rune read from input
	items []item // array of scanned items
	line  int    // 1+number of newlines seen
}

// next returns the next rune in the input.
func (l *lexer) next() rune {
	if int(l.pos) >= len(l.input) {
		l.width = 0
		return eof
	}
	r, w := utf8.DecodeRuneInString(l.input[l.pos:])
	l.width = Pos(w)
	l.pos += l.width
	if r == '\n' {
		l.line++
	}
	return r
}

// peek returns but does not consume the next rune in the input.
func (l *lexer) peek() rune {
	r := l.next()
	l.backup()
	return r
}

// backup steps back one rune. Can only be called once per call of next.
func (l *lexer) backup() {
	l.pos -= l.width
	// Correct newline count.
	if l.width == 1 && l.input[l.pos] == '\n' {
		l.line--
	}
}

func (l *lexer) current() string {
	return l.input[l.start:l.pos]
}

// emit passes an item back to the client.
func (l *lexer) emit(t itemType) {
	l.items = append(l.items, item{t, l.start, l.input[l.start:l.pos], l.line})
	// Some items contain text internally. If so, count their newlines.
	switch t {
	case itemName:
		l.line += strings.Count(l.input[l.start:l.pos], "\n")
	}
	l.start = l.pos
}

// ignore skips over the pending input before this point.
func (l *lexer) ignore() {
	l.line += strings.Count(l.input[l.start:l.pos], "\n")
	l.start = l.pos
}

// accept consumes the next rune if it's from the valid set.
func (l *lexer) accept(valid string) bool {
	if strings.ContainsRune(valid, l.next()) {
		return true
	}
	l.backup()
	return false
}

// accept onsumes a run of runes while they are alpha nums
func (l *lexer) acceptAlphaNum() bool {
	n := 0
	for r := l.next(); isAlphaNumeric(r); r = l.next() {
		n++
	}
	l.backup()
	return (n != 0)
}

// acceptRun consumes a run of runes from the valid set.
func (l *lexer) acceptRun(valid string) {
	for strings.ContainsRune(valid, l.next()) {
	}
	l.backup()
}

// errorf returns an error token and terminates the scan by passing
// back a nil pointer that will be the next state, terminating l.nextItem.
func (l *lexer) errorf(format string, args ...interface{}) stateFn {
	l.items = append(l.items, item{itemError, l.start,
		fmt.Sprintf(format, args...), l.line})
	return nil
}

// lex creates a new scanner for the input string.
func lex(input string) (*lexer, error) {
	l := &lexer{
		input: input,
		items: make([]item, 0, 100),
		line:  1,
	}
	l.run()

	if last := l.items[len(l.items)-1]; last.typ == itemError {
		return nil, fmt.Errorf(last.val)
	}
	return l, nil
}

// run runs the state machine for the lexer.
func (l *lexer) run() {
	for state := lexRoot; state != nil; {
		state = state(l)
	}
}

// lexInsideAction scans the elements inside action delimiters.
func lexRoot(l *lexer) stateFn {
	r := l.next()

	switch {
	case r == eof:
		l.emit(itemEOF)
		return nil
	case isEndOfLine(r):
		l.ignore()
	case isSpace(r):
		l.ignore()
	case r == '@':
		l.ignore()
		if l.acceptAlphaNum() {
			l.emit(itemDirective)
		}
	case r == '$':
		l.ignore()
		if l.acceptAlphaNum() {
			l.emit(itemVariable)
		}
	case strings.ContainsRune("!():=[]{|}", r):
		if item, ok := punctuators[r]; ok {
			l.emit(item)
		} else {
			l.emit(itemPunctuator)
		}
	case r == '"' || r == '\'':
		l.backup()
		return lexString
	case r == '.':
		if len(l.input) >= 3 {
			if strings.HasSuffix(l.input[:l.pos], "...") {
				l.emit(itemSpread)
				return lexRoot
			}
		}
		fallthrough // '.' can start a number.
	case r == '+' || r == '-' || ('0' <= r && r <= '9'):
		l.backup()
		return lexNumber
	case isAlphaNumeric(r):
		l.backup()
		return lexName
	default:
		return l.errorf("unrecognized character in action: %#U", r)
	}
	return lexRoot
}

// lexName scans a name.
func lexName(l *lexer) stateFn {
	for {
		r := l.next()
		if r == eof {
			l.emit(itemEOF)
			return nil
		}
		if !isAlphaNumeric(r) {
			l.backup()
			v := l.current()

			if len(v) == 0 {
				switch {
				case strings.EqualFold(v, "query"):
					l.emit(itemQuery)
					break
				case strings.EqualFold(v, "mutation"):
					l.emit(itemMutation)
					break
				case strings.EqualFold(v, "subscription"):
					l.emit(itemSub)
					break
				}
			}

			switch {
			case strings.EqualFold(v, "true"):
				l.emit(itemBoolVal)
			case strings.EqualFold(v, "false"):
				l.emit(itemBoolVal)
			default:
				l.emit(itemName)
			}
			break
		}
	}
	return lexRoot
}

// lexString scans a string.
func lexString(l *lexer) stateFn {
	if l.accept("\"'") {
		l.ignore()

		for {
			r := l.next()
			if r == eof {
				l.emit(itemEOF)
				return nil
			}
			if r == '\'' || r == '"' {
				l.backup()
				l.emit(itemStringVal)
				if l.accept("\"'") {
					l.ignore()
				}
				break
			}
		}
	}
	return lexRoot
}

// lexNumber scans a number: decimal, octal, hex, float, or imaginary. This
// isn't a perfect number scanner - for instance it accepts "." and "0x0.2"
// and "089" - but when it's wrong the input is invalid and the parser (via
// strconv) will notice.
func lexNumber(l *lexer) stateFn {
	var it itemType
	// Optional leading sign.
	l.accept("+-")

	// Is it integer
	digits := "0123456789"
	if l.accept(digits) {
		l.acceptRun(digits)
		it = itemIntVal
	}

	// Is it float
	if l.peek() == '.' {
		if l.accept(".") {
			if l.accept(digits) {
				l.acceptRun(digits)
				it = itemFloatVal
			}
		} else {
			l.backup()
		}
	}

	// Next thing mustn't be alphanumeric.
	if isAlphaNumeric(l.peek()) {
		l.next()
		return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
	}

	if it != 0 {
		l.emit(it)
	}

	return lexRoot
}

// isSpace reports whether r is a space character.
func isSpace(r rune) bool {
	return r == ',' || r == ' ' || r == '\t'
}

// isEndOfLine reports whether r is an end-of-line character.
func isEndOfLine(r rune) bool {
	return r == '\r' || r == '\n'
}

// isAlphaNumeric reports whether r is an alphabetic, digit, or underscore.
func isAlphaNumeric(r rune) bool {
	return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
}