package lexer

import (
	"encoding/hex"
	"monkey/token"
	"strings"
)

type Lexer struct {
	input        string
	position     int  // current position in input (point to current char)
	readPosition int  // current reading position in input (after current char)
	ch           byte // current char under exclamation
	prevCh       byte // previous char read
}

func New(input string) *Lexer {
	l := &Lexer{input: input}
	l.readChar()
	return l
}

func (l *Lexer) readChar() {
	l.prevCh = l.ch
	if l.readPosition >= len(l.input) {
		l.ch = 0
	} else {
		l.ch = l.input[l.readPosition]
	}
	l.position = l.readPosition
	l.readPosition += 1
}

func (l *Lexer) peekChar() byte {
	if l.readPosition >= len(l.input) {
		return 0
	} else {
		return l.input[l.readPosition]
	}
}

func (l *Lexer) NextToken() token.Token {
	var tok token.Token

	l.skipWhitespace()

	switch l.ch {
	case '#':
		tok.Type = token.COMMENT
		tok.Literal = l.readLine()
	case '=':
		if l.peekChar() == '=' {
			ch := l.ch
			l.readChar()
			literal := string(ch) + string(l.ch)
			tok = token.Token{
				Type:    token.EQ,
				Literal: literal,
			}
		} else {
			tok = newToken(token.ASSIGN, l.ch)
		}
	case '+':
		tok = newToken(token.PLUS, l.ch)
	case '-':
		tok = newToken(token.MINUS, l.ch)
	case '!':
		if l.peekChar() == '=' {
			ch := l.ch
			l.readChar()
			literal := string(ch) + string(l.ch)
			tok = token.Token{
				Type:    token.NOT_EQ,
				Literal: literal,
			}
		} else {
			tok = newToken(token.BANG, l.ch)
		}

	case '/':
		if l.peekChar() == '/' {
			l.readChar()
			tok.Type = token.COMMENT
			tok.Literal = l.readLine()
		} else {
			tok = newToken(token.SLASH, l.ch)
		}
	case '*':
		tok = newToken(token.ASTERISK, l.ch)
	case '<':
		if l.peekChar() == '=' {
			l.readChar()
			tok = newToken(token.LTE, l.ch)
			tok.Literal = "<="
		} else {
			tok = newToken(token.LT, l.ch)
		}
	case '>':
		if l.peekChar() == '=' {
			l.readChar()
			tok = newToken(token.GTE, l.ch)
			tok.Literal = ">="
		} else {
			tok = newToken(token.GT, l.ch)
		}
	case ';':
		tok = newToken(token.SEMICOLON, l.ch)
	case ',':
		tok = newToken(token.COMMA, l.ch)
	case '(':
		tok = newToken(token.LPAREN, l.ch)
	case ')':
		tok = newToken(token.RPAREN, l.ch)
	case '{':
		tok = newToken(token.LBRACE, l.ch)
	case '}':
		tok = newToken(token.RBRACE, l.ch)
	case 0:
		tok.Literal = ""
		tok.Type = token.EOF
	case '"':
		tok.Type = token.STRING
		str, err := l.readString()
		if err != nil {
			tok = newToken(token.ILLEGAL, l.prevCh)
		} else {
			tok.Type = token.STRING
			tok.Literal = str
		}
	case '[':
		tok = newToken(token.LBRACKET, l.ch)
	case ']':
		tok = newToken(token.RBRACKET, l.ch)
	case ':':
		tok = newToken(token.COLON, l.ch)
	default:
		if isLetter(l.ch) {
			tok.Literal = l.readIdentifier()
			tok.Type = token.LookupIdent(tok.Literal)
			return tok
		} else if isDigit(l.ch) {
			tok.Type = token.INT
			tok.Literal = l.readNumber()
			return tok
		} else {
			tok = newToken(token.ILLEGAL, l.ch)
		}
	}

	l.readChar()
	return tok
}

func newToken(tokenType token.TokenType, ch byte) token.Token {
	return token.Token{
		Type:    tokenType,
		Literal: string(ch),
	}
}

func (l *Lexer) readIdentifier() string {
	position := l.position
	for isLetter(l.ch) {
		l.readChar()
	}
	return l.input[position:l.position]
}

func (l *Lexer) readNumber() string {
	position := l.position
	for isDigit(l.ch) {
		l.readChar()
	}
	return l.input[position:l.position]
}

func isLetter(ch byte) bool {
	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_'
}

func isDigit(ch byte) bool {
	return '0' <= ch && ch <= '9'
}

func (l *Lexer) skipWhitespace() {
	for l.ch == ' ' || l.ch == '\t' || l.ch == '\n' || l.ch == '\r' {
		l.readChar()
	}
}

func (l *Lexer) readString() (string, error) {
	b := &strings.Builder{}
	for {
		l.readChar()
		if l.ch == '\\' {
			switch l.peekChar() {
			case '"':
				b.WriteByte('"')
			case 'n':
				b.WriteString("\n")
			case 'r':
				b.WriteString("\r")
			case 't':
				b.WriteString("\t")
			case '\\':
				b.WriteString("\\")
			case 'x':
				// Skip over the '\\', 'x' and the next two bytes (hex)
				l.readChar()
				l.readChar()
				l.readChar()
				src := string([]byte{l.prevCh, l.ch})
				dst, err := hex.DecodeString(src)
				if err != nil {
					return "", err
				}
				b.Write(dst)
				continue
			}

			// Skip over the '\\' and the matched single escape char
			l.readChar()
			continue
		} else {
			if l.ch == '"' || l.ch == 0 {
				break
			}
		}

		b.WriteByte(l.ch)
	}
	return b.String(), nil
}

func (l *Lexer) readLine() string {
	position := l.position + 1
	for {
		l.readChar()
		if l.ch == '\r' || l.ch == '\n' || l.ch == 0 {
			break
		}
	}
	return l.input[position:l.position]
}