package parser

import (
	"bufio"
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"strconv"
	"strings"
	"unicode/utf16"
	"unicode/utf8"
)

type File struct {
	Commands []Command
}

func (f File) String() string {
	var sb strings.Builder
	for _, cmd := range f.Commands {
		fmt.Fprintln(&sb, cmd.String())
	}

	return sb.String()
}

type Command struct {
	Name string
	Args string
}

func (c Command) String() string {
	var sb strings.Builder
	switch c.Name {
	case "model":
		fmt.Fprintf(&sb, "FROM %s", c.Args)
	case "license", "template", "system", "adapter":
		fmt.Fprintf(&sb, "%s %s", strings.ToUpper(c.Name), quote(c.Args))
	case "message":
		role, message, _ := strings.Cut(c.Args, ": ")
		fmt.Fprintf(&sb, "MESSAGE %s %s", role, quote(message))
	default:
		fmt.Fprintf(&sb, "PARAMETER %s %s", c.Name, quote(c.Args))
	}

	return sb.String()
}

type state int

const (
	stateNil state = iota
	stateName
	stateValue
	stateParameter
	stateMessage
	stateComment
)

var (
	errMissingFrom        = errors.New("no FROM line")
	errInvalidMessageRole = errors.New("message role must be one of \"system\", \"user\", or \"assistant\"")
	errInvalidCommand     = errors.New("command must be one of \"from\", \"license\", \"template\", \"system\", \"adapter\", \"parameter\", or \"message\"")
)

func ParseFile(r io.Reader) (*File, error) {
	var cmd Command
	var curr state
	var b bytes.Buffer
	var role string

	var f File

	br := bufio.NewReader(r)

	var sc scannerDecoder = utf8ScannerDecoder{}
	if bom, err := br.Peek(2); err != nil {
		slog.Warn("error reading byte-order mark", "error", err)
	} else if bytes.Equal(bom, []byte{0xFE, 0xFF}) {
		sc = utf16ScannerDecoder{binary.LittleEndian}
		//nolint:errcheck
		br.Discard(2)
	} else if bytes.Equal(bom, []byte{0xFF, 0xFE}) {
		sc = utf16ScannerDecoder{binary.BigEndian}
		//nolint:errcheck
		br.Discard(2)
	}

	scanner := bufio.NewScanner(br)
	scanner.Split(sc.ScanBytes)
	for scanner.Scan() {
		r, err := sc.DecodeRune(scanner.Bytes())
		if err != nil {
			return nil, err
		}

		next, r, err := parseRuneForState(r, curr)
		if errors.Is(err, io.ErrUnexpectedEOF) {
			return nil, fmt.Errorf("%w: %s", err, b.String())
		} else if err != nil {
			return nil, err
		}

		// process the state transition, some transitions need to be intercepted and redirected
		if next != curr {
			switch curr {
			case stateName:
				if !isValidCommand(b.String()) {
					return nil, errInvalidCommand
				}

				// next state sometimes depends on the current buffer value
				switch s := strings.ToLower(b.String()); s {
				case "from":
					cmd.Name = "model"
				case "parameter":
					// transition to stateParameter which sets command name
					next = stateParameter
				case "message":
					// transition to stateMessage which validates the message role
					next = stateMessage
					fallthrough
				default:
					cmd.Name = s
				}
			case stateParameter:
				cmd.Name = b.String()
			case stateMessage:
				if !isValidMessageRole(b.String()) {
					return nil, errInvalidMessageRole
				}

				role = b.String()
			case stateComment, stateNil:
				// pass
			case stateValue:
				s, ok := unquote(b.String())
				if !ok || isSpace(r) {
					if _, err := b.WriteRune(r); err != nil {
						return nil, err
					}

					continue
				}

				if role != "" {
					s = role + ": " + s
					role = ""
				}

				cmd.Args = s
				f.Commands = append(f.Commands, cmd)
			}

			b.Reset()
			curr = next
		}

		if strconv.IsPrint(r) {
			if _, err := b.WriteRune(r); err != nil {
				return nil, err
			}
		}
	}

	// flush the buffer
	switch curr {
	case stateComment, stateNil:
		// pass; nothing to flush
	case stateValue:
		s, ok := unquote(b.String())
		if !ok {
			return nil, io.ErrUnexpectedEOF
		}

		if role != "" {
			s = role + ": " + s
		}

		cmd.Args = s
		f.Commands = append(f.Commands, cmd)
	default:
		return nil, io.ErrUnexpectedEOF
	}

	for _, cmd := range f.Commands {
		if cmd.Name == "model" {
			return &f, nil
		}
	}

	return nil, errMissingFrom
}

func parseRuneForState(r rune, cs state) (state, rune, error) {
	switch cs {
	case stateNil:
		switch {
		case r == '#':
			return stateComment, 0, nil
		case isSpace(r), isNewline(r):
			return stateNil, 0, nil
		default:
			return stateName, r, nil
		}
	case stateName:
		switch {
		case isAlpha(r):
			return stateName, r, nil
		case isSpace(r):
			return stateValue, 0, nil
		default:
			return stateNil, 0, errInvalidCommand
		}
	case stateValue:
		switch {
		case isNewline(r):
			return stateNil, r, nil
		case isSpace(r):
			return stateNil, r, nil
		default:
			return stateValue, r, nil
		}
	case stateParameter:
		switch {
		case isAlpha(r), isNumber(r), r == '_':
			return stateParameter, r, nil
		case isSpace(r):
			return stateValue, 0, nil
		default:
			return stateNil, 0, io.ErrUnexpectedEOF
		}
	case stateMessage:
		switch {
		case isAlpha(r):
			return stateMessage, r, nil
		case isSpace(r):
			return stateValue, 0, nil
		default:
			return stateNil, 0, io.ErrUnexpectedEOF
		}
	case stateComment:
		switch {
		case isNewline(r):
			return stateNil, 0, nil
		default:
			return stateComment, 0, nil
		}
	default:
		return stateNil, 0, errors.New("")
	}
}

func quote(s string) string {
	if strings.Contains(s, "\n") || strings.HasPrefix(s, " ") || strings.HasSuffix(s, " ") {
		if strings.Contains(s, "\"") {
			return `"""` + s + `"""`
		}

		return `"` + s + `"`
	}

	return s
}

func unquote(s string) (string, bool) {
	// TODO: single quotes
	if len(s) >= 3 && s[:3] == `"""` {
		if len(s) >= 6 && s[len(s)-3:] == `"""` {
			return s[3 : len(s)-3], true
		}

		return "", false
	}

	if len(s) >= 1 && s[0] == '"' {
		if len(s) >= 2 && s[len(s)-1] == '"' {
			return s[1 : len(s)-1], true
		}

		return "", false
	}

	return s, true
}

func isAlpha(r rune) bool {
	return r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z'
}

func isNumber(r rune) bool {
	return r >= '0' && r <= '9'
}

func isSpace(r rune) bool {
	return r == ' ' || r == '\t'
}

func isNewline(r rune) bool {
	return r == '\r' || r == '\n'
}

func isValidMessageRole(role string) bool {
	return role == "system" || role == "user" || role == "assistant"
}

func isValidCommand(cmd string) bool {
	switch strings.ToLower(cmd) {
	case "from", "license", "template", "system", "adapter", "parameter", "message":
		return true
	default:
		return false
	}
}

type scannerDecoder interface {
	ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error)
	DecodeRune([]byte) (rune, error)
}

type utf8ScannerDecoder struct{}

func (utf8ScannerDecoder) ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
	return scanBytesN(data, 1, atEOF)
}

func (utf8ScannerDecoder) DecodeRune(data []byte) (rune, error) {
	r, _ := utf8.DecodeRune(data)
	return r, nil
}

type utf16ScannerDecoder struct {
	binary.ByteOrder
}

func (utf16ScannerDecoder) ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
	return scanBytesN(data, 2, atEOF)
}

func (e utf16ScannerDecoder) DecodeRune(data []byte) (rune, error) {
	return utf16.Decode([]uint16{e.ByteOrder.Uint16(data)})[0], nil
}

func scanBytesN(data []byte, n int, atEOF bool) (int, []byte, error) {
	if atEOF && len(data) == 0 {
		return 0, nil, nil
	}

	return n, data[:n], nil
}
