package bytes

import (
	"errors"
	"fmt"
	"io"
	"regexp"
	"strings"
	"unicode/utf8"

	"git.codemonkeysoftware.net/b/gigaparsec"
	"git.codemonkeysoftware.net/b/gigaparsec/cursor"
)

// RuneReader is an io.RuneReader backed by a Cursor, for compatibility
// with the regexp package.
type RuneReader struct {
	cursor cursor.Cursor[byte]
}

func NewRuneReader(c cursor.Cursor[byte]) *RuneReader {
	return &RuneReader{cursor: c}
}

func (rr *RuneReader) ReadRune() (r rune, size int, err error) {
	var b [4]byte
	s := b[:]
	n, next, err := rr.cursor.Read(s)
	if err != nil && !errors.Is(err, io.EOF) {
		rr.cursor = next
		return 0, 0, fmt.Errorf("ReadRune: %w", err)
	}
	s = s[:n]
	r, size = utf8.DecodeRune(s)
	rr.cursor = rr.cursor.At(rr.cursor.Pos() + uint64(size))
	return r, size, err
}

func (rr *RuneReader) Cursor() cursor.Cursor[byte] {
	return rr.cursor
}

func Regexp(str string) gigaparsec.Parser[byte, []byte] {
	if !strings.HasPrefix(str, "^") {
		str = "^" + str
	}
	re := regexp.MustCompile(str)
	expected := fmt.Sprintf("match `%s`", str)
	return func(input gigaparsec.State[byte]) (consumed bool, result gigaparsec.Result[byte, []byte], err error) {
		r := NewRuneReader(input.Cursor())
		idx := re.FindReaderIndex(r)
		// TODO Check error from r; this requires an Error() method on cursor.RuneReader.
		if idx == nil {
			return false, gigaparsec.Result[byte, []byte]{}, gigaparsec.ParseError{
				Pos:      input.Pos(),
				Expected: []string{expected},
				// TODO Not having a Got is unsatisfactory, but how do I extract useful information?
				// Maybe just read a fixed number of bytes or to the end, whichever comes first?
				// I could add extra methods to cursor.RuneReader to figure out how much it had read.
			}
		}
		// Alas, this is a little wasteful because a Regexp can only return indices
		// when searching a RuneReader.
		dst := make([]byte, idx[1]-idx[0])
		n, _, err := input.Cursor().Read(dst)
		if err != nil {
			// If we can't access those same bytes again, something is wrong.
			return false, gigaparsec.Result[byte, []byte]{}, fmt.Errorf("Regex: unexpected error: %w", err)
		}
		result = gigaparsec.Result[byte, []byte]{
			State:   input.At(input.Pos() + n),
			Value:   dst,
			Message: gigaparsec.MessageOK(input.Pos()),
		}
		return true, result, nil
	}
}