Some suggestions, this could be improved still.

[arran4]

Hey, this is a pull request INTO your pull request.

There still might be some good work which could be done here.

It also occurred to me that if you were to put a "read counter" in:
https://github.com/arran4/golang-ical/blob/84e95bcb9f6760d1b288e655977a3fc5e2ae6fa9/calendar.go#L523-L533

golang-ical/calendar.go

Lines 523 to 533 in 84e95bc

	type CalendarStream struct {
	r io.Reader
	b *bufio.Reader
	}
	func NewCalendarStream(r io.Reader) *CalendarStream {
	return &CalendarStream{
	r: r,
	b: bufio.NewReader(r),
	}
	}

And put it in the function here;
https://github.com/arran4/golang-ical/blob/84e95bcb9f6760d1b288e655977a3fc5e2ae6fa9/calendar.go#L535-L590

golang-ical/calendar.go

Lines 535 to 590 in 84e95bc

	func (cs *CalendarStream) ReadLine() (*ContentLine, error) {
	r := []byte{}
	c := true
	var err error
	for c {
	var b []byte
	b, err = cs.b.ReadBytes('\n')
	if len(b) == 0 {
	if err == nil {
	continue
	} else {
	c = false
	}
	} else if b[len(b)-1] == '\n' {
	o := 1
	if len(b) > 1 && b[len(b)-2] == '\r' {
	o = 2
	}
	p, err := cs.b.Peek(1)
	r = append(r, b[:len(b)-o]...)
	if err == io.EOF {
	c = false
	}
	if len(p) == 0 {
	c = false
	} else if p[0] == ' ' || p[0] == '\t' {
	_, _ = cs.b.Discard(1) // nolint:errcheck
	} else {
	c = false
	}
	} else {
	r = append(r, b...)
	}
	switch err {
	case nil:
	if len(r) == 0 {
	c = true
	}
	case io.EOF:
	c = false
	default:
	if err != nil {
	err = fmt.Errorf("readline: %w", err)
	}
	return nil, err
	}
	}
	if len(r) == 0 && err != nil {
	return nil, fmt.Errorf("readline: %w", err)
	}
	cl := ContentLine(r)
	if err != nil {
	err = fmt.Errorf("readline: %w", err)
	}
	return &cl, err
	}

You could a line offset and with some smart refactoring a character offset inserted in the errors raised in:
https://github.com/arran4/golang-ical/blob/84e95bcb9f6760d1b288e655977a3fc5e2ae6fa9/calendar.go#L438-L521

golang-ical/calendar.go

Lines 438 to 521 in 84e95bc

	func ParseCalendar(r io.Reader) (*Calendar, error) {
	state := "begin"
	c := &Calendar{}
	cs := NewCalendarStream(r)
	cont := true
	for ln := 0; cont; ln++ {
	l, err := cs.ReadLine()
	if err != nil {
	switch err {
	case io.EOF:
	cont = false
	default:
	return c, err
	}
	}
	if l == nil || len(*l) == 0 {
	continue
	}
	line, err := ParseProperty(*l)
	if err != nil {
	return nil, fmt.Errorf("%w %d: %w", ErrParsingLine, ln, err)
	}
	if line == nil {
	return nil, fmt.Errorf("%w %d", ErrParsingCalendarLine, ln)
	}
	switch state {
	case "begin":
	switch line.IANAToken {
	case "BEGIN":
	switch line.Value {
	case "VCALENDAR":
	state = "properties"
	default:
	return nil, ErrMalformedCalendarExpectedVCalendar
	}
	default:
	return nil, ErrMalformedCalendarExpectedBegin
	}
	case "properties":
	switch line.IANAToken {
	case "END":
	switch line.Value {
	case "VCALENDAR":
	state = "end"
	default:
	return nil, ErrMalformedCalendarExpectedEnd
	}
	case "BEGIN":
	state = "components"
	default: // TODO put in all the supported types for type switching etc.
	c.CalendarProperties = append(c.CalendarProperties, CalendarProperty{*line})
	}
	if state != "components" {
	break
	}
	fallthrough
	case "components":
	switch line.IANAToken {
	case "END":
	switch line.Value {
	case "VCALENDAR":
	state = "end"
	default:
	return nil, fmt.Errorf("%w at '%s': %w", ErrMalformedCalendarExpectedEnd, line.IANAToken, err)
	}
	case "BEGIN":
	co, err := GeneralParseComponent(cs, line)
	if err != nil {
	return nil, fmt.Errorf("%w at '%s': %w", ErrMalformedCalendarExpectedEnd, line.IANAToken, err)
	}
	if co != nil {
	c.Components = append(c.Components, co)
	}
	default:
	return nil, fmt.Errorf("%w at '%s'", ErrMalformedCalendarExpectedBeginOrEnd, line.IANAToken)
	}
	case "end":
	return nil, ErrMalformedCalendarUnexpectedEnd
	default:
	return nil, ErrMalformedCalendarBadState
	}
	}
	return c, nil
	}

A bit like any other parser.

Anyway. Please review this, then make some improvements if you can find any and then we can merge upstream into master once done.

[arran4]

@ManoloTonto1 This is my "reverse" pull request, a PR into your PR to fix some changes. I haven't reviewed if they are still relevant just fixed the conflicts so you can. This PR goes into arran4#73 which has stagnated.

I have replaced a couple of PRs with my own which tries to resolve issues with them and I will wait a short while before merging them myself (feel free to review them if you like.) I can take over yours ( arran4#73 ) too and replace it with my own (I will be keeping your commits so in essence I will be duplicating this pr #1 into mine. )

[arran4]

Actually rebased it with master too. This should improve things. You will have to enable actions on your fork to see if the tests works before accepting the merge into your pull request sorry these things tend to be a bit confusing. I wish github supported "suggest a change" like gitlab has

[arran4]

@ManoloTonto1 Fixed the failing test b/c of not picking up on the error wrapping of EOF. There are potentially some other of these issues around too.

[arran4]

Did a quick spot check. But there could be more issues here too. I'm not 100% on the format strings, but they will work.

[ManoloTonto1]

@arran4 shall we merge it?
Did a quick spot check also, looks good. Thanks for the patience, I haven't had enough time to sit down and fix more things.

[arran4]

THis is a PR into your PR, so it's up to you. I suggest yes. But I can go around you and just merge it my branch into the "origin" repo.

I recommend that you merge it into yours for process sake. So feel free to request changes into your pr. :P

[arran4]

But thanks. Happy to wait for you.

[ManoloTonto1]

@arran4 Merged 🚀

[arran4]

Alright, looks like some new conflicts have occurred. I will see where this leaves us.