Add Marquee decorator for long message scrolling

[DeathKing]

Add Marquee decorator that scrolls text from right to
left, it is useful when displaying long messages. The codes are not carefully written, modifications are welcomed.

[vbauerster]

Implementation assumes that (string width = bytes width), therefore it's going to fail with non Latin strings. Try it for example with Россия великая страна! string to see what I mean.

[DeathKing]

@vbauerster Da, I know what you mean, I noticed that issue too. After opened this PR and some investigation of mpb source code, I found the go-runewidth library that you used may be the solution.

I implement this feature in my project like this (https://github.com/DeathKing/pico/blob/master/cmd/pdf2image/bar.go#L14-L28):

func Marquee(textGetter func() string, ws uint, wcc ...decor.WC) decor.Decorator {
	var count uint
	f := func(s decor.Statistics) string {
		text := textGetter()
		runes := []rune(text)

		msg := string(runes[int(count)%len(runes):])
		count++

		return runewidth.FillRight(
			runewidth.Truncate(msg, int(ws), ""),
			int(ws))
	}
	return decor.Any(f, wcc...)
}

bar.mov

For this specific implementation, the API requires a getter function to obtain the text for display, which may not suitable for most case, a string parameter should be used instead as I suppose.

I'd like to know your opinion about this feature.

[RikaCelery]

Try this?🤔
It works well for me.

func Marquee(t string, ws int, divider string, wcc ...decor.WC) decor.Decorator {
	var count int
	var f = func(s decor.Statistics) string {
		length := runewidth.StringWidth(t + divider)
		if runewidth.StringWidth(t) < ws {
			return runewidth.FillRight(t, ws)
		}
		text := t + divider
		var msg string

		if count-ws > length {
			msg = TruncateLeft(Truncate(text, count+ws), ws)
		} else {
			msg = TruncateLeft(Truncate(text, count+ws), count)
		}
		if count+ws > length {
			msg += Truncate(text, count+ws-length)
		}
		count++
		if count+ws >= len(t)+len(divider)-1 {
			count = 0
		}
		return runewidth.FillRight(msg, ws)
	}
	return decor.Any(f, wcc...)
}

func Truncate(s string, size int) string {
	return runewidth.Truncate(s, size, "")
}
func TruncateLeft(s string, size int) string {
	return runewidth.TruncateLeft(s, size, "")
}

[Solaris-star]

Nice idea — a scrolling marquee decorator for long filenames in progress bars.

A few things to address:

1. Byte-level scrolling breaks multi-byte UTF-8: bytes := []byte(text) and indexing by byte offset will split multi-byte characters (CJK, emoji, accented chars), producing garbled output. Consider []rune instead, or use github.com/mattn/go-runewidth (already a dependency) for display-width-aware scrolling.

2. No wrap-around: when start + i >= len(bytes) the remaining window is padded with spaces. A true marquee wraps around so the text scrolls continuously. Something like buf[i] = bytes[(start+i) % uint(len(bytes))] would fix this.

3. count increments on every render call, not on a timer. Since mpb re-renders at ~10fps by default, the scroll speed is tied to the render rate. This is probably fine but worth documenting.

4. Example go.mod pins mpb v8.3.0 — should use a replace directive pointing to ../.. like the other examples do, so it builds against the local checkout.

5. The example uses deprecated rand.New(rand.NewSource(...)) — minor, but the other examples in the repo use the same pattern so this is consistent at least.

The core Marquee function is clean. With the UTF-8 and wrap-around fixes this would be a welcome addition to the decorator library.

[vbauerster]

I'm ok with updated PR.