What is the intended behavior of GenericLineTransformer:SetTextStyle when a value of zero is passed for the length parameter?

[udlose]

I'm writing unit tests for GenericLineTransformer and have a question about the intended behavior for SetTextStyle when a value of zero is passed for the length parameter.

AvaloniaEdit/src/AvaloniaEdit.TextMate/GenericLineTransformer.cs

Lines 32 to 69 in e88d73a

	public void SetTextStyle(
	DocumentLine line,
	int startIndex,
	int length,
	IBrush foreground,
	IBrush background,
	FontStyle fontStyle,
	FontWeight fontWeigth,
	bool isUnderline)
	{
	int startOffset = 0;
	int endOffset = 0;
	if (startIndex >= 0 && length > 0)
	{
	if ((line.Offset + startIndex + length) > line.EndOffset)
	{
	length = (line.EndOffset - startIndex) - line.Offset - startIndex;
	}
	startOffset = line.Offset + startIndex;
	endOffset = line.Offset + startIndex + length;
	}
	else
	{
	startOffset = line.Offset;
	endOffset = line.EndOffset;
	}
	if (startOffset > CurrentContext.Document.TextLength ||
	endOffset > CurrentContext.Document.TextLength)
	return;
	ChangeLinePart(
	startOffset,
	endOffset,
	visualLine => ChangeVisualLine(visualLine, foreground, background, fontStyle, fontWeigth, isUnderline));
	}

When length == 0 is passed, the condition length > 0 is false, so execution falls into the else branch and styles the entire line — startOffset and endOffset are set to line.Offset and line.EndOffset respectively.

This seems to be unintentional. There are two plausible caller intents when passing length = 0, and neither of them is "style the whole line":

1. No-op — the caller computed a zero-length span and expects nothing to happen
2. Programming error — the caller made a mistake and should be told immediately via an exception

The else branch silently does something completely different from what either intent would expect,

[danipen]

I'd say 1.

[udlose]

so if length == 0, then early return from method...

[danipen]

Yes. Early return for length == 0

[udlose]

What is the intended behavior when length or startIndex is negative? The current behavior styles the entire line if either are negative. I don't know if that was intentional or not.

[danipen]

Good question. I don't really remember. Probably is death path. I can't remember a case covering it, but I cannot confirm 100%.

[udlose]

Who would know for sure?

[danipen]

I think Dan made the initial implementation for the transformers but he's not on GH anymore.