From: Phillip Wood <phillip.wood123@gmail.com>
Date: Fri, 25 Sep 2020 19:01:07 GMT
Subject: Re: [RFC 0/1] Leading whitespace as a function identification heuristic?
Message-ID: <005de229-ad05-437c-7b83-8f6502b4e27d@gmail.com>
In-Reply-To: <20200925184319.GA660343@coredump.intra.peff.net>

Hi Peff

On 25/09/2020 19:43, Jeff King wrote:
> On Fri, Sep 25, 2020 at 10:11:33AM +0100, Phillip Wood wrote:
> 
>> If I've understood correctly when ...code 2... contains changes that are
>> themselves indented then we'll pick the wrong function header for example
>>
>> 	void foo() {
>> 		void bar() {
>> 			...code 1...
>> 		}
>> 		for (...) {
>> 			// if this line is changed we pick bar rather
>> 			// than foo because it is the first function
>> 			// header with a smaller indentation than the
>> 			// first changed line.
>> 		}
>> 	}
>>
>> Unfortunately I suspect code like that is not uncommon and the diff would
>> regress with this simple heuristic. It might be possible to recalculate the
>> required indentation as we walk backwards up the file though, so when we hit
>> the "for" line we reduce the maximum indentation allowed for a match and so
>> skip "bar" as a function header.
> 
> Thanks, that's a great counter-example I hadn't considered.
> 
> Yes, I agree that adjusting the desired indentation as we walk backwards
> would work. That's assuming indentation is hierarchical, but I think
> that's implied by the existence of this feature at all.
> 
> Another possible corner case: tabs vs spaces. If I have:
> 
>    <space><space><space><space><space><space><space><space>foo
>    <tab><tab>bar
> 
> which is more indented? Counting isspace(), it is the first one. But
> visually, it would _usually_ be the second one. But of course it would
> depend on your tabstops.
> 
> The above example is obviously stupid and contrived, but I wonder if
> there are legitimate confusing cases where people mix tabs and spaces
> (e.g., mixed tabs and spaces to align function parameters, etc).

To calculate the indentation for diff 
--color-moved-ws=allow-indentation-change in fill_es_indent_data() we 
use the tabwidth whitespace attribute to calculate the width of a tab in 
spaces

	int tab_width = es->flags & WS_TAB_WIDTH_MASK;
	/* calculate the visual width of indentation */
	while(1) {
		if (s[off] == ' ') {
			width++;
			off++;
		} else if (s[off] == '\t') {
			width += tab_width - (width % tab_width);
			while (s[++off] == '\t')
				width += tab_width;
		} else {
			break;
		}
	}

I think we could probably do something similar here assuming it is the 
visual width of the indentation that we care about.

Best Wishes

Phillip

> -Peff
>