Re: [PATCH] diff compaction heuristic: favor shortest neighboring blank lines

[Junio C Hamano <gitster@pobox.com>]

Stefan Beller <sbeller@google.com> writes:

> +        def bal
> +                do_bal_stuff()
> +
> +                common_ending()
> +        end
> +
>          def baz
>                  do_baz_stuff()
>
>                  common_ending()
>          end
>
> whereas before we had:
>
>   WIP (TODO: ask peff to provide an example that actually triggers, I seem to be
>        unable to write one, though I thought the above was one)
>
>
> The way we do it, is by inspecting the neighboring lines and see how
> much indent there is and we choose the line that has the shortest amount
> of blanks in the neighboring lines.
> ...
> because there is less space between line start and {end, def bal}
> than for {do_bal_stuff, common_ending}.

I haven't thought this carefully yet, but would this equally work
well for Python, where it does not have the "end" or does the lack
of "end" pose a problem?  You'll still find "def bal" is a good
boundary (but you cannot tell if it is the beginning or the end of a
block, unless you understand the language), though.

> +static unsigned int leading_blank(const char *line)
> +{
> +	unsigned int ret = 0;
> +	while (*line) {
> +		if (*line == '\t')
> +			ret += 8;

This will be broken with a line with space-before-tab whitespace
breakage, I suspect...

> +		else if (*line == ' ')
> +			ret ++;
> +		else
> +			break;
> +		line++;
> +	}
> +	return ret;
> +}
> +
> +static unsigned int surrounding_leading_blank(xrecord_t **recs, long ix,
> +		long flags, long nrec)
> +{
> +	unsigned int i, ret = UINT_MAX;
> +	if (ix > 0)
> +		ret = leading_blank(recs[ix - 1]->ptr);
> +	if (ix < nrec - 1) {
> +		i = leading_blank(recs[ix + 1]->ptr);
> +		if (i < ret)
> +			ret = i;
> +	}
> +	return ret;
> +}
> +
>  static int recs_match(xrecord_t **recs, long ixs, long ix, long flags)
>  {
>  	return (recs[ixs]->ha == recs[ix]->ha &&
> @@ -416,7 +445,7 @@ static int recs_match(xrecord_t **recs, long ixs, long ix, long flags)
>  int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
>  	long ix, ixo, ixs, ixref, grpsiz, nrec = xdf->nrec;
>  	char *rchg = xdf->rchg, *rchgo = xdfo->rchg;
> -	unsigned int blank_lines;
> +	unsigned int blank_lines, min_bl_neigh_indent;
>  	xrecord_t **recs = xdf->recs;
>  
>  	/*
> @@ -451,6 +480,7 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
>  		do {
>  			grpsiz = ix - ixs;
>  			blank_lines = 0;
> +			min_bl_neigh_indent = UINT_MAX;
>  
>  			/*
>  			 * If the line before the current change group, is equal to
> @@ -485,7 +515,13 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
>  			 * the group.
>  			 */
>  			while (ix < nrec && recs_match(recs, ixs, ix, flags)) {
> -				blank_lines += is_blank_line(recs, ix, flags);
> +				if (is_blank_line(recs, ix, flags)) {
> +					unsigned int bl_neigh_indent =
> +						surrounding_leading_blank(recs, ix, flags, nrec);
> +					if (min_bl_neigh_indent > bl_neigh_indent)
> +						min_bl_neigh_indent = min_bl_neigh_indent;
> +					blank_lines++;
> +				}
>  
>  				rchg[ixs++] = 0;
>  				rchg[ix++] = 1;
> @@ -525,6 +561,7 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
>  		if ((flags & XDF_COMPACTION_HEURISTIC) && blank_lines) {
>  			while (ixs > 0 &&
>  			       !is_blank_line(recs, ix - 1, flags) &&
> +			       surrounding_leading_blank(recs, ix - 1, flags, nrec) > min_bl_neigh_indent &&
>  			       recs_match(recs, ixs - 1, ix - 1, flags)) {
>  				rchg[--ixs] = 1;
>  				rchg[--ix] = 0;