patchxdiff: unignore changes in function context

> Changes involving only blank lines are hidden with --ignore-blank-lines,
> unless they appear in the context lines of other changes.  This is
> handled by xdl_get_hunk() for context added by --inter-hunk-context, -u
> and -U.
>
> Function context for -W and --function-context added by xdl_emit_diff()
> doesn't pay attention to such ignored changes; it relies fully on
> xdl_get_hunk() and shows just the post-image of ignored changes
> appearing in function context.  That's inconsistent and confusing.
>
> Improve the result of using --ignore-blank-lines and --function-context
> together by fully showing ignored changes if they happen to fall within
> function context.
>
> Signed-off-by: René Scharfe <l.s.r@web.de>
> ---
>  t/t4015-diff-whitespace.sh |  6 +-----
>  xdiff/xemit.c              | 17 +++++++++++++++++
>  2 files changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
> index eadaf57262..5888ae5ed3 100755
> --- a/t/t4015-diff-whitespace.sh
> +++ b/t/t4015-diff-whitespace.sh
> @@ -2025,11 +2025,6 @@ test_expect_success 'compare mixed whitespace delta across moved blocks' '
>  	test_cmp expected actual
>  '
>
> -# Note that the "6" in the expected hunk header below is funny, since we only
> -# show 5 lines (the missing one was blank and thus ignored). This is how
> -# --ignore-blank-lines behaves even without --function-context, and this test
> -# is just checking the interaction of the two features. Don't take it as an
> -# endorsement of that output.

> @@ -2039,6 +2034,7 @@ test_expect_success 'combine --ignore-blank-lines with --function-context' '
>  	cat <<-\EOF >expect &&
>  	@@ -1,6 +1,4 @@
>  	 1
> +	-
>  	 2
>  	 3
>  	 4

> diff --git a/xdiff/xemit.c b/xdiff/xemit.c
> index 30713ae9a9..9d7d6c5087 100644
> --- a/xdiff/xemit.c
> +++ b/xdiff/xemit.c
> @@ -172,10 +172,12 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
>  	struct func_line func_line = { 0 };
>
>  	for (xch = xscr; xch; xch = xche->next) {
> +		xdchange_t *xchp = xch;
>  		xche = xdl_get_hunk(&xch, xecfg);
>  		if (!xch)
>  			break;
>
> +pre_context_calculation:
>  		s1 = XDL_MAX(xch->i1 - xecfg->ctxlen, 0);
>  		s2 = XDL_MAX(xch->i2 - xecfg->ctxlen, 0);
>
> @@ -212,6 +214,21 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
>  			if (fs1 < s1) {
>  				s2 = XDL_MAX(s2 - (s1 - fs1), 0);
>  				s1 = fs1;
> +
> +				/*
> +				 * Did we extend context upwards into an
> +				 * ignored change?
> +				 */
> +				while (xchp != xch &&
> +				       xchp->i1 + xchp->chg1 <= s1 &&
> +				       xchp->i2 + xchp->chg2 <= s2)
> +					xchp = xchp->next;
> +
> +				/* If so, show it after all. */
> +				if (xchp != xch) {
> +					xch = xchp;
> +					goto pre_context_calculation;
> +				}
>  			}
>  		}
>
> --
> 2.24.0

> > -# Note that the "6" in the expected hunk header below is funny, since we only
> > -# show 5 lines (the missing one was blank and thus ignored). This is how
> > -# --ignore-blank-lines behaves even without --function-context, and this test
> > -# is just checking the interaction of the two features. Don't take it as an
> > -# endorsement of that output.
> 
> Nice to see that somebody anticipated that we may fix this some day.