Re: [PATCH v4 05/15] diff --color-moved=zebra: fix alternate coloring

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Phillip,

On Tue, 16 Nov 2021, Phillip Wood via GitGitGadget wrote:

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>
> b0a2ba4776 ("diff --color-moved=zebra: be stricter with color
> alternation", 2018-11-23) sought to avoid using the alternate colors
> unless there are two adjacent moved blocks of the same
> sign. Unfortunately it contains two bugs that prevented it from fixing
> the problem properly. Firstly `last_symbol` is reset at the start of
> each iteration of the loop losing the symbol of the last line and
> secondly when deciding whether to use the alternate color it should be
> checking if the current line is the same sign of the last line, not a
> different sign. The combination of the two errors means that we still
> use the alternate color when we should do but we also use it when we
> shouldn't. This is most noticable when using
> --color-moved-ws=allow-indentation-change with hunks like
>
> -this line gets indented
> +    this line gets indented
>
> where the post image is colored with newMovedAlternate rather than
> newMoved. While this does not matter much, the next commit will change
> the coloring to be correct in this case, so lets fix the bug here to
> make it clear why the output is changing and add a regression test.

What an excellent commit message!

Thank you,
Dscho

>
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> ---
>  diff.c                     |  4 +--
>  t/t4015-diff-whitespace.sh | 72 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 74 insertions(+), 2 deletions(-)
>
> diff --git a/diff.c b/diff.c
> index 1e1b5127d15..53f0df75329 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -1176,6 +1176,7 @@ static void mark_color_as_moved(struct diff_options *o,
>  	struct moved_block *pmb = NULL; /* potentially moved blocks */
>  	int pmb_nr = 0, pmb_alloc = 0;
>  	int n, flipped_block = 0, block_length = 0;
> +	enum diff_symbol last_symbol = 0;
>
>
>  	for (n = 0; n < o->emitted_symbols->nr; n++) {
> @@ -1183,7 +1184,6 @@ static void mark_color_as_moved(struct diff_options *o,
>  		struct moved_entry *key;
>  		struct moved_entry *match = NULL;
>  		struct emitted_diff_symbol *l = &o->emitted_symbols->buf[n];
> -		enum diff_symbol last_symbol = 0;
>
>  		switch (l->s) {
>  		case DIFF_SYMBOL_PLUS:
> @@ -1251,7 +1251,7 @@ static void mark_color_as_moved(struct diff_options *o,
>  							    &pmb, &pmb_alloc,
>  							    &pmb_nr);
>
> -			if (contiguous && pmb_nr && last_symbol != l->s)
> +			if (contiguous && pmb_nr && last_symbol == l->s)
>  				flipped_block = (flipped_block + 1) % 2;
>  			else
>  				flipped_block = 0;
> diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
> index 308dc136596..4e0fd76c6c5 100755
> --- a/t/t4015-diff-whitespace.sh
> +++ b/t/t4015-diff-whitespace.sh
> @@ -1442,6 +1442,78 @@ test_expect_success 'detect permutations inside moved code -- dimmed-zebra' '
>  	test_cmp expected actual
>  '
>
> +test_expect_success 'zebra alternate color is only used when necessary' '
> +	cat >old.txt <<-\EOF &&
> +	line 1A should be marked as oldMoved newMovedAlternate
> +	line 1B should be marked as oldMoved newMovedAlternate
> +	unchanged
> +	line 2A should be marked as oldMoved newMovedAlternate
> +	line 2B should be marked as oldMoved newMovedAlternate
> +	line 3A should be marked as oldMovedAlternate newMoved
> +	line 3B should be marked as oldMovedAlternate newMoved
> +	unchanged
> +	line 4A should be marked as oldMoved newMovedAlternate
> +	line 4B should be marked as oldMoved newMovedAlternate
> +	line 5A should be marked as oldMovedAlternate newMoved
> +	line 5B should be marked as oldMovedAlternate newMoved
> +	line 6A should be marked as oldMoved newMoved
> +	line 6B should be marked as oldMoved newMoved
> +	EOF
> +	cat >new.txt <<-\EOF &&
> +	  line 1A should be marked as oldMoved newMovedAlternate
> +	  line 1B should be marked as oldMoved newMovedAlternate
> +	unchanged
> +	  line 3A should be marked as oldMovedAlternate newMoved
> +	  line 3B should be marked as oldMovedAlternate newMoved
> +	  line 2A should be marked as oldMoved newMovedAlternate
> +	  line 2B should be marked as oldMoved newMovedAlternate
> +	unchanged
> +	  line 6A should be marked as oldMoved newMoved
> +	  line 6B should be marked as oldMoved newMoved
> +	    line 4A should be marked as oldMoved newMovedAlternate
> +	    line 4B should be marked as oldMoved newMovedAlternate
> +	  line 5A should be marked as oldMovedAlternate newMoved
> +	  line 5B should be marked as oldMovedAlternate newMoved
> +	EOF
> +	test_expect_code 1 git diff --no-index --color --color-moved=zebra \
> +		 --color-moved-ws=allow-indentation-change \
> +		 old.txt new.txt >output &&
> +	grep -v index output | test_decode_color >actual &&
> +	cat >expected <<-\EOF &&
> +	<BOLD>diff --git a/old.txt b/new.txt<RESET>
> +	<BOLD>--- a/old.txt<RESET>
> +	<BOLD>+++ b/new.txt<RESET>
> +	<CYAN>@@ -1,14 +1,14 @@<RESET>
> +	<BOLD;MAGENTA>-line 1A should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;MAGENTA>-line 1B should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 1A should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 1B should be marked as oldMoved newMovedAlternate<RESET>
> +	 unchanged<RESET>
> +	<BOLD;MAGENTA>-line 2A should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;MAGENTA>-line 2B should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;BLUE>-line 3A should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;BLUE>-line 3B should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 3A should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 3B should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>  line 2A should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>  line 2B should be marked as oldMoved newMovedAlternate<RESET>
> +	 unchanged<RESET>
> +	<BOLD;MAGENTA>-line 4A should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;MAGENTA>-line 4B should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;BLUE>-line 5A should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;BLUE>-line 5B should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;MAGENTA>-line 6A should be marked as oldMoved newMoved<RESET>
> +	<BOLD;MAGENTA>-line 6B should be marked as oldMoved newMoved<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 6A should be marked as oldMoved newMoved<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 6B should be marked as oldMoved newMoved<RESET>
> +	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>    line 4A should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>    line 4B should be marked as oldMoved newMovedAlternate<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 5A should be marked as oldMovedAlternate newMoved<RESET>
> +	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 5B should be marked as oldMovedAlternate newMoved<RESET>
> +	EOF
> +	test_cmp expected actual
> +'
> +
>  test_expect_success 'cmd option assumes configured colored-moved' '
>  	test_config color.diff.oldMoved "magenta" &&
>  	test_config color.diff.newMoved "cyan" &&
> --
> gitgitgadget
>
>