Re: [PATCH v1 8/9] diff --color-moved-ws: modify allow-indentation-change

[Stefan Beller <sbeller@google.com>]

On Fri, Nov 16, 2018 at 3:04 AM Phillip Wood <phillip.wood@talktalk.net> wrote:
>
> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>
> Currently diff --color-moved-ws=allow-indentation-change does not
> support indentation that contains a mix of tabs and spaces. For
> example in commit 546f70f377 ("convert.h: drop 'extern' from function
> declaration", 2018-06-30) the function parameters in the following
> lines are not colored as moved [1].
>
> -extern int stream_filter(struct stream_filter *,
> -                        const char *input, size_t *isize_p,
> -                        char *output, size_t *osize_p);
> +int stream_filter(struct stream_filter *,
> +                 const char *input, size_t *isize_p,
> +                 char *output, size_t *osize_p);
>
> This commit changes the way the indentation is handled to track the
> visual size of the indentation rather than the characters in the
> indentation. This has they benefit that any whitespace errors do not

s/they/the/

> interfer with the move detection (the whitespace errors will still be
> highlighted according to --ws-error-highlight). During the discussion
> of this feature there were concerns about the correct detection of
> indentation for python. However those concerns apply whether or not
> we're detecting moved lines so no attempt is made to determine if the
> indentation is 'pythonic'.
>
> [1] Note that before the commit to fix the erroneous coloring of moved
>     lines each line was colored as a different block, since that commit
>     they are uncolored.
>
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> ---
>
> Notes:
>     Changes since rfc:
>      - It now replaces the existing implementation rather than adding a new
>        mode.
>      - The indentation deltas are now calculated once for each line and
>        cached.
>      - Optimized the whitespace delta comparison to compare string lengths
>        before comparing the actual strings.
>      - Modified the calculation of tabs as suggested by Stefan.
>      - Split out the blank line handling into a separate commit as suggest
>        by Stefan.
>      - Fixed some comments pointed out by Stefan.
>
>  diff.c                     | 130 +++++++++++++++++++++----------------
>  t/t4015-diff-whitespace.sh |  56 ++++++++++++++++
>  2 files changed, 129 insertions(+), 57 deletions(-)
>
> diff --git a/diff.c b/diff.c
> index c378ce3daf..89559293e7 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -750,6 +750,8 @@ struct emitted_diff_symbol {
>         const char *line;
>         int len;
>         int flags;
> +       int indent_off;
> +       int indent_width;

So this is the trick how we compute the ws related
data only once per line. :-)

On the other hand, we do not save memory by disabling
the ws detection, but I guess that is not a problem for now.

Would it make sense to have the new variables be
unsigned? (Also a comment on what they are, I
needed to read the code to understand off to be
offset into the line, where the content starts, and
width to be the visual width, as I did not recall
the RFC.)

> +static void fill_es_indent_data(struct emitted_diff_symbol *es)
> [...]

> +               if (o->color_moved_ws_handling &
> +                   COLOR_MOVED_WS_ALLOW_INDENTATION_CHANGE)
> +                       fill_es_indent_data(&o->emitted_symbols->buf[n]);

Nice.

By reducing the information kept around to ints, we also do not need
to alloc/free
memory for each line.

> +++ b/t/t4015-diff-whitespace.sh
> @@ -1901,4 +1901,60 @@ test_expect_success 'compare whitespace delta incompatible with other space opti
>         test_i18ngrep allow-indentation-change err
>  '
>
> +test_expect_success 'compare mixed whitespace delta across moved blocks' '

Looks good,

Thanks!
Stefan