On Fri, Apr 15, 2016 at 4:01 PM, Stefan Beller <sbeller@xxxxxxxxxx> wrote:
> In order to produce the smallest possible diff and combine several diff
> hunks together, we implement a heuristic from GNU Diff which moves diff
> hunks forward as far as possible when we find common context above and
> below a diff hunk. This sometimes produces less readable diffs when
> writing C, Shell, or other programming languages, ie:
>
> ...
> /*
> + *
> + *
> + */
> +
> +/*
> ...
>
> instead of the more readable equivalent of
>
> ...
> +/*
> + *
> + *
> + */
> +
> /*
> ...
>
> Original discussion and testing found the following heuristic to be
> producing the desired output:
>
> If there are diff chunks which can be shifted around, shift each hunk
> such that the last common empty line is below the chunk with the rest
> of the context above.
>
> This heuristic appears to resolve the above example and several other
> common issues without producing significantly weird results. When
> implementing this heuristic the handling of empty lines was awkward as
> it is unclear what an empty line is. ('\n' or do we include "\r\n" as it
> is common on Windows?) Instead we implement a slightly different heuristic:
>
> If there are diff chunks which can be shifted around, find the shortest
> line in the overlapping parts. Use the line with the shortest length that
> occurs last as the last line of the chunk with the rest
> of the context above.
>
> However, as with any heuristic it is not really known whether this will
> always be more optimal. Thus, leave the heuristic disabled by default.
>
> Add an XDIFF flag to enable this heuristic only conditionally. Add
> a diff command line option and diff configuration option to allow users
> to enable this option when desired.
>
> TODO:
> * Add tests
> * Add better/more documentation explaining the heuristic, possibly with
> examples(?)
> * better name(?)
>
There's a few places that will need cleaning up (comments and such)
that mention empty line still, but that's not surprising. I am going
to test this for a bit on my local repos, and see if it makes any
difference to the old heuristic as well.
Thanks,
Jake
> Signed-off-by: Stefan Beller <sbeller@xxxxxxxxxx>
> Signed-off-by: Jacob Keller <jacob.e.keller@xxxxxxxxx>
> Signed-off-by: Stefan Beller <sbeller@xxxxxxxxxx>
> ---
> Documentation/diff-config.txt | 6 ++++++
> Documentation/diff-options.txt | 6 ++++++
> diff.c | 11 +++++++++++
> xdiff/xdiff.h | 2 ++
> xdiff/xdiffi.c | 29 +++++++++++++++++++++++++++++
> 5 files changed, 54 insertions(+)
>
> diff --git a/Documentation/diff-config.txt b/Documentation/diff-config.txt
> index edba565..3d99a90 100644
> --- a/Documentation/diff-config.txt
> +++ b/Documentation/diff-config.txt
> @@ -170,6 +170,12 @@ diff.tool::
>
> include::mergetools-diff.txt[]
>
> +diff.shortestLineHeuristic::
> + Set this option to true to enable the shortest line chunk heuristic when
> + producing diff output. This heuristic will attempt to shift hunks such
> + that the last shortest common line occurs below the hunk with the rest of
> + the context above it.
> +
> diff.algorithm::
> Choose a diff algorithm. The variants are as follows:
> +
> diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
> index 4b0318e..b1ca83d 100644
> --- a/Documentation/diff-options.txt
> +++ b/Documentation/diff-options.txt
> @@ -63,6 +63,12 @@ ifndef::git-format-patch[]
> Synonym for `-p --raw`.
> endif::git-format-patch[]
>
> +--shortest-line-heuristic::
> +--no-shortest-line-heuristic::
> + When possible, shift common shortest line in diff hunks below the hunk
> + such that the last common shortest line for each hunk is below, with the
> + rest of the context above the hunk.
> +
> --minimal::
> Spend extra time to make sure the smallest possible
> diff is produced.
> diff --git a/diff.c b/diff.c
> index 4dfe660..a02aff9 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -26,6 +26,7 @@
> #endif
>
> static int diff_detect_rename_default;
> +static int diff_shortest_line_heuristic = 0;
> static int diff_rename_limit_default = 400;
> static int diff_suppress_blank_empty;
> static int diff_use_color_default = -1;
> @@ -189,6 +190,10 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
> diff_detect_rename_default = git_config_rename(var, value);
> return 0;
> }
> + if (!strcmp(var, "diff.shortestlineheuristic")) {
> + diff_shortest_line_heuristic = git_config_bool(var, value);
> + return 0;
> + }
> if (!strcmp(var, "diff.autorefreshindex")) {
> diff_auto_refresh_index = git_config_bool(var, value);
> return 0;
> @@ -3278,6 +3283,8 @@ void diff_setup(struct diff_options *options)
> options->use_color = diff_use_color_default;
> options->detect_rename = diff_detect_rename_default;
> options->xdl_opts |= diff_algorithm;
> + if (diff_shortest_line_heuristic)
> + DIFF_XDL_SET(options, SHORTEST_LINE_HEURISTIC);
>
> options->orderfile = diff_order_file_cfg;
>
> @@ -3798,6 +3805,10 @@ int diff_opt_parse(struct diff_options *options,
> DIFF_XDL_SET(options, IGNORE_WHITESPACE_AT_EOL);
> else if (!strcmp(arg, "--ignore-blank-lines"))
> DIFF_XDL_SET(options, IGNORE_BLANK_LINES);
> + else if (!strcmp(arg, "--shortest-line-heuristic"))
> + DIFF_XDL_SET(options, SHORTEST_LINE_HEURISTIC);
> + else if (!strcmp(arg, "--no-shortest-line-heuristic"))
> + DIFF_XDL_CLR(options, SHORTEST_LINE_HEURISTIC);
> else if (!strcmp(arg, "--patience"))
> options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
> else if (!strcmp(arg, "--histogram"))
> diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
> index 4fb7e79..e1f8ec0 100644
> --- a/xdiff/xdiff.h
> +++ b/xdiff/xdiff.h
> @@ -41,6 +41,8 @@ extern "C" {
>
> #define XDF_IGNORE_BLANK_LINES (1 << 7)
>
> +#define XDF_SHORTEST_LINE_HEURISTIC (1 << 8)
> +
> #define XDL_EMIT_FUNCNAMES (1 << 0)
> #define XDL_EMIT_FUNCCONTEXT (1 << 2)
>
> diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c
> index 748eeb9..7d15b26 100644
> --- a/xdiff/xdiffi.c
> +++ b/xdiff/xdiffi.c
> @@ -400,6 +400,12 @@ static xdchange_t *xdl_add_change(xdchange_t *xscr, long i1, long i2, long chg1,
> }
>
>
> +static int line_length(const char *recs)
> +{
> + char *s = strchr(recs, '\n');
> + return s ? s - recs : strlen(recs);
> +}
> +
> static int recs_match(xrecord_t **recs, long ixs, long ix, long flags)
> {
> return (recs[ixs]->ha == recs[ix]->ha &&
> @@ -411,6 +417,7 @@ 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 shortest_line;
> xrecord_t **recs = xdf->recs;
>
> /*
> @@ -444,6 +451,7 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
>
> do {
> grpsiz = ix - ixs;
> + shortest_line = UINT_MAX;
>
> /*
> * If the line before the current change group, is equal to
> @@ -478,6 +486,10 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
> * the group.
> */
> while (ix < nrec && recs_match(recs, ixs, ix, flags)) {
> + int l = line_length(recs[ix]->ptr);
> + if (l < shortest_line)
> + shortest_line = l;
> +
> rchg[ixs++] = 0;
> rchg[ix++] = 1;
>
> @@ -504,6 +516,23 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
> rchg[--ix] = 0;
> while (rchgo[--ixo]);
> }
> +
> + /*
> + * If a group can be moved back and forth, see if there is an
> + * empty line in the moving space. If there is an empty line,
> + * make sure the last empty line is the end of the group.
> + *
> + * As we shifted the group forward as far as possible, we only
> + * need to shift it back if at all.
> + */
> + if ((flags & XDF_SHORTEST_LINE_HEURISTIC)) {
> + while (ixs > 0 &&
> + line_length(recs[ix - 1]->ptr) > shortest_line &&
> + recs_match(recs, ixs - 1, ix - 1, flags)) {
> + rchg[--ixs] = 1;
> + rchg[--ix] = 0;
> + }
> + }
> }
>
> return 0;
> --
> 2.8.1.189.gd13d43c
>
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