[PATCH 0/3] Rework git-diff algorithm selection

[PATCH 0/3] Rework git-diff algorithm selection

[Michal Privoznik]

It's been a while I was trying to get this in. Recently, I realized how
important this is.

Please keep me CC'ed as I am not subscribed to the list.

Michal Privoznik (3):
  git-completion.bash: Autocomplete --minimal and --histogram for
    git-diff
  config: Introduce diff.algorithm variable
  diff: Introduce --diff-algorithm command line option

 Documentation/diff-config.txt          | 17 +++++++++++++++++
 Documentation/diff-options.txt         | 22 ++++++++++++++++++++++
 contrib/completion/git-completion.bash | 14 +++++++++++++-
 diff.c                                 | 33 +++++++++++++++++++++++++++++++++
 diff.h                                 |  2 ++
 merge-recursive.c                      |  6 ++++++
 6 files changed, 93 insertions(+), 1 deletion(-)

-- 
1.8.0.2

[PATCH 1/3] git-completion.bash: Autocomplete --minimal and --histogram for git-diff

[Michal Privoznik]

Even though --patience was already there, we missed --minimal and
--histogram for some reason.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 contrib/completion/git-completion.bash | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index a4c48e1..383518c 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1031,7 +1031,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--no-ext-diff
 			--no-prefix --src-prefix= --dst-prefix=
 			--inter-hunk-context=
-			--patience
+			--patience --histogram --minimal
 			--raw
 			--dirstat --dirstat= --dirstat-by-file
 			--dirstat-by-file= --cumulative
-- 
1.8.0.2

[PATCH 2/3] config: Introduce diff.algorithm variable

[Michal Privoznik]

Some users or projects prefer different algorithms over others, e.g.
patience over myers or similar. However, specifying appropriate
argument every time diff is to be used is impractical. Moreover,
creating an alias doesn't play nicely with other tools based on diff
(git-show for instance). Hence, a configuration variable which is able
to set specific algorithm is needed. For now, these four values are
accepted: 'myers' (which has the same effect as not setting the config
variable at all), 'minimal', 'patience' and 'histogram'.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 Documentation/diff-config.txt          | 17 +++++++++++++++++
 contrib/completion/git-completion.bash |  1 +
 diff.c                                 | 23 +++++++++++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/Documentation/diff-config.txt b/Documentation/diff-config.txt
index 4314ad0..be31276 100644
--- a/Documentation/diff-config.txt
+++ b/Documentation/diff-config.txt
@@ -155,3 +155,20 @@ diff.tool::
 	"kompare".  Any other value is treated as a custom diff tool,
 	and there must be a corresponding `difftool.<tool>.cmd`
 	option.
+
+diff.algorithm::
+	Choose a diff algorithm.  The variants are as follows:
++
+--
+`myers`;;
+	The basic greedy diff algorithm.
+`minimal`;;
+	Spend extra time to make sure the smallest possible diff is
+	produced.
+`patience`;;
+	Use "patience diff" algorithm when generating patches.
+`histogram`;;
+	This algorithm extends the patience algorithm to "support
+	low-occurrence common elements".
+--
++
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 383518c..33e25dc 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1839,6 +1839,7 @@ _git_config ()
 		diff.suppressBlankEmpty
 		diff.tool
 		diff.wordRegex
+		diff.algorithm
 		difftool.
 		difftool.prompt
 		fetch.recurseSubmodules
diff --git a/diff.c b/diff.c
index 732d4c2..ddae5c4 100644
--- a/diff.c
+++ b/diff.c
@@ -36,6 +36,7 @@ static int diff_no_prefix;
 static int diff_stat_graph_width;
 static int diff_dirstat_permille_default = 30;
 static struct diff_options default_diff_options;
+static long diff_algorithm = 0;
 
 static char diff_colors[][COLOR_MAXLEN] = {
 	GIT_COLOR_RESET,
@@ -143,6 +144,20 @@ static int git_config_rename(const char *var, const char *value)
 	return git_config_bool(var,value) ? DIFF_DETECT_RENAME : 0;
 }
 
+static long parse_algorithm_value(const char *value)
+{
+	if (!value || !strcasecmp(value, "myers"))
+		return 0;
+	else if (!strcasecmp(value, "minimal"))
+		return XDF_NEED_MINIMAL;
+	else if (!strcasecmp(value, "patience"))
+		return XDF_PATIENCE_DIFF;
+	else if (!strcasecmp(value, "histogram"))
+		return XDF_HISTOGRAM_DIFF;
+	else
+		return -1;
+}
+
 /*
  * These are to give UI layer defaults.
  * The core-level commands such as git-diff-files should
@@ -196,6 +211,13 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "diff.algorithm")) {
+		diff_algorithm = parse_algorithm_value(value);
+		if (diff_algorithm < 0)
+			return -1;
+		return 0;
+	}
+
 	if (git_color_config(var, value, cb) < 0)
 		return -1;
 
@@ -3213,6 +3235,7 @@ void diff_setup(struct diff_options *options)
 	options->add_remove = diff_addremove;
 	options->use_color = diff_use_color_default;
 	options->detect_rename = diff_detect_rename_default;
+	options->xdl_opts |= diff_algorithm;
 
 	if (diff_no_prefix) {
 		options->a_prefix = options->b_prefix = "";
-- 
1.8.0.2

[PATCH 3/3] diff: Introduce --diff-algorithm command line option

[Michal Privoznik]

Since command line options have higher priority than config file
variables and taking previous commit into account, we need a way
how to specify myers algorithm on command line. However,
inventing `--myers` is not the right answer. We need far more
general option, and that is `--diff-algorithm`. The older options
(`--minimal`, `--patience` and `--histogram`) are kept for
backward compatibility.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 Documentation/diff-options.txt         | 22 ++++++++++++++++++++++
 contrib/completion/git-completion.bash | 11 +++++++++++
 diff.c                                 | 12 +++++++++++-
 diff.h                                 |  2 ++
 merge-recursive.c                      |  6 ++++++
 5 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 39f2c50..4091f52 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -45,6 +45,9 @@ ifndef::git-format-patch[]
 	Synonym for `-p --raw`.
 endif::git-format-patch[]
 
+The next three options are kept for compatibility reason. You should use the
+`--diff-algorithm` option instead.
+
 --minimal::
 	Spend extra time to make sure the smallest possible
 	diff is produced.
@@ -55,6 +58,25 @@ endif::git-format-patch[]
 --histogram::
 	Generate a diff using the "histogram diff" algorithm.
 
+--diff-algorithm={patience|minimal|histogram|myers}::
+	Choose a diff algorithm. The variants are as follows:
++
+--
+`myers`;;
+	The basic greedy diff algorithm.
+`minimal`;;
+	Spend extra time to make sure the smallest possible diff is
+	produced.
+`patience`;;
+	Use "patience diff" algorithm when generating patches.
+`histogram`;;
+	This algorithm extends the patience algorithm to "support
+	low-occurrence common elements".
+--
++
+You should prefer this option over the `--minimal`, `--patience` and
+`--histogram` which are kept just for backwards compatibility.
+
 --stat[=<width>[,<name-width>[,<count>]]]::
 	Generate a diffstat. By default, as much space as necessary
 	will be used for the filename part, and the rest for the graph
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 33e25dc..d592cf9 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1021,6 +1021,8 @@ _git_describe ()
 	__gitcomp_nl "$(__git_refs)"
 }
 
+__git_diff_algorithms="myers minimal patience histogram"
+
 __git_diff_common_options="--stat --numstat --shortstat --summary
 			--patch-with-stat --name-only --name-status --color
 			--no-color --color-words --no-renames --check
@@ -1035,6 +1037,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--raw
 			--dirstat --dirstat= --dirstat-by-file
 			--dirstat-by-file= --cumulative
+			--diff-algorithm=
 "
 
 _git_diff ()
@@ -1042,6 +1045,10 @@ _git_diff ()
 	__git_has_doubledash && return
 
 	case "$cur" in
+	--diff-algorithm=*)
+		__gitcomp "$__git_diff_algorithms" "" "${cur##--diff-algorithm=}"
+		return
+		;;
 	--*)
 		__gitcomp "--cached --staged --pickaxe-all --pickaxe-regex
 			--base --ours --theirs --no-index
@@ -2114,6 +2121,10 @@ _git_show ()
 			" "" "${cur#*=}"
 		return
 		;;
+	--diff-algorithm=*)
+		__gitcomp "$__git_diff_algorithms" "" "${cur##--diff-algorithm=}"
+		return
+		;;
 	--*)
 		__gitcomp "--pretty= --format= --abbrev-commit --oneline
 			$__git_diff_common_options
diff --git a/diff.c b/diff.c
index ddae5c4..6418076 100644
--- a/diff.c
+++ b/diff.c
@@ -144,7 +144,7 @@ static int git_config_rename(const char *var, const char *value)
 	return git_config_bool(var,value) ? DIFF_DETECT_RENAME : 0;
 }
 
-static long parse_algorithm_value(const char *value)
+long parse_algorithm_value(const char *value)
 {
 	if (!value || !strcasecmp(value, "myers"))
 		return 0;
@@ -3633,6 +3633,16 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
 	else if (!strcmp(arg, "--histogram"))
 		options->xdl_opts = DIFF_WITH_ALG(options, HISTOGRAM_DIFF);
+	else if (!prefixcmp(arg, "--diff-algorithm=")) {
+		long value = parse_algorithm_value(arg+17);
+		if (value < 0)
+			return error("option diff-algorithm accepts \"myers\", "
+				     "\"minimal\", \"patience\" and \"histogram\"");
+		/* clear out previous settings */
+		DIFF_XDL_CLR(options, NEED_MINIMAL);
+		options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
+		options->xdl_opts |= value;
+	}
 
 	/* flags options */
 	else if (!strcmp(arg, "--binary")) {
diff --git a/diff.h b/diff.h
index a47bae4..54c2590 100644
--- a/diff.h
+++ b/diff.h
@@ -333,6 +333,8 @@ extern struct userdiff_driver *get_textconv(struct diff_filespec *one);
 
 extern int parse_rename_score(const char **cp_p);
 
+extern long parse_algorithm_value(const char *value);
+
 extern int print_stat_summary(FILE *fp, int files,
 			      int insertions, int deletions);
 extern void setup_diff_pager(struct diff_options *);
diff --git a/merge-recursive.c b/merge-recursive.c
index d882060..53d8feb 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2068,6 +2068,12 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
 	else if (!strcmp(s, "histogram"))
 		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
+	else if (!strcmp(s, "diff-algorithm=")) {
+		long value = parse_algorithm_value(s+15);
+		if (value < 0)
+			return -1;
+		o->xdl_opts |= value;
+	}
 	else if (!strcmp(s, "ignore-space-change"))
 		o->xdl_opts |= XDF_IGNORE_WHITESPACE_CHANGE;
 	else if (!strcmp(s, "ignore-all-space"))
-- 
1.8.0.2

Re: [PATCH 2/3] config: Introduce diff.algorithm variable

[Junio C Hamano]

Michal Privoznik <mprivozn@redhat.com> writes:

> Some users or projects prefer different algorithms over others, e.g.
> patience over myers or similar. However, specifying appropriate
> argument every time diff is to be used is impractical. Moreover,
> creating an alias doesn't play nicely with other tools based on diff
> (git-show for instance). Hence, a configuration variable which is able
> to set specific algorithm is needed. For now, these four values are
> accepted: 'myers' (which has the same effect as not setting the config
> variable at all), 'minimal', 'patience' and 'histogram'.
>
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> ---
>  Documentation/diff-config.txt          | 17 +++++++++++++++++
>  contrib/completion/git-completion.bash |  1 +
>  diff.c                                 | 23 +++++++++++++++++++++++
>  3 files changed, 41 insertions(+)
>
> diff --git a/Documentation/diff-config.txt b/Documentation/diff-config.txt
> index 4314ad0..be31276 100644
> --- a/Documentation/diff-config.txt
> +++ b/Documentation/diff-config.txt
> @@ -155,3 +155,20 @@ diff.tool::
>  	"kompare".  Any other value is treated as a custom diff tool,
>  	and there must be a corresponding `difftool.<tool>.cmd`
>  	option.
> +
> +diff.algorithm::
> +	Choose a diff algorithm.  The variants are as follows:
> ++
> +--
> +`myers`;;
> +	The basic greedy diff algorithm.
> +`minimal`;;
> +	Spend extra time to make sure the smallest possible diff is
> +	produced.
> +`patience`;;
> +	Use "patience diff" algorithm when generating patches.
> +`histogram`;;
> +	This algorithm extends the patience algorithm to "support
> +	low-occurrence common elements".
> +--
> ++

Sounds sensible.

> diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
> index 383518c..33e25dc 100644
> --- a/contrib/completion/git-completion.bash
> +++ b/contrib/completion/git-completion.bash
> @@ -1839,6 +1839,7 @@ _git_config ()
>  		diff.suppressBlankEmpty
>  		diff.tool
>  		diff.wordRegex
> +		diff.algorithm
>  		difftool.
>  		difftool.prompt
>  		fetch.recurseSubmodules
> diff --git a/diff.c b/diff.c
> index 732d4c2..ddae5c4 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -36,6 +36,7 @@ static int diff_no_prefix;
>  static int diff_stat_graph_width;
>  static int diff_dirstat_permille_default = 30;
>  static struct diff_options default_diff_options;
> +static long diff_algorithm = 0;

Please do not initialize a static explicitly to a zero and instead
let BSS do its thing.

>  static char diff_colors[][COLOR_MAXLEN] = {
>  	GIT_COLOR_RESET,
> @@ -143,6 +144,20 @@ static int git_config_rename(const char *var, const char *value)
>  	return git_config_bool(var,value) ? DIFF_DETECT_RENAME : 0;
>  }
>  
> +static long parse_algorithm_value(const char *value)
> +{
> +	if (!value || !strcasecmp(value, "myers"))
> +		return 0;
> +	else if (!strcasecmp(value, "minimal"))
> +		return XDF_NEED_MINIMAL;
> +	else if (!strcasecmp(value, "patience"))
> +		return XDF_PATIENCE_DIFF;
> +	else if (!strcasecmp(value, "histogram"))
> +		return XDF_HISTOGRAM_DIFF;
> +	else
> +		return -1;
> +}
> +
>  /*
>   * These are to give UI layer defaults.
>   * The core-level commands such as git-diff-files should
> @@ -196,6 +211,13 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
>  		return 0;
>  	}
>  
> +	if (!strcmp(var, "diff.algorithm")) {
> +		diff_algorithm = parse_algorithm_value(value);
> +		if (diff_algorithm < 0)
> +			return -1;
> +		return 0;
> +	}
> +
>  	if (git_color_config(var, value, cb) < 0)
>  		return -1;
>  
> @@ -3213,6 +3235,7 @@ void diff_setup(struct diff_options *options)
>  	options->add_remove = diff_addremove;
>  	options->use_color = diff_use_color_default;
>  	options->detect_rename = diff_detect_rename_default;
> +	options->xdl_opts |= diff_algorithm;
>  
>  	if (diff_no_prefix) {
>  		options->a_prefix = options->b_prefix = "";

Re: [PATCH 3/3] diff: Introduce --diff-algorithm command line option

[Junio C Hamano]

Michal Privoznik <mprivozn@redhat.com> writes:

> Since command line options have higher priority than config file
> variables and taking previous commit into account, we need a way
> how to specify myers algorithm on command line.

Yes.  We cannot stop at [2/3] without this patch.

> However,
> inventing `--myers` is not the right answer. We need far more
> general option, and that is `--diff-algorithm`.

Yes, --diff-algorithm=default would let people defeat a configured
algo without knowing how exactly to spell myers.

> The older options
> (`--minimal`, `--patience` and `--histogram`) are kept for
> backward compatibility.

That is phrasing it too strongly to be acceptable.

People who do not care any configuration can keep using --histogram
without having to retrain their fingers to type a much longer form
you introduced (i.e. --diff-algorithm=histogram).  It is and will
always be just as valid a way to ask for --histogram as the new
longhand is.

> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> ---
>  Documentation/diff-options.txt         | 22 ++++++++++++++++++++++
>  contrib/completion/git-completion.bash | 11 +++++++++++
>  diff.c                                 | 12 +++++++++++-
>  diff.h                                 |  2 ++
>  merge-recursive.c                      |  6 ++++++
>  5 files changed, 52 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
> index 39f2c50..4091f52 100644
> --- a/Documentation/diff-options.txt
> +++ b/Documentation/diff-options.txt
> @@ -45,6 +45,9 @@ ifndef::git-format-patch[]
>  	Synonym for `-p --raw`.
>  endif::git-format-patch[]
>  
> +The next three options are kept for compatibility reason. You should use the
> +`--diff-algorithm` option instead.
> +

Drop this.

> @@ -55,6 +58,25 @@ endif::git-format-patch[]
>  --histogram::
>  	Generate a diff using the "histogram diff" algorithm.
>  
> +--diff-algorithm={patience|minimal|histogram|myers}::
> +	Choose a diff algorithm. The variants are as follows:
> ++
> +--
> +`myers`;;
> +	The basic greedy diff algorithm.
> +`minimal`;;
> +	Spend extra time to make sure the smallest possible diff is
> +	produced.
> +`patience`;;
> +	Use "patience diff" algorithm when generating patches.
> +`histogram`;;
> +	This algorithm extends the patience algorithm to "support
> +	low-occurrence common elements".
> +--
> ++
> +You should prefer this option over the `--minimal`, `--patience` and
> +`--histogram` which are kept just for backwards compatibility.

Drop the last one, and replace it with something like:

	If you configured diff.algorithm to a non-default value and
	want to use the default one, you have to use this form and
	choose myers, i.e. --diff-algorithm=myers, as we do not have
	a short-and-sweet --myers option.

(but the above is a bit too verbose, so please shorten it appropriately).

> diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
> index 33e25dc..d592cf9 100644
> --- a/contrib/completion/git-completion.bash
> +++ b/contrib/completion/git-completion.bash
> @@ -1021,6 +1021,8 @@ _git_describe ()
>  	__gitcomp_nl "$(__git_refs)"
>  }
>  
> +__git_diff_algorithms="myers minimal patience histogram"
> +
>  __git_diff_common_options="--stat --numstat --shortstat --summary
>  			--patch-with-stat --name-only --name-status --color
>  			--no-color --color-words --no-renames --check
> @@ -1035,6 +1037,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
>  			--raw
>  			--dirstat --dirstat= --dirstat-by-file
>  			--dirstat-by-file= --cumulative
> +			--diff-algorithm=
>  "
>  
>  _git_diff ()
> @@ -1042,6 +1045,10 @@ _git_diff ()
>  	__git_has_doubledash && return
>  
>  	case "$cur" in
> +	--diff-algorithm=*)
> +		__gitcomp "$__git_diff_algorithms" "" "${cur##--diff-algorithm=}"
> +		return
> +		;;
>  	--*)
>  		__gitcomp "--cached --staged --pickaxe-all --pickaxe-regex
>  			--base --ours --theirs --no-index
> @@ -2114,6 +2121,10 @@ _git_show ()
>  			" "" "${cur#*=}"
>  		return
>  		;;
> +	--diff-algorithm=*)
> +		__gitcomp "$__git_diff_algorithms" "" "${cur##--diff-algorithm=}"
> +		return
> +		;;
>  	--*)
>  		__gitcomp "--pretty= --format= --abbrev-commit --oneline
>  			$__git_diff_common_options
> diff --git a/diff.c b/diff.c
> index ddae5c4..6418076 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -144,7 +144,7 @@ static int git_config_rename(const char *var, const char *value)
>  	return git_config_bool(var,value) ? DIFF_DETECT_RENAME : 0;
>  }
>  
> -static long parse_algorithm_value(const char *value)
> +long parse_algorithm_value(const char *value)
>  {
>  	if (!value || !strcasecmp(value, "myers"))
>  		return 0;
> @@ -3633,6 +3633,16 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
>  		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
>  	else if (!strcmp(arg, "--histogram"))
>  		options->xdl_opts = DIFF_WITH_ALG(options, HISTOGRAM_DIFF);
> +	else if (!prefixcmp(arg, "--diff-algorithm=")) {
> +		long value = parse_algorithm_value(arg+17);
> +		if (value < 0)
> +			return error("option diff-algorithm accepts \"myers\", "
> +				     "\"minimal\", \"patience\" and \"histogram\"");
> +		/* clear out previous settings */
> +		DIFF_XDL_CLR(options, NEED_MINIMAL);
> +		options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
> +		options->xdl_opts |= value;
> +	}
>  
>  	/* flags options */
>  	else if (!strcmp(arg, "--binary")) {
> diff --git a/diff.h b/diff.h
> index a47bae4..54c2590 100644
> --- a/diff.h
> +++ b/diff.h
> @@ -333,6 +333,8 @@ extern struct userdiff_driver *get_textconv(struct diff_filespec *one);
>  
>  extern int parse_rename_score(const char **cp_p);
>  
> +extern long parse_algorithm_value(const char *value);
> +
>  extern int print_stat_summary(FILE *fp, int files,
>  			      int insertions, int deletions);
>  extern void setup_diff_pager(struct diff_options *);
> diff --git a/merge-recursive.c b/merge-recursive.c
> index d882060..53d8feb 100644
> --- a/merge-recursive.c
> +++ b/merge-recursive.c
> @@ -2068,6 +2068,12 @@ int parse_merge_opt(struct merge_options *o, const char *s)
>  		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
>  	else if (!strcmp(s, "histogram"))
>  		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
> +	else if (!strcmp(s, "diff-algorithm=")) {
> +		long value = parse_algorithm_value(s+15);
> +		if (value < 0)
> +			return -1;
> +		o->xdl_opts |= value;

Isn't this new hunk wrong?  DIFF_WITH_ALG() removes the previously
chosen algorithm choice before OR'ing the new one in, so that

	diff --histogram --patience

would not end up with a value PATIENCE|HISTOGRAM OR'ed together in
the algo field.

Re: [PATCH 3/3] diff: Introduce --diff-algorithm command line option

[Junio C Hamano]

Junio C Hamano <gitster@pobox.com> writes:

> Michal Privoznik <mprivozn@redhat.com> writes:
> ...
>> diff --git a/merge-recursive.c b/merge-recursive.c
>> index d882060..53d8feb 100644
>> --- a/merge-recursive.c
>> +++ b/merge-recursive.c
>> @@ -2068,6 +2068,12 @@ int parse_merge_opt(struct merge_options *o, const char *s)
>>  		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
>>  	else if (!strcmp(s, "histogram"))
>>  		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
>> +	else if (!strcmp(s, "diff-algorithm=")) {
>> +		long value = parse_algorithm_value(s+15);
>> +		if (value < 0)
>> +			return -1;
>> +		o->xdl_opts |= value;
>
> Isn't this new hunk wrong?  DIFF_WITH_ALG() removes the previously
> chosen algorithm choice before OR'ing the new one in, so that
>
> 	diff --histogram --patience
>
> would not end up with a value PATIENCE|HISTOGRAM OR'ed together in
> the algo field.

I misspoke; this is not "diff", but "merge-recursive".  The issue
may be the same here, though.