config: Introduce diff.algorithm variable

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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:
++
+--
+`default`, `myers`;;
+	The basic greedy diff algorithm. Currently, this is the default.
+`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".
+--
++

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.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;
 
 static char diff_colors[][COLOR_MAXLEN] = {
 	GIT_COLOR_RESET,
@@ -143,6 +144,21 @@ 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)
+		return -1;
+	else if (!strcasecmp(value, "myers") || !strcasecmp(value, "default"))
+		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;
+	return -1;
+}
+
 /*
  * These are to give UI layer defaults.
  * The core-level commands such as git-diff-files should
@@ -196,6 +212,13 @@ 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 +3236,7 @@ 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 = "";