[PATCH 3/5] ref-filter: add a "detached_head_first" sorting option

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add a "detached_head_first" sorting option for eventual use by the
"git branch" command. When listing branches we want to list the
detached HEAD "ref" at the start of the list. As shown in
28438e84e04 (ref-filter: sort detached HEAD lines firstly, 2019-06-18)
this currently relies on "(" sorting before any other refname by
strcmp().

This boxes translators into using ASCII parentheses, a subsequent
commit will amend get_head_description() to get rid of this
limitation.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 ref-filter.c | 23 ++++++++++++++++++++++-
 ref-filter.h |  3 +++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/ref-filter.c b/ref-filter.c
index aa260bfd099..94ab3f86a53 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -2350,6 +2350,16 @@ int filter_refs(struct ref_array *array, struct ref_filter *filter, unsigned int
 	return ret;
 }
 
+static int compare_detached_head(struct ref_array_item *a, struct ref_array_item *b)
+{
+	if (a->kind & FILTER_REFS_DETACHED_HEAD)
+		return -1;
+	else if (b->kind & FILTER_REFS_DETACHED_HEAD)
+		return 1;
+	BUG("compare_detached_head() is guarded by an xor on [ab]->kind & FILTER_REFS_DETACHED_HEAD");
+	return 0;
+}
+
 static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, struct ref_array_item *b)
 {
 	struct atom_value *va, *vb;
@@ -2364,7 +2374,12 @@ static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, stru
 		die("%s", err.buf);
 	strbuf_release(&err);
 	cmp_fn = s->ignore_case ? strcasecmp : strcmp;
-	if (s->version)
+	if (s->detached_head_first &&
+	    ((a->kind & FILTER_REFS_DETACHED_HEAD)
+	     ^
+	     (b->kind & FILTER_REFS_DETACHED_HEAD))) {
+		cmp = compare_detached_head(a, b);
+	} else if (s->version)
 		cmp = versioncmp(va->s, vb->s);
 	else if (cmp_type == FIELD_STR)
 		cmp = cmp_fn(va->s, vb->s);
@@ -2403,6 +2418,12 @@ void ref_sorting_icase_all(struct ref_sorting *sorting, int flag)
 		sorting->ignore_case = !!flag;
 }
 
+void ref_sorting_detached_head_first_all(struct ref_sorting *sorting, int flag)
+{
+	for (; sorting; sorting = sorting->next)
+		sorting->detached_head_first = !!flag;
+}
+
 void ref_array_sort(struct ref_sorting *sorting, struct ref_array *array)
 {
 	QSORT_S(array->items, array->nr, compare_refs, sorting);
diff --git a/ref-filter.h b/ref-filter.h
index feaef4a8fde..3b92e0f2696 100644
--- a/ref-filter.h
+++ b/ref-filter.h
@@ -30,6 +30,7 @@ struct ref_sorting {
 	int atom; /* index into used_atom array (internal) */
 	unsigned reverse : 1,
 		ignore_case : 1,
+		detached_head_first : 1,
 		version : 1;
 };
 
@@ -111,6 +112,8 @@ int verify_ref_format(struct ref_format *format);
 void ref_array_sort(struct ref_sorting *sort, struct ref_array *array);
 /*  Set the ignore_case flag for all elements of a sorting list */
 void ref_sorting_icase_all(struct ref_sorting *sorting, int flag);
+/*  Set the detached_head_first flag for all elements of a sorting list */
+void ref_sorting_detached_head_first_all(struct ref_sorting *sorting, int flag);
 /*  Based on the given format and quote_style, fill the strbuf */
 int format_ref_array_item(struct ref_array_item *info,
 			  const struct ref_format *format,
-- 
2.29.2.222.g5d2a92d10f8