oidset: introduce `oidset_equal()`

Introduce a new function that allows the caller to verify whether two
oidsets contain the exact same object IDs.

Note that this change requires us to change `oidset_iter_init()` to
accept a `const struct oidset`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

oidset.c

diff --git a/oidset.c b/oidset.c
index 8d36aef8..c8ff0b3 100644
--- a/oidset.c
+++ b/oidset.c
@@ -16,6 +16,22 @@ int oidset_contains(const struct oidset *set, const struct object_id *oid)
 	return pos != kh_end(&set->set);
 }
 
+bool oidset_equal(const struct oidset *a, const struct oidset *b)
+{
+	struct oidset_iter iter;
+	struct object_id *a_oid;
+
+	if (oidset_size(a) != oidset_size(b))
+		return false;
+
+	oidset_iter_init(a, &iter);
+	while ((a_oid = oidset_iter_next(&iter)))
+		if (!oidset_contains(b, a_oid))
+			return false;
+
+	return true;
+}
+
 int oidset_insert(struct oidset *set, const struct object_id *oid)
 {
 	int added;

oidset.h

diff --git a/oidset.h b/oidset.h
index 0106b6f..e0f1a6f 100644
--- a/oidset.h
+++ b/oidset.h
@@ -39,6 +39,11 @@ void oidset_init(struct oidset *set, size_t initial_size);
 int oidset_contains(const struct oidset *set, const struct object_id *oid);
 
 /**
+ * Returns true iff `a` and `b` contain the exact same OIDs.
+ */
+bool oidset_equal(const struct oidset *a, const struct oidset *b);
+
+/**
  * Insert the oid into the set; a copy is made, so "oid" does not need
  * to persist after this function is called.
  *
@@ -94,11 +99,11 @@ void oidset_parse_file_carefully(struct oidset *set, const char *path,
 				 oidset_parse_tweak_fn fn, void *cbdata);
 
 struct oidset_iter {
-	kh_oid_set_t *set;
+	const kh_oid_set_t *set;
 	khiter_t iter;
 };
 
-static inline void oidset_iter_init(struct oidset *set,
+static inline void oidset_iter_init(const struct oidset *set,
 				    struct oidset_iter *iter)
 {
 	iter->set = &set->set;