ndb_filter_is_subset_of

subset testing for filters. Can be used to see if one subset is
redundant in the presence of a another in the local relay model

Changelog-Added: Add ndb_filter_is_subset_of
Signed-off-by: William Casarin <[email protected]>

src/nostrdb.c

@@ -2672,6 +2672,47 @@ ndb_filter_find_elements(struct ndb_filter *filter, enum ndb_filter_fieldtype ty
 	return NULL;
 }
 
+int ndb_filter_is_subset_of(struct ndb_filter *a, struct ndb_filter *b)
+{
+	int i;
+	struct ndb_filter_elements *b_field, *a_field;
+
+        // Everything is always a subset of {}
+	if (b->num_elements == 0)
+		return 1;
+
+        // We can't be a subset if the number of elements in the other
+        // filter is larger then the number of elements we have.
+	if (b->num_elements > a->num_elements)
+		return 0;
+
+        // If our filter count matches, we can only be a subset if we are
+        // equal
+	if (b->num_elements == a->num_elements)
+		return ndb_filter_eq(a, b);
+
+        // If our element count is larger than the other filter, then we
+        // must see if every element in the other filter exists in ours. If
+        // so, then we are a subset of the other.
+        //
+        // eg: B={k:1, a:b} <- A={t:x, k:1, a:b}
+        //
+        // A is a subset of B because `k:1` and `a:b` both exist in A
+
+	for (i = 0; i < b->num_elements; i++) {
+		b_field = ndb_filter_get_elements(b, i);
+		a_field = ndb_filter_find_elements(a, b_field->field.type);
+
+		if (a_field == NULL)
+			return 0;
+
+		if (!ndb_filter_field_eq(a, a_field, b, b_field))
+			return 0;
+	}
+
+	return 1;
+}
+
 int ndb_filter_eq(struct ndb_filter *a, struct ndb_filter *b)
 {
 	int i;

src/nostrdb.h

@@ -497,6 +497,9 @@ int ndb_filter_add_int_element(struct ndb_filter *, uint64_t integer);
 int ndb_filter_add_str_element(struct ndb_filter *, const char *str);
 int ndb_filter_eq(struct ndb_filter *, struct ndb_filter *);
 
+/// is `a` a subset of `b`
+int ndb_filter_is_subset_of(struct ndb_filter *a, struct ndb_filter *b);
+
 // filters from json
 int ndb_filter_from_json(const char *, int len, struct ndb_filter *filter, unsigned char *buf, int bufsize);
 

test.c

@@ -1689,9 +1689,45 @@ static void test_filter_eq() {
 	assert(ndb_filter_eq(f, f2));
 }
 
+static void test_filter_is_subset() {
+	struct ndb_filter global, *g = &global;
+	struct ndb_filter kind, *k = &kind;
+	struct ndb_filter kind_and_id, *ki = &kind_and_id;
+
+	const unsigned char id[] = {
+	  0x03, 0x36, 0x94, 0x8b, 0xdf, 0xbf, 0x5f, 0x93, 0x98, 0x02, 0xeb, 0xa0,
+	  0x3a, 0xa7, 0x87, 0x35, 0xc8, 0x28, 0x25, 0x21, 0x1e, 0xec, 0xe9, 0x87,
+	  0xa6, 0xd2, 0xe2, 0x0e, 0x3c, 0xff, 0xf9, 0x30
+	};
+
+	ndb_filter_init(g);
+	ndb_filter_end(g);
+
+	ndb_filter_init(k);
+	assert(ndb_filter_start_field(k, NDB_FILTER_KINDS));
+	assert(ndb_filter_add_int_element(k, 1));
+	ndb_filter_end_field(k);
+	ndb_filter_end(k);
+
+	ndb_filter_init(ki);
+	assert(ndb_filter_start_field(ki, NDB_FILTER_KINDS));
+	assert(ndb_filter_add_int_element(ki, 1));
+	ndb_filter_end_field(ki);
+	assert(ndb_filter_start_field(ki, NDB_FILTER_IDS));
+	assert(ndb_filter_add_id_element(ki, id));
+	ndb_filter_end_field(ki);
+	ndb_filter_end(ki);
+
+	assert(ndb_filter_is_subset_of(g, k) == 0);
+	assert(ndb_filter_is_subset_of(k, g) == 1);
+	assert(ndb_filter_is_subset_of(ki, k) == 1);
+	assert(ndb_filter_is_subset_of(k, ki) == 0);
+}
+
 int main(int argc, const char *argv[]) {
 	test_parse_filter_json();
 	test_filter_eq();
+	test_filter_is_subset();
 	test_filter_json();
 	test_bech32_parsing();
 	test_single_url_parsing();