features: add 'feature_bit_sub', which will subtract/unset bits

Given a two sets of featurebits, unset the featurebits in the first set
that are set in the second set

common/features.c

@@ -205,6 +205,35 @@ bool feature_set_or(struct feature_set *a,
 	return true;
 }
 
+bool feature_set_sub(struct feature_set *a,
+		     const struct feature_set *b TAKES)
+{
+	/* Check first, before we change anything! */
+	for (size_t i = 0; i < ARRAY_SIZE(b->bits); i++) {
+		for (size_t j = 0; j < tal_bytelen(b->bits[i])*8; j++) {
+			if (feature_is_set(b->bits[i], j)
+			    && !feature_offered(a->bits[i], j)) {
+				if (taken(b))
+					tal_free(b);
+				return false;
+			}
+		}
+	}
+
+	for (size_t i = 0; i < ARRAY_SIZE(a->bits); i++) {
+		for (size_t j = 0; j < tal_bytelen(b->bits[i])*8; j++) {
+			if (feature_is_set(b->bits[i], j))
+				clear_feature_bit(a->bits[i], j);
+		}
+		trim_features(&a->bits[i]);
+	}
+
+
+	if (taken(b))
+		tal_free(b);
+	return true;
+}
+
 /* BOLT #1:
  *
  * All data fields are unsigned big-endian unless otherwise specified.

common/features.h

@@ -32,6 +32,10 @@ void towire_feature_set(u8 **pptr, const struct feature_set *fset);
 bool feature_set_or(struct feature_set *a,
 		    const struct feature_set *b TAKES);
 
+/* a - b, or returns false if features not already in a */
+bool feature_set_sub(struct feature_set *a,
+		     const struct feature_set *b TAKES);
+
 /* Returns -1 if we're OK with all these offered features, otherwise first
  * unsupported (even) feature. */
 int features_unsupported(const struct feature_set *our_features,

common/test/run-features.c

@@ -179,6 +179,38 @@ static void test_feature_set_or(void)
 	}
 }
 
+static void test_feature_set_sub(void)
+{
+	struct feature_set *f1, *f2, *control;
+	for (size_t i = 0; i < ARRAY_SIZE(f1->bits); i++) {
+		f1 = talz(tmpctx, struct feature_set);
+		f2 = talz(tmpctx, struct feature_set);
+		control = talz(tmpctx, struct feature_set);
+
+		f1->bits[i] = tal_arr(f1, u8, 0);
+		f2->bits[i] = tal_arr(f2, u8, 0);
+		control->bits[i] = tal_arr(control, u8, 0);
+
+		/* sub with nothing is a nop */
+		set_feature_bit(&f1->bits[i], 0);
+		set_feature_bit(&control->bits[i], 0);
+		assert(feature_set_eq(f1, control));
+		assert(feature_set_sub(f1, f2));
+
+		/* can't sub feature bit that's not set */
+		set_feature_bit(&f2->bits[i], 2);
+		assert(!feature_set_sub(f1, f2));
+		assert(feature_set_eq(f1, control));
+		assert(!feature_set_sub(f2, f1));
+
+		/* sub does the right thing */
+		set_feature_bit(&f1->bits[i], 2);
+		assert(feature_set_sub(f1, f2));
+		assert(feature_set_eq(f1, control));
+		assert(!feature_set_sub(f1, f2));
+	}
+}
+
 static void test_feature_trim(void)
 {
 	struct feature_set *f;
@@ -299,6 +331,7 @@ int main(void)
 	test_featurebits_or();
 	test_feature_set_or();
 	test_feature_trim();
+	test_feature_set_sub();
 
 	wally_cleanup(0);
 	tal_free(tmpctx);