common/features: helper to pretty-print feature bits.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/features.c

@@ -497,3 +497,18 @@ void towire_feature_set(u8 **pptr, const struct feature_set *fset)
 		towire_u8_array(pptr, fset->bits[i], tal_bytelen(fset->bits[i]));
 	}
 }
+
+const char *fmt_featurebits(const tal_t *ctx, const u8 *featurebits)
+{
+	size_t size = tal_count(featurebits);
+	char *fmt = tal_strdup(ctx, "");
+	const char *prefix = "";
+
+	for (size_t i = 0; i < size * 8; i++) {
+		if (feature_is_set(featurebits, i)) {
+			tal_append_fmt(&fmt, "%s%zu", prefix, i);
+			prefix = ",";
+		}
+	}
+	return fmt;
+}

common/features.h

@@ -71,6 +71,9 @@ void set_feature_bit(u8 **ptr, u32 bit);
 /* Given two featurebit vectors, combine them by applying a logical OR. */
 u8 *featurebits_or(const tal_t *ctx, const u8 *f1 TAKES, const u8 *f2 TAKES);
 
+/* Good for debugging: returns comma-separated string of bits. */
+const char *fmt_featurebits(const tal_t *ctx, const u8 *featurebits);
+
 /* BOLT #9:
  *
  * Flags are numbered from the least-significant bit, at bit 0 (i.e. 0x1,