Functions for encoding reversed hex

ccan/ccan/str/hex/hex.c

@@ -64,3 +64,21 @@ bool hex_encode(const void *buf, size_t bufsize, char *dest, size_t destsize)
 
 	return true;
 }
+
+bool hex_encode_reversed(const void *buf, size_t bufsize, char *dest, size_t destsize)
+{
+	size_t i;
+
+	if (destsize < hex_str_size(bufsize))
+		return false;
+
+    // Start at end of data, go to index 0.
+	for (i = bufsize; i--;) {
+        unsigned int c = ((const unsigned char *)buf)[i];
+        *(dest++) = hexchar(c >> 4);
+        *(dest++) = hexchar(c & 0xF);
+	}
+	*dest = '\0';
+
+	return true;
+}

ccan/ccan/str/hex/hex.h

@@ -41,6 +41,24 @@ bool hex_decode(const char *str, size_t slen, void *buf, size_t bufsize);
  */
 bool hex_encode(const void *buf, size_t bufsize, char *dest, size_t destsize);
 
+/**
+ * hex_encode_reversed - Create a nul-terminated reversed hex string
+ * @buf: the buffer to read the data from
+ * @bufsize: the length of @buf
+ * @dest: the string to fill
+ * @destsize: the max size of the string
+ *
+ * Returns true if the string, including terminator, fit in @destsize;
+ *
+ * Example:
+ *	unsigned char buf[] = { 0x1F, 0x2F };
+ *	char str[5];
+ *
+ *	if (!hex_encode(buf, sizeof(buf), str, sizeof(str)))
+ *		abort();
+ */
+bool hex_encode_reversed(const void *buf, size_t bufsize, char *dest, size_t destsize);
+
 /**
  * hex_str_size - Calculate how big a nul-terminated hex string is
  * @bytes: bytes of data to represent

common/json.c

@@ -471,6 +471,15 @@ void json_add_hex(struct json_result *result, const char *fieldname,
 	json_add_string(result, fieldname, hex);
 }
 
+void json_add_hex_reversed(struct json_result *result, const char *fieldname,
+		  const void *data, size_t len)
+{
+	char hex[hex_str_size(len)];
+
+	hex_encode_reversed(data, len, hex, sizeof(hex));
+	json_add_string(result, fieldname, hex);
+}
+
 void json_add_object(struct json_result *result, ...)
 {
 	va_list ap;

common/json.h

@@ -102,6 +102,9 @@ void json_add_null(struct json_result *result, const char *fieldname);
 /* '"fieldname" : "0189abcdef..."' or "0189abcdef..." if fieldname is NULL */
 void json_add_hex(struct json_result *result, const char *fieldname,
 		  const void *data, size_t len);
+/* '"fieldname" : "0189abcdef..."' or "0189abcdef..." if fieldname is NULL */
+void json_add_hex_reversed(struct json_result *result, const char *fieldname,
+		  const void *data, size_t len);
 void json_add_object(struct json_result *result, ...);
 
 const char *json_result_string(const struct json_result *result);