json: Add top-level array decoding support

The library supports encoding JSON objects and arrays as well as
parsing JSON objects. Introduce a new function json_arr_parse() adding
support for parsing top-level JSON arrays.

Signed-off-by: Markus Fuchs <markus.fuchs@ch.sauter-bc.com>

include/data/json.h

@@ -528,7 +528,7 @@ typedef int (*json_append_bytes_t)(const char *bytes, size_t len,
 	}
 
 /**
- * @brief Parses the JSON-encoded object pointer to by @a json, with
+ * @brief Parses the JSON-encoded object pointed to by @a json, with
  * size @a len, according to the descriptor pointed to by @a descr.
  * Values are stored in a struct pointed to by @a val.  Set up the
  * descriptor like this:
@@ -561,6 +561,41 @@ int json_obj_parse(char *json, size_t len,
 	const struct json_obj_descr *descr, size_t descr_len,
 	void *val);
 
+/**
+ * @brief Parses the JSON-encoded array pointed to by @a json, with
+ * size @a len, according to the descriptor pointed to by @a descr.
+ * Values are stored in a struct pointed to by @a val.  Set up the
+ * descriptor like this:
+ *
+ *    struct s { int foo; char *bar; }
+ *    struct json_obj_descr descr[] = {
+ *       JSON_OBJ_DESCR_PRIM(struct s, foo, JSON_TOK_NUMBER),
+ *       JSON_OBJ_DESCR_PRIM(struct s, bar, JSON_TOK_STRING),
+ *    };
+ *    struct a { struct s baz[10]; size_t count; }
+ *    struct json_obj_descr array[] = {
+ *       JSON_OBJ_DESCR_OBJ_ARRAY(struct a, baz, 10, count,
+ *                                descr, ARRAY_SIZE(descr)),
+ *    };
+ *
+ * Since this parser is designed for machine-to-machine communications, some
+ * liberties were taken to simplify the design:
+ * (1) strings are not unescaped (but only valid escape sequences are
+ * accepted);
+ * (2) no UTF-8 validation is performed; and
+ * (3) only integer numbers are supported (no strtod() in the minimal libc).
+ *
+ * @param json Pointer to JSON-encoded array to be parsed
+ * @param len Length of JSON-encoded array
+ * @param descr Pointer to the descriptor array
+ * @param val Pointer to the struct to hold the decoded values
+ *
+ * @return 0 if array has been successfully parsed. A negative value
+ * indicates an error (as defined on errno.h).
+ */
+int json_arr_parse(char *json, size_t len,
+	const struct json_obj_descr *descr, void *val);
+
 /**
  * @brief Escapes the string so it can be used to encode JSON objects
  *

lib/os/json.c

@@ -302,6 +302,23 @@ static int obj_init(struct json_obj *json, char *data, size_t len)
 	return 0;
 }
 
+static int arr_init(struct json_obj *json, char *data, size_t len)
+{
+	struct token token;
+
+	lexer_init(&json->lexer, data, len);
+
+	if (!lexer_next(&json->lexer, &token)) {
+		return -EINVAL;
+	}
+
+	if (token.type != JSON_TOK_ARRAY_START) {
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int element_token(enum json_tokens token)
 {
 	switch (token) {
@@ -607,6 +624,23 @@ int json_obj_parse(char *payload, size_t len,
 	return obj_parse(&obj, descr, descr_len, val);
 }
 
+int json_arr_parse(char *payload, size_t len,
+		   const struct json_obj_descr *descr, void *val)
+{
+	struct json_obj arr;
+	int ret;
+
+	ret = arr_init(&arr, payload, len);
+	if (ret < 0) {
+		return ret;
+	}
+
+	void *ptr = (char *)val + descr->offset;
+
+	return arr_parse(&arr, descr->array.element_descr,
+			 descr->array.n_elements, ptr, val);
+}
+
 static char escape_as(char chr)
 {
 	switch (chr) {