[PATCH 2/4] lib: extend jsmn with simple JSONPath lookup helpers

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Board code may not be interested in iterating fully over the JSON blob
and instead just wants to lookup a value at a specific JSONPath.

Add helpers to facilitate this.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 include/jsmn.h | 57 +++++++++++++++++++++++++++++++++
 lib/jsmn.c     | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 143 insertions(+)

diff --git a/include/jsmn.h b/include/jsmn.h
index 394ffc467487..8f6db8d534f4 100644
--- a/include/jsmn.h
+++ b/include/jsmn.h
@@ -84,6 +84,63 @@ JSMN_API void jsmn_init(jsmn_parser *parser);
 JSMN_API int jsmn_parse(jsmn_parser *parser, const char *js, const size_t len,
 			jsmntok_t *tokens, const unsigned int num_tokens);
 
+/** Returns `true` if `token` is a string and equal to `str`. */
+JSMN_API bool jsmn_str_eq(const char *str, const char *json, const jsmntok_t *token);
+
+/** Returns `true` if `token` is to `str`. */
+JSMN_API bool jsmn_eq(const char *val, const char *json, const jsmntok_t *token);
+
+/** Returns the token after the value at `tokens[0]`. */
+JSMN_API const jsmntok_t *jsmn_skip_value(const jsmntok_t *tokens);
+
+/**
+ * Returns a pointer to the value associated with `key` inside `json` starting
+ * at `tokens[0]`, which is expected to be an object, or returns `NULL` if `key`
+ * cannot be found.
+ */
+JSMN_API const jsmntok_t *jsmn_find_value(const char *key, const char *json,
+					  const jsmntok_t *tokens);
+
+/**
+ * Locate the token at `path` inside `json` in a manner similar to JSONPath.
+ *
+ * Example:
+ * \code
+ * {
+ *   "date": "...",
+ *   "product": {
+ *	 "serial": "1234",
+ *   }
+ * }
+ * \endcode
+ *
+ * To locate the serial number in the JSON object above, you would use the
+ * JSONPath expression "$.product.serial". The same thing can be accomplished
+ * with this call:
+ *
+ * \code
+ * const char *path[] = {"product", "serial", NULL};
+ * const jsmntok_t *token = jsmn_locate(path, json, tokens);
+ * \endcode
+ *
+ * \remark This function cannot search inside arrays.
+ *
+ * @param path		Null-terminated list of path elements.
+ * @param json		JSON string to search in.
+ * @param tokens	Tokens for `json`.
+ *
+ * @return Pointer to the value token or `NULL` if the token could not be found.
+ */
+JSMN_API const jsmntok_t *jsmn_locate(const char *path[], const char *json,
+				      const jsmntok_t *tokens);
+
+/**
+ * Similar to `jsmn_locate` but returns a copy of the value or `NULL` if the
+ * value does not exist or is not a string. The caller takes ownership of the
+ * pointer returned.
+ */
+JSMN_API char *jsmn_strcpy(const char *path[], const char *json, const jsmntok_t *tokens);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/jsmn.c b/lib/jsmn.c
index 3a68f89337fc..7bdcc90f2f4b 100644
--- a/lib/jsmn.c
+++ b/lib/jsmn.c
@@ -369,3 +369,89 @@ JSMN_API void jsmn_init(jsmn_parser *parser) {
 	parser->toknext = 0;
 	parser->toksuper = -1;
 }
+
+JSMN_API bool jsmn_eq(const char *val, const char *json, const jsmntok_t *token)
+{
+	size_t token_size = token->end - token->start;
+	return strlen(val) == token_size
+		&& strncmp(json + token->start, val, token_size) == 0;
+}
+
+JSMN_API bool jsmn_str_eq(const char *str, const char *json, const jsmntok_t *token)
+{
+	return token->type == JSMN_STRING && jsmn_eq(str, json, token);
+}
+
+JSMN_API const jsmntok_t *jsmn_skip_value(const jsmntok_t *tokens)
+{
+	int max_index = tokens[0].end;
+	do {
+		++tokens;
+	} while (tokens[0].start < max_index);
+	return &tokens[0];
+}
+
+JSMN_API const jsmntok_t *jsmn_find_value(const char *key, const char *json,
+					  const jsmntok_t *tokens)
+{
+	int items;
+	if (tokens[0].type != JSMN_OBJECT || tokens[0].size == 0)
+		return NULL;
+
+	items = tokens[0].size;
+	++tokens;
+
+	do {
+		if (jsmn_str_eq(key, json, tokens))
+			return &tokens[1];
+		tokens = --items ? jsmn_skip_value(&tokens[1]) : NULL;
+	} while (tokens);
+
+	return NULL;
+}
+
+JSMN_API const jsmntok_t *jsmn_locate(const char *path[], const char *json,
+				      const jsmntok_t *tokens)
+{
+	int i = 0;
+	while (path[i] != NULL) {
+		const jsmntok_t *value = jsmn_find_value(path[i], json, tokens);
+		if (!value)
+			return NULL;
+
+		switch (value->type) {
+		case JSMN_OBJECT:
+		case JSMN_ARRAY:
+			tokens = value;
+			++i;
+			break;
+		case JSMN_UNDEFINED:
+		case JSMN_STRING:
+		case JSMN_PRIMITIVE:
+			return value;
+		}
+	}
+
+	return tokens;
+}
+
+JSMN_API char *jsmn_strcpy(const char *path[], const char *json,
+			   const jsmntok_t *tokens)
+{
+	const jsmntok_t *node;
+	int value_size;
+	char *value;
+
+	node = jsmn_locate(path, json, tokens);
+	if (!node || node->type != JSMN_STRING)
+		return NULL;
+
+	value_size = node->end - node->start;
+	value = malloc(value_size + 1);
+	if (value) {
+		strncpy(value, json + node->start, value_size);
+		value[value_size] = '\0';
+	}
+
+	return value;
+}
-- 
2.30.2