Re: [PATCH v4 08/14] lib: implement JSON tree class for json library

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Thu, Oct 11, 2018 at 10:58:53AM +0300, Kirill Shcherbatov wrote:
> +/**
> + * JSON tree is a hierarchical data structure that organize JSON
> + * nodes produced by parser.
> + * 1. Numeric records are sorted in ascending order in sibling rlist.
> + * 2. Structure key records point to source strings memory.
> + * 3. Lookup by path complexity is O(path_len):
> + * 	path = path_token1 + .. + path_tokenN
> + * 	- Complexity of hash(path_tokenI) - O(strlen(path_tokenN))
> + * 	- Complexity of hash lookup - O(1) + strcmp on collision

I doubt that an unaware reader would figure out how the tree organizes
json path tokens after reading this comment...

> + */
> +struct json_tree_node {
> +	/** Path component represented by this node. */
> +	struct json_path_node key;
> +	/** Hash calculated for key field. */
> +	uint32_t key_hash;
> +	/**
> +	 * Children hashtable:
> +	 * json_path_node -> json_tree_node.
> +	 */
> +	struct mh_json_tree_node_t *child_by_path_node;
> +	/**
> +	 * Link in json_tree_node::children of the parent node.
> +	 */
> +	struct rlist sibling;
> +	/**
> +	 * List of child nodes, linked by
> +	 * json_tree_node::sibling.
> +	 */
> +	struct rlist children;
> +	/** Pointer to parent json_tree_node record. */
> +	struct json_tree_node *parent;
> +};

For the record. After discussion with Kostja we agreed that the json
tree class needs to be reworked so that there's the only hash map per a
tree that contains all descendants. To make it possible to look up a
descendant of a non-root node, rolling hashes should be used:

  hash(child node) = hash(parent node) xor hash(path token)

Also, child nodes should be stored in an array rather than in a list,
because that would allow fast access of json array elements.