WriterHead

The WriterHead is the second on-chain PDA used by Solstory. This is a mostlly internal data structure, used to locate the first off-chain item. It's essentially a stub of an item, it contains no writer data, but does contain information for locating the next (first) item in the linked list as well as the information required to verify it. This structure gets updated every time an item is appended, and the rules around updating it are what enforce the cryptographical verification promises that Solstory delivers.

• authorized a bool that means that the NFT creator has approved this chain. There's no similar flag for the writer, since if the writer doesn't approve a chain it'll just be empty
• visibilityIndex this i8 serves a dual purpose, both allowing the NFT holder to override external permissioning. First, a value of -1 will hide a story from an NFT. A value of 0 will leave everything as default (and is the default value). A value of 1+ signifies a holder both prioritizing how an story should be displayed (think z-index in css) and, if the story wouldn't be display otherwise, "aftermarket" attaching the story and overriding other permissions. See the permissions page for more info.
• accessType determins how objId should be used to locate a resource.
• objIdis 32 bytes that, together with accessType allow a client to locate where the first block is located.
• currentHash this is h(itemhash, previtem, timestamp) of the first node. See the items standard or either the rust or JS implementations for reference.

Updating

When someone appends an item to Solstory, the api calculates a new current hash. This hash includes both a timestamp and the previous current hash, as well as a data hash. Those three components get uploaded as part of the update process, and the Solstory program verifies them. Fooling this requires finding a sha256 collision, which is both a well studied problem and extremely difficult–exactly what we want in a security schema.