Function validate_message_value_hash_chain 

pub fn validate_message_value_hash_chain<T: AsRef<[u8]>, U: AsRef<[u8]>>(
    message_bytes: T,
    previous_msg_bytes: Option<U>,
) -> Result<()>

Validate a message value in relation to the previous message value.

It expects the messages to be the JSON encoded message value of shape: { previous: "", author: "", sequence: ..., timestamp: ..., content: {}, signature: "" }

This checks that:

• the sequence starts at one if it’s the first message
• the previous is correctly set to null if it’s the first message
• the sequence increments correctly
• the author has not changed
• the feed is not forked

This does not check:

• the signature. See ssb-verify-signatures which lets you to batch verification of signatures.

previous_msg_bytes will be None only when message_bytes is the first message by that author.

Example

use ssb_validate::message_value::validate_message_value_hash_chain;
let valid_message_1 = r##"{
  "previous": null,
  "author": "@U5GvOKP/YUza9k53DSXxT0mk3PIrnyAmessvNfZl5E0=.ed25519",
  "sequence": 1,
  "timestamp": 1470186877575,
  "hash": "sha256",
  "content": {
    "type": "about",
    "about": "@U5GvOKP/YUza9k53DSXxT0mk3PIrnyAmessvNfZl5E0=.ed25519",
    "name": "Piet"
  },
  "signature": "QJKWui3oyK6r5dH13xHkEVFhfMZDTXfK2tW21nyfheFClSf69yYK77Itj1BGcOimZ16pj9u3tMArLUCGSscqCQ==.sig.ed25519"
}"##;
let valid_message_2 = r##"{
  "previous": "%/v5mCnV/kmnVtnF3zXtD4tbzoEQo4kRq/0d/bgxP1WI=.sha256",
  "author": "@U5GvOKP/YUza9k53DSXxT0mk3PIrnyAmessvNfZl5E0=.ed25519",
  "sequence": 2,
  "timestamp": 1470187292812,
  "hash": "sha256",
  "content": {
    "type": "about",
    "about": "@U5GvOKP/YUza9k53DSXxT0mk3PIrnyAmessvNfZl5E0=.ed25519",
    "image": {
      "link": "&MxwsfZoq7X6oqnEX/TWIlAqd6S+jsUA6T1hqZYdl7RM=.sha256",
      "size": 642763,
      "type": "image/png",
      "width": 512,
      "height": 512
    }
  },
  "signature": "j3C7Us3JDnSUseF4ycRB0dTMs0xC6NAriAFtJWvx2uyz0K4zSj6XL8YA4BVqv+AHgo08+HxXGrpJlZ3ADwNnDw==.sig.ed25519"
}"##;

 let result = validate_message_value_hash_chain(valid_message_2.as_bytes(),
 Some(valid_message_1.as_bytes()));
 assert!(result.is_ok());