sort a collection of messages by their causal order.

A hash can be interpreted as a link, (except it is what link, not a where link. Like cinderella's glass slipper, a hash lets you confirm when you have found a thing, but doesn't clue you in to where that thing is)

An interesting thing about hashes as links, is that they always point backwards in time. That is because you can't know the hash of something you've seen it, or seen evidence of it. You could make up a random hash - but then the chance that something with that hash will actually turn up is basically impossible.

This means that hashes represent causation, in some sense,


var sort = require('ssb-sort')

all functions will throw if there is a duplicate message in the input.

sort (set) => Array

sort a set (array of unique messages) by causal order, sorts first by causal order, but if two messages are concurrent, breaks the tie by their received, then self-stated timestamps, then order by keys.

mutates the original set

sort.heads (set) => Array

returns the most "recent keys" in the set (furtherest down the causation chain). sort.heads is used to calculate the branch property is patchwork threads.

Usually, a single key is returned, but if there have been concurrent responses heads will return multiple values. If two or more peers respond while not having the latest messages, (for example because they are offline, or they respond before all the messages have reached them)

You can think of "concurrent" as meaning both "at the same time" or "not knowing about the other".

sort.roots (set) => Array

returns the earliest keys in the set, since this operates on an arbitary set of messages, there may be more than one root. but usually you'll select a set by getting messages that point to a particular root node, in which case this will return one key.

sort.missingContext (set) => Object

with scuttlebutt, it's possible for people to post message simultaneously (or even at different times) and not know about other messages that were written. this method tells you which messages were on 'different branches', as in did not know about other messages at the time of writing.

    A     // first message
   / \
  B1  B2  // messages which were posted without know about each other
   \ /
    C     // this message was posted and had see B1+B2 (and A)
    D     // most recent message

here sort.missingContext([A, B1, B2, C, D]) returns:

  B1.key: [ B2 ],
  B2.key: [ B1 ]

There are more complicated examples (with diagrams!) in the tests.



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