Add ordering option to getPath

[hannahhoward]

Goals

This adds support for depth first traversals to Dagula. It's designed to be in conformance with ipfs/specs#412.

(will update if parameter names change)

Implementation

• adds order parameter to options struct for getPath and get
• renames breadFirstSearch to blockLinks (it isn't really "bread first search" so much as "all the links in this block").
• handles ordering change inside of get, using recursion for depth-first-search
• adds tests for both orders.

[olizilla]

Ah right, the half-formed idea here was that the search fn could be used to let a caller pass in a stateful depth first search equivalent to the default "just return all the links in the current block", but i didn't try and implement it to try out that idea.

...all i really needed at the time was a way to pass in a rule for filtering out some links so i could implement car-scope: file dag-scope: entity for a hamt and got carried away.

search could go away in favour of linkFilter: ([name, cid]) => boolean or similar, and we could just have this get fn always do depth first. There was no strong reason for the previous default, and we can return depth-first for rnd as well.

[olizilla]

news just in! the transform function used here doesn't guarantee order of results!
https://github.com/web3-storage/dagula/blob/ceca20a7da7f976777d4e77297eb6aada45f67d9/index.js#L52

Order is determined by when func resolves.
– https://github.com/bustle/streaming-iterables#transform

so for us to guarantee dfs that would need to switch out for parallelMap. I didn't check that when I named the default search impl breadth-first, so it was already misleading.

[hannahhoward]

@olizilla used parallelMap for DFS. left transform in place otherwise -- I dunno if the speed optimization matters but I figured as little substantive change to the existing workflow the better :)

[alanshaw]

This LGTM but we should remove the search param (as mentioned here), switch to dfs by default and rename carScope to dagScope before release.

I'll send a follow up PR(s) to get those things done.

[alanshaw]

@hannahhoward thank you for the contribution ❤️