Add unist-util-ancestor to list of utilities

[gorango]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

Adding a small utility to readme that helps with finding common ancestors of one or more unist nodes. It has come in handy a few times while working with ast trees so I thought I'd package it up. Here's hoping it might be helpful to someone else too!

P.S. I am open to renaming the package if desired.

[wooorm]

Nice!

For performance reasons you might want to not use unist-util-find (in this way). And perhaps also getParents like this.
Right now, for every node, the whole tree is walked many many times.

If possible, it would be muuuch faster to walk the tree once/twice.
Here’s some pseudocode:

const stacks = new Map()

visitParents(tree, (node, parents) => {
  if (nodesToFind.includes(node)) {
    stacks.set(node, parents)
  }
})

nodesToFind.forEach(node => {
  if (!stacks.has(node)) {
    throw new Error('unist-util-ancestor requires all nodes be contained in the tree')
  }
})

// Somethign like this?
let index = -1
let ancestor = tree

// This is broken, maybe you get the gist?
while (++index) {
  const nextAncestor = stacks.get(nodesToFind[0])[index]

  const shared = nodesToFind.every(node => stacks.get(node)[index] === nextAncestor)

  // To do: exit if there are no nodes left on stacks.

  if (shared) {
    ancestor = nextAncestor
  } else {
    break
  }
}

[gorango]

Thank you so much for taking the time to review and provide this great feedback!

I have implemented your suggestions and removed the find util completely. Way cleaner :)

[wooorm]

That looks great! Nice find as well on the things that I didn't get to. Glad it worked!