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// Perform a breadth-first walk of a tree, either logical or physical
// This one only visits, it doesn't leave. That's because
// in a breadth-first traversal, children may be visited long
// after their parent, so the "exit" pass ends up being just
// another breadth-first walk.
//
// Breadth-first traversals are good for either creating a tree (ie,
// reifying a dep graph based on a package.json without a node_modules
// or package-lock), or mutating it in-place. For a map-reduce type of
// walk, it doesn't make a lot of sense, and is very expensive.
const breadth = ({
visit,
filter = () => true,
getChildren,
tree,
}) => {
const queue = []
const seen = new Map()
const next = () => {
while (queue.length) {
const node = queue.shift()
const res = visitNode(node)
if (isPromise(res)) {
return res.then(() => next())
}
}
return seen.get(tree)
}
const visitNode = (visitTree) => {
if (seen.has(visitTree)) {
return seen.get(visitTree)
}
seen.set(visitTree, null)
const res = visit ? visit(visitTree) : visitTree
if (isPromise(res)) {
const fullResult = res.then(resThen => {
seen.set(visitTree, resThen)
return kidNodes(visitTree)
})
seen.set(visitTree, fullResult)
return fullResult
} else {
seen.set(visitTree, res)
return kidNodes(visitTree)
}
}
const kidNodes = (kidTree) => {
const kids = getChildren(kidTree, seen.get(kidTree))
return isPromise(kids) ? kids.then(processKids) : processKids(kids)
}
const processKids = (kids) => {
kids = (kids || []).filter(filter)
queue.push(...kids)
}
queue.push(tree)
return next()
}
const isPromise = p => p && typeof p.then === 'function'
module.exports = breadth