update

package/node_modules/@npmcli/arborist/lib/peer-entry-sets.js

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+// Given a node in a tree, return all of the peer dependency sets that
+// it is a part of, with the entry (top or non-peer) edges into the sets
+// identified.
+//
+// With this information, we can determine whether it is appropriate to
+// replace the entire peer set with another (and remove the old one),
+// push the set deeper into the tree, and so on.
+//
+// Returns a Map of { edge => Set(peerNodes) },
+
+const peerEntrySets = node => {
+  // this is the union of all peer groups that the node is a part of
+  // later, we identify all of the entry edges, and create a set of
+  // 1 or more overlapping sets that this node is a part of.
+  const unionSet = new Set([node])
+  for (const node of unionSet) {
+    for (const edge of node.edgesOut.values()) {
+      if (edge.valid && edge.peer && edge.to) {
+        unionSet.add(edge.to)
+      }
+    }
+    for (const edge of node.edgesIn) {
+      if (edge.valid && edge.peer) {
+        unionSet.add(edge.from)
+      }
+    }
+  }
+  const entrySets = new Map()
+  for (const peer of unionSet) {
+    for (const edge of peer.edgesIn) {
+      // if not valid, it doesn't matter anyway.  either it's been previously
+      // peerConflicted, or it's the thing we're interested in replacing.
+      if (!edge.valid) {
+        continue
+      }
+      // this is the entry point into the peer set
+      if (!edge.peer || edge.from.isTop) {
+        // get the subset of peer brought in by this peer entry edge
+        const sub = new Set([peer])
+        for (const peer of sub) {
+          for (const edge of peer.edgesOut.values()) {
+            if (edge.valid && edge.peer && edge.to) {
+              sub.add(edge.to)
+            }
+          }
+        }
+        // if this subset does not include the node we are focused on,
+        // then it is not relevant for our purposes.  Example:
+        //
+        // a -> (b, c, d)
+        // b -> PEER(d) b -> d -> e -> f <-> g
+        // c -> PEER(f, h) c -> (f <-> g, h -> g)
+        // d -> PEER(e) d -> e -> f <-> g
+        // e -> PEER(f)
+        // f -> PEER(g)
+        // g -> PEER(f)
+        // h -> PEER(g)
+        //
+        // The unionSet(e) will include c, but we don't actually care about
+        // it.  We only expanded to the edge of the peer nodes in order to
+        // find the entry edges that caused the inclusion of peer sets
+        // including (e), so we want:
+        //   Map{
+        //     Edge(a->b) => Set(b, d, e, f, g)
+        //     Edge(a->d) => Set(d, e, f, g)
+        //   }
+        if (sub.has(node)) {
+          entrySets.set(edge, sub)
+        }
+      }
+    }
+  }
+
+  return entrySets
+}
+
+module.exports = peerEntrySets