Given a message (b of type B), nested in another (a of type A); how does one get ‘a’? I was hoping for something like ‘b.getParent()’.

message B { optional string name = 1;  repeated B b = 2; }
message A { optional string name = 1;  repeated B b = 1; }

Here is an example instance of an ‘a’ with textual serialization.

name: "a"
b { name: “foo”
    b { name: “fred”
        b { name: “flintstone” }}}
b { name: “bar” }
b { name: “baz” }

The issue is that I am navigating the collection 'a' with a visitor and I need to be able to reconstruct a full name composed of names of all the ancestors.

Refs:

• Get the parent message of a protobuf message (in python)

I am fine with internal representation as I will wrap it in a kotlin extension.

CodePudding user response：

You cannot do this. A B has no need of, or attachment to, an A. If you want to track the parents, you must do so explicitly yourself.

CodePudding user response：

There is no good (efficient) way to initialize a backing data structure.

An alternative approach is for the visitor to maintain an Array rather than just a B. Here is an example:

fun depthFirstVisitor(root: A, fragFilter: (frag: B) -> Boolean): Sequence<Array<B>> {
    return sequence {
        root.fragmentsList.forEach { child ->
            if (fragFilter(child)) {
                yield(arrayOf(child))
                dfs(emptyArray(), child, fragFilter)
            }
        }
    }
}

private suspend fun SequenceScope<Array<B>>.dfs(
    ancestry: Array<B>,
    base: B,
    fragFilter: (frag: B) -> Boolean
) {
    val ancestryNew = ancestry   base
    base.fragmentsList.forEach { child ->
        if (fragFilter(child)) {
            yield(ancestryNew   child)
            dfs(ancestryNew, child, fragFilter)
        }
    }
}

The current B is the last element of the array. I am using an Array because it does not suffer from type erasure like other containers.