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Tree Grafts/Transplants
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Ryan Watts  
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 More options Oct 18 2012, 3:28 pm
From: Ryan Watts <rtwatt...@gmail.com>
Date: Thu, 18 Oct 2012 12:28:37 -0700 (PDT)
Local: Thurs, Oct 18 2012 3:28 pm
Subject: Tree Grafts/Transplants
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I would think this has been done before but I've been looking around for a
straightforward solution and haven't come across anything yet.

The problem is this:

I have a Tree[A] and I want to take all the children of a node designated
by a certain path P1 and move them to another node designated by path P2.
 I want this to be recursive so if the children have children, they are
moved as well.

Can anyone point me in the right direction?


 
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Tony Morris  
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 More options Oct 18 2012, 8:58 pm
From: Tony Morris <tonymor...@gmail.com>
Date: Fri, 19 Oct 2012 10:58:01 +1000
Local: Thurs, Oct 18 2012 8:58 pm
Subject: Re: [scalaz] Tree Grafts/Transplants
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TreeLoc#cobind? Or something like it, would have to do some twiddling to
figure it out for sure, but I would start there.
 On Oct 19, 2012 10:56 AM, "Ryan Watts" <rtwatt...@gmail.com> wrote:


 
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Runar Bjarnason  
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 More options Oct 18 2012, 9:21 pm
From: Runar Bjarnason <runaror...@gmail.com>
Date: Thu, 18 Oct 2012 21:21:06 -0400
Local: Thurs, Oct 18 2012 9:21 pm
Subject: Re: [scalaz] Tree Grafts/Transplants
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TreeLoc will definitely do this for you. You can use findChild
repeatedly to walk into the tree along a path. Do this with both
paths, then use setTree to replace one with the other.

def walkPath[A](t: Tree[A], path: List[A]): TreeLoc[A] =
  path.foldLeft(tree.loc)((t, e) => t.findChild(_.rootLabel == e))

def graft[A](t: Tree[A], p1: List[A], p2: List[A]): Tree[A] =
  walkPath(t, p1).setTree(walkPath(t, p2).tree).root

Something like that.

Runar


 
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Ryan Watts  
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 More options Oct 22 2012, 2:04 pm
From: Ryan Watts <rtwatt...@gmail.com>
Date: Mon, 22 Oct 2012 11:04:13 -0700 (PDT)
Local: Mon, Oct 22 2012 2:04 pm
Subject: Re: [scalaz] Tree Grafts/Transplants
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Thanks for the help.  I ended up using a slight variation on your code:

def walkPath[A](t: Tree[A], path: List[A]): TreeLoc[A] =
    path.tail.foldLeft(t.loc)((t, e) => t.findChild(_.rootLabel ==
e).getOrElse(t))

  def graft[A](t: Tree[A], to: List[A], from: List[A]): Tree[A] =
    walkPath(t, to).insertDownLast(walkPath(t, from).tree).root.tree

Basically the same except for the getOrElse and turning the graft result
into a tree.  In order to get the "transplant" behavior I was hoping for --
i.e. "take this branch and move it here" I had to add one additional method:

def prune[A](t: Tree[A], path: List[A]): Tree[A] =
    walkPath(t, path).delete.map(_.root.tree).getOrElse(t)

By combining graft and prune I was able to create my transplant method:

def transplant[A](t: Tree[A], to: List[A], from: List[A]): Tree[A] =
    prune(graft(t, to, from), from)

I will probably change the syntax around a little bit, but that's basically
the functionality I was looking for.

Thanks again for the help!


 
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