Looking for smarter way to build directory tree - json

Question

I have large Json object. The object describes among other things a tree type relation of how its components objects are connected hierarchally. The object knows who its children are, but does not know (directly) who its parent is. "my_hash" below exemplifies the structure. Every object has an id 101, 102, etc., a name "one", "two" etc and it can have 0, 1 or more children. I am trying to build the "path" to the every object. E.g. object name "five" should a result have a path of "/one/two/four" as a result of the code. Basically I am trying to build a sort of directory structure of the hierarchy of the objects.
The code below works, but it looks quite long'ish, not very elegant, not very Ruby'ish.
I would be grate for suggestion on how to to do this more efficiently and elegantly. And I got hunch that my code may not be very robust, ie deal well with exceptions.
Any thoughts or help ar appreciated.
On a side note, I am just learning Ruby and so far have mainly programmed in Perl.
class Tree
def initialize
#my_hash = {
101 => ["one", [102, 107]],
102 => ["two", [103, 104]],
103 => ["three", []],
104 => ["four", [105, 106]],
105 => ["five", []],
106 => ["six", []],
107 => ["seven", [108]],
108 => ["eight", []],
}
#child_to_parent_node = {}
#id_to_name = {}
#my_path_hash = {}
#my_hash.keys.each do |key|
#my_path_hash[key] = ""
end
#parent_path_id = []
end
def map_child_to_parent
#my_hash.each do |key, value|
#id_to_name.store(key, value[0])
node_name, children = value[0], value[1]
children.each do |child_id|
#child_to_parent_node.store(child_id, node_name)
end
end
end
def build_path(id)
parent = #child_to_parent_node[id]
parent.nil? ? return : #parent_path_id << parent
id = #id_to_name.key(parent)
build_path(id)
#parent_path_id
end
def update_tree
#id_to_name.keys.each do |id|
tmp_array = self.build_path(id)
path = ""
if (tmp_array.nil?)
path = "/"
else
tmp_array.reverse.each do
path = path + "/" + tmp_array.pop
end
end
puts "id: #{id} path: #{path}"
end
end
end
my_tree = Tree.new
my_tree.map_child_to_parent
my_tree.update_tree

Answer 1

In fact, to solve this task you have to traverse the tree from the leaf to root, right? So, your representation of the tree is very inconvenient for this particular task. If you are ok to trade off some memory for a more clean solution, I'd create an auxiliary structure that contains parents for each node. Let's say
#parents = #my_hash.each_with_object({}) do |(pid, props), acc|
_, children = props
children.each { |cid| acc[cid] = pid }
end
#=> {102=>101, 107=>101, 103=>102, 104=>102, 105=>104, 106=>104, 108=>107}
Now, the task can be solved in a quite concise way with a couple of auxiliary functions. For example:
def id_by_name(name)
id, _ = #my_hash.find { |k, v| v.first == name }
id
end
def name_by_id(id)
#my_hash[id].first
end
def path_to(node)
path = [node]
id = id_by_name(node)
path.unshift(name_by_id(id)) while id = #parents[id]
path.join("/")
end
path_to "five" #=> "one/two/four/five"
Please, note: the solution is still very inefficient - mostly because to fetch a node's id by its name we have to iterate over the whole initial hash in the worst case. This is the price we pay for the data structure that doesn't fit the task well.