Create a Petgraph graph from a Vec<(String, String)> loaded from JSON

Question

I'm trying to create a petgraph Graph from JSON data. The JSON contains the edges of the graph, the key represents the starting vertex and the value is a list of adjacent vertices. It's possible to generate a graph with a vector of edges.

I managed to create a Vec<(String, String))> but not a Vec<(&str, &str)> as expected.

extern crate petgraph;
extern crate serde_json;

use petgraph::prelude::*;
use serde_json::{Value, Error};

fn main() {
    let data = r#"{
      "A": [ "B" ],
      "B": [ "C", "D" ],
      "D": [ "E", "F" ]
    }"#;
    let json_value: Value = serde_json::from_str(data).unwrap();
    let mut edges: Vec<(String, String)> = vec![];
    if let Value::Object(map) = json_value {
        for (from_edge, array) in &map {
            if let &Value::Array(ref array_value) = array {
                for edge in array_value {
                    if let &Value::String(ref to_edge) = edge {
                        edges.push((from_edge.clone(), to_edge.clone()))
                    }
                }
            }
        }
    }
    // let graph = DiGraphMap::<&str, ()>::from_edges(edges);
    //             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct
    //             `std::string::String`, found &str

}

I tried different things:

• Change the graph type to DiGraphMap::<String, ()>, however it does not accept it.
• Transform a Vec<(String, String)> into a Vec<(&str, &str)>. I read this post but it didn't help.
• edges.push((&"a", &"b")) works but not edges.push((&from.clone(), &to.clone())).

There is probably a better way to extract the edges here.

Answer 1

Change the graph type to DiGraphMap::<String, ()>, however it does not accept it.

A GraphMap requires that the node type be copyable. String does not implement Copy.

Transform a Vec<(String, String)> into a Vec<(&str, &str)>

As mentioned in the question you linked, this is impossible. What you can do is create a second Vec with &str that reference the original Strings:

let a: Vec<(String, String)> = vec![("a".into(), "b".into())];

let b: Vec<(&str, &str)> = a.iter()
    .map(|&(ref x, ref y)| (x.as_str(), y.as_str()))
    .collect();

---

However, that's not needed in this case. Instead, read the JSON data into a data structure that models a map (I chose BTreeMap) and leave the Strings there. You can then construct an iterator of pairs of references to those Strings, building the graph from that:

extern crate petgraph;
extern crate serde_json;

use petgraph::prelude::*;
use std::collections::BTreeMap;
use std::iter;

fn main() {
    let data = r#"{
      "A": [ "B" ],
      "B": [ "C", "D" ],
      "D": [ "E", "F" ]
    }"#;

    let json_value: BTreeMap<String, Vec<String>> =
        serde_json::from_str(data).unwrap();

    let edges = json_value
        .iter()
        .flat_map(|(k, vs)| {
            let vs = vs.iter().map(|v| v.as_str());
            iter::repeat(k.as_str()).zip(vs)
        });

    let graph: DiGraphMap<_, ()> = edges.collect();
}

I need to encapsulate this into a function

This is barely possible to do. Since JSON strings contain UTF-8 data, Serde allows you to get references to original input strings. You need to remember that your graph cannot outlive input:

fn main() {
    let data = r#"{
      "A": [ "B" ],
      "B": [ "C", "D" ],
      "D": [ "E", "F" ]
    }"#;

    let graph = example(data);
}

fn example(data: &str) -> serde_json::Result<DiGraphMap<&str, ()>> {
    let json_value: BTreeMap<&str, Vec<&str>> = serde_json::from_str(data)?;

    let edges = json_value
        .into_iter()
        .flat_map(|(k, vs)| iter::repeat(k).zip(vs));

    Ok(edges.collect())
}