How to convert a map keyed by pair to a map of maps

help
1

I would like to write a function to convert from a map whose key is a pair to a map whose key is the first from the pair and value is a map whose key is the second from the pair, with the types as follows:

fn tuple_key_to_map_of_map(tupled: BTreeMap<(String, String), i32>) -> BTreeMap<String, BTreeMap<String, i32>> {
  // ?
}

What is an idiomatic way to code this function?

2

Here's the way to do it.

fn tuple_key_to_map_of_map(tupled: BTreeMap<(String, String), i32>) -> BTreeMap<String, BTreeMap<String, i32>> {
    let mut new = BTreeMap::new();
    
    for ((first, second), value) in tupled {
        new.entry(first).or_insert_with(BTreeMap::new).insert(second, value);
    }
    
    new
}
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