Function group 

pub fn group<'a, T, K, F>(data: &'a [T], key: F) -> HashMap<K, Vec<&'a T>>
where
    K: Eq + Hash,
    F: Fn(&T) -> K,

Groups elements by a key function.

Returns a HashMap where each key maps to a Vec of elements with that key.

Example

use d3rs::array::group;

let data = vec!["apple", "apricot", "banana", "blueberry", "cherry"];
let grouped = group(&data, |s| s.chars().next().unwrap());

assert_eq!(grouped[&'a'].len(), 2);
assert_eq!(grouped[&'b'].len(), 2);
assert_eq!(grouped[&'c'].len(), 1);