#![cfg(feature = "use_std")]

use std::collections::HashMap;

use std::hash::Hash;

use std::iter::Iterator;

/// Return a `HashMap` of keys mapped to a list of their corresponding values.

///

/// See [`.into_group_map()`](crate::Itertools::into_group_map)

/// for more information.

pub fn into_group_map<I, K, V>(iter: I) -> HashMap<K, Vec<V>>

    where I: Iterator<Item=(K, V)>,

          K: Hash + Eq,

{

    let mut lookup = HashMap::new();

    iter.for_each(|(key, val)| {

        lookup.entry(key).or_insert_with(Vec::new).push(val);

    });

    lookup

}

pub fn into_group_map_by<I, K, V>(iter: I, f: impl Fn(&V) -> K) -> HashMap<K, Vec<V>>

    where

        I: Iterator<Item=V>,

        K: Hash + Eq,

{

    into_group_map(

        iter.map(|v| (f(&v), v))

    )

}