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

use core::hash::BuildHasher;

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_with_hasher<I, K, V, S>(iter: I, hash_builder: S) -> HashMap<K, Vec<V>, S>

where

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

    K: Hash + Eq,

    S: BuildHasher,

{

    let mut lookup = HashMap::<K, Vec<V>, S>::with_hasher(hash_builder);

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

        lookup.entry(key).or_default().push(val);

    });

    lookup

}

pub fn into_group_map_by_with_hasher<I, K, V, F, S>(

    iter: I,

    mut f: F,

    hash_builder: S,

) -> HashMap<K, Vec<V>, S>

where

    I: Iterator<Item = V>,

    K: Hash + Eq,

    F: FnMut(&V) -> K,

    S: BuildHasher,

{

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

}