easier 0.3.0

making rust easier
Documentation 
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use std::collections::HashMap;

/// Struct that stores a vec of values and keeps the order of insertion
/// Used for group by
pub(crate) struct OrderedVecMap<K, V> {
    map: HashMap<K, Vec<V>>,
    keys: Vec<K>,
}
impl<K, V> OrderedVecMap<K, V> {
    pub fn new() -> Self {
        Self {
            map: HashMap::new(),
            keys: Vec::new(),
        }
    }

    pub fn into_iter(self) -> IntoIter<K, V> {
        IntoIter { map: self }
    }

    //this adds
    pub fn insert_push(&mut self, key: K, item: V)
    where
        K: Eq + std::hash::Hash + Clone,
    {
        let entry = self.map.entry(key.clone()).or_default();

        if entry.is_empty() {
            self.keys.push(key);
        }
        entry.push(item);
    }
}

impl<K, V> IntoIterator for OrderedVecMap<K, V>
where
    K: Eq + std::hash::Hash,
{
    type Item = (K, Vec<V>);
    type IntoIter = IntoIter<K, V>;

    fn into_iter(self) -> Self::IntoIter {
        IntoIter { map: self }
    }
}

pub struct IntoIter<K, V> {
    map: OrderedVecMap<K, V>,
}

impl<K, V> Iterator for IntoIter<K, V>
where
    K: Eq + std::hash::Hash,
{
    type Item = (K, Vec<V>);

    fn next(&mut self) -> Option<Self::Item> {
        if self.map.keys.is_empty() {
            return None;
        }
        let key = self.map.keys.remove(0);
        let value = self.map.map.remove(&key).expect("guaranteed key exists");
        Some((key, value))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_ordered_map() {
        let mut map = OrderedVecMap::new();
        map.insert_push(1, 1);
        map.insert_push(1, 2);
        map.insert_push(2, 3);
        map.insert_push(2, 4);
        map.insert_push(3, 5);
        map.insert_push(3, 6);

        let mut iter = map.into_iter();
        assert_eq!(iter.next(), Some((1, vec![1, 2])));
        assert_eq!(iter.next(), Some((2, vec![3, 4])));
        assert_eq!(iter.next(), Some((3, vec![5, 6])));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn into_iter() {
        let mut map = OrderedVecMap::new();
        map.insert_push(1, 1);
        map.insert_push(1, 2);
        map.insert_push(2, 3);
        map.insert_push(2, 4);
        map.insert_push(3, 5);
        map.insert_push(3, 6);
        for (key, value) in map {
            println!("{:?} {:?}", key, value);
        }
    }
}