use std::collections::HashMap;

use std::collections::hash_map::Entry;

use std::hash::Hash;

use std::fmt;

use std::iter::FusedIterator;

/// An iterator adapter to filter out duplicate elements.

///

/// See [`.unique_by()`](crate::Itertools::unique) for more information.

#[derive(Clone)]

#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]

pub struct UniqueBy<I: Iterator, V, F> {

    iter: I,

    // Use a Hashmap for the Entry API in order to prevent hashing twice.

    // This can maybe be replaced with a HashSet once `get_or_insert_with`

    // or a proper Entry API for Hashset is stable and meets this msrv

    used: HashMap<V, ()>,

    f: F,

}

impl<I, V, F> fmt::Debug for UniqueBy<I, V, F>

    where I: Iterator + fmt::Debug,

          V: fmt::Debug + Hash + Eq,

{

    debug_fmt_fields!(UniqueBy, iter, used);

}

/// Create a new `UniqueBy` iterator.

pub fn unique_by<I, V, F>(iter: I, f: F) -> UniqueBy<I, V, F>

    where V: Eq + Hash,

          F: FnMut(&I::Item) -> V,

          I: Iterator,

{

    UniqueBy {

        iter,

        used: HashMap::new(),

        f,

    }

}

// count the number of new unique keys in iterable (`used` is the set already seen)

fn count_new_keys<I, K>(mut used: HashMap<K, ()>, iterable: I) -> usize

    where I: IntoIterator<Item=K>,

          K: Hash + Eq,

{

    let iter = iterable.into_iter();

    let current_used = used.len();

    used.extend(iter.map(|key| (key, ())));

    used.len() - current_used

}

impl<I, V, F> Iterator for UniqueBy<I, V, F>

    where I: Iterator,

          V: Eq + Hash,

          F: FnMut(&I::Item) -> V

{

    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {

        while let Some(v) = self.iter.next() {

            let key = (self.f)(&v);

            if self.used.insert(key, ()).is_none() {

                return Some(v);

            }

        }

        None

    }

    #[inline]

    fn size_hint(&self) -> (usize, Option<usize>) {

        let (low, hi) = self.iter.size_hint();

        ((low > 0 && self.used.is_empty()) as usize, hi)

    }

    fn count(self) -> usize {

        let mut key_f = self.f;

        count_new_keys(self.used, self.iter.map(move |elt| key_f(&elt)))

    }

}

impl<I, V, F> DoubleEndedIterator for UniqueBy<I, V, F>

    where I: DoubleEndedIterator,

          V: Eq + Hash,

          F: FnMut(&I::Item) -> V

{

    fn next_back(&mut self) -> Option<Self::Item> {

        while let Some(v) = self.iter.next_back() {

            let key = (self.f)(&v);

            if self.used.insert(key, ()).is_none() {

                return Some(v);

            }

        }

        None

    }

}

impl<I, V, F> FusedIterator for UniqueBy<I, V, F>

    where I: FusedIterator,

          V: Eq + Hash,

          F: FnMut(&I::Item) -> V

{}

impl<I> Iterator for Unique<I>

    where I: Iterator,

          I::Item: Eq + Hash + Clone

{

    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {

        while let Some(v) = self.iter.iter.next() {

            if let Entry::Vacant(entry) = self.iter.used.entry(v) {

                let elt = entry.key().clone();

                entry.insert(());

                return Some(elt);

            }

        }

        None

    }

    #[inline]

    fn size_hint(&self) -> (usize, Option<usize>) {

        let (low, hi) = self.iter.iter.size_hint();

        ((low > 0 && self.iter.used.is_empty()) as usize, hi)

    }

    fn count(self) -> usize {

        count_new_keys(self.iter.used, self.iter.iter)

    }

}

impl<I> DoubleEndedIterator for Unique<I>

    where I: DoubleEndedIterator,

          I::Item: Eq + Hash + Clone

{

    fn next_back(&mut self) -> Option<Self::Item> {

        while let Some(v) = self.iter.iter.next_back() {

            if let Entry::Vacant(entry) = self.iter.used.entry(v) {

                let elt = entry.key().clone();

                entry.insert(());

                return Some(elt);

            }

        }

        None

    }

}

impl<I> FusedIterator for Unique<I>

    where I: FusedIterator,

          I::Item: Eq + Hash + Clone

{}

/// An iterator adapter to filter out duplicate elements.

///

/// See [`.unique()`](crate::Itertools::unique) for more information.

#[derive(Clone)]

#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]

pub struct Unique<I: Iterator> {

    iter: UniqueBy<I, I::Item, ()>,

}

impl<I> fmt::Debug for Unique<I>

    where I: Iterator + fmt::Debug,

          I::Item: Hash + Eq + fmt::Debug,

{

    debug_fmt_fields!(Unique, iter);

}

pub fn unique<I>(iter: I) -> Unique<I>

    where I: Iterator,

          I::Item: Eq + Hash,

{

    Unique {

        iter: UniqueBy {

            iter,

            used: HashMap::new(),

            f: (),

        }

    }

}