use std::fmt;

use std::iter::FusedIterator;

use super::lazy_buffer::LazyBuffer;

use alloc::vec::Vec;

/// An iterator to iterate through all the `k`-length combinations in an iterator.

///

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

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

pub struct Combinations<I: Iterator> {

    indices: Vec<usize>,

    pool: LazyBuffer<I>,

    first: bool,

}

impl<I> Clone for Combinations<I>

    where I: Clone + Iterator,

          I::Item: Clone,

{

    clone_fields!(indices, pool, first);

}

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

    where I: Iterator + fmt::Debug,

          I::Item: fmt::Debug,

{

    debug_fmt_fields!(Combinations, indices, pool, first);

}

/// Create a new `Combinations` from a clonable iterator.

pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>

    where I: Iterator

{

    let mut pool = LazyBuffer::new(iter);

    pool.prefill(k);

    Combinations {

        indices: (0..k).collect(),

        pool,

        first: true,

    }

}

impl<I: Iterator> Combinations<I> {

    /// Returns the length of a combination produced by this iterator.

    #[inline]

    pub fn k(&self) -> usize { self.indices.len() }

    /// Returns the (current) length of the pool from which combination elements are

    /// selected. This value can change between invocations of [`next`](Combinations::next).

    #[inline]

    pub fn n(&self) -> usize { self.pool.len() }

    /// Returns a reference to the source iterator.

    #[inline]

    pub(crate) fn src(&self) -> &I { &self.pool.it }

    /// Resets this `Combinations` back to an initial state for combinations of length

    /// `k` over the same pool data source. If `k` is larger than the current length

    /// of the data pool an attempt is made to prefill the pool so that it holds `k`

    /// elements.

    pub(crate) fn reset(&mut self, k: usize) {

        self.first = true;

        if k < self.indices.len() {

            self.indices.truncate(k);

            for i in 0..k {

                self.indices[i] = i;

            }

        } else {

            for i in 0..self.indices.len() {

                self.indices[i] = i;

            }

            self.indices.extend(self.indices.len()..k);

            self.pool.prefill(k);

        }

    }

}

impl<I> Iterator for Combinations<I>

    where I: Iterator,

          I::Item: Clone

{

    type Item = Vec<I::Item>;

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

        if self.first {

            if self.k() > self.n() {

                return None;

            }

            self.first = false;

        } else if self.indices.is_empty() {

            return None;

        } else {

            // Scan from the end, looking for an index to increment

            let mut i: usize = self.indices.len() - 1;

            // Check if we need to consume more from the iterator

            if self.indices[i] == self.pool.len() - 1 {

                self.pool.get_next(); // may change pool size

            }

            while self.indices[i] == i + self.pool.len() - self.indices.len() {

                if i > 0 {

                    i -= 1;

                } else {

                    // Reached the last combination

                    return None;

                }

            }

            // Increment index, and reset the ones to its right

            self.indices[i] += 1;

            for j in i+1..self.indices.len() {

                self.indices[j] = self.indices[j - 1] + 1;

            }

        }

        // Create result vector based on the indices

        Some(self.indices.iter().map(|i| self.pool[*i].clone()).collect())

    }

}

impl<I> FusedIterator for Combinations<I>

    where I: Iterator,

          I::Item: Clone

{}