use alloc::vec::Vec;

use std::fmt;

use std::iter::once;

use super::lazy_buffer::LazyBuffer;

/// An iterator adaptor that iterates through all the `k`-permutations of the

/// elements from an iterator.

///

/// See [`.permutations()`](crate::Itertools::permutations) for

/// more information.

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

pub struct Permutations<I: Iterator> {

    vals: LazyBuffer<I>,

    state: PermutationState,

}

impl<I> Clone for Permutations<I>

    where I: Clone + Iterator,

          I::Item: Clone,

{

    clone_fields!(vals, state);

}

#[derive(Clone, Debug)]

enum PermutationState {

    StartUnknownLen {

        k: usize,

    },

    OngoingUnknownLen {

        k: usize,

        min_n: usize,

    },

    Complete(CompleteState),

    Empty,

}

#[derive(Clone, Debug)]

enum CompleteState {

    Start {

        n: usize,

        k: usize,

    },

    Ongoing {

        indices: Vec<usize>,

        cycles: Vec<usize>,

    }

}

enum CompleteStateRemaining {

    Known(usize),

    Overflow,

}

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

    where I: Iterator + fmt::Debug,

          I::Item: fmt::Debug,

{

    debug_fmt_fields!(Permutations, vals, state);

}

pub fn permutations<I: Iterator>(iter: I, k: usize) -> Permutations<I> {

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

    if k == 0 {

        // Special case, yields single empty vec; `n` is irrelevant

        let state = PermutationState::Complete(CompleteState::Start { n: 0, k: 0 });

        return Permutations {

            vals,

            state

        };

    }

    let mut enough_vals = true;

    while vals.len() < k {

        if !vals.get_next() {

            enough_vals = false;

            break;

        }

    }

    let state = if enough_vals {

        PermutationState::StartUnknownLen { k }

    } else {

        PermutationState::Empty

    };

    Permutations {

        vals,

        state

    }

}

impl<I> Iterator for Permutations<I>

where

    I: Iterator,

    I::Item: Clone

{

    type Item = Vec<I::Item>;

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

        self.advance();

        let &mut Permutations { ref vals, ref state } = self;

        match *state {

            PermutationState::StartUnknownLen { .. } => panic!("unexpected iterator state"),

            PermutationState::OngoingUnknownLen { k, min_n } => {

                let latest_idx = min_n - 1;

                let indices = (0..(k - 1)).chain(once(latest_idx));

                Some(indices.map(|i| vals[i].clone()).collect())

            }

            PermutationState::Complete(CompleteState::Ongoing { ref indices, ref cycles }) => {

                let k = cycles.len();

                Some(indices[0..k].iter().map(|&i| vals[i].clone()).collect())

            },

            PermutationState::Complete(CompleteState::Start { .. }) | PermutationState::Empty => None

        }

    }

    fn count(self) -> usize {

        fn from_complete(complete_state: CompleteState) -> usize {

            match complete_state.remaining() {

                CompleteStateRemaining::Known(count) => count,

                CompleteStateRemaining::Overflow => {

                    panic!("Iterator count greater than usize::MAX");

                }

            }

        }

        let Permutations { vals, state } = self;

        match state {

            PermutationState::StartUnknownLen { k } => {

                let n = vals.len() + vals.it.count();

                let complete_state = CompleteState::Start { n, k };

                from_complete(complete_state)

            }

            PermutationState::OngoingUnknownLen { k, min_n } => {

                let prev_iteration_count = min_n - k + 1;

                let n = vals.len() + vals.it.count();

                let complete_state = CompleteState::Start { n, k };

                from_complete(complete_state) - prev_iteration_count

            },

            PermutationState::Complete(state) => from_complete(state),

            PermutationState::Empty => 0

        }

    }

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

        match self.state {

            PermutationState::StartUnknownLen { .. } |

            PermutationState::OngoingUnknownLen { .. } => (0, None), // TODO can we improve this lower bound?

            PermutationState::Complete(ref state) => match state.remaining() {

                CompleteStateRemaining::Known(count) => (count, Some(count)),

                CompleteStateRemaining::Overflow => (::std::usize::MAX, None)

            }

            PermutationState::Empty => (0, Some(0))

        }

    }

}

impl<I> Permutations<I>

where

    I: Iterator,

    I::Item: Clone

{

    fn advance(&mut self) {

        let &mut Permutations { ref mut vals, ref mut state } = self;

        *state = match *state {

            PermutationState::StartUnknownLen { k } => {

                PermutationState::OngoingUnknownLen { k, min_n: k }

            }

            PermutationState::OngoingUnknownLen { k, min_n } => {

                if vals.get_next() {

                    PermutationState::OngoingUnknownLen { k, min_n: min_n + 1 }

                } else {

                    let n = min_n;

                    let prev_iteration_count = n - k + 1;

                    let mut complete_state = CompleteState::Start { n, k };

                    // Advance the complete-state iterator to the correct point

                    for _ in 0..(prev_iteration_count + 1) {

                        complete_state.advance();

                    }

                    PermutationState::Complete(complete_state)

                }

            }

            PermutationState::Complete(ref mut state) => {

                state.advance();

                return;

            }

            PermutationState::Empty => { return; }

        };

    }

}

impl CompleteState {

    fn advance(&mut self) {

        *self = match *self {

            CompleteState::Start { n, k } => {

                let indices = (0..n).collect();

                let cycles = ((n - k)..n).rev().collect();

                CompleteState::Ongoing {

                    cycles,

                    indices

                }

            },

            CompleteState::Ongoing { ref mut indices, ref mut cycles } => {

                let n = indices.len();

                let k = cycles.len();

                for i in (0..k).rev() {

                    if cycles[i] == 0 {

                        cycles[i] = n - i - 1;

                        let to_push = indices.remove(i);

                        indices.push(to_push);

                    } else {

                        let swap_index = n - cycles[i];

                        indices.swap(i, swap_index);

                        cycles[i] -= 1;

                        return;

                    }

                }

                CompleteState::Start { n, k }

            }

        }

    }

    fn remaining(&self) -> CompleteStateRemaining {

        use self::CompleteStateRemaining::{Known, Overflow};

        match *self {

            CompleteState::Start { n, k } => {

                if n < k {

                    return Known(0);

                }

                let count: Option<usize> = (n - k + 1..n + 1).fold(Some(1), |acc, i| {

                    acc.and_then(|acc| acc.checked_mul(i))

                });

                match count {

                    Some(count) => Known(count),

                    None => Overflow

                }

            }

            CompleteState::Ongoing { ref indices, ref cycles } => {

                let mut count: usize = 0;

                for (i, &c) in cycles.iter().enumerate() {

                    let radix = indices.len() - i;

                    let next_count = count.checked_mul(radix)

                        .and_then(|count| count.checked_add(c));

                    count = match next_count {

                        Some(count) => count,

                        None => { return Overflow; }

                    };

                }

                Known(count)

            }

        }

    }

}