use alloc::vec::Vec;

use std::iter::Fuse;

use std::ops::Index;

use crate::size_hint::{self, SizeHint};

#[derive(Debug, Clone)]

pub struct LazyBuffer<I: Iterator> {

    it: Fuse<I>,

    buffer: Vec<I::Item>,

}

impl<I> LazyBuffer<I>

where

    I: Iterator,

{

    pub fn new(it: I) -> Self {

        Self {

            it: it.fuse(),

            buffer: Vec::new(),

        }

    }

    pub fn len(&self) -> usize {

        self.buffer.len()

    }

    pub fn size_hint(&self) -> SizeHint {

        size_hint::add_scalar(self.it.size_hint(), self.len())

    }

    pub fn count(self) -> usize {

        self.len() + self.it.count()

    }

    pub fn get_next(&mut self) -> bool {

        if let Some(x) = self.it.next() {

            self.buffer.push(x);

            true

        } else {

            false

        }

    }

    pub fn prefill(&mut self, len: usize) {

        let buffer_len = self.buffer.len();

        if len > buffer_len {

            let delta = len - buffer_len;

            self.buffer.extend(self.it.by_ref().take(delta));

        }

    }

}

impl<I> LazyBuffer<I>

where

    I: Iterator,

    I::Item: Clone,

{

    pub fn get_at(&self, indices: &[usize]) -> Vec<I::Item> {

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

    }

    pub fn get_array<const K: usize>(&self, indices: [usize; K]) -> [I::Item; K] {

        indices.map(|i| self.buffer[i].clone())

    }

}

impl<I, J> Index<J> for LazyBuffer<I>

where

    I: Iterator,

    I::Item: Sized,

    Vec<I::Item>: Index<J>,

{

    type Output = <Vec<I::Item> as Index<J>>::Output;

    fn index(&self, index: J) -> &Self::Output {

        self.buffer.index(index)

    }

}