use crate::{
    iter::{
        atomic_iter::{AtomicIter, AtomicIterWithInitialLen},
        buffered::{buffered_chunk::BufferedChunk, buffered_iter::BufferedIter, vec::BufferedVec},
        no_leak_iter::NoLeakIter,
    },
    next::NextChunk,
    AtomicCounter, ConcurrentIter, Next,
};
use std::{
    cmp::Ordering,
    mem::{ManuallyDrop, MaybeUninit},
    ops::Range,
};

/// A concurrent iterator over a vector, consuming the vector and yielding its elements.
pub struct ConIterOfVec<T: Send + Sync> {
    ptr: *mut T,
    vec_len: usize,
    vec_cap: usize,
    counter: AtomicCounter,
}

impl<T: Send + Sync> Drop for ConIterOfVec<T> {
    fn drop(&mut self) {
        // # SAFETY
        // null ptr indicates that the data is already taken out of this iterator
        // by a consuming method such as `into_seq_iter`
        if !self.ptr.is_null() {
            unsafe { self.drop_elements_in_place(self.num_taken()..self.vec_len) };
            let _vec_to_drop = unsafe { Vec::from_raw_parts(self.ptr, 0, self.vec_cap) };
        }
    }
}

impl<T: Send + Sync> std::fmt::Debug for ConIterOfVec<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        super::helpers::fmt_iter(f, "ConIterOfVec", Some(self.initial_len()), &self.counter)
    }
}

impl<T: Send + Sync> From<Vec<T>> for ConIterOfVec<T> {
    /// Consumes and creates a concurrent iterator of the given `vec`.
    fn from(vec: Vec<T>) -> Self {
        Self::new(vec)
    }
}

impl<T: Send + Sync> ConIterOfVec<T> {
    /// Consumes and creates a concurrent iterator of the given `vec`.
    pub fn new(mut vec: Vec<T>) -> Self {
        let (vec_len, vec_cap) = (vec.len(), vec.capacity());
        let ptr = vec.as_mut_ptr();
        let _ = ManuallyDrop::new(vec);
        Self {
            ptr,
            vec_len,
            vec_cap,
            counter: AtomicCounter::new(),
        }
    }

    unsafe fn take_one(&self, item_idx: usize) -> T {
        let src_ptr = self.ptr.add(item_idx);

        let mut value = MaybeUninit::<T>::uninit();
        value.as_mut_ptr().swap(src_ptr);

        value.assume_init()
    }

    unsafe fn drop_elements_in_place(&self, range: Range<usize>) {
        for i in range {
            self.ptr.add(i).drop_in_place();
        }
    }

    fn num_taken(&self) -> usize {
        self.counter.current().min(self.vec_len)
    }

    pub(crate) unsafe fn take_slice(
        &self,
        begin_idx: usize,
        len: usize,
    ) -> impl ExactSizeIterator<Item = T> + '_ {
        let end_idx = (begin_idx + len).min(self.vec_len);
        let iter = (begin_idx..end_idx).map(|i| self.take_one(i));
        NoLeakIter::from(iter)
    }
}

impl<T: Send + Sync> AtomicIter<T> for ConIterOfVec<T> {
    #[inline(always)]
    fn counter(&self) -> &AtomicCounter {
        &self.counter
    }

    #[inline(always)]
    fn progress_and_get_begin_idx(&self, number_to_fetch: usize) -> Option<usize> {
        let begin_idx = self.counter().fetch_and_add(number_to_fetch);
        match begin_idx.cmp(&self.vec_len) {
            Ordering::Less => Some(begin_idx),
            _ => None,
        }
    }

    fn get(&self, item_idx: usize) -> Option<T> {
        match item_idx.cmp(&self.vec_len) {
            // SAFETY: only one thread can access the `item_idx`-th position and `item_idx` is in bounds
            Ordering::Less => Some(unsafe { self.take_one(item_idx) }),
            _ => None,
        }
    }

    fn fetch_n(&self, n: usize) -> Option<NextChunk<T, impl ExactSizeIterator<Item = T>>> {
        let begin_idx = self.progress_and_get_begin_idx(n).unwrap_or(self.vec_len);
        let end_idx = (begin_idx + n).min(self.vec_len).max(begin_idx);

        match begin_idx.cmp(&end_idx) {
            Ordering::Equal => None,
            _ => {
                let values = unsafe { self.take_slice(begin_idx, n) };
                Some(NextChunk { begin_idx, values })
            }
        }
    }

    fn early_exit(&self) {
        let num_taken_before = self.counter.get_current_max_value(self.vec_len);
        if num_taken_before < self.vec_len {
            unsafe { self.drop_elements_in_place(num_taken_before..self.vec_len) };
        }
    }
}

impl<T: Send + Sync> AtomicIterWithInitialLen<T> for ConIterOfVec<T> {
    #[inline(always)]
    fn initial_len(&self) -> usize {
        self.vec_len
    }
}

unsafe impl<T: Send + Sync> Sync for ConIterOfVec<T> {}

unsafe impl<T: Send + Sync> Send for ConIterOfVec<T> {}

// AtomicIter -> ConcurrentIter

impl<T: Send + Sync> ConcurrentIter for ConIterOfVec<T> {
    type Item = T;

    type BufferedIter = BufferedVec<T>;

    type SeqIter = std::vec::IntoIter<T>;

    /// Converts the concurrent iterator back to the original wrapped type which is the source of the elements to be iterated.
    /// Already progressed elements are skipped.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use orx_concurrent_iter::*;
    ///
    /// let vec: Vec<_> = (0..1024).map(|x| x.to_string()).collect();
    /// let con_iter = vec.into_con_iter();
    ///
    /// std::thread::scope(|s| {
    ///     s.spawn(|| {
    ///         for _ in 0..42 {
    ///             _ = con_iter.next();
    ///         }
    ///
    ///         let mut buffered = con_iter.buffered_iter(32);
    ///         let _chunk = buffered.next().unwrap();
    ///     });
    /// });
    ///
    /// let num_used = 42 + 32;
    ///
    /// // converts the remaining elements into a sequential iterator
    /// let seq_iter = con_iter.into_seq_iter();
    ///
    /// assert_eq!(seq_iter.len(), 1024 - num_used);
    /// for (i, x) in seq_iter.enumerate() {
    ///     assert_eq!(x, (num_used + i).to_string());
    /// }
    /// ```
    fn into_seq_iter(mut self) -> Self::SeqIter {
        let num_taken = self.counter.current().min(self.vec_len);
        let ptr = self.ptr;

        self.ptr = std::ptr::null_mut(); // to avoid double free on drop

        match num_taken {
            0 => {
                let vec = unsafe { Vec::from_raw_parts(ptr, self.vec_len, self.vec_cap) };
                vec.into_iter()
            }
            _ => {
                let right_len = self.vec_len - num_taken;
                for i in 0..right_len {
                    let dst = unsafe { ptr.add(i) };
                    let src = unsafe { ptr.add(i + num_taken) };
                    unsafe { dst.swap(src) };
                }
                let vec = unsafe { Vec::from_raw_parts(ptr, right_len, self.vec_cap) };
                vec.into_iter()
            }
        }
    }

    #[inline(always)]
    fn next_id_and_value(&self) -> Option<Next<Self::Item>> {
        self.fetch_one()
    }

    #[inline(always)]
    fn next_chunk(
        &self,
        chunk_size: usize,
    ) -> Option<NextChunk<Self::Item, impl ExactSizeIterator<Item = Self::Item>>> {
        self.fetch_n(chunk_size)
    }

    fn buffered_iter(&self, chunk_size: usize) -> BufferedIter<Self::Item, Self::BufferedIter> {
        let buffered_iter = Self::BufferedIter::new(chunk_size);
        BufferedIter::new(buffered_iter, self)
    }

    #[inline(always)]
    fn try_get_len(&self) -> Option<usize> {
        let current = <Self as AtomicIter<_>>::counter(self).current();
        let initial_len = <Self as AtomicIterWithInitialLen<_>>::initial_len(self);
        let len = match current.cmp(&initial_len) {
            std::cmp::Ordering::Less => initial_len - current,
            _ => 0,
        };
        Some(len)
    }

    fn skip_to_end(&self) {
        self.early_exit()
    }
}