use crate::MiniVec;

extern crate alloc;

/// `Splice` is an iterator that removes a sub-section of the backing `MiniVec` and then replaces it with the contents
/// of another iterator. The removed sub-section and the iterator used to replace it can have independent lengths.
///
pub struct Splice<'a, I>
where
  I: 'a + Iterator,
{
  vec_: core::ptr::NonNull<MiniVec<I::Item>>,
  drain_pos_: core::ptr::NonNull<I::Item>,
  drain_end_: core::ptr::NonNull<I::Item>,
  remaining_pos_: core::ptr::NonNull<I::Item>,
  remaining_: usize,
  marker_: core::marker::PhantomData<&'a I::Item>,
  fill_: I,
}

pub fn make_splice_iterator<'a, I: 'a + Iterator>(
  vec: &mut MiniVec<I::Item>,
  data: *mut I::Item,
  remaining: usize,
  start_idx: usize,
  end_idx: usize,
  fill: I,
) -> Splice<'a, I> {
  if data.is_null() {
    let dangling = core::ptr::NonNull::<I::Item>::dangling();

    Splice {
      vec_: core::ptr::NonNull::from(vec),
      drain_pos_: dangling,
      drain_end_: dangling,
      remaining_pos_: dangling,
      remaining_: 0,
      marker_: core::marker::PhantomData,
      fill_: fill,
    }
  } else {
    Splice {
      vec_: core::ptr::NonNull::from(vec),
      drain_pos_: unsafe { core::ptr::NonNull::new_unchecked(data.add(start_idx)) },
      drain_end_: unsafe { core::ptr::NonNull::new_unchecked(data.add(end_idx)) },
      remaining_pos_: unsafe { core::ptr::NonNull::new_unchecked(data.add(end_idx)) },
      remaining_: remaining,
      marker_: core::marker::PhantomData,
      fill_: fill,
    }
  }
}

impl<I> Iterator for Splice<'_, I>
where
  I: Iterator,
{
  type Item = I::Item;

  fn next(&mut self) -> Option<Self::Item> {
    if self.drain_pos_ >= self.drain_end_ {
      return None;
    }

    let p = self.drain_pos_.as_ptr();
    let tmp = unsafe { core::ptr::read(p) };
    self.drain_pos_ = unsafe { core::ptr::NonNull::new_unchecked(p.add(1)) };
    Some(tmp)
  }

  fn size_hint(&self) -> (usize, Option<usize>) {
    let len = (self.drain_end_.as_ptr() as *const _ as usize
      - self.drain_pos_.as_ptr() as *const _ as usize)
      / core::mem::size_of::<I::Item>();

    (len, Some(len))
  }
}

impl<I: Iterator> ExactSizeIterator for Splice<'_, I> {}

impl<I> DoubleEndedIterator for Splice<'_, I>
where
  I: Iterator,
{
  fn next_back(&mut self) -> Option<Self::Item> {
    let pos = unsafe { self.drain_end_.as_ptr().sub(1) };
    if pos < self.drain_pos_.as_ptr() {
      return None;
    }

    let tmp = unsafe { core::ptr::read(pos) };
    self.drain_end_ = unsafe { core::ptr::NonNull::new_unchecked(pos) };
    Some(tmp)
  }
}

struct DropGuard<'b, 'a, I>
where
  I: Iterator,
{
  splice: &'b mut Splice<'a, I>,
}

impl<'b, 'a, I> Drop for DropGuard<'b, 'a, I>
where
  I: Iterator,
{
  fn drop(&mut self) {
    // much like Drain, remove the rest of the elements from the splice range if they
    // haven't already been exhausted
    //
    for x in &mut self.splice {
      core::mem::drop(x);
    }

    let vec = unsafe { self.splice.vec_.as_mut() };
    if vec.is_default() {
      for x in &mut self.splice.fill_ {
        vec.push(x);
      }
      return;
    }

    // first, figure out where our draining operation started
    // this is at offset vec.len() from the start of [T]'s data
    //
    let drain_begin = unsafe { vec.as_mut_ptr().add(vec.len()) };

    // infer the number of items we drained by where the remaining_pos_ is
    //
    let num_drained = (self.splice.remaining_pos_.as_ptr() as usize - drain_begin as usize)
      / core::mem::size_of::<I::Item>();

    // fill the drained sub-section using the iterator the user supplied
    // if the iterator, for example, has more elements than the draiend region allows,
    // we need to know this so we can reallocate the vector accordingly
    //
    let needs_more = {
      let mut needs_more = true;
      for idx in 0..num_drained {
        if let Some(val) = self.splice.fill_.next() {
          unsafe {
            core::ptr::write(drain_begin.add(idx), val);
            vec.set_len(vec.len() + 1);
          };
        } else {
          needs_more = false;
        }
      }

      needs_more
    };

    // if we don't have any more elements in the iterator the user supplied, we can
    // go ahead and shift the tail down
    //
    if !needs_more {
      // if the supplied iterator had exactly the number of elements that we drained,
      // we don't need to memcpy and can instead just adjust the length of the vector
      // and return
      //
      if unsafe { vec.as_ptr().add(vec.len()) == self.splice.remaining_pos_.as_ptr() } {
        unsafe {
          vec.set_len(vec.len() + self.splice.remaining_);
        }
        return;
      }

      // we need to copy things down from Drain's "tail" to where our iterator left
      // off in the drained range
      // this basically downshifts the elements from right-to-left so it's safe to
      // call `core::ptr::copy`
      //
      let src = self.splice.remaining_pos_.as_ptr();
      let dst = unsafe { vec.as_mut_ptr().add(vec.len()) };
      let count = self.splice.remaining_;
      unsafe {
        core::ptr::copy(src, dst, count);
        vec.set_len(vec.len() + self.splice.remaining_);
      };

      return;
    }

    // we need to handle the rest of the iterator's elements now
    // pool them into a temporary vector for storage
    //
    let mut tmp: MiniVec<_> = (&mut self.splice.fill_).collect();

    // reserve extra capacity if required
    // note, this will invalidate all of our previously cached pointers in the Splice
    // iterator so we have to store the offset of the drain tail manually
    //
    let capacity = vec.capacity();
    let remaining_offset = (self.splice.remaining_pos_.as_ptr() as usize - vec.as_ptr() as usize)
      / core::mem::size_of::<I::Item>();

    // if our vector's length + the remaining elements + the extra tmp length exceeds
    // our capacity we need to reallocate
    //
    let total_elements = vec.len() + self.splice.remaining_ + tmp.len();

    if total_elements > capacity {
      vec.grow(total_elements, vec.alignment());
    }

    // let's first move the Drain tail over to the right
    // we know our Drain's tail starts at the `remaining_offset` and we have to copy
    // self.splice.remaining_ elements over starting at the offset of our current len
    // plus the tail len
    //
    if self.splice.remaining_ > 0 {
      unsafe {
        let src = vec.as_ptr().add(remaining_offset);
        let dst = vec.as_mut_ptr().add(vec.len() + tmp.len());
        let count = self.splice.remaining_;
        core::ptr::copy(src, dst, count);
      };
    }

    // finally we copy the remaining tmp elements into the vector and then we make sure
    // to set its length to 0 to prevent any sort of double-frees
    //
    if !tmp.is_empty() {
      unsafe {
        let src = tmp.as_ptr();
        let dst = vec.as_mut_ptr().add(vec.len());
        let count = tmp.len();
        core::ptr::copy(src, dst, count);
      };
    }

    unsafe {
      vec.set_len(vec.len() + self.splice.remaining_ + tmp.len());
      if !tmp.is_empty() {
        tmp.set_len(0);
      }
    };
  }
}

impl<I: Iterator> Drop for Splice<'_, I> {
  fn drop(&mut self) {
    while let Some(item) = self.next() {
      let guard = DropGuard { splice: self };
      drop(item);
      core::mem::forget(guard);
    }

    DropGuard { splice: self };
  }
}