use bevy_reflect::Reflect;
use core::iter;
use core::iter::FusedIterator;
use thiserror::Error;
use wgpu_types::IndexFormat;

/// A disjunction of four iterators. This is necessary to have a well-formed type for the output
/// of [`Mesh::triangles`](super::Mesh::triangles), which produces iterators of four different types depending on the
/// branch taken.
pub(crate) enum FourIterators<A, B, C, D> {
    First(A),
    Second(B),
    Third(C),
    Fourth(D),
}

impl<A, B, C, D, I> Iterator for FourIterators<A, B, C, D>
where
    A: Iterator<Item = I>,
    B: Iterator<Item = I>,
    C: Iterator<Item = I>,
    D: Iterator<Item = I>,
{
    type Item = I;

    fn next(&mut self) -> Option<Self::Item> {
        match self {
            FourIterators::First(iter) => iter.next(),
            FourIterators::Second(iter) => iter.next(),
            FourIterators::Third(iter) => iter.next(),
            FourIterators::Fourth(iter) => iter.next(),
        }
    }
}

/// An error that occurred while trying to invert the winding of a [`Mesh`](super::Mesh).
#[derive(Debug, Error)]
pub enum MeshWindingInvertError {
    /// This error occurs when you try to invert the winding for a mesh with [`PrimitiveTopology::PointList`](super::PrimitiveTopology::PointList).
    #[error("Mesh winding inversion does not work for primitive topology `PointList`")]
    WrongTopology,

    /// This error occurs when you try to invert the winding for a mesh with
    /// * [`PrimitiveTopology::TriangleList`](super::PrimitiveTopology::TriangleList), but the indices are not in chunks of 3.
    /// * [`PrimitiveTopology::LineList`](super::PrimitiveTopology::LineList), but the indices are not in chunks of 2.
    #[error("Indices weren't in chunks according to topology")]
    AbruptIndicesEnd,
}

/// An error that occurred while trying to extract a collection of triangles from a [`Mesh`](super::Mesh).
#[derive(Debug, Error)]
pub enum MeshTrianglesError {
    #[error("Source mesh does not have primitive topology TriangleList or TriangleStrip")]
    WrongTopology,

    #[error("Source mesh lacks position data")]
    MissingPositions,

    #[error("Source mesh position data is not Float32x3")]
    PositionsFormat,

    #[error("Source mesh lacks face index data")]
    MissingIndices,

    #[error("Face index data references vertices that do not exist")]
    BadIndices,
}

/// An array of indices into the [`VertexAttributeValues`](super::VertexAttributeValues) for a mesh.
///
/// It describes the order in which the vertex attributes should be joined into faces.
#[derive(Debug, Clone, Reflect)]
#[reflect(Clone)]
pub enum Indices {
    U16(Vec<u16>),
    U32(Vec<u32>),
}

impl Indices {
    /// Returns an iterator over the indices.
    pub fn iter(&self) -> impl Iterator<Item = usize> + '_ {
        match self {
            Indices::U16(vec) => IndicesIter::U16(vec.iter()),
            Indices::U32(vec) => IndicesIter::U32(vec.iter()),
        }
    }

    /// Returns the number of indices.
    pub fn len(&self) -> usize {
        match self {
            Indices::U16(vec) => vec.len(),
            Indices::U32(vec) => vec.len(),
        }
    }

    /// Returns `true` if there are no indices.
    pub fn is_empty(&self) -> bool {
        match self {
            Indices::U16(vec) => vec.is_empty(),
            Indices::U32(vec) => vec.is_empty(),
        }
    }

    /// Add an index. If the index is greater than `u16::MAX`,
    /// the storage will be converted to `u32`.
    pub fn push(&mut self, index: u32) {
        self.extend([index]);
    }
}

/// Extend the indices with indices from an iterator.
/// Semantically equivalent to calling [`push`](Indices::push) for each element in the iterator,
/// but more efficient.
impl Extend<u32> for Indices {
    fn extend<T: IntoIterator<Item = u32>>(&mut self, iter: T) {
        let mut iter = iter.into_iter();
        match self {
            Indices::U32(indices) => indices.extend(iter),
            Indices::U16(indices) => {
                indices.reserve(iter.size_hint().0);
                while let Some(index) = iter.next() {
                    match u16::try_from(index) {
                        Ok(index) => indices.push(index),
                        Err(_) => {
                            let new_vec = indices
                                .iter()
                                .map(|&index| u32::from(index))
                                .chain(iter::once(index))
                                .chain(iter)
                                .collect::<Vec<u32>>();
                            *self = Indices::U32(new_vec);
                            break;
                        }
                    }
                }
            }
        }
    }
}

/// An Iterator for the [`Indices`].
enum IndicesIter<'a> {
    U16(core::slice::Iter<'a, u16>),
    U32(core::slice::Iter<'a, u32>),
}

impl Iterator for IndicesIter<'_> {
    type Item = usize;

    fn next(&mut self) -> Option<Self::Item> {
        match self {
            IndicesIter::U16(iter) => iter.next().map(|val| *val as usize),
            IndicesIter::U32(iter) => iter.next().map(|val| *val as usize),
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        match self {
            IndicesIter::U16(iter) => iter.size_hint(),
            IndicesIter::U32(iter) => iter.size_hint(),
        }
    }
}

impl<'a> ExactSizeIterator for IndicesIter<'a> {}
impl<'a> FusedIterator for IndicesIter<'a> {}

impl From<&Indices> for IndexFormat {
    fn from(indices: &Indices) -> Self {
        match indices {
            Indices::U16(_) => IndexFormat::Uint16,
            Indices::U32(_) => IndexFormat::Uint32,
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::Indices;
    use wgpu_types::IndexFormat;

    #[test]
    fn test_indices_push() {
        let mut indices = Indices::U16(Vec::new());
        indices.push(10);
        assert_eq!(IndexFormat::Uint16, IndexFormat::from(&indices));
        assert_eq!(vec![10], indices.iter().collect::<Vec<_>>());

        // Add a value that is too large for `u16` so the storage should be converted to `U32`.
        indices.push(0x10000);
        assert_eq!(IndexFormat::Uint32, IndexFormat::from(&indices));
        assert_eq!(vec![10, 0x10000], indices.iter().collect::<Vec<_>>());

        indices.push(20);
        indices.push(0x20000);
        assert_eq!(IndexFormat::Uint32, IndexFormat::from(&indices));
        assert_eq!(
            vec![10, 0x10000, 20, 0x20000],
            indices.iter().collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_indices_extend() {
        let mut indices = Indices::U16(Vec::new());
        indices.extend([10, 11]);
        assert_eq!(IndexFormat::Uint16, IndexFormat::from(&indices));
        assert_eq!(vec![10, 11], indices.iter().collect::<Vec<_>>());

        // Add a value that is too large for `u16` so the storage should be converted to `U32`.
        indices.extend([12, 0x10013, 0x10014]);
        assert_eq!(IndexFormat::Uint32, IndexFormat::from(&indices));
        assert_eq!(
            vec![10, 11, 12, 0x10013, 0x10014],
            indices.iter().collect::<Vec<_>>()
        );

        indices.extend([15, 0x10016]);
        assert_eq!(IndexFormat::Uint32, IndexFormat::from(&indices));
        assert_eq!(
            vec![10, 11, 12, 0x10013, 0x10014, 15, 0x10016],
            indices.iter().collect::<Vec<_>>()
        );
    }
}