use crate::render::{mesh::Quad, pipelines::AtlasData, Vertex};
use common_base::{prof_span, span};
use vek::*;

type TodoRect = (
    Vec3<i32>,
    Vec2<Vec3<u16>>,
    guillotiere::Rectangle,
    Vec3<i32>,
);

pub struct GreedyConfig<D, FA, FL, FG, FO, FS, FP, FT> {
    pub data: D,
    /// The minimum position to mesh, in the coordinate system used
    /// for queries against the volume.
    pub draw_delta: Vec3<i32>,
    /// For each dimension i, for faces drawn in planes *parallel* to i,
    /// represents the number of voxels considered along dimension i in those
    /// planes, starting from `draw_delta`.
    pub greedy_size: Vec3<usize>,
    /// For each dimension i, represents the number of planes considered
    /// *orthogonal* to dimension i, starting from `draw_delta`.  This should
    /// usually be the same as greedy_size.
    ///
    /// An important exception is during chunk rendering (where vertical faces
    /// at chunk boundaries would otherwise be rendered twice, and also
    /// force us to use more than 5 bits to represent x and y
    /// positions--though there may be a clever way around the latter).
    /// Thus, for chunk rendering we set the number of *vertical* planes to
    /// one less than the chunk size along the x and y dimensions, but keep
    /// the number of *horizontal* planes large enough to cover the whole
    /// chunk.
    pub greedy_size_cross: Vec3<usize>,
    /// Given a position, return the AO information for the voxel at that
    /// position (0.0 - 1.0).
    pub get_ao: FA,
    /// Given a position, return the lighting information for the voxel at that
    /// position.
    pub get_light: FL,
    /// Given a position, return the glow information for the voxel at that
    /// position (i.e: additional non-sun light).
    pub get_glow: FG,
    /// Given a position, return the opacity information for the voxel at that
    /// position. Currently, we don't support real translucent lighting, so the
    /// value should either be `false` (for opaque blocks) or `true`
    /// (otherwise).
    pub get_opacity: FO,
    /// Given a position and a normal, should we draw the face between the
    /// position and position - normal (i.e. the voxel "below" this vertex)?
    /// If so, provide its orientation, together with any other meta
    /// information required for the mesh that needs to split up faces.  For
    /// example, terrain faces currently record a bit indicating whether
    /// they are exposed to water or not, so we should not merge faces where
    /// one is submerged in water and the other is not, even if they
    /// otherwise have the same orientation, dimensions, and are
    /// next to each other.
    pub should_draw: FS,
    /// Create an opaque quad (used for only display rendering) from its
    /// top-left atlas position, the rectangle's dimensions in (2D) atlas
    /// space, a world position, the u and v axes of the rectangle in (3D)
    /// world space, the normal facing out frmo the rectangle in world
    /// space, and meta information common to every voxel in this rectangle.
    pub push_quad: FP,
    /// Given a position and the lighting information for a face at that
    /// position, return the texel for the face at that position.
    pub make_face_texel: FT,
}

/// A suspended greedy mesh, with enough information to recover color data.
///
/// The reason this exists is that greedy meshing is split into two parts.
/// First, the meshing itself needs to be performed; secondly, we generate a
/// texture atlas.  We do things in this order to avoid having to copy all the
/// old vertices to the correct location.  However, when trying to use the same
/// texture atlas for more than one model, this approach runs into the
/// problem that enough model information needs to be remembered to be able to
/// generate the colors after the function returns, so we box up the actual
/// coloring part as a continuation.  When called with a final tile size and
/// vector, the continuation will consume the color data and write it to the
/// vector.
pub type SuspendedMesh<'a, A> = dyn for<'r> FnOnce(&'r mut A, Vec2<u16>) + 'a;

/// Abstraction over different atlas allocators. Useful to swap out the
/// allocator implementation for specific cases (e.g. sprites).
pub trait AtlasAllocator {
    type Config;

    /// Creates a new instance of this atlas allocator taking into account the
    /// provided max size;
    fn with_max_size(max_size: Vec2<u16>, config: Self::Config) -> Self;

    /// Allocates a rectangle of the given size.
    // TODO: don't use guillotiere type here
    fn allocate(&mut self, size: Vec2<u16>) -> Option<guillotiere::Rectangle>;

    /// Retrieves the current size of the atlas being allocated from.
    fn size(&self) -> Vec2<u16>;

    /// Grows the size of the atlas to the provided size.
    fn grow(&mut self, new_size: Vec2<u16>);
}

fn guillotiere_size<T: Into<i32>>(size: Vec2<T>) -> guillotiere::Size {
    guillotiere::Size::new(size.x.into(), size.y.into())
}

/// Currently used by terrain/particles/figures
pub fn general_config() -> guillotiere::AllocatorOptions {
    // TODO: Collect information to see if we can choose a good value here. These
    // current values were optimized for sprites, but we are using a
    // different allocator for them so different values might be better
    // here.
    let large_size_threshold = 8; //256.min(min_max_dim / 2 + 1);
    let small_size_threshold = 3; //33.min(large_size_threshold / 2 + 1);

    guillotiere::AllocatorOptions {
        alignment: guillotiere::Size::new(1, 1),
        small_size_threshold,
        large_size_threshold,
    }
}

pub fn sprite_config() -> guillotiere::AllocatorOptions {
    // TODO: Collect information to see if we can choose a better value here (these
    // values were picked before switching to this tiled implementation). I
    // suspect these are still near optimal though.
    let large_size_threshold = 8;
    let small_size_threshold = 3;

    guillotiere::AllocatorOptions {
        alignment: guillotiere::Size::new(1, 1),
        small_size_threshold,
        large_size_threshold,
    }
}

impl AtlasAllocator for guillotiere::SimpleAtlasAllocator {
    type Config = guillotiere::AllocatorOptions;

    fn with_max_size(max_size: Vec2<u16>, config: Self::Config) -> Self {
        let size = guillotiere_size(Vec2::new(32, 32)).min(guillotiere_size(max_size));
        guillotiere::SimpleAtlasAllocator::with_options(size, &config)
    }

    /// Allocates a rectangle of the given size.
    fn allocate(&mut self, size: Vec2<u16>) -> Option<guillotiere::Rectangle> {
        self.allocate(guillotiere_size(size))
    }

    /// Retrieves the current size of the atlas being allocated from.
    fn size(&self) -> Vec2<u16> {
        // NOTE: with_max_size / grow take a u16 so the size will never be larger than
        // u16::MAX
        Vec2::<i32>::from(self.size().to_array()).map(|e| e as u16)
    }

    /// Grows the size of the atlas to the provided size.
    fn grow(&mut self, new_size: Vec2<u16>) { self.grow(guillotiere_size(new_size)) }
}

pub struct GuillotiereTiled {
    options: guillotiere::AllocatorOptions,
    // Each tile is Self::TILE_SIZE (unless max size is not aligned to this, in which case the
    // tiles that reach the max size are truncated below this value).
    allocator: guillotiere::SimpleAtlasAllocator,
    // offset in tiles
    free_tiles: Vec<Vec2<usize>>,
    // Total width and height in tiles (in case this isn't a square).
    // Not zero
    size: Vec2<usize>,
    // Offset (in tiles) of current tile being allocated from (others returned `None` on last
    // allocation attempt)
    current: Option<Vec2<usize>>,
    // Efficiency history for filled tiles (total area, used area)
    //
    // This is useful to examine packing efficiency.
    history: Vec<(u32, u32)>,
    used_in_current_tile: u32,
}

impl GuillotiereTiled {
    // We can potentially further optimize packing by deferring the allocations
    // until all rectangles are available for packing. We could also cache this
    // for sprites if we get to the point of having the rest of start up times
    // fast enough for this to be helpful (e.g. for iterative work).
    //
    // Tested with sprites:
    // 64 1.63s 1.109 packing
    // 128 1.65s 1.083 packing
    // 256 1.77s 1.070 packing
    // 512 2.27s 1.055 packing
    // 1024 5.32s 1.045 packing
    // 2048 10.49s n/a packing (didn't fill up)
    const TILE_SIZE: u16 = 512;

    fn next_tile(&mut self) {
        if self.current.is_some() {
            prof_span!("stats");
            let size = self.allocator.size();
            // NOTE: TILE_SIZE is small enough that this won't overflow.
            let area = size.width as u32 * size.height as u32;
            let used = self.used_in_current_tile;
            self.history.push((area, used));
        }

        self.current = if let Some(offset) = self.free_tiles.pop() {
            self.allocator.reset(
                guillotiere_size(Vec2::broadcast(Self::TILE_SIZE)),
                &self.options,
            );
            self.used_in_current_tile = 0;
            Some(offset)
        } else {
            None
        };
    }
}

impl AtlasAllocator for GuillotiereTiled {
    type Config = guillotiere::AllocatorOptions;

    fn with_max_size(max_size: Vec2<u16>, config: Self::Config) -> Self {
        let size =
            guillotiere_size(Vec2::broadcast(Self::TILE_SIZE)).min(guillotiere_size(max_size));

        let allocator = guillotiere::SimpleAtlasAllocator::with_options(size, &config);

        Self {
            options: config,
            allocator,
            free_tiles: Vec::new(),
            size: Vec2::new(1, 1),
            current: Some(Vec2::new(0, 0)),
            history: Vec::new(),
            used_in_current_tile: 0,
        }
    }

    /// Allocates a rectangle of the given size.
    fn allocate(&mut self, size: Vec2<u16>) -> Option<guillotiere::Rectangle> {
        let size = guillotiere_size(size);

        while let Some(current) = self.current {
            match self.allocator.allocate(size) {
                Some(r) => {
                    // NOTE: The offset will always be smaller or equal to the `u16`s passed into
                    // `with_max_size`/`grow` so this won't overflow.
                    let offset = guillotiere_size(current.map(|e| e as u16 * Self::TILE_SIZE));

                    let offset_rect = guillotiere::Rectangle {
                        min: r.min.add_size(&offset),
                        max: r.max.add_size(&offset),
                    };
                    // NOTE: `i32` -> `u32` conversion is fine since these will always be positive.
                    self.used_in_current_tile += size.width as u32 * size.height as u32;

                    return Some(offset_rect);
                },
                None => self.next_tile(),
            }
        }

        None
    }

    /// Retrieves the current size of the atlas being allocated from.
    fn size(&self) -> Vec2<u16> {
        // NOTE: The size will always be smaller or equal to the `u16`s passed into
        // `with_max_size`/`grow` so this won't overflow.
        self.size.map(|e| e as u16 * Self::TILE_SIZE)
    }

    /// Grows the size of the atlas to the provided size.
    fn grow(&mut self, new_size: Vec2<u16>) {
        if tracing::enabled!(tracing::Level::TRACE) {
            tracing::trace!(
                "Tile count: {}",
                self.history.len() + self.free_tiles.len() + self.current.is_some() as usize
            );
            let mut total_area = 0;
            let mut total_used = 0;
            for (area, used) in self.history.iter() {
                total_area += area;
                total_used += used;
            }
            tracing::trace!("Packing ratio: {}", total_area as f32 / total_used as f32);
        }

        let diff = new_size.map2(self.size(), |n, s| n.saturating_sub(s));
        // NOTE: Growing only occurs in increments of TILE_SIZE so any remaining size is
        // ignored. Max size is not known here so this must truncate instead of rounding
        // up.
        let diff_tiles = diff.map(|e| usize::from(e) / usize::from(Self::TILE_SIZE));
        let old_size = self.size;
        self.size += diff_tiles;

        // Add new tiles to free tile list
        for x in old_size.x..self.size.x {
            for y in 0..old_size.y {
                self.free_tiles.push(Vec2::new(x, y));
            }
        }
        for y in old_size.y..self.size.y {
            for x in 0..self.size.x {
                self.free_tiles.push(Vec2::new(x, y));
            }
        }
        if self.current.is_none() {
            self.next_tile();
        }
    }
}

pub type SpriteAtlasAllocator = GuillotiereTiled;

/// Shared state for a greedy mesh, potentially passed along to multiple models.
///
/// For an explanation of why we want this, see `SuspendedMesh`.
pub struct GreedyMesh<
    'a,
    A: AtlasData,
    Allocator: AtlasAllocator = guillotiere::SimpleAtlasAllocator,
> {
    //atlas: guillotiere::SimpleAtlasAllocator,
    atlas: Allocator,
    tex_size: Vec2<u16>,
    max_size: Vec2<u16>,
    suspended: Vec<Box<SuspendedMesh<'a, A>>>,
}

impl<'a, A: AtlasData, Allocator: AtlasAllocator> GreedyMesh<'a, A, Allocator> {
    /// Construct a new greedy mesher.
    ///
    /// Takes as input the maximum allowable size of the texture atlas used to
    /// store the light/color data for this mesh.
    ///
    /// NOTE: It is an error to pass any size > u16::MAX (this is now enforced
    /// by the type being `u16`).
    ///
    /// Even aside from the above limitation, this will not necessarily always
    /// be the same as the maximum atlas size supported by the hardware.
    /// For instance, since we want to reserve 4 bits for a bone index for
    /// figures in their shadow vertex, the atlas parameter for figures has
    /// to have at least 2 bits of the normal; thus, it can only take up at
    /// most 30 bits total, meaning we are restricted to "only" at most 2^15
    /// × 2^15 atlases even if the hardware supports larger ones.
    pub fn new(max_size: Vec2<u16>, config: Allocator::Config) -> Self {
        span!(_guard, "new", "GreedyMesh::new");
        let min_max_dim = max_size.reduce_min();
        assert!(
            min_max_dim >= 4,
            "min_max_dim={:?} >= 4 ({:?}",
            min_max_dim,
            max_size
        );
        let atlas = Allocator::with_max_size(max_size, config);
        let tex_size = Vec2::new(1, 1);
        Self {
            atlas,
            tex_size,
            max_size,
            suspended: Vec::new(),
        }
    }

    /// Perform greedy meshing on a model, separately producing "pure" model
    /// data (the opaque mesh, together with atlas positions connecting
    /// each rectangle with texture information), and raw light and color
    /// data ready to be used as a texture (accessible with `finalize`).
    /// Texture data built up within the same greedy mesh will be inserted
    /// into the same atlas, which can be used to group texture data for
    /// things like figures that are the result of meshing multiple models.
    ///
    /// Returns an estimate of the bounds of the current meshed model.
    ///
    /// For more information on the config parameter, see [GreedyConfig].
    pub fn push<M: PartialEq, D: 'a, FA, FL, FG, FO, FS, FP, FT>(
        &mut self,
        config: GreedyConfig<D, FA, FL, FG, FO, FS, FP, FT>,
    ) where
        FA: for<'r> FnMut(&'r mut D, Vec3<i32>) -> f32 + 'a,
        FL: for<'r> FnMut(&'r mut D, Vec3<i32>) -> f32 + 'a,
        FG: for<'r> FnMut(&'r mut D, Vec3<i32>) -> f32 + 'a,
        FO: for<'r> FnMut(&'r mut D, Vec3<i32>) -> bool + 'a,
        FS: for<'r> FnMut(&'r mut D, Vec3<i32>, Vec3<i32>, Vec2<Vec3<i32>>) -> Option<(bool, M)>,
        FP: FnMut(Vec2<u16>, Vec2<Vec2<u16>>, Vec3<f32>, Vec2<Vec3<f32>>, Vec3<f32>, &M),
        FT: for<'r> FnMut(<A::SliceMut<'_> as Iterator>::Item, &'r mut D, Vec3<i32>, u8, u8, bool)
            + 'a,
    {
        span!(_guard, "push", "GreedyMesh::push");
        let cont = greedy_mesh(&mut self.atlas, &mut self.tex_size, self.max_size, config);
        self.suspended.push(cont);
    }

    /// Finalize the mesh, producing texture color data for the whole model.
    ///
    /// By delaying finalization until the contents of the whole texture atlas
    /// are known, we can perform just a single allocation to construct a
    /// precisely fitting atlas.  This will also let us (in the future)
    /// suspend meshing partway through in order to meet frame budget, and
    /// potentially use a single staged upload to the GPU.
    ///
    /// Returns the ColLightsInfo corresponding to the constructed atlas.
    pub fn finalize(self) -> (A, Vec2<u16>) {
        span!(_guard, "finalize", "GreedyMesh::finalize");
        let mut atlas_texture_data = A::blank_with_size(self.tex_size);
        self.suspended.into_iter().for_each(|cont| {
            cont(&mut atlas_texture_data, self.tex_size);
        });
        (atlas_texture_data, self.tex_size)
    }

    pub fn max_size(&self) -> Vec2<u16> { self.max_size }
}

fn greedy_mesh<
    'a,
    M: PartialEq,
    D: 'a,
    FA,
    FL,
    FG,
    FO,
    FS,
    FP,
    FT,
    A: AtlasData,
    Allocator: AtlasAllocator,
>(
    atlas: &mut Allocator,
    atlas_size: &mut Vec2<u16>,
    max_size: Vec2<u16>,
    GreedyConfig {
        mut data,
        draw_delta,
        greedy_size,
        greedy_size_cross,
        get_ao,
        get_light,
        get_glow,
        get_opacity,
        mut should_draw,
        mut push_quad,
        make_face_texel,
    }: GreedyConfig<D, FA, FL, FG, FO, FS, FP, FT>,
) -> Box<SuspendedMesh<'a, A>>
where
    FA: for<'r> FnMut(&'r mut D, Vec3<i32>) -> f32 + 'a,
    FL: for<'r> FnMut(&'r mut D, Vec3<i32>) -> f32 + 'a,
    FG: for<'r> FnMut(&'r mut D, Vec3<i32>) -> f32 + 'a,
    FO: for<'r> FnMut(&'r mut D, Vec3<i32>) -> bool + 'a,
    FS: for<'r> FnMut(&'r mut D, Vec3<i32>, Vec3<i32>, Vec2<Vec3<i32>>) -> Option<(bool, M)>,
    FP: FnMut(Vec2<u16>, Vec2<Vec2<u16>>, Vec3<f32>, Vec2<Vec3<f32>>, Vec3<f32>, &M),
    FT: for<'r> FnMut(<A::SliceMut<'_> as Iterator>::Item, &'r mut D, Vec3<i32>, u8, u8, bool) + 'a,
{
    span!(_guard, "greedy_mesh");
    // TODO: Collect information to see if we can choose a good value here.
    let mut todo_rects = Vec::with_capacity(1024);

    // x (u = y, v = z)
    greedy_mesh_cross_section(
        Vec3::new(greedy_size.y, greedy_size.z, greedy_size_cross.x),
        |pos| {
            should_draw(
                &mut data,
                draw_delta + Vec3::new(pos.z, pos.x, pos.y),
                Vec3::unit_x(),
                Vec2::new(Vec3::unit_y(), Vec3::unit_z()),
            )
        },
        |pos, dim, &(faces_forward, ref meta)| {
            let pos = Vec3::new(pos.z, pos.x, pos.y);
            let uv = Vec2::new(Vec3::unit_y(), Vec3::unit_z());
            let norm = Vec3::unit_x();
            let atlas_pos = add_to_atlas(
                atlas,
                &mut todo_rects,
                pos,
                uv,
                dim,
                norm,
                faces_forward,
                max_size,
                atlas_size,
            );
            create_quad_greedy(
                pos,
                dim,
                uv,
                norm,
                faces_forward,
                meta,
                atlas_pos,
                |atlas_pos, dim, pos, draw_dim, norm, meta| {
                    push_quad(atlas_pos, dim, pos, draw_dim, norm, meta)
                },
            );
        },
    );

    // y (u = z, v = x)
    greedy_mesh_cross_section(
        Vec3::new(greedy_size.z, greedy_size.x, greedy_size_cross.y),
        |pos| {
            should_draw(
                &mut data,
                draw_delta + Vec3::new(pos.y, pos.z, pos.x),
                Vec3::unit_y(),
                Vec2::new(Vec3::unit_z(), Vec3::unit_x()),
            )
        },
        |pos, dim, &(faces_forward, ref meta)| {
            let pos = Vec3::new(pos.y, pos.z, pos.x);
            let uv = Vec2::new(Vec3::unit_z(), Vec3::unit_x());
            let norm = Vec3::unit_y();
            let atlas_pos = add_to_atlas(
                atlas,
                &mut todo_rects,
                pos,
                uv,
                dim,
                norm,
                faces_forward,
                max_size,
                atlas_size,
            );
            create_quad_greedy(
                pos,
                dim,
                uv,
                norm,
                faces_forward,
                meta,
                atlas_pos,
                |atlas_pos, dim, pos, draw_dim, norm, meta| {
                    push_quad(atlas_pos, dim, pos, draw_dim, norm, meta)
                },
            );
        },
    );

    // z (u = x, v = y)
    greedy_mesh_cross_section(
        Vec3::new(greedy_size.x, greedy_size.y, greedy_size_cross.z),
        |pos| {
            should_draw(
                &mut data,
                draw_delta + Vec3::new(pos.x, pos.y, pos.z),
                Vec3::unit_z(),
                Vec2::new(Vec3::unit_x(), Vec3::unit_y()),
            )
        },
        |pos, dim, &(faces_forward, ref meta)| {
            let pos = Vec3::new(pos.x, pos.y, pos.z);
            let uv = Vec2::new(Vec3::unit_x(), Vec3::unit_y());
            let norm = Vec3::unit_z();
            let atlas_pos = add_to_atlas(
                atlas,
                &mut todo_rects,
                pos,
                uv,
                dim,
                norm,
                faces_forward,
                max_size,
                atlas_size,
            );
            create_quad_greedy(
                pos,
                dim,
                uv,
                norm,
                faces_forward,
                meta,
                atlas_pos,
                |atlas_pos, dim, pos, draw_dim, norm, meta| {
                    push_quad(atlas_pos, dim, pos, draw_dim, norm, meta)
                },
            );
        },
    );

    Box::new(move |atlas_texture_data, cur_size| {
        let mut data = data;
        draw_texels::<_, A>(
            atlas_texture_data,
            cur_size,
            &mut data,
            todo_rects,
            draw_delta,
            get_ao,
            get_light,
            get_glow,
            get_opacity,
            make_face_texel,
        );
    })
}

/// Greedy meshing a single cross-section.
// TODO: See if we can speed a lot of this up using SIMD.
fn greedy_mesh_cross_section<M: PartialEq>(
    dims: Vec3<usize>,
    // Should we draw a face here (below this vertex)?  If so, provide its meta information.
    mut draw_face: impl FnMut(Vec3<i32>) -> Option<M>,
    // Vertex, width and height, and meta information about the block.
    mut push_quads: impl FnMut(Vec3<usize>, Vec2<usize>, &M),
) {
    span!(_guard, "greedy_mesh_cross_section");
    // mask represents which faces are either set while the other is unset, or unset
    // while the other is set.
    let mut mask = (0..dims.y * dims.x).map(|_| None).collect::<Vec<_>>();
    (0..dims.z + 1).for_each(|d| {
        // Compute mask
        mask.iter_mut().enumerate().for_each(|(posi, mask)| {
            let i = posi % dims.x;
            let j = posi / dims.x;
            // NOTE: Safe because dims.z actually fits in a u16.
            *mask = draw_face(Vec3::new(i as i32, j as i32, d as i32));
        });

        (0..dims.y).for_each(|j| {
            let mut i = 0;
            while i < dims.x {
                // Compute width (number of set x bits for this row and layer, starting at the
                // current minimum column).
                if let Some(ori) = &mask[j * dims.x + i] {
                    let width = 1 + mask[j * dims.x + i + 1..j * dims.x + dims.x]
                        .iter()
                        .take_while(move |&mask| mask.as_ref() == Some(ori))
                        .count();
                    let max_x = i + width;
                    // Compute height (number of rows having w set x bits for this layer, starting
                    // at the current minimum column and row).
                    let height = 1
                        + (j + 1..dims.y)
                            .take_while(|h| {
                                mask[h * dims.x + i..h * dims.x + max_x]
                                    .iter()
                                    .all(|mask| mask.as_ref() == Some(ori))
                            })
                            .count();
                    let max_y = j + height;
                    // Add quad.
                    push_quads(Vec3::new(i, j, d), Vec2::new(width, height), ori);
                    // Unset mask bits in drawn region, so we don't try to re-draw them.
                    (j..max_y).for_each(|l| {
                        mask[l * dims.x + i..l * dims.x + max_x]
                            .iter_mut()
                            .for_each(|mask| {
                                *mask = None;
                            });
                    });
                    // Update x value.
                    i = max_x;
                } else {
                    i += 1;
                }
            }
        });
    });
}

fn add_to_atlas<Allocator: AtlasAllocator>(
    atlas: &mut Allocator,
    todo_rects: &mut Vec<TodoRect>,
    pos: Vec3<usize>,
    uv: Vec2<Vec3<u16>>,
    dim: Vec2<usize>,
    norm: Vec3<i16>,
    faces_forward: bool,
    max_size: Vec2<u16>,
    cur_size: &mut Vec2<u16>,
) -> guillotiere::Rectangle {
    // TODO: Check this conversion.
    let atlas_rect = loop {
        // NOTE: Conversion to u16 is safe because he x, y, and z dimensions for any
        // chunk index must fit in at least an i16 (lower for x and y, probably
        // lower for z) and at least x and y are not negative.
        let res = atlas.allocate(Vec2::new(dim.x as u16 + 1, dim.y as u16 + 1));
        if let Some(atlas_rect) = res {
            break atlas_rect;
        }
        // Allocation failure.
        let current_size = atlas.size();
        if current_size == max_size {
            // NOTE: Currently, if we fail to allocate a terrain chunk in the atlas and we
            // have already reached the maximum texture size, we choose to just skip the
            // geometry and log a warning, rather than panicking or trying to use a fallback
            // technique (e.g. a texture array).
            //
            // FIXME: Either make more robust, or explicitly document that limits on texture
            // size need to be respected for terrain data (the OpenGL minimum requirement is
            // 1024 × 1024, but in practice almost all computers support 4096 × 4096 or
            // higher; see
            // https://feedback.wildfiregames.com/report/opengl/feature/GL_MAX_TEXTURE_SIZE).
            panic!(
                "Could not add texture to atlas using simple allocator (pos={:?}, dim={:?});we \
                 could not fit the whole model into a single texture on this machine
                        (max texture size={:?}, so we are discarding this rectangle.",
                pos, dim, max_size
            );
        }
        // Otherwise, we haven't reached max size yet, so double the size (or reach the
        // max texture size) and try again.
        let new_size = max_size.map2(current_size, |max, current| {
            max.min(current.saturating_mul(2))
        });
        atlas.grow(new_size);
    };
    // NOTE: Conversion is correct because our initial max size for the atlas was a
    // u16 and we never grew the atlas past the max size, meaning all valid
    // coordinates within the atlas also fit into a u16.
    *cur_size = Vec2::new(
        cur_size.x.max(atlas_rect.max.x as u16),
        cur_size.y.max(atlas_rect.max.y as u16),
    );

    // NOTE: pos can be converted safely from usize to i32 because all legal block
    // coordinates in this chunk must fit in an i32 (actually we have the much
    // stronger property that this holds across the whole map).
    let norm = norm.map(i32::from);
    todo_rects.push((
        pos.map(|e| e as i32) + if faces_forward { -norm } else { Vec3::zero() },
        uv,
        atlas_rect,
        if faces_forward { norm } else { -norm },
    ));
    atlas_rect
}

/// We deferred actually recording the colors within the rectangles in order to
/// generate a texture of minimal size; we now proceed to create and populate
/// it.
// TODO: Consider using the heavier interface (not the simple one) which seems
// to provide builtin support for what we're doing here.
//
// TODO: See if we can speed this up using SIMD.
fn draw_texels<D, A: AtlasData>(
    atlas_texture_data: &mut A,
    cur_size: Vec2<u16>,
    data: &mut D,
    todo_rects: Vec<TodoRect>,
    draw_delta: Vec3<i32>,
    mut get_ao: impl FnMut(&mut D, Vec3<i32>) -> f32,
    mut get_light: impl FnMut(&mut D, Vec3<i32>) -> f32,
    mut get_glow: impl FnMut(&mut D, Vec3<i32>) -> f32,
    mut get_opacity: impl FnMut(&mut D, Vec3<i32>) -> bool,
    mut make_face_texel: impl FnMut(
        <A::SliceMut<'_> as Iterator>::Item,
        &mut D,
        Vec3<i32>,
        u8,
        u8,
        bool,
    ),
) {
    todo_rects.into_iter().for_each(|(pos, uv, rect, delta)| {
        // NOTE: Conversions are safe because width, height, and offset must be
        // non-negative, and because every allocated coordinate in the atlas must be in
        // bounds for the original size, max_texture_size, which fit into a u16.
        let width = (rect.max.x - rect.min.x) as u16;
        let height = (rect.max.y - rect.min.y) as u16;
        let left = rect.min.x as u16;
        let top = rect.min.y as u16;
        let uv = uv.map(|e| e.map(i32::from));
        let pos = pos + draw_delta;
        (0..height).for_each(|v| {
            let start = cur_size.x as usize * usize::from(top + v) + usize::from(left);
            (0..width)
                .zip(atlas_texture_data.slice_mut(start..start + usize::from(width)))
                .for_each(|(u, texel)| {
                    let pos = pos + uv.x * i32::from(u) + uv.y * i32::from(v);
                    // TODO: Consider optimizing to take advantage of the fact that this whole
                    // face should be facing nothing but air (this is not currently true, but
                    // could be if we used the right AO strategy).
                    // Each indirect light needs to come in through the direct light.
                    // Thus, we assign each light a score based on opacity (currently just 0 or
                    // 1, but it could support translucent lights in the future).
                    // Thus, indirect_u_opacity and indirect_v_opacity are multiplied by
                    // direct_opacity, and indirect_uv_opacity is multiplied by
                    // the maximum of both of u and v's indirect opacities (since there are
                    // two choices for how to get to the direct surface).
                    let pos = pos
                        + if u + 1 == width { -uv.x } else { Vec3::zero() }
                        + if v + 1 == height { -uv.y } else { Vec3::zero() };
                    let uv = Vec2::new(
                        if u + 1 == width { -uv.x } else { uv.x },
                        if v + 1 == height { -uv.y } else { uv.y },
                    );

                    let light_pos = pos + delta;

                    // Currently, we assume that direct_opacity is 1 (if it's 0, you can't see
                    // the face anyway, since it's blocked by the block directly in front of it).
                    // TODO: If we add non-0/1 opacities, fix this.
                    // bottom-left block
                    let direct_u_opacity = get_opacity(data, light_pos - uv.x);
                    // top-right block
                    let direct_v_opacity = get_opacity(data, light_pos - uv.y);

                    // NOTE: Since we only support 0/1 opacities currently, we assume
                    // direct_opacity is  1, and the light value will be zero anyway for objects
                    // with opacity 0, we only "multiply" by indirect_uv_opacity for now (since
                    // it's the only one that could be 0 even if its light value is not).
                    // However, "spiritually" these light values are all being multiplied by
                    // their opacities.
                    let darkness = (
                        // Light from the bottom-right-front block to this vertex always
                        // appears on this face, since it's the block this face is facing (so
                        // it can't be blocked by anything).
                        get_light(data, light_pos)
                            + get_light(data, light_pos - uv.x)
                            + get_light(data, light_pos - uv.y)
                            + if direct_u_opacity || direct_v_opacity {
                                get_light(data, light_pos - uv.x - uv.y)
                            } else {
                                0.0
                            }
                    ) / 4.0;
                    let ao = (get_ao(data, light_pos)
                        + get_ao(data, light_pos - uv.x)
                        + get_ao(data, light_pos - uv.y)
                        + if direct_u_opacity || direct_v_opacity {
                            get_ao(data, light_pos - uv.x - uv.y)
                        } else {
                            0.0
                        })
                        / 4.0;
                    let glowiness = (get_glow(data, light_pos)
                        + get_glow(data, light_pos - uv.x)
                        + get_glow(data, light_pos - uv.y)
                        + if direct_u_opacity || direct_v_opacity {
                            get_glow(data, light_pos - uv.x - uv.y)
                        } else {
                            0.0
                        })
                        / 4.0;
                    let light = (darkness * 31.5) as u8;
                    let glow = (glowiness * 31.5) as u8;
                    let ao = ao > 0.7;
                    make_face_texel(texel, data, pos, light, glow, ao);
                });
        });
    });
}

/// Precondition: when this function is called, atlas_pos should reflect an
/// actual valid position in a texture atlas (meaning it should fit into a u16).
// TODO: See if we can speed a lot of this up using SIMD.
fn create_quad_greedy<M>(
    origin: Vec3<usize>,
    dim: Vec2<usize>,
    uv: Vec2<Vec3<u16>>,
    norm: Vec3<i16>,
    faces_forward: bool,
    meta: &M,
    atlas_pos: guillotiere::Rectangle,
    mut push_quad: impl FnMut(Vec2<u16>, Vec2<Vec2<u16>>, Vec3<f32>, Vec2<Vec3<f32>>, Vec3<f32>, &M),
) {
    let origin = origin.map(|e| e as f32);
    // NOTE: Conversion to f32 safe by function precondition (u16 can losslessly
    // cast to f32, and dim fits in a u16).
    let draw_dim = uv.map2(dim.map(|e| e as f32), |e, f| e.map(f32::from) * f);
    let dim = Vec2::new(Vec2::new(dim.x as u16, 0), Vec2::new(0, dim.y as u16));
    let (draw_dim, dim, /* uv, */ norm) = if faces_forward {
        (draw_dim, dim, norm)
    } else {
        (
            Vec2::new(draw_dim.y, draw_dim.x),
            Vec2::new(dim.y, dim.x),
            -norm,
        )
    };
    let norm = norm.map(f32::from);
    // NOTE: Conversion to u16 safe by function precondition.
    let atlas_pos = Vec2::new(atlas_pos.min.x as u16, atlas_pos.min.y as u16);
    push_quad(atlas_pos, dim, origin, draw_dim, norm, meta);
}

pub fn create_quad<O: Vertex, M>(
    atlas_pos: Vec2<u16>,
    dim: Vec2<Vec2<u16>>,
    origin: Vec3<f32>,
    draw_dim: Vec2<Vec3<f32>>,
    norm: Vec3<f32>,
    meta: &M,
    mut create_vertex: impl FnMut(Vec2<u16>, Vec3<f32>, Vec3<f32>, &M) -> O,
) -> Quad<O> {
    Quad::new(
        create_vertex(atlas_pos, origin, norm, meta),
        create_vertex(atlas_pos + dim.x, origin + draw_dim.x, norm, meta),
        create_vertex(
            atlas_pos + dim.x + dim.y,
            origin + draw_dim.x + draw_dim.y,
            norm,
            meta,
        ),
        create_vertex(atlas_pos + dim.y, origin + draw_dim.y, norm, meta),
    )
}