18243240671

Committed 04 Oct 2025 10:36AM UTC coverage: 66.58% (+0.1%) from 66.455%

Build # 18243240671

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push

github

Committed by

web-flow

Commit Message

Split extraction and render into unit systems (#499)

Reorganize most of the extraction and render systems into smaller,
unit-like systems with limited (ideally, a single) responsibility. Split
most of the data into separate, smaller components too. This not only
enable better multithreading, but also greatly simplify maintenance by
clarifying the logic and responsibility of each system and component.

As part of this change, add a "ready state" to the effect, which is read
back from the render world and informs the main world about whether an
effect is ready for simulation and rendering. This includes:

- All GPU resources being allocated, and in particular the PSOs
  (pipelines) which in Bevy are compiled asynchronously and can be very
  slow (many frames of delay).
- The ready state of all descendant effects, recursively. This ensures a
  child is ready to _e.g._ receive GPU spawn events before its parent,
  which emits those events, starts simulating.

This new ready state is accessed via
`CompiledParticleEffect::is_ready()`. Note that the state is updated
during the extract phase with the information collected from the
previous render frame, so by the time `is_ready()` returns `true`,
already one frame of simulation and rendering generally occurred.

Remove the outdated `copyless` dependency.

Run Details

594 of 896 new or added lines in 12 files covered. (66.29%)

21 existing lines in 3 files now uncovered.

5116 of 7684 relevant lines covered (66.58%)

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Source File
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64.07

/src/render/buffer_table.rs

use std::{
    borrow::Cow,
    num::{NonZeroU32, NonZeroU64},
    ops::Range,
};

use bevy::{
    log::trace,
    render::{
        render_resource::{
            Buffer, BufferAddress, BufferDescriptor, BufferUsages, CommandEncoder, ShaderSize,
            ShaderType,
        },
        renderer::{RenderDevice, RenderQueue},
    },
};
use bytemuck::{cast_slice, Pod};

/// Round a range start down to a given alignment, and return the new range and
/// the start offset inside the new range of the old range.
fn round_range_start_down(range: Range<u64>, align: u64) -> (Range<u64>, u64) {
    assert!(align > 0);
    let delta = align - 1;
    if range.start >= delta {
        // Snap range start to previous multiple of align
        let old_start = range.start;
        let new_start = (range.start - delta).next_multiple_of(align);
        let offset = old_start - new_start;
        (new_start..range.end, offset)
    } else {
        // Snap range start to 0
        (0..range.end, range.start)
    }
}

/// Index of a row in a [`BufferTable`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct BufferTableId(pub(crate) u32); // TEMP: pub(crate)

impl BufferTableId {
    /// An invalid value, often used as placeholder.
    pub const INVALID: BufferTableId = BufferTableId(u32::MAX);

    /// Check if the current ID is valid, that is, is different from
    /// [`INVALID`].
    ///
    /// [`INVALID`]: Self::INVALID
    #[inline]
    pub fn is_valid(&self) -> bool {
        *self != Self::INVALID
    }

    /// Compute a new buffer table ID by offseting an existing one by `count`
    /// rows.
    #[inline]
    #[allow(dead_code)]
    pub fn offset(&self, count: u32) -> BufferTableId {
        debug_assert!(self.is_valid());
        BufferTableId(self.0 + count)
    }
}

impl Default for BufferTableId {
    fn default() -> Self {
        Self::INVALID
    }
}

#[derive(Debug)]
struct AllocatedBuffer {
    /// Currently allocated buffer, of size equal to `size`.
    buffer: Buffer,
    /// Size of the currently allocated buffer, in number of rows.
    count: u32,
    /// Previously allocated buffer if any, cached until the next buffer write
    /// so that old data can be copied into the newly-allocated buffer.
    old_buffer: Option<Buffer>,
    /// Size of the old buffer if any, in number of rows.
    old_count: u32,
}

impl AllocatedBuffer {
    /// Get the number of rows of the currently allocated GPU buffer.
    ///
    /// On capacity grow, the count is valid until the next buffer swap.
    pub fn allocated_count(&self) -> u32 {
        if self.old_buffer.is_some() {
            self.old_count
        } else {
            self.count
        }
    }
}

/// GPU buffer holding a table with concurrent interleaved CPU/GPU access.
///
/// The buffer table data structure represents a GPU buffer holding a table made
/// of individual rows. Each row of the table has the same layout (same size),
/// and can be allocated (assigned to an existing index) or free (available for
/// future allocation). The data structure manages a free list of rows, and copy
/// of rows modified on CPU to the GPU without touching other rows. This ensures
/// that existing rows in the GPU buffer can be accessed and modified by the GPU
/// without being overwritten by the CPU and without the need for the CPU to
/// read the data back from GPU into CPU memory.
///
/// The element type `T` needs to implement the following traits:
/// - [`Pod`] to allow copy.
/// - [`ShaderType`] because it needs to be mapped for a shader.
/// - [`ShaderSize`] to ensure a fixed footprint, to allow packing multiple
///   instances inside a single buffer. This therefore excludes any
///   runtime-sized array.
///
/// This is similar to a [`BufferVec`] or [`AlignedBufferVec`], but unlike those
/// data structures a buffer table preserves rows modified by the GPU without
/// overwriting. This is useful when the buffer is also modified by GPU shaders,
/// so neither the CPU side nor the GPU side have an up-to-date view of the
/// entire table, and so the CPU cannot re-upload the entire table on changes.
///
/// # Usage
///
/// - During the [`RenderStage::Prepare`] stage, call
///   [`clear_previous_frame_resizes()`] to clear any stale buffer from the
///   previous frame. Then insert new rows with [`insert()`] and if you made
///   changes call [`allocate_gpu()`] at the end to allocate any new buffer
///   needed.
/// - During the [`RenderStage::Render`] stage, call [`write_buffer()`] from a
///   command encoder before using any row, to perform any buffer resize copy
///   pending.
///
/// [`BufferVec`]: bevy::render::render_resource::BufferVec
/// [`AlignedBufferVec`]: crate::render::aligned_buffer_vec::AlignedBufferVec
#[derive(Debug)]
pub struct BufferTable<T: Pod + ShaderSize> {
    /// GPU buffer if already allocated, or `None` otherwise.
    buffer: Option<AllocatedBuffer>,
    /// GPU buffer usages.
    buffer_usage: BufferUsages,
    /// Optional GPU buffer name, for debugging.
    label: Option<String>,
    /// Size of a single buffer element, in bytes, in CPU memory (Rust layout).
    item_size: usize,
    /// Size of a single buffer element, in bytes, aligned to GPU memory
    /// constraints.
    aligned_size: usize,
    /// Capacity of the buffer, in number of rows.
    ///
    /// This is the expected capacity, as requested by CPU side allocations and
    /// deallocations. The GPU buffer might not have been resized yet to handle
    /// it, so might be allocated with a different size.
    capacity: u32,
    /// Size of the "active" portion of the table, which includes allocated rows
    /// and any row in the free list. All other rows in the
    /// `active_size..capacity` range are implicitly unallocated.
    active_count: u32,
    /// Free list of rows available in the GPU buffer for a new allocation. This
    /// only contains indices in the `0..active_size` range; all row indices in
    /// `active_size..capacity` are assumed to be unallocated.
    free_indices: Vec<u32>,
    /// Pending values accumulated on CPU and not yet written to GPU, and their
    /// rows.
    pending_values: Vec<(u32, T)>,
    /// Extra pending values accumulated on CPU like `pending_values`, but for
    /// which there's not enough space in the current GPU buffer. Those values
    /// are sorted in index order, occupying the range `buffer.size..`.
    extra_pending_values: Vec<T>,
}

impl<T: Pod + ShaderSize> Default for BufferTable<T> {
    fn default() -> Self {
        let item_size = std::mem::size_of::<T>();
        let aligned_size = <T as ShaderSize>::SHADER_SIZE.get() as usize;
        assert!(aligned_size >= item_size);
        Self {
            buffer: None,
            buffer_usage: BufferUsages::all(),
            label: None,
            item_size,
            aligned_size,
            capacity: 0,
            active_count: 0,
            free_indices: Vec::new(),
            pending_values: Vec::new(),
            extra_pending_values: Vec::new(),
        }
    }
}

impl<T: Pod + ShaderSize> BufferTable<T> {
    /// Create a new collection.
    ///
    /// `item_align` is an optional additional alignment for items in the
    /// collection. If greater than the natural alignment dictated by WGSL
    /// rules, this extra alignment is enforced. Otherwise it's ignored (so you
    /// can pass `None` to ignore). This is useful if for example you want to
    /// bind individual rows or any subset of the table, to ensure each row is
    /// aligned to the device constraints.
    ///
    /// # Panics
    ///
    /// Panics if `buffer_usage` contains [`BufferUsages::UNIFORM`] and the
    /// layout of the element type `T` does not meet the requirements of the
    /// uniform address space, as tested by
    /// [`ShaderType::assert_uniform_compat()`].
    ///
    /// [`BufferUsages::UNIFORM`]: bevy::render::render_resource::BufferUsages::UNIFORM
    pub fn new(
        buffer_usage: BufferUsages,
        item_align: Option<NonZeroU64>,
        label: Option<String>,
    ) -> Self {
        // GPU-aligned item size, compatible with WGSL rules
        let item_size = <T as ShaderSize>::SHADER_SIZE.get() as usize;
        // Extra manual alignment for device constraints
        let aligned_size = if let Some(item_align) = item_align {
            let item_align = item_align.get() as usize;
            let aligned_size = item_size.next_multiple_of(item_align);
            assert!(aligned_size >= item_size);
            assert!(aligned_size % item_align == 0);
            aligned_size
        } else {
            item_size
        };
        trace!(
            "BufferTable[\"{}\"]: item_size={} aligned_size={}",
            label.as_ref().unwrap_or(&String::new()),
            item_size,
            aligned_size
        );
        if buffer_usage.contains(BufferUsages::UNIFORM) {
            <T as ShaderType>::assert_uniform_compat();
        }
        Self {
            // Need COPY_SRC and COPY_DST to copy from old to new buffer on resize
            buffer_usage: buffer_usage | BufferUsages::COPY_SRC | BufferUsages::COPY_DST,
            aligned_size,
            label,
            ..Default::default()
        }
    }

    /// Get a safe buffer label for debug display.
    ///
    /// Falls back to an empty string if no label was specified.
    pub fn safe_label(&self) -> Cow<'_, str> {
        self.label
            .as_ref()
            .map(|s| Cow::Borrowed(&s[..]))
            .unwrap_or(Cow::Borrowed(""))
    }

    /// Get a safe buffer name for debug display.
    ///
    /// Same as [`safe_label()`] but includes the buffer ID as well.
    ///
    /// [`safe_label()`]: self::BufferTable::safe_label
    pub fn safe_name(&self) -> String {
        let id = self
            .buffer
            .as_ref()
            .map(|ab| {
                let id: NonZeroU32 = ab.buffer.id().into();
                id.get()
            })
            .unwrap_or(0);
        format!("#{}:{}", id, self.safe_label())
    }

    /// Reference to the GPU buffer, if already allocated.
    ///
    /// This reference corresponds to the currently allocated GPU buffer, which
    /// may not contain all data since the last [`insert()`] call, and could
    /// become invalid if a new larger buffer needs to be allocated to store the
    /// pending values inserted with [`insert()`].
    ///
    /// [`insert()]`: BufferTable::insert
    #[inline]
    pub fn buffer(&self) -> Option<&Buffer> {
        self.buffer.as_ref().map(|ab| &ab.buffer)
    }

    /// Maximum number of rows the table can hold without reallocation.
    ///
    /// This is the maximum number of rows that can be added to the table
    /// without forcing a new GPU buffer to be allocated and a copy from the old
    /// to the new buffer.
    ///
    /// Note that this doesn't imply that no GPU buffer allocation will ever
    /// occur; if a GPU buffer was never allocated, and there are pending
    /// CPU rows to insert, then a new buffer will be allocated on next
    /// update with this capacity.
    #[inline]
    #[allow(dead_code)]
    pub fn capacity(&self) -> u32 {
        self.capacity
    }

    /// Current number of rows in use in the table.
    ///
    /// Note that rows in use are not necessarily contiguous. There may be gaps
    /// between used rows.
    #[inline]
    #[allow(dead_code)]
    pub fn len(&self) -> u32 {
        self.active_count - self.free_indices.len() as u32
    }

    /// Size of a single row in the table, in bytes, aligned to GPU constraints.
    #[inline]
    #[allow(dead_code)]
    pub fn aligned_size(&self) -> usize {
        self.aligned_size
    }

    /// Is the table empty?
    #[inline]
    #[allow(dead_code)]
    pub fn is_empty(&self) -> bool {
        self.active_count == 0
    }

    /// Clear all rows of the table without deallocating any existing GPU
    /// buffer.
    ///
    /// This operation only updates the CPU cache of the table, without touching
    /// any GPU buffer. On next GPU buffer update, the GPU buffer will be
    /// deallocated.
    #[allow(dead_code)]
    pub fn clear(&mut self) {
        self.pending_values.clear();
        self.extra_pending_values.clear();
        self.free_indices.clear();
        self.active_count = 0;
    }

    /// Clear any stale buffer used for resize in the previous frame during
    /// rendering while the data structure was immutable.
    ///
    /// This must be called before any new [`insert()`].
    ///
    /// [`insert()`]: crate::BufferTable::insert
    pub fn clear_previous_frame_resizes(&mut self) {
        if let Some(ab) = self.buffer.as_mut() {
            ab.old_buffer = None;
            ab.old_count = 0;
        }
    }

    /// Calculate the size in byte of `count` rows.
    #[inline]
    fn to_byte_size(&self, count: u32) -> usize {
        count as usize * self.aligned_size
    }

    /// Insert a new row into the table.
    ///
    /// For performance reasons, this buffers the row content on the CPU until
    /// the next GPU update, to minimize the number of CPU to GPU transfers.
    pub fn insert(&mut self, value: T) -> BufferTableId {
        trace!(
            "Inserting into table buffer '{}' with {} free indices, capacity: {}, active_size: {}",
            self.safe_name(),
            self.free_indices.len(),
            self.capacity,
            self.active_count
        );
        let index = if self.free_indices.is_empty() {
            let index = self.active_count;
            if index == self.capacity {
                self.capacity += 1;
            }
            debug_assert!(index < self.capacity);
            self.active_count += 1;
            index
        } else {
            self.free_indices.pop().unwrap()
        };
        let allocated_count = self
            .buffer
            .as_ref()
            .map(|ab| ab.allocated_count())
            .unwrap_or(0);
        trace!(
            "Found free index {}, capacity: {}, active_count: {}, allocated_count: {}",
            index,
            self.capacity,
            self.active_count,
            allocated_count
        );
        if index < allocated_count {
            self.pending_values.push((index, value));
        } else {
            let extra_index = index - allocated_count;
            if extra_index < self.extra_pending_values.len() as u32 {
                self.extra_pending_values[extra_index as usize] = value;
            } else {
                self.extra_pending_values.push(value);
            }
        }
        BufferTableId(index)
    }

    /// Calculate a dynamic byte offset for a bind group from a table entry.
    ///
    /// This returns the product of `id` by the internal [`aligned_size()`].
    ///
    /// # Panic
    ///
    /// Panics if the `index` is too large, producing a byte offset larger than
    /// `u32::MAX`.
    ///
    /// [`aligned_size()`]: Self::aligned_size
    #[inline]
    pub fn dynamic_offset(&self, id: BufferTableId) -> u32 {
        let offset = self.aligned_size * id.0 as usize;
        assert!(offset <= u32::MAX as usize);
        u32::try_from(offset).expect("BufferTable index out of bounds")
    }

    /// Update an existing row in the table.
    ///
    /// For performance reasons, this buffers the row content on the CPU until
    /// the next GPU update, to minimize the number of CPU to GPU transfers.
    ///
    /// Calling this function multiple times overwrites the previous value. Only
    /// the last value recorded each frame will be uploaded to GPU.
    ///
    /// # Panics
    ///
    /// Panics if the `id` is invalid.
    pub fn update(&mut self, id: BufferTableId, value: T) {
        assert!(id.is_valid());
        trace!(
            "Updating row {} of table buffer '{}'",
            id.0,
            self.safe_name(),
        );
        let allocated_count = self
            .buffer
            .as_ref()
            .map(|ab| ab.allocated_count())
            .unwrap_or(0);
        if id.0 < allocated_count {
            if let Some(idx) = self
                .pending_values
                .iter()
                .position(|&(index, _)| index == id.0)
            {
                // Overwrite a previous update. This ensures we never upload more than one
                // update per row, which would waste GPU bandwidth.
                self.pending_values[idx] = (id.0, value);
            } else {
                self.pending_values.push((id.0, value));
            }
        } else {
            let extra_index = (id.0 - allocated_count) as usize;
            assert!(extra_index < self.extra_pending_values.len());
            // Overwrite a previous update. This ensures we never upload more than one
            // update per row, which would waste GPU bandwidth.
            self.extra_pending_values[extra_index] = value;
        }
    }

    /// Insert several new contiguous rows into the table.
    ///
    /// For performance reasons, this buffers the row content on the CPU until
    /// the next GPU update, to minimize the number of CPU to GPU transfers.
    ///
    /// # Returns
    ///
    /// Returns the index of the first entry. Other entries follow right after
    /// it.
    #[allow(dead_code)] // unused but annoying to write, so keep if we need in the future
    pub fn insert_contiguous(&mut self, values: impl ExactSizeIterator<Item = T>) -> BufferTableId {
        let count = values.len() as u32;
        trace!(
            "Inserting {} contiguous values into table buffer '{}' with {} free indices, capacity: {}, active_size: {}",
            count,
            self.safe_name(),
            self.free_indices.len(),
            self.capacity,
            self.active_count
        );
        let first_index = if self.free_indices.is_empty() {
            let index = self.active_count;
            if index == self.capacity {
                self.capacity += count;
            }
            debug_assert!(index < self.capacity);
            self.active_count += count;
            index
        } else {
            let mut s = 0;
            let mut n = 1;
            let mut i = 1;
            while i < self.free_indices.len() {
                debug_assert!(self.free_indices[i] > self.free_indices[i - 1]); // always sorted
                if self.free_indices[i] == self.free_indices[i - 1] + 1 {
                    // contiguous
                    n += 1;
                    if n == count {
                        break;
                    }
                } else {
                    // non-contiguous; restart a new sequence
                    debug_assert!(n < count);
                    s = i;
                }
                i += 1;
            }
            if n == count {
                // Found a range of 'count' consecutive entries. Consume it.
                let index = self.free_indices[s];
                self.free_indices.splice(s..=i, []);
                index
            } else {
                // No free range for 'count' consecutive entries. Allocate at end instead.
                let index = self.active_count;
                if index == self.capacity {
                    self.capacity += count;
                }
                debug_assert!(index < self.capacity);
                self.active_count += count;
                index
            }
        };
        let allocated_count = self
            .buffer
            .as_ref()
            .map(|ab| ab.allocated_count())
            .unwrap_or(0);
        trace!(
            "Found {} free indices {}..{}, capacity: {}, active_count: {}, allocated_count: {}",
            count,
            first_index,
            first_index + count,
            self.capacity,
            self.active_count,
            allocated_count
        );
        for (i, value) in values.enumerate() {
            let index = first_index + i as u32;
            if index < allocated_count {
                self.pending_values.push((index, value));
            } else {
                let extra_index = index - allocated_count;
                if extra_index < self.extra_pending_values.len() as u32 {
                    self.extra_pending_values[extra_index as usize] = value;
                } else {
                    self.extra_pending_values.push(value);
                }
            }
        }
        BufferTableId(first_index)
    }

    /// Remove a row from the table.
    #[allow(dead_code)]
    pub fn remove(&mut self, id: BufferTableId) {
        let index = id.0;
        assert!(index < self.active_count);

        // If this is the last item in the active zone, just shrink the active zone
        // (implicit free list).
        if index == self.active_count - 1 {
            self.active_count -= 1;
            self.capacity -= 1;
        } else {
            // This is very inefficient but we need to apply the same logic as the
            // EffectCache because we rely on indices being in sync.
            let pos = self
                .free_indices
                .binary_search(&index) // will fail
                .unwrap_or_else(|e| e); // will get position of insertion
            self.free_indices.insert(pos, index);
        }
    }

    /// Remove a range of rows from the table.
    #[allow(dead_code)]
    pub fn remove_range(&mut self, first: BufferTableId, count: u32) {
        let index = first.0;
        assert!(index + count <= self.active_count);

        // If this is the last item in the active zone, just shrink the active zone
        // (implicit free list).
        if index == self.active_count - count {
            self.active_count -= count;
            self.capacity -= count;

            // Also try to remove free indices
            if self.free_indices.len() as u32 == self.active_count {
                // Easy case: everything is free, clear everything
                self.free_indices.clear();
                self.active_count = 0;
                self.capacity = 0;
            } else {
                // Some rows are still allocated. Dequeue from end while we have a contiguous
                // tail of free indices.
                let mut num_popped = 0;
                while let Some(idx) = self.free_indices.pop() {
                    if idx < self.active_count - 1 - num_popped {
                        self.free_indices.push(idx);
                        break;
                    }
                    num_popped += 1;
                }
                self.active_count -= num_popped;
                self.capacity -= num_popped;
            }
        } else {
            // This is very inefficient but we need to apply the same logic as the
            // EffectCache because we rely on indices being in sync.
            let pos = self
                .free_indices
                .binary_search(&index) // will fail
                .unwrap_or_else(|e| e); // will get position of insertion
            self.free_indices.splice(pos..pos, index..(index + count));
        }

        debug_assert!(
            (self.free_indices.is_empty() && self.active_count == 0)
                || (self.free_indices.len() as u32) < self.active_count
        );
    }

    /// Allocate any GPU buffer if needed, based on the most recent capacity
    /// requested.
    ///
    /// This should be called only once per frame after all allocation requests
    /// have been made via [`insert()`] but before the GPU buffer is actually
    /// updated. This is an optimization to enable allocating the GPU buffer
    /// earlier than it's actually needed. Calling this multiple times is not
    /// supported, and might assert. Not calling it is safe, as the next
    /// update will call it just-in-time anyway.
    ///
    /// # Returns
    ///
    /// Returns `true` if a new buffer was (re-)allocated, to indicate any bind
    /// group needs to be re-created.
    ///
    /// [`insert()]`: crate::render::BufferTable::insert
    pub fn allocate_gpu(&mut self, device: &RenderDevice, queue: &RenderQueue) -> bool {
        // The allocated capacity is the capacity of the currently allocated GPU buffer,
        // which can be different from the expected capacity (self.capacity) for next
        // update.
        let allocated_count = self.buffer.as_ref().map(|ab| ab.count).unwrap_or(0);
        let reallocated = if self.capacity > allocated_count {
            let byte_size = self.to_byte_size(self.capacity);
            trace!(
                "reserve('{}'): increase capacity from {} to {} elements, old size {} bytes, new size {} bytes",
                self.safe_name(),
                allocated_count,
                self.capacity,
                self.to_byte_size(allocated_count),
                byte_size
            );

            // Create the new buffer, swapping with the old one if any
            let has_init_data = !self.extra_pending_values.is_empty();
            let new_buffer = device.create_buffer(&BufferDescriptor {
                label: self.label.as_ref().map(|s| &s[..]),
                size: byte_size as BufferAddress,
                usage: self.buffer_usage,
                mapped_at_creation: has_init_data,
            });

            // Use any pending data to initialize the buffer. We only use CPU-available
            // data, which was inserted after the buffer was (re-)allocated and
            // has not been uploaded to GPU yet.
            if has_init_data {
                // Leave some space to copy the old buffer if any
                let base_size = self.to_byte_size(allocated_count) as u64;
                let extra_count = self.extra_pending_values.len() as u32;
                let extra_size = self.to_byte_size(extra_count) as u64;

                // Scope get_mapped_range_mut() to force a drop before unmap()
                {
                    // Note: get_mapped_range_mut() requires 8-byte alignment of the start offset.
                    let unaligned_range = base_size..(base_size + extra_size);
                    let (range, byte_offset) = round_range_start_down(unaligned_range, 8);

                    let dst_slice = &mut new_buffer.slice(range).get_mapped_range_mut();

                    let base_offset = byte_offset as usize;
                    let byte_size = self.aligned_size; // single row
                    for (index, content) in self.extra_pending_values.drain(..).enumerate() {
                        let byte_offset = base_offset + byte_size * index;

                        // Copy Rust value into a GPU-ready format, including GPU padding.
                        let src: &[u8] = cast_slice(std::slice::from_ref(&content));
                        let dst_range = byte_offset..(byte_offset + self.item_size);
                        trace!(
                            "+ init_copy: index={} src={:?} dst={:?} byte_offset={} byte_size={}",
                            index,
                            src.as_ptr(),
                            dst_range,
                            byte_offset,
                            byte_size
                        );
                        let dst = &mut dst_slice[dst_range];
                        dst.copy_from_slice(src);
                    }
                }

                new_buffer.unmap();
            }

            if let Some(ab) = self.buffer.as_mut() {
                // If there's any data currently in the GPU buffer, we need to copy it on next
                // update to preserve it.
                if self.active_count > 0 {
                    // Current buffer has value to preserve, save it into old_buffer before
                    // replacing it with the newly-allocated one.

                    // By design we can't have all active entries as free ones; we should have
                    // updated active_count=0 and cleared the free list if that was the case.
                    debug_assert!(self.free_indices.len() < self.active_count as usize);

                    // If we already have an old buffer, that means we already have scheduled a copy
                    // to preserve some values. And we can't do that twice per frame.
                    assert!(
                        ab.old_buffer.is_none(),
                        "allocate_gpu() called twice before write_buffer() took effect."
                    );

                    // Swap old <-> new
                    let mut old_buffer = new_buffer;
                    std::mem::swap(&mut old_buffer, &mut ab.buffer);
                    ab.old_buffer = Some(old_buffer);
                    ab.old_count = ab.count;
                } else {
                    // Current buffer is unused, so we don't need to preserve anything.

                    // It could happen we reallocate during the frame then immediately free the rows
                    // to preserve, such that we don't need in the end to preserve anything.
                    if let Some(old_buffer) = ab.old_buffer.take() {
                        old_buffer.destroy();
                    }

                    ab.buffer.destroy();
                    ab.buffer = new_buffer;
                }
                ab.count = self.capacity;
            } else {
                self.buffer = Some(AllocatedBuffer {
                    buffer: new_buffer,
                    count: self.capacity,
                    old_buffer: None,
                    old_count: 0,
                });
            }

            true
        } else {
            false
        };

        // Immediately schedule a copy of old rows.
        // - For old rows, copy into the old buffer because the old-to-new buffer copy
        //   will be executed during a command queue while any CPU to GPU upload is
        //   prepended before the next command queue. To ensure things do get out of
        //   order with the CPU upload overwriting the GPU-to-GPU copy, make sure those
        //   two are disjoint.
        if let Some(ab) = self.buffer.as_ref() {
            let buffer = ab.old_buffer.as_ref().unwrap_or(&ab.buffer);
            for (index, content) in self.pending_values.drain(..) {
                let byte_size = self.aligned_size;
                let byte_offset = byte_size * index as usize;

                // Copy Rust value into a GPU-ready format, including GPU padding.
                // TODO - Do that in insert()!
                let mut aligned_buffer: Vec<u8> = vec![0; self.aligned_size];
                let src: &[u8] = cast_slice(std::slice::from_ref(&content));
                let dst_range = ..self.item_size;
                trace!(
                    "+ old_copy: index={} src={:?} dst={:?} byte_offset={} byte_size={}",
                    index,
                    src.as_ptr(),
                    dst_range,
                    byte_offset,
                    byte_size
                );
                let dst = &mut aligned_buffer[dst_range];
                dst.copy_from_slice(src);

                // Upload to GPU
                // TODO - Merge contiguous blocks into a single write_buffer()
                let bytes: &[u8] = cast_slice(&aligned_buffer);
                queue.write_buffer(buffer, byte_offset as u64, bytes);
            }
        } else {
            debug_assert!(self.pending_values.is_empty());
            debug_assert!(self.extra_pending_values.is_empty());
        }

        reallocated
    }

    /// Write CPU data to the GPU buffer, (re)allocating it as needed.
    pub fn write_buffer(&self, encoder: &mut CommandEncoder) {
        // Check if there's any work to do: either some pending values to upload or some
        // existing buffer to copy into a newly-allocated one.
        if self.pending_values.is_empty()
            && self
                .buffer
                .as_ref()
                .map(|ab| ab.old_buffer.is_none())
                .unwrap_or(true)
        {
            trace!("write_buffer({}): nothing to do", self.safe_name());
            return;
        }

        trace!(
            "write_buffer({}): pending_values.len={} item_size={} aligned_size={} buffer={:?}",
            self.safe_name(),
            self.pending_values.len(),
            self.item_size,
            self.aligned_size,
            self.buffer,
        );

        // If there's no more GPU buffer, there's nothing to do
        let Some(ab) = self.buffer.as_ref() else {
            return;
        };

        // Copy any old buffer into the new one, and clear the old buffer. Note that we
        // only clear the ref-counted reference to the buffer, not the actual buffer,
        // which stays alive until the copy is done (but we don't need to care about
        // keeping it alive, wgpu does that for us).
        if let Some(old_buffer) = ab.old_buffer.as_ref() {
            let old_size = self.to_byte_size(ab.old_count) as u64;
            trace!("Copy old buffer id {:?} of size {} bytes into newly-allocated buffer {:?} of size {} bytes.", old_buffer.id(), old_size, ab.buffer.id(), self.to_byte_size(ab.count));
            encoder.copy_buffer_to_buffer(old_buffer, 0, &ab.buffer, 0, old_size);
        }
    }
}

#[cfg(test)]
mod tests {
    use bevy::math::Vec3;
    use bytemuck::{Pod, Zeroable};

    use super::*;

    #[test]
    fn test_round_range_start_down() {
        // r8(0..7) : no-op
        {
            let (r, o) = round_range_start_down(0..7, 8);
            assert_eq!(r, 0..7);
            assert_eq!(o, 0);
        }

        // r8(2..7) = 0..7, +2
        {
            let (r, o) = round_range_start_down(2..7, 8);
            assert_eq!(r, 0..7);
            assert_eq!(o, 2);
        }

        // r8(7..32) = 0..32, +7
        {
            let (r, o) = round_range_start_down(7..32, 8);
            assert_eq!(r, 0..32);
            assert_eq!(o, 7);
        }

        // r8(8..32) = no-op
        {
            let (r, o) = round_range_start_down(8..32, 8);
            assert_eq!(r, 8..32);
            assert_eq!(o, 0);
        }
    }

    #[repr(C)]
    #[derive(Debug, Default, Clone, Copy, Pod, Zeroable, ShaderType)]
    pub(crate) struct GpuDummy {
        pub v: Vec3,
    }

    #[repr(C)]
    #[derive(Debug, Default, Clone, Copy, Pod, Zeroable, ShaderType)]
    pub(crate) struct GpuDummyComposed {
        pub simple: GpuDummy,
        pub tag: u32,
        // GPU padding to 16 bytes due to GpuDummy forcing align to 16 bytes
    }

    #[repr(C)]
    #[derive(Debug, Clone, Copy, Pod, Zeroable, ShaderType)]
    pub(crate) struct GpuDummyLarge {
        pub simple: GpuDummy,
        pub tag: u32,
        pub large: [f32; 128],
    }

    #[test]
    fn buffer_table_sizes() {
        // Rust
        assert_eq!(std::mem::size_of::<GpuDummy>(), 12);
        assert_eq!(std::mem::align_of::<GpuDummy>(), 4);
        assert_eq!(std::mem::size_of::<GpuDummyComposed>(), 16); // tight packing
        assert_eq!(std::mem::align_of::<GpuDummyComposed>(), 4);
        assert_eq!(std::mem::size_of::<GpuDummyLarge>(), 132 * 4); // tight packing
        assert_eq!(std::mem::align_of::<GpuDummyLarge>(), 4);

        // GPU
        assert_eq!(<GpuDummy as ShaderType>::min_size().get(), 16); // Vec3 gets padded to 16 bytes
        assert_eq!(<GpuDummy as ShaderSize>::SHADER_SIZE.get(), 16);
        assert_eq!(<GpuDummyComposed as ShaderType>::min_size().get(), 32); // align is 16 bytes, forces padding
        assert_eq!(<GpuDummyComposed as ShaderSize>::SHADER_SIZE.get(), 32);
        assert_eq!(<GpuDummyLarge as ShaderType>::min_size().get(), 544); // align is 16 bytes, forces padding
        assert_eq!(<GpuDummyLarge as ShaderSize>::SHADER_SIZE.get(), 544);

        for (item_align, expected_aligned_size) in [
            (0, 16),
            (4, 16),
            (8, 16),
            (16, 16),
            (32, 32),
            (256, 256),
            (512, 512),
        ] {
            let mut table = BufferTable::<GpuDummy>::new(
                BufferUsages::STORAGE,
                NonZeroU64::new(item_align),
                None,
            );
            assert_eq!(table.aligned_size(), expected_aligned_size);
            assert!(table.is_empty());
            table.insert(GpuDummy::default());
            assert!(!table.is_empty());
            assert_eq!(table.len(), 1);
        }

        for (item_align, expected_aligned_size) in [
            (0, 32),
            (4, 32),
            (8, 32),
            (16, 32),
            (32, 32),
            (256, 256),
            (512, 512),
        ] {
            let mut table = BufferTable::<GpuDummyComposed>::new(
                BufferUsages::STORAGE,
                NonZeroU64::new(item_align),
                None,
            );
            assert_eq!(table.aligned_size(), expected_aligned_size);
            assert!(table.is_empty());
            table.insert(GpuDummyComposed::default());
            assert!(!table.is_empty());
            assert_eq!(table.len(), 1);
        }

        for (item_align, expected_aligned_size) in [
            (0, 544),
            (4, 544),
            (8, 544),
            (16, 544),
            (32, 544),
            (256, 768),
            (512, 1024),
        ] {
            let mut table = BufferTable::<GpuDummyLarge>::new(
                BufferUsages::STORAGE,
                NonZeroU64::new(item_align),
                None,
            );
            assert_eq!(table.aligned_size(), expected_aligned_size);
            assert!(table.is_empty());
            table.insert(GpuDummyLarge {
                simple: Default::default(),
                tag: 0,
                large: [0.; 128],
            });
            assert!(!table.is_empty());
            assert_eq!(table.len(), 1);
        }
    }

    #[test]
    fn buffer_table_insert() {
        let mut table =
            BufferTable::<GpuDummy>::new(BufferUsages::STORAGE, NonZeroU64::new(32), None);

        // [x]
        let id1 = table.insert(GpuDummy::default());
        assert_eq!(id1.0, 0);
        assert_eq!(table.active_count, 1);
        assert!(table.free_indices.is_empty());

        // [x x]
        let id2 = table.insert(GpuDummy::default());
        assert_eq!(id2.0, 1);
        assert_eq!(table.active_count, 2);
        assert!(table.free_indices.is_empty());

        // [- x]
        table.remove(id1);
        assert_eq!(table.active_count, 2);
        assert_eq!(table.free_indices.len(), 1);
        assert_eq!(table.free_indices[0], 0);

        // [- x x x]
        let id3 = table.insert_contiguous([GpuDummy::default(); 2].into_iter());
        assert_eq!(id3.0, 2); // at the end (doesn't fit in free slot #0)
        assert_eq!(table.active_count, 4);
        assert_eq!(table.free_indices.len(), 1);
        assert_eq!(table.free_indices[0], 0);

        // [- - x x]
        table.remove(id2);
        assert_eq!(table.active_count, 4);
        assert_eq!(table.free_indices.len(), 2);
        assert_eq!(table.free_indices[0], 0);
        assert_eq!(table.free_indices[1], 1);

        // [x x x x]
        let id4 = table.insert_contiguous([GpuDummy::default(); 2].into_iter());
        assert_eq!(id4.0, 0); // this times it fit into slot #0-#1
        assert_eq!(table.active_count, 4);
        assert!(table.free_indices.is_empty());

        // [- - x x]
        table.remove_range(id4, 2);
        assert_eq!(table.active_count, 4);
        assert_eq!(table.free_indices.len(), 2);
        assert_eq!(table.free_indices[0], 0);
        assert_eq!(table.free_indices[1], 1);

        // []
        table.remove_range(id3, 2);
        assert_eq!(table.active_count, 0);
        assert!(table.free_indices.is_empty());

        // [x x]
        let id5 = table.insert_contiguous([GpuDummy::default(); 2].into_iter());
        assert_eq!(id5.0, 0);
        assert_eq!(table.active_count, 2);
        assert!(table.free_indices.is_empty());

        // [x x x x]
        let id6 = table.insert_contiguous([GpuDummy::default(); 2].into_iter());
        assert_eq!(id6.0, 2);
        assert_eq!(table.active_count, 4);
        assert!(table.free_indices.is_empty());

        // [x x x x x x]
        let id7 = table.insert_contiguous([GpuDummy::default(); 2].into_iter());
        assert_eq!(id7.0, 4);
        assert_eq!(table.active_count, 6);
        assert!(table.free_indices.is_empty());

        // [x x - - x x]
        table.remove_range(id6, 2);
        assert_eq!(table.active_count, 6);
        assert_eq!(table.free_indices.len(), 2);
        assert_eq!(table.free_indices[0], 2);
        assert_eq!(table.free_indices[1], 3);

        // [x x]
        table.remove_range(id7, 2);
        assert_eq!(table.active_count, 2);
        assert!(table.free_indices.is_empty());
    }
}

#[cfg(all(test, feature = "gpu_tests"))]
mod gpu_tests {
    use std::fmt::Write;

    use bevy::render::render_resource::BufferSlice;
    use tests::*;
    use wgpu::{BufferView, CommandBuffer};

    use super::*;
    use crate::test_utils::MockRenderer;

    /// Read data from GPU back into CPU memory.
    ///
    /// This call blocks until the data is available on CPU. Used for testing
    /// only.
    fn read_back_gpu<'a>(device: &RenderDevice, slice: BufferSlice<'a>) -> BufferView<'a> {
        let (tx, rx) = futures::channel::oneshot::channel();
        slice.map_async(wgpu::MapMode::Read, move |result| {
            tx.send(result).unwrap();
        });
        device.poll(wgpu::Maintain::Wait);
        let result = futures::executor::block_on(rx);
        assert!(result.is_ok());
        slice.get_mapped_range()
    }

    /// Submit a command buffer to GPU and wait for completion.
    ///
    /// This call blocks until the GPU executed the command buffer. Used for
    /// testing only.
    fn submit_gpu_and_wait(
        device: &RenderDevice,
        queue: &RenderQueue,
        command_buffer: CommandBuffer,
    ) {
        // Queue command
        queue.submit([command_buffer]);

        // Register callback to observe completion
        let (tx, rx) = futures::channel::oneshot::channel();
        queue.on_submitted_work_done(move || {
            tx.send(()).unwrap();
        });

        // Poll device, checking for completion and raising callback
        device.poll(wgpu::Maintain::Wait);

        // Wait for callback to be raised. This was need in previous versions, however
        // it's a bit unclear if it's still needed or if device.poll() is enough to
        // guarantee that the command was executed.
        let _ = futures::executor::block_on(rx);
    }

    /// Convert a byte slice to a string of hexadecimal values separated by a
    /// blank space.
    fn to_hex_string(slice: &[u8]) -> String {
        let len = slice.len();
        let num_chars = len * 3 - 1;
        let mut s = String::with_capacity(num_chars);
        for b in &slice[..len - 1] {
            write!(&mut s, "{:02x} ", *b).unwrap();
        }
        write!(&mut s, "{:02x}", slice[len - 1]).unwrap();
        debug_assert_eq!(s.len(), num_chars);
        s
    }

    fn write_buffers_and_wait<T: Pod + ShaderSize>(
        table: &BufferTable<T>,
        device: &RenderDevice,
        queue: &RenderQueue,
    ) {
        let mut encoder = device.create_command_encoder(&wgpu::CommandEncoderDescriptor {
            label: Some("test"),
        });
        table.write_buffer(&mut encoder);
        let command_buffer = encoder.finish();
        submit_gpu_and_wait(device, queue, command_buffer);
        println!("Buffer written to GPU");
    }

    #[test]
    fn table_write() {
        let renderer = MockRenderer::new();
        let device = renderer.device();
        let queue = renderer.queue();

        let item_align = device.limits().min_storage_buffer_offset_alignment as u64;
        println!("min_storage_buffer_offset_alignment = {item_align}");
        let mut table = BufferTable::<GpuDummyComposed>::new(
            BufferUsages::STORAGE | BufferUsages::MAP_READ,
            NonZeroU64::new(item_align),
            None,
        );
        let final_align = item_align.max(<GpuDummyComposed as ShaderSize>::SHADER_SIZE.get());
        assert_eq!(table.aligned_size(), final_align as usize);

        // Initial state
        assert!(table.is_empty());
        assert_eq!(table.len(), 0);
        assert_eq!(table.capacity(), 0);
        assert!(table.buffer.is_none());

        // This has no effect while the table is empty
        table.clear_previous_frame_resizes();
        table.allocate_gpu(&device, &queue);
        write_buffers_and_wait(&table, &device, &queue);
        assert!(table.is_empty());
        assert_eq!(table.len(), 0);
        assert_eq!(table.capacity(), 0);
        assert!(table.buffer.is_none());

        // New frame
        table.clear_previous_frame_resizes();

        // Insert some entries
        let len = 3;
        for i in 0..len {
            let row = table.insert(GpuDummyComposed {
                tag: i + 1,
                ..Default::default()
            });
            assert_eq!(row.0, i);
        }
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len); // contract: could over-allocate...
        assert!(table.buffer.is_none()); // not yet allocated on GPU

        // Allocate GPU buffer for current requested state
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert!(ab.old_buffer.is_none()); // no previous copy
        assert_eq!(ab.count, len);
        println!(
            "Allocated buffer #{:?} of {} rows",
            ab.buffer.id(),
            ab.count
        );
        let ab_buffer = ab.buffer.clone();

        // Another allocate_gpu() is a no-op
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert!(ab.old_buffer.is_none()); // no previous copy
        assert_eq!(ab.count, len);
        assert_eq!(ab_buffer.id(), ab.buffer.id()); // same buffer

        // Write buffer (CPU -> GPU)
        write_buffers_and_wait(&table, &device, &queue);

        {
            // Read back (GPU -> CPU)
            let buffer = table.buffer().expect("Buffer was not allocated").clone(); // clone() for lifetime
            {
                let slice = buffer.slice(..);
                let view = read_back_gpu(&device, slice);
                println!(
                    "GPU data read back to CPU for validation: {} bytes",
                    view.len()
                );

                // Validate content
                assert_eq!(view.len(), final_align as usize * table.capacity() as usize);
                for i in 0..len as usize {
                    let offset = i * final_align as usize;
                    let item_size = std::mem::size_of::<GpuDummyComposed>();
                    let src = &view[offset..offset + 16];
                    println!("{}", to_hex_string(src));
                    let dummy_composed: &[GpuDummyComposed] =
                        cast_slice(&view[offset..offset + item_size]);
                    assert_eq!(dummy_composed[0].tag, (i + 1) as u32);
                }
            }
            buffer.unmap();
        }

        // New frame
        table.clear_previous_frame_resizes();

        // Insert more entries
        let old_capacity = table.capacity();
        let mut len = len;
        while table.capacity() == old_capacity {
            let row = table.insert(GpuDummyComposed {
                tag: len + 1,
                ..Default::default()
            });
            assert_eq!(row.0, len);
            len += 1;
        }
        println!(
            "Added {} rows to grow capacity from {} to {}.",
            len - 3,
            old_capacity,
            table.capacity()
        );

        // This re-allocates a new GPU buffer because the capacity changed
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert_eq!(ab.count, len);
        assert!(ab.old_buffer.is_some()); // old buffer to copy
        assert_ne!(ab.old_buffer.as_ref().unwrap().id(), ab.buffer.id());
        println!(
            "Allocated new buffer #{:?} of {} rows",
            ab.buffer.id(),
            ab.count
        );

        // Write buffer (CPU -> GPU)
        write_buffers_and_wait(&table, &device, &queue);

        {
            // Read back (GPU -> CPU)
            let buffer = table.buffer().expect("Buffer was not allocated").clone(); // clone() for lifetime
            {
                let slice = buffer.slice(..);
                let view = read_back_gpu(&device, slice);
                println!(
                    "GPU data read back to CPU for validation: {} bytes",
                    view.len()
                );

                // Validate content
                assert_eq!(view.len(), final_align as usize * table.capacity() as usize);
                for i in 0..len as usize {
                    let offset = i * final_align as usize;
                    let item_size = std::mem::size_of::<GpuDummyComposed>();
                    let src = &view[offset..offset + 16];
                    println!("{}", to_hex_string(src));
                    let dummy_composed: &[GpuDummyComposed] =
                        cast_slice(&view[offset..offset + item_size]);
                    assert_eq!(dummy_composed[0].tag, (i + 1) as u32);
                }
            }
            buffer.unmap();
        }

        // New frame
        table.clear_previous_frame_resizes();

        // Delete the last allocated row
        let old_capacity = table.capacity();
        let len = len - 1;
        table.remove(BufferTableId(len));
        println!(
            "Removed last row to shrink capacity from {} to {}.",
            old_capacity,
            table.capacity()
        );

        // This doesn't do anything since we removed a row only
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert_eq!(ab.count, len + 1); // GPU buffer kept its size
        assert!(ab.old_buffer.is_none());

        // Write buffer (CPU -> GPU)
        write_buffers_and_wait(&table, &device, &queue);

        {
            // Read back (GPU -> CPU)
            let buffer = table.buffer().expect("Buffer was not allocated").clone(); // clone() for lifetime
            {
                let slice = buffer.slice(..);
                let view = read_back_gpu(&device, slice);
                println!(
                    "GPU data read back to CPU for validation: {} bytes",
                    view.len()
                );

                // Validate content
                assert!(view.len() >= final_align as usize * table.capacity() as usize); // note the >=, the buffer is over-allocated
                for i in 0..len as usize {
                    let offset = i * final_align as usize;
                    let item_size = std::mem::size_of::<GpuDummyComposed>();
                    let src = &view[offset..offset + 16];
                    println!("{}", to_hex_string(src));
                    let dummy_composed: &[GpuDummyComposed] =
                        cast_slice(&view[offset..offset + item_size]);
                    assert_eq!(dummy_composed[0].tag, (i + 1) as u32);
                }
            }
            buffer.unmap();
        }

        // New frame
        table.clear_previous_frame_resizes();

        // Delete the first allocated row
        let old_capacity = table.capacity();
        let mut len = len - 1;
        table.remove(BufferTableId(0));
        assert_eq!(old_capacity, table.capacity());
        println!(
            "Removed first row to shrink capacity from {} to {} (no change).",
            old_capacity,
            table.capacity()
        );

        // This doesn't do anything since we only removed a row
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert_eq!(ab.count, len + 2); // GPU buffer kept its size
        assert!(ab.old_buffer.is_none());

        // Write buffer (CPU -> GPU)
        write_buffers_and_wait(&table, &device, &queue);

        {
            // Read back (GPU -> CPU)
            let buffer = table.buffer().expect("Buffer was not allocated").clone(); // clone() for lifetime
            {
                let slice = buffer.slice(..);
                let view = read_back_gpu(&device, slice);
                println!(
                    "GPU data read back to CPU for validation: {} bytes",
                    view.len()
                );

                // Validate content
                assert!(view.len() >= final_align as usize * table.capacity() as usize); // note the >=, the buffer is over-allocated
                for i in 0..len as usize {
                    let offset = i * final_align as usize;
                    let item_size = std::mem::size_of::<GpuDummyComposed>();
                    let src = &view[offset..offset + 16];
                    println!("{}", to_hex_string(src));
                    if i > 0 {
                        let dummy_composed: &[GpuDummyComposed] =
                            cast_slice(&view[offset..offset + item_size]);
                        assert_eq!(dummy_composed[0].tag, (i + 1) as u32);
                    }
                }
            }
            buffer.unmap();
        }

        // New frame
        table.clear_previous_frame_resizes();

        // Insert a row; this should get into row #0 in the buffer
        let row = table.insert(GpuDummyComposed {
            tag: 1,
            ..Default::default()
        });
        assert_eq!(row.0, 0);
        len += 1;
        println!(
            "Added 1 row to grow capacity from {} to {}.",
            old_capacity,
            table.capacity()
        );

        // This doesn't reallocate the GPU buffer since we used a free list entry
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert_eq!(ab.count, 4); // 4 == last time we grew
        assert!(ab.old_buffer.is_none());

        // Write buffer (CPU -> GPU)
        write_buffers_and_wait(&table, &device, &queue);

        {
            // Read back (GPU -> CPU)
            let buffer = table.buffer().expect("Buffer was not allocated").clone(); // clone() for lifetime
            {
                let slice = buffer.slice(..);
                let view = read_back_gpu(&device, slice);
                println!(
                    "GPU data read back to CPU for validation: {} bytes",
                    view.len()
                );

                // Validate content
                assert!(view.len() >= final_align as usize * table.capacity() as usize);
                for i in 0..len as usize {
                    let offset = i * final_align as usize;
                    let item_size = std::mem::size_of::<GpuDummyComposed>();
                    let src = &view[offset..offset + 16];
                    println!("{}", to_hex_string(src));
                    let dummy_composed: &[GpuDummyComposed] =
                        cast_slice(&view[offset..offset + item_size]);
                    assert_eq!(dummy_composed[0].tag, (i + 1) as u32);
                }
            }
            buffer.unmap();
        }

        // New frame
        table.clear_previous_frame_resizes();

        // Insert a row; this should get into row #3 at the end of the allocated buffer
        let row = table.insert(GpuDummyComposed {
            tag: 4,
            ..Default::default()
        });
        assert_eq!(row.0, 3);
        len += 1;
        println!(
            "Added 1 row to grow capacity from {} to {}.",
            old_capacity,
            table.capacity()
        );

        // This doesn't reallocate the GPU buffer since we used an implicit free entry
        table.allocate_gpu(&device, &queue);
        assert!(!table.is_empty());
        assert_eq!(table.len(), len);
        assert!(table.capacity() >= len);
        let ab = table
            .buffer
            .as_ref()
            .expect("GPU buffer should be allocated after allocate_gpu()");
        assert_eq!(ab.count, 4); // 4 == last time we grew
        assert!(ab.old_buffer.is_none());

        // Write buffer (CPU -> GPU)
        write_buffers_and_wait(&table, &device, &queue);

        {
            // Read back (GPU -> CPU)
            let buffer = table.buffer().expect("Buffer was not allocated").clone(); // clone() for lifetime
            {
                let slice = buffer.slice(..);
                let view = read_back_gpu(&device, slice);
                println!(
                    "GPU data read back to CPU for validation: {} bytes",
                    view.len()
                );

                // Validate content
                assert!(view.len() >= final_align as usize * table.capacity() as usize);
                for i in 0..len as usize {
                    let offset = i * final_align as usize;
                    let item_size = std::mem::size_of::<GpuDummyComposed>();
                    let src = &view[offset..offset + 16];
                    println!("{}", to_hex_string(src));
                    let dummy_composed: &[GpuDummyComposed] =
                        cast_slice(&view[offset..offset + item_size]);
                    assert_eq!(dummy_composed[0].tag, (i + 1) as u32);
                }
            }
            buffer.unmap();
        }
    }
}

djeedai / bevy_hanabi / 18243240671

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