frust_engine/cache/

images.rs

//! Image residency: which decoded images own atlas space, and for how long.
//!
//! A widget hands the display list the *same* decoded image every frame — the
//! `peniko::ImageData` in a [`Command::Image`](frust_scene::Command::Image) is a
//! reference-counted handle cloned per frame, never a re-decode — so the pixels
//! behind it are stable across frames and belong on the GPU once rather than
//! per draw. [`ImageResidency`] is what makes that "once" true: it keys an
//! image on its blob's process-unique id ([`peniko::Blob::id`]) and hands back
//! the same [`ImageId`] and the same atlas rectangle for every later frame that
//! draws it.
//!
//! Allocation itself is pure — a rectangle packer over
//! [`vello_common::multi_atlas::MultiAtlasManager`], no device in the loop — so
//! everything here is host-testable. The two GPU-side halves it *describes*
//! (writing a newly allocated region's texels, clearing an evicted one's) are
//! carried out by [`crate::gpu::atlas`] against the plan this module produces:
//! [`ImageResidency::evictions`] first, [`ImageResidency::uploads`] second. That
//! order is a contract, not a preference — a rectangle freed this frame can be
//! re-allocated in the same frame, so clearing after uploading would erase the
//! image that just moved in.
//!
//! ## Residency is committed only once the plan is serviced
//!
//! The cache decides residency long before anything reaches a queue: a
//! compiled frame can still be refused afterwards (a layer shape the scheduler
//! will not serve, a page past the pool's ceiling, coverage or paints outgrowing
//! their textures), and a frame refused after compiling is never drawn and never
//! uploaded. So the plan is *pending* rather than taken: [`ImageResidency::plan`]
//! hands out a copy, [`begin_frame`](ImageResidency::begin_frame) leaves it
//! alone, and only [`acknowledge_plan`](ImageResidency::acknowledge_plan) —
//! called by a consumer that has actually written the regions into a live atlas
//! — clears it.
//!
//! That is what makes the entry map and the atlas's real contents agree. Without
//! it, a refused frame leaves an image *recorded as resident* whose texels were
//! never written: every later frame resolves the entry, schedules no upload
//! (residency is a hit), and either drops the draw for the life of the process
//! or samples a rectangle nothing ever wrote. Re-emitting the same plan until it
//! is acknowledged costs a vector copy per frame in the steady state — where the
//! plan is empty — and is what makes "an image drawn on a thousand frames
//! uploads once" true of the *atlas* rather than only of the cache.
//!
//! ## An image larger than the atlas is minified, not dropped
//!
//! A source whose own extent will not fit one atlas layer is downsampled to fit
//! ([`fit_extent`], [`minify`]) instead of being refused. A photograph decoded
//! at its capture resolution is routinely several times the atlas budget while
//! its destination rectangle is a thumbnail, so refusing it drops a draw the
//! display list plainly describes; and every texel past the destination's own
//! scale is one the sampler would have thrown away anyway.
//!
//! The filter is a plain box average over the premultiplied texels, hand-written
//! rather than pulled in: the workspace's version pins are law, an image decoder
//! is not a dependency this crate carries, and a box average is exactly right
//! for the minification direction (every output texel is the mean of the source
//! texels it covers). The aspect ratio is preserved, so the paint transform's
//! two axes stay in step; what changes is the resident rectangle's extent, which
//! [`ResidentImage::natural`] records alongside it so the consumer can rescale
//! the encoded paint's own transform into the smaller rectangle.
//!
//! ## Two things are refused rather than attempted
//!
//! 1. **A format or size `vello_common` panics on.**
//!    [`vello_common::paint::ImageSource::from_peniko_image_data`] asserts on a
//!    dimension past `u16::MAX` and `unimplemented!()`s on a format outside
//!    `Rgba8`/`Bgra8`, and a pixel buffer whose length disagrees with the
//!    declared extent trips [`Pixmap::from_parts_with_opacity`]'s own assertion.
//!    All three are checked here, *before* the call, so an oversized or
//!    malformed image is an [`ImageSkip`] the frame path reports rather than a
//!    panic it takes (E17). These, plus an atlas with no room left in it, are
//!    now the only things [`ImageResidency::skipped`] counts: an image that is
//!    merely *big* is minified and drawn.
//! 2. **Anything at all, when `FRUST_ENGINE_NO_ATLAS` is set.** The kill switch
//!    ([`crate::config::atlas_disabled`]) makes every resolution answer
//!    [`ImageSkip::AtlasDisabled`], so no atlas is allocated, nothing is
//!    uploaded, and image draws fall back to painting nothing — the switch's
//!    whole point being to take the atlas out of a frame under diagnosis.
//!
//! ## Residency is bounded by age, and by pressure
//!
//! An entry unseen for [`MAX_UNSEEN_FRAMES`] consecutive frames is deallocated
//! and its rectangle reported for clearing — the same age-based reap
//! `frust-render`'s shader-effect cache uses, and for the same reason: a screen
//! that stops drawing an image should give its texels back promptly, while an
//! image drawn every other frame must never be mistaken for gone.
//!
//! Age alone is not a bound on *population*, though: it says nothing about how
//! many distinct images a screen draws inside one age window. A list scrolling
//! faster than [`MAX_UNSEEN_FRAMES`] streams more of them past the viewport
//! than the atlas has rectangles for, and every one past the last free
//! rectangle used to be an [`ImageSkip::NoAtlasSpace`] — a draw that painted
//! nothing, on a screen whose solid fills all still landed, which reads as a
//! hole rather than as a cache miss. So an allocation the packer refuses now
//! *evicts* instead of giving up: [`ImageResidency::resolve`] works out which
//! least-recently-seen rectangles would make room, frees exactly those, and
//! each one whose rectangle was taken re-uploads on the next frame that draws
//! it. A working set larger than the atlas therefore degrades to re-uploads —
//! bandwidth — rather than to missing content, and
//! [`ImageResidency::pressure_evictions`] counts how often that trade was made.
//!
//! ## An eviction is planned on paper before anything is freed
//!
//! An eviction that does not produce the allocation is pure loss. The draw is
//! skipped anyway, so the frame pays a clear per freed rectangle *and* the
//! re-upload of every displaced image *and* the hole it was trying to avoid.
//! And the condition recurs: a request the atlas can never satisfy would strip
//! and rebuild the residency on every frame that asks for it, which on a mobile
//! budget is megabytes of texture traffic each way, per frame, buying nothing.
//!
//! So the pressure path commits nothing speculatively. `allocate_under_pressure`
//! walks the candidates in least-recently-seen order over a *model* of the
//! layers — the rectangles this residency knows are occupied, and each layer's
//! free area as the packer itself reports it — and stops at the first prefix
//! that would admit the request. Only then is anything released, and only the
//! candidates on the layer the fit was found in. A request no bounded set of
//! candidates can satisfy therefore stays the cheap [`ImageSkip::NoAtlasSpace`]
//! it was before eviction existed, with the residency untouched.
//!
//! The model is deliberately *optimistic*: it can see this residency's own
//! rectangles but not the glyph pages sharing the same allocator (see below),
//! so it never refuses a request eviction could have satisfied on a given
//! layer — a placement that clears the real occupants clears the model's
//! subset of them too. The reverse can happen: the packer may still refuse a
//! plan the model approved, which costs the planned evictions and no
//! allocation. That case is bounded by the same plan budget rather than
//! open-ended, and it is the one residual the planner cannot rule out without
//! knowing where every glyph page sits.
//!
//! The bounded search itself is scoped *per layer*: each atlas layer gets its
//! own candidate-count and area allowance rather than the whole residency
//! sharing one pool, so a request only one layer could ever satisfy is not
//! starved by candidates on layers that were never going to host it — each
//! layer is either cleared within its own allowance or ruled out on its own
//! terms, never on account of what another layer's candidates already cost.
//!
//! The one entry never freed on either route is one *this frame* has already
//! resolved. The plan clears before it uploads, so handing back a rectangle the
//! frame is about to sample would erase exactly the image the eviction was
//! meant to make room for. A working set that outgrows the atlas inside a
//! single frame is therefore still a skip, and honestly so: nothing is
//! evictable. The same guard covers the *extent-mismatch* release in
//! [`ImageResidency::resolve`], where a blob redrawn at a new extent gives its
//! old rectangle back: on a frame that already sampled that rectangle the
//! second draw is skipped instead, as
//! [`ImageSkip::SameFrameExtentConflict`] rather than as a capacity refusal —
//! the atlas may hold nothing at all beside the first extent's own rectangle.
//!
//! ## One allocator for images *and* glyphs
//!
//! The [`ImageCache`] this module holds is the process's only atlas allocator:
//! the glyph policy (`crate::text::atlas_policy`) owns no cache of its own and
//! allocates its slots through [`ImageResidency::allocator_mut`].
//!
//! That is a correctness requirement, not tidiness. An [`ImageId`] is a *slot
//! index into one cache* — `ImageCache::allocate` hands out the next free index
//! of its own `slots` vector — and the strip shader has exactly one atlas
//! texture array to sample from. Two caches over the same page geometry would
//! therefore mint the same small integers for unrelated occupants and pack them
//! into overlapping rectangles of the same layers, so a glyph and an image would
//! address each other's texels by construction. Sharing one cache makes both
//! collisions unrepresentable: one id space, one packer, one set of live
//! rectangles.
//!
//! It is also what upstream does — `vello_hybrid`'s `Resources` holds a single
//! `image_cache` and hands it to its glyph atlas at every call — and it is what
//! keeps the budget honest: a glyph page and an image page come out of the same
//! [`AtlasBudget::max_atlases`] allowance, and [`ImageResidency::layers`] counts
//! whichever of the two created a layer.
//!
//! The one rule a borrower must keep is ownership of its own handles: each side
//! deallocates only the ids it allocated. Nothing enforces that beyond the two
//! call sites, and it is the reason [`ImageResidency::allocator_mut`] is
//! documented as a contract rather than a plain accessor.

use std::collections::{HashMap, HashSet};
use std::sync::Arc;
#[cfg(feature = "perf-trace")]
use std::sync::atomic::{AtomicBool, Ordering};

use frust_gpu::{DownlevelProfile, TierCaps};
use peniko::color::PremulRgba8;
use peniko::{ImageData, ImageFormat};
use thiserror::Error;
use vello_common::image_cache::ImageCache;
use vello_common::multi_atlas::{AllocationStrategy, AtlasConfig};
use vello_common::paint::{ImageId, ImageSource};
use vello_common::pixmap::Pixmap;

use crate::config;

/// Consecutive frames an image may go undrawn before its atlas rectangle is
/// reclaimed.
///
/// 60 frames is half a second at 120Hz and a whole one at 60Hz: long enough
/// that an image drawn intermittently (an every-other-frame animation, a brief
/// scene-diff hiccup) never loses its residency, short enough that navigating
/// away from an image-heavy screen returns its texels within a frame budget's
/// worth of frames rather than at surface teardown.
pub const MAX_UNSEEN_FRAMES: u64 = 60;

/// Transparent padding pixels placed around each image in the atlas.
///
/// Zero, deliberately. Padding exists to stop a filtered sample from reading a
/// neighbour's texels, and the strip shader's atlas samplers cannot do that:
/// both the bilinear and the bicubic path clamp every tap into
/// `[offset, offset + size - 1]` — the image's own rectangle — before it
/// reaches `textureLoad`. Paying a two-pixel border per image would buy nothing
/// and cost atlas space that a mobile budget does not have.
pub const ATLAS_PADDING: u16 = 0;

// The clear skip in `uncovered_clears` compares reported (padded) eviction
// rectangles with raw upload rectangles; a non-zero padding would make them
// differ and silently turn the skip off, or worse, mis-pair rectangles.
const _: () = assert!(
    ATLAS_PADDING == 0,
    "uncovered_clears matches padded evictions against raw uploads, which only holds with zero padding"
);

/// The largest image edge that can be made resident at all.
///
/// `vello_common`'s image cache addresses an offset and an extent in `u16`, and
/// its `ImageSource` conversion asserts on anything larger, so this is the
/// ceiling the pre-check enforces rather than a policy of ours.
pub const MAX_IMAGE_DIMENSION: u32 = u16::MAX as u32;

/// The most atlas layers the shader's encoded-image record can name.
///
/// `GpuEncodedImage::image_params` gives `atlas_index` eight bits, so a layer
/// past 255 could not be addressed even if the adapter allowed it.
pub const MAX_ATLAS_LAYERS: usize = 256;

/// Why one image could not be made resident.
///
/// Every variant is a *skip*, never a frame failure: the draw paints nothing
/// and the rest of the frame proceeds. They are distinguished because the
/// remedies differ — a too-large image is an application decision, an exhausted
/// atlas is a budget one, and a disabled atlas is the operator's own doing.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Error)]
pub enum ImageSkip {
    /// `FRUST_ENGINE_NO_ATLAS` is set; the atlas is out of the frame entirely.
    #[error("image residency is disabled by FRUST_ENGINE_NO_ATLAS")]
    AtlasDisabled,
    /// The image declares a format the atlas does not store.
    #[error("unsupported image format {0:?}: only Rgba8 and Bgra8 are uploaded")]
    UnsupportedFormat(ImageFormat),
    /// The image has no area, so there is nothing to make resident.
    #[error("degenerate image extent {width}x{height}")]
    DegenerateExtent {
        /// Declared width in pixels.
        width: u32,
        /// Declared height in pixels.
        height: u32,
    },
    /// The image is larger than [`MAX_IMAGE_DIMENSION`] on at least one axis.
    #[error("image {width}x{height} exceeds the {max}-pixel residency ceiling")]
    TooLarge {
        /// Declared width in pixels.
        width: u32,
        /// Declared height in pixels.
        height: u32,
        /// The per-axis ceiling that refused it.
        max: u32,
    },
    /// The pixel buffer's length disagrees with the declared extent and format.
    #[error("image pixel buffer is {actual} bytes; {width}x{height} needs exactly {expected}")]
    MalformedPixels {
        /// Declared width in pixels.
        width: u32,
        /// Declared height in pixels.
        height: u32,
        /// Bytes the declared extent and format require.
        expected: usize,
        /// Bytes the blob actually holds.
        actual: usize,
    },
    /// The paint transform cannot be inverted, so there is no device-to-image
    /// mapping for the shader to sample through.
    #[error("image paint transform is singular or non-finite")]
    SingularTransform,
    /// The image fits the ceiling but not the configured atlas budget, and no
    /// resident image could be evicted to make room for it — either the atlas
    /// holds nothing but images this same frame drew, or what it holds beside
    /// them still leaves no rectangle this one fits in.
    #[error("image {width}x{height} does not fit the {atlas_width}x{atlas_height} atlas budget")]
    NoAtlasSpace {
        /// Declared width in pixels.
        width: u32,
        /// Declared height in pixels.
        height: u32,
        /// Configured atlas width.
        atlas_width: u32,
        /// Configured atlas height.
        atlas_height: u32,
    },
    /// The same blob was already resolved earlier in this same frame at a
    /// different extent.
    ///
    /// Not an atlas-capacity refusal: the atlas may be nearly empty. The plan
    /// clears before it uploads (see the module doc), so releasing this
    /// blob's rectangle to make room for its own second extent would schedule
    /// a clear over texels this frame already sampled — the first extent's
    /// draw keeps its texels and the second is skipped until the next frame
    /// instead.
    #[error(
        "image {width}x{height} already resolved this frame at a different extent; the first \
         extent's texels are kept until the next frame"
    )]
    SameFrameExtentConflict {
        /// The refused extent's width, after [`fit_extent`] the same way
        /// [`NoAtlasSpace`](Self::NoAtlasSpace) reports it.
        width: u32,
        /// The refused extent's height, after [`fit_extent`].
        height: u32,
    },
}

/// The atlas geometry a tier of adapter is given.
///
/// Never `vello_common`'s own [`AtlasConfig`] default: that is 4096 square
/// across eight layers, 64 MiB of `Rgba8Unorm` per layer and half a gigabyte in
/// total, which is a desktop-only figure that a phone would pay for the first
/// image it drew.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct AtlasBudget {
    /// Extent of each atlas layer, in pixels.
    pub atlas_size: (u32, u32),
    /// Maximum number of layers the array may grow to.
    pub max_atlases: usize,
}

impl AtlasBudget {
    /// The budget a tile-based or downlevel-clamped adapter gets: 4 MiB a
    /// layer, 16 MiB in total.
    pub const MOBILE: Self = Self {
        atlas_size: (1024, 1024),
        max_atlases: 4,
    };

    /// The budget a full-profile immediate-mode adapter gets: 16 MiB a layer,
    /// 128 MiB in total.
    pub const DESKTOP: Self = Self {
        atlas_size: (2048, 2048),
        max_atlases: 8,
    };

    /// The large-mobile budget's preferred shape: four 2048-square layers,
    /// 64 MiB in total — a quarter as many layers as
    /// [`MOBILE_LARGE_DEEP`](Self::MOBILE_LARGE_DEEP) for the same texels, so
    /// the array grows (and copies itself) a quarter as often.
    pub const MOBILE_LARGE_WIDE: Self = Self {
        atlas_size: (2048, 2048),
        max_atlases: 4,
    };

    /// The large-mobile budget's shape on an adapter whose texture edge stops
    /// short of 2048: sixteen 1024-square layers, the same 64 MiB.
    pub const MOBILE_LARGE_DEEP: Self = Self {
        atlas_size: (1024, 1024),
        max_atlases: 16,
    };

    /// The large-mobile shape `caps`' adapter can create unclamped, or `None`
    /// when it can create neither — the tier then falls back to
    /// [`MOBILE`](Self::MOBILE) rather than to a large budget its ceilings
    /// would have narrowed below the plain one's.
    #[must_use]
    pub fn mobile_large(caps: &TierCaps) -> Option<Self> {
        [Self::MOBILE_LARGE_WIDE, Self::MOBILE_LARGE_DEEP]
            .into_iter()
            .find(|shape| shape.clamped(caps) == *shape)
    }

    /// The unclamped budget `tier` stands for on `caps`' adapter.
    #[must_use]
    pub fn for_tier(tier: AtlasTier, caps: &TierCaps) -> Self {
        match tier {
            AtlasTier::Mobile => Self::MOBILE,
            AtlasTier::MobileLarge => Self::mobile_large(caps).unwrap_or(Self::MOBILE),
            AtlasTier::Desktop => Self::DESKTOP,
        }
    }

    /// The budget for `caps`' adapter, clamped to what it can actually create.
    ///
    /// The tier choice is [`choose_tier`] over the adapter and the device's
    /// own memory ([`device_memory_bytes`]); the clamps that follow are the
    /// adapter's own texture-edge and array-layer ceilings plus the eight-bit
    /// [`MAX_ATLAS_LAYERS`] the encoded-image record can address.
    ///
    /// `FRUST_ENGINE_ATLAS_SIZE` overrides the tier's extent, and the override
    /// is **budgeted, not unbudgeted**: it passes through
    /// [`within_total_bytes`](Self::within_total_bytes) against the tier's own
    /// [`total_bytes`](Self::total_bytes) before the adapter clamps. So the knob
    /// redistributes a tier's memory between extent and depth — one large layer
    /// instead of eight small ones — and cannot spend more of it than the tier
    /// itself would. Clamped to the adapter last, because an override is a
    /// request and a request the adapter cannot honour is still narrowed to what
    /// it can.
    ///
    /// The resolved budget is reported once per process through
    /// `atlas_tier_line`, because which tier a given phone takes is otherwise
    /// an inference from behaviour rather than an observation: the adapter
    /// name, the driver's own capability answers and the memory reading are
    /// all device-reported, and a device answering differently from the one
    /// it resembles is exactly the case a benchmark capture has to be able to
    /// name. That line is `perf-trace`-only, like every other `frust-perf`
    /// line the engine emits — a build without the feature resolves the same
    /// budget and writes nothing.
    #[must_use]
    pub fn for_caps(caps: &TierCaps) -> Self {
        let choice = choose_tier(caps, device_memory_bytes());
        let resolved = Self::for_choice(choice, caps);

        #[cfg(feature = "perf-trace")]
        ATLAS_TIER_LOG.emit(caps, choice, resolved);
        resolved
    }

    /// The budget `choice` resolves to on `caps`' adapter — [`for_caps`]
    /// without reading the device's memory or writing the tier line, so the
    /// table's outcome is testable for any adapter and memory size.
    ///
    /// [`for_caps`]: Self::for_caps
    #[must_use]
    pub fn for_choice(choice: TierChoice, caps: &TierCaps) -> Self {
        let tier = Self::for_tier(choice.tier, caps);
        match config::atlas_size() {
            None => tier.clamped(caps),
            Some(atlas_size) => Self {
                atlas_size,
                max_atlases: tier.max_atlases,
            }
            .within_total_bytes(tier.total_bytes())
            .clamped(caps),
        }
    }

    /// This budget narrowed so a fully grown array costs at most `total_bytes`.
    ///
    /// The extent goes first, scaled uniformly until one layer fits inside the
    /// whole allowance; the layer count then takes whatever multiple of that
    /// layer is left, never below one. Scaling the extent rather than only the
    /// depth is what makes the ceiling hold at all: a single 16384-square layer
    /// is a gigabyte on its own, so a clamp that could only reduce the layer
    /// *count* would have nothing left to reduce.
    ///
    /// Both axes are floored after the uniform scale, which can only take the
    /// product below the allowance — except where an extreme aspect ratio floors
    /// one axis to nothing and it is raised back to a texel, so each axis is
    /// then held against what the other leaves.
    #[must_use]
    pub fn within_total_bytes(self, total_bytes: u64) -> Self {
        // A single texel is the smallest atlas that exists, so an allowance
        // below one is treated as one rather than producing a zero extent the
        // packer could not address.
        let allowance = total_bytes.max(ATLAS_FORMAT_BYTES);
        let max_texels = allowance / ATLAS_FORMAT_BYTES;

        let (width, height) = self.atlas_size;
        let texels = u64::from(width).saturating_mul(u64::from(height));
        let atlas_size = if texels <= max_texels {
            (width, height)
        } else {
            // `as` on a float saturates rather than wrapping, and both factors
            // are finite and positive, so the narrowing is a plain floor.
            let scale = (max_texels as f64 / texels.max(1) as f64).sqrt();
            let scaled_w = ((f64::from(width) * scale) as u32).max(1);
            let scaled_h = ((f64::from(height) * scale) as u32).max(1);
            let fitted_w = fit_axis(scaled_w, max_texels / u64::from(scaled_h));
            let fitted_h = fit_axis(scaled_h, max_texels / u64::from(fitted_w));
            (fitted_w, fitted_h)
        };

        let layer_bytes = u64::from(atlas_size.0)
            .saturating_mul(u64::from(atlas_size.1))
            .saturating_mul(ATLAS_FORMAT_BYTES)
            .max(1);
        let layers = usize::try_from(allowance / layer_bytes).unwrap_or(MAX_ATLAS_LAYERS);

        Self {
            atlas_size,
            max_atlases: self.max_atlases.min(layers).max(1),
        }
    }

    /// Whether `region` lies inside an atlas array of `layers` layers at this
    /// budget's per-layer extent.
    ///
    /// The pure counterpart of [`crate::gpu::atlas::AtlasArray::contains`],
    /// answerable with no device: a consumer that records where an image lives
    /// before the array exists checks the rectangle here, so it can never come
    /// to name a region the array will refuse to write and then sample it as
    /// whatever the texture happened to hold.
    #[must_use]
    pub fn contains(self, region: AtlasRegion, layers: u32) -> bool {
        !region.is_empty()
            && region.layer < layers
            && region.offset[0].saturating_add(region.size[0]) <= self.atlas_size.0
            && region.offset[1].saturating_add(region.size[1]) <= self.atlas_size.1
    }

    /// This budget narrowed to what `caps`' adapter can create.
    #[must_use]
    pub fn clamped(self, caps: &TierCaps) -> Self {
        let edge = caps.max_texture_dimension_2d.max(1);
        let layers = usize::try_from(caps.max_texture_array_layers).unwrap_or(MAX_ATLAS_LAYERS);

        Self {
            atlas_size: (
                self.atlas_size.0.clamp(1, edge),
                self.atlas_size.1.clamp(1, edge),
            ),
            max_atlases: self.max_atlases.clamp(1, layers.min(MAX_ATLAS_LAYERS)),
        }
    }

    /// The `vello_common` configuration this budget describes.
    ///
    /// `initial_atlas_count` is zero so the first layer is created by the first
    /// allocation that needs one: an application drawing no images pays for no
    /// atlas at all, and [`vello_common::multi_atlas::MultiAtlasManager::new`]'s
    /// own `expect` on eager creation can never be reached.
    #[must_use]
    pub fn config(self) -> AtlasConfig {
        AtlasConfig {
            initial_atlas_count: 0,
            max_atlases: self.max_atlases,
            atlas_size: self.atlas_size,
            auto_grow: true,
            allocation_strategy: AllocationStrategy::FirstFit,
        }
    }

    /// The bytes one fully populated layer costs at [`ATLAS_FORMAT_BYTES`].
    #[must_use]
    pub fn layer_bytes(self) -> u64 {
        u64::from(self.atlas_size.0)
            .saturating_mul(u64::from(self.atlas_size.1))
            .saturating_mul(ATLAS_FORMAT_BYTES)
    }

    /// The bytes every layer of this budget costs once fully grown.
    #[must_use]
    pub fn total_bytes(self) -> u64 {
        self.layer_bytes()
            .saturating_mul(self.max_atlases.try_into().unwrap_or(u64::MAX))
    }
}

/// Bytes per texel of the atlas array (`Rgba8Unorm`).
pub const ATLAS_FORMAT_BYTES: u64 = 4;

/// `evictions` less every rectangle one of `uploads` covers exactly.
///
/// Exact cover only — same layer, offset and extent: an upload writes every
/// texel of its own rectangle (its pixels are exactly the region's extent), so
/// a clear of that same rectangle ordered ahead of it is overwritten in full.
/// A rectangle covered only partly, or only by several uploads together, keeps
/// its clear: whatever texels no upload reaches must still read as zero to the
/// next tenant. Order is preserved, so the clears stay in the order the
/// residency reported them.
///
/// The exact-cover match compares the reported eviction rectangle with the
/// upload's allocated rectangle. Evictions are reported padded by
/// [`ATLAS_PADDING`] while uploads carry the raw allocation, so the two only
/// agree while the padding is zero; the assertion below ties the skip to that.
#[must_use]
pub fn uncovered_clears(evictions: &[AtlasRegion], uploads: &[ImageUpload]) -> Vec<AtlasRegion> {
    if evictions.is_empty() || uploads.is_empty() {
        return evictions.to_vec();
    }
    let written: HashSet<AtlasRegion> = uploads.iter().map(|upload| upload.region).collect();
    evictions
        .iter()
        .copied()
        .filter(|region| !written.contains(region))
        .collect()
}

/// Whether `caps`' adapter reads as a mobile part by its capability answers
/// alone.
///
/// Two signals, either of which is enough:
///
/// - its usable limits are clamped to the GLES-3.0/WebGL2 downlevel defaults,
///   which is the profile every GL-backed target reports; or
/// - it reports `transient_saves_memory`, wgpu's own answer to "is this a
///   tile-based renderer keeping attachments on-chip" — true on the mobile
///   TBDR parts and false on desktop immediate-mode ones.
///
/// This is the device-class read text hinting keys on, and the atlas tier's
/// fallback for an adapter [`ADAPTER_FAMILIES`] does not name. It is not the
/// atlas tier itself: some mobile drivers answer `transient_saves_memory`
/// false (Adreno 618/620 do), which this read alone would put on the desktop
/// budget — see [`choose_tier`].
#[must_use]
pub fn is_mobile_tier(caps: &TierCaps) -> bool {
    caps.downlevel_profile != DownlevelProfile::Full || caps.transient_saves_memory
}

/// Which class of part an adapter's name places it in.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AdapterFamily {
    /// A phone/tablet GPU family: the atlas competes with the rest of a
    /// shared-memory device's budget.
    Mobile,
    /// A discrete or desktop-integrated GPU family.
    Desktop,
}

/// The adapter-name table the atlas tier is read from: a lowercase fragment
/// and the family an adapter whose name contains it belongs to, first match
/// winning.
///
/// A name table rather than capability answers alone because the capability
/// answers disagree with the hardware: Adreno 618 and 620 report a full
/// profile and `transient_saves_memory` false — the desktop answers — while
/// sharing their device's memory with everything else on it. An adapter the
/// table does not name keeps the capability read ([`is_mobile_tier`]), so a
/// family missing here costs the old behaviour rather than a wrong one.
///
/// Apple is listed whole: Apple-silicon Macs already took the mobile budget
/// through `transient_saves_memory`, so naming them changes nothing there.
pub const ADAPTER_FAMILIES: &[(&str, AdapterFamily)] = &[
    ("adreno", AdapterFamily::Mobile),
    ("mali", AdapterFamily::Mobile),
    ("immortalis", AdapterFamily::Mobile),
    ("powervr", AdapterFamily::Mobile),
    ("xclipse", AdapterFamily::Mobile),
    ("maleoon", AdapterFamily::Mobile),
    ("apple", AdapterFamily::Mobile),
    ("tegra", AdapterFamily::Mobile),
    ("nvidia", AdapterFamily::Desktop),
    ("geforce", AdapterFamily::Desktop),
    ("quadro", AdapterFamily::Desktop),
    ("radeon", AdapterFamily::Desktop),
    ("amd", AdapterFamily::Desktop),
    ("intel", AdapterFamily::Desktop),
];

/// The family [`ADAPTER_FAMILIES`] places `adapter_name` in, matched
/// case-insensitively; `None` for a name the table does not know.
#[must_use]
pub fn adapter_family(adapter_name: &str) -> Option<AdapterFamily> {
    let name = adapter_name.to_ascii_lowercase();
    ADAPTER_FAMILIES
        .iter()
        .find(|(fragment, _)| name.contains(fragment))
        .map(|(_, family)| *family)
}

/// The atlas budget tiers.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AtlasTier {
    /// [`AtlasBudget::MOBILE`].
    Mobile,
    /// [`AtlasBudget::mobile_large`]: a mobile family part on a device with at
    /// least [`MOBILE_LARGE_MIN_MEMORY`] of memory.
    MobileLarge,
    /// [`AtlasBudget::DESKTOP`].
    Desktop,
}

impl AtlasTier {
    /// The tier's name as the tier line spells it.
    #[must_use]
    pub fn label(self) -> &'static str {
        match self {
            Self::Mobile => "mobile",
            Self::MobileLarge => "mobile_large",
            Self::Desktop => "desktop",
        }
    }
}

/// Which rule of [`choose_tier`] decided a tier.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum TierRule {
    /// The adapter's limits are clamped to the WebGL2 downlevel defaults.
    Downlevel,
    /// [`ADAPTER_FAMILIES`] named the adapter.
    Table,
    /// The table did not; [`is_mobile_tier`]'s capability read decided.
    Heuristic,
}

impl TierRule {
    /// The rule's name as the tier line spells it.
    #[must_use]
    pub fn label(self) -> &'static str {
        match self {
            Self::Downlevel => "downlevel",
            Self::Table => "table",
            Self::Heuristic => "heuristic",
        }
    }
}

/// The resolved tier, with what decided it.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TierChoice {
    /// The tier.
    pub tier: AtlasTier,
    /// The rule that chose it.
    pub rule: TierRule,
    /// The device memory the choice was made against, when it was known.
    pub device_memory: Option<u64>,
}

/// The device memory from which a mobile family part takes the large budget:
/// 4 GiB.
///
/// The large budget is 64 MiB of atlas, a sixteenth of that; below it the
/// plain 16 MiB budget stays.
pub const MOBILE_LARGE_MIN_MEMORY: u64 = 4 << 30;

/// The atlas tier for `caps`' adapter on a device with `device_memory` bytes
/// of memory (`None` when unknown).
///
/// In order:
///
/// 1. A downlevel (WebGL2/GLES) profile is [`AtlasTier::Mobile`], whatever the
///    name: its limits are the browser ceiling, and the budget follows them.
/// 2. An adapter [`ADAPTER_FAMILIES`] names takes its family's tier — a
///    desktop family [`AtlasTier::Desktop`]; a mobile family
///    [`AtlasTier::MobileLarge`] when the device reports at least
///    [`MOBILE_LARGE_MIN_MEMORY`] and the adapter can create a large shape
///    unclamped ([`AtlasBudget::mobile_large`]), else [`AtlasTier::Mobile`].
///    Unknown memory is the small tier: the large one is earned, not assumed.
/// 3. Any other adapter keeps [`is_mobile_tier`]'s capability read, mobile or
///    desktop, exactly as before the table existed.
#[must_use]
pub fn choose_tier(caps: &TierCaps, device_memory: Option<u64>) -> TierChoice {
    let (tier, rule) = if caps.downlevel_profile != DownlevelProfile::Full {
        (AtlasTier::Mobile, TierRule::Downlevel)
    } else {
        match adapter_family(&caps.adapter_name) {
            Some(AdapterFamily::Desktop) => (AtlasTier::Desktop, TierRule::Table),
            Some(AdapterFamily::Mobile) => {
                let large = device_memory.is_some_and(|bytes| bytes >= MOBILE_LARGE_MIN_MEMORY)
                    && AtlasBudget::mobile_large(caps).is_some();
                let tier = if large {
                    AtlasTier::MobileLarge
                } else {
                    AtlasTier::Mobile
                };
                (tier, TierRule::Table)
            }
            None if is_mobile_tier(caps) => (AtlasTier::Mobile, TierRule::Heuristic),
            None => (AtlasTier::Desktop, TierRule::Heuristic),
        }
    };
    TierChoice {
        tier,
        rule,
        device_memory,
    }
}

/// The device's total memory in bytes, when this platform reports it.
///
/// Read once per process from `/proc/meminfo`'s `MemTotal` on Linux and
/// Android — the kernel's own figure, readable without a permission on both —
/// and `None` everywhere else, which keeps every other platform on the tier
/// it took before the large one existed.
#[must_use]
pub fn device_memory_bytes() -> Option<u64> {
    #[cfg(any(target_os = "linux", target_os = "android"))]
    {
        use std::sync::OnceLock;
        static MEMORY: OnceLock<Option<u64>> = OnceLock::new();
        *MEMORY.get_or_init(|| {
            std::fs::read_to_string("/proc/meminfo")
                .ok()
                .and_then(|text| parse_mem_total(&text))
        })
    }
    #[cfg(not(any(target_os = "linux", target_os = "android")))]
    {
        None
    }
}

/// `MemTotal` from a `/proc/meminfo` text, in bytes; `None` when the line is
/// missing or not the `<n> kB` the kernel writes.
#[must_use]
pub fn parse_mem_total(meminfo: &str) -> Option<u64> {
    let line = meminfo
        .lines()
        .find_map(|line| line.strip_prefix("MemTotal:"))?;
    let mut fields = line.split_whitespace();
    let kib: u64 = fields.next()?.parse().ok()?;
    match fields.next() {
        Some("kB") => kib.checked_mul(1024),
        _ => None,
    }
}

/// The process's latch for the resolved-tier line, held by
/// [`AtlasBudget::for_caps`].
#[cfg(feature = "perf-trace")]
static ATLAS_TIER_LOG: TierLogOnce = TierLogOnce::new();

/// A one-shot latch for the resolved-tier line.
///
/// Once per *process* rather than once per call: a budget is resolved once per
/// surface, and the question the line answers — which tier this adapter took —
/// is about the device, so one line per run answers it completely while a line
/// per surface would repeat it.
///
/// A latch value rather than a bare [`std::sync::Once`] because the rule it
/// keeps is worth testing: [`emit`](Self::emit) answers whether *this* call was
/// the one that wrote the line, so a test constructs a latch of its own and
/// pins "the first call writes and no later one does" without installing a
/// process-global logger.
///
/// Compiled only under `perf-trace`, with the line it writes: the release-lean
/// gate asserts a shipping binary carries no `frust-perf` bytes at all, and a
/// `#[cfg]` is what makes that the compiler's answer rather than the
/// optimizer's.
#[cfg(feature = "perf-trace")]
#[derive(Debug, Default)]
pub struct TierLogOnce {
    emitted: AtomicBool,
}

#[cfg(feature = "perf-trace")]
impl TierLogOnce {
    /// A latch that has not emitted yet.
    #[must_use]
    pub const fn new() -> Self {
        Self {
            emitted: AtomicBool::new(false),
        }
    }

    /// Log [`atlas_tier_line`] for `caps`, `choice` and `budget` if this latch
    /// has not fired, answering whether this call was the one that did.
    pub fn emit(&self, caps: &TierCaps, choice: TierChoice, budget: AtlasBudget) -> bool {
        if self.emitted.swap(true, Ordering::Relaxed) {
            return false;
        }

        log::info!("{}", atlas_tier_line(caps, choice, budget));
        true
    }
}

/// The one line a run records the resolved atlas tier as.
///
/// `perf-trace`-only, with [`TierLogOnce`] — see its own doc for why the gate
/// is a `#[cfg]` rather than a branch.
///
/// `frust-perf`-prefixed deliberately: a benchmark capture keeps every line
/// carrying that prefix and drops the rest, so this is what lets a run's own
/// log say which budget the device was given rather than leaving it to be
/// inferred from whether images went missing. `rule` names which of
/// [`choose_tier`]'s rules decided, and `memory_mib` the device memory it was
/// decided against (`unknown` where the platform does not report it). The
/// adapter name is last because it is the one field that can contain spaces,
/// which keeps every `key=value` field ahead of it parseable by splitting on
/// whitespace.
#[cfg(feature = "perf-trace")]
#[must_use]
pub fn atlas_tier_line(caps: &TierCaps, choice: TierChoice, budget: AtlasBudget) -> String {
    let memory = choice
        .device_memory
        .map_or_else(|| "unknown".to_string(), |bytes| (bytes >> 20).to_string());

    format!(
        "frust-perf atlas tier={} rule={} budget={}x{}x{} downlevel={:?} \
         transient_saves_memory={} memory_mib={memory} adapter={}",
        choice.tier.label(),
        choice.rule.label(),
        budget.atlas_size.0,
        budget.atlas_size.1,
        budget.max_atlases,
        caps.downlevel_profile,
        caps.transient_saves_memory,
        caps.adapter_name,
    )
}

/// Where one image's texels live in the atlas array.
///
/// Hashable because the pending-clear list carries a set index beside it: the
/// same rectangle can reach [`ImageResidency::evictions`] twice before a single
/// clear has been serviced, and the dedup that keeps it reported once must not
/// be a linear scan of a list that grows with the churn rate.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct AtlasRegion {
    /// Array layer holding the region.
    pub layer: u32,
    /// Pixel offset of the region's top-left corner within that layer.
    pub offset: [u32; 2],
    /// Region extent in pixels.
    pub size: [u32; 2],
}

impl AtlasRegion {
    /// The bytes this region's texels occupy at [`ATLAS_FORMAT_BYTES`].
    #[must_use]
    pub fn byte_len(self) -> usize {
        (self.size[0] as usize)
            .saturating_mul(self.size[1] as usize)
            .saturating_mul(ATLAS_FORMAT_BYTES as usize)
    }

    /// The row stride a texel copy of this region uses.
    #[must_use]
    pub fn bytes_per_row(self) -> u32 {
        self.size[0].saturating_mul(ATLAS_FORMAT_BYTES as u32)
    }

    /// Whether this region has any texels at all.
    #[must_use]
    pub fn is_empty(self) -> bool {
        self.size[0] == 0 || self.size[1] == 0
    }
}

/// A resident image: the handle a paint names it by, and where its texels are.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct ResidentImage {
    /// The handle an [`ImageSource::OpaqueId`] carries.
    pub id: ImageId,
    /// The atlas rectangle holding the image's own pixels, padding excluded.
    ///
    /// Equal to [`natural`](Self::natural) unless the source was minified to
    /// fit the atlas, in which case it is the smaller rectangle actually
    /// uploaded.
    pub region: AtlasRegion,
    /// The source's own declared extent, before any minification.
    ///
    /// Carried because it is the extent a paint transform was composed against
    /// — `compile::paint::encode_image_command` derives its natural-to-dest
    /// mapping from the `ImageData`'s declared size, which it reads before this
    /// residency is consulted. A consumer that ends up sampling a *minified*
    /// rectangle has to fold [`minify_scale`](Self::minify_scale) into that
    /// transform, and this is the half of the ratio the atlas rectangle does
    /// not carry.
    pub natural: [u32; 2],
    /// Transparent padding pixels around `region` in the atlas.
    pub padding: u32,
    /// Whether the image's premultiplied pixels include any non-opaque texel.
    pub may_have_transparency: bool,
}

impl ResidentImage {
    /// This image as the paint-side source a `vello_common` encoding carries.
    #[must_use]
    pub fn source(&self) -> ImageSource {
        ImageSource::opaque_id_with_transparency_hint(self.id, self.may_have_transparency)
    }

    /// The per-axis factor mapping a natural-space texel coordinate onto the
    /// resident rectangle, or `None` when the source was stored at full size.
    ///
    /// `None` rather than `Some((1.0, 1.0))` so a caller can skip the rescale
    /// entirely on the overwhelmingly common path, and so "was this image
    /// minified?" is one question rather than a float comparison.
    #[must_use]
    pub fn minify_scale(&self) -> Option<(f32, f32)> {
        if self.region.size == self.natural {
            return None;
        }
        let natural_w = self.natural[0].max(1) as f32;
        let natural_h = self.natural[1].max(1) as f32;
        Some((
            self.region.size[0] as f32 / natural_w,
            self.region.size[1] as f32 / natural_h,
        ))
    }
}

/// One region's texels, waiting to be written into the atlas array.
///
/// The pixels are the premultiplied `Rgba8Unorm` the atlas stores, produced
/// once at allocation — an image made resident on frame 1 and drawn on frames
/// 1..1000 is converted exactly once.
#[derive(Debug, Clone)]
pub struct ImageUpload {
    /// The handle the paint naming these pixels carries.
    ///
    /// Carried so a consumer rebuilding its own view of residency can key this
    /// upload directly rather than pairing the frame's uploads positionally
    /// against its encoded paints. A plan re-emitted until it is acknowledged
    /// breaks that pairing (an upload can outlive the frame whose draw order
    /// produced it), and an upload that names itself needs no ordering to be
    /// read correctly.
    pub id: ImageId,
    /// Where the texels go.
    pub region: AtlasRegion,
    /// The source's own declared extent, before any minification — see
    /// [`ResidentImage::natural`], whose value this is.
    ///
    /// Carried on the upload because a consumer rebuilding its own view of
    /// residency from a compiled frame has the upload and nothing else, and
    /// `region` alone cannot say whether it holds a minified copy.
    pub natural: [u32; 2],
    /// The premultiplied pixels, row-major and exactly `region`'s extent.
    pub pixels: Arc<Pixmap>,
    /// Whether those pixels include any non-opaque texel — the same hint
    /// [`ResidentImage::may_have_transparency`] carries, so a consumer can
    /// rebuild the whole residency record from this upload alone.
    pub may_have_transparency: bool,
}

/// One cache entry: the handle, its rectangle, and when it was last drawn.
#[derive(Debug, Clone, Copy)]
struct Entry {
    id: ImageId,
    region: AtlasRegion,
    /// The source extent this entry was allocated for, which is what a later
    /// frame's `ImageData` is matched against — `region` may be the minified
    /// rectangle and so says nothing about the source.
    natural: [u32; 2],
    may_have_transparency: bool,
    last_seen: u64,
}

/// The images resident in the atlas array, keyed by blob identity.
///
/// One per compiler (and so one per surface). Its frame clock is advanced by
/// [`begin_frame`](Self::begin_frame), which is also where the age-based reap
/// runs — before any of the frame's own resolutions, so a rectangle freed this
/// frame is available to this frame's allocations and its clear is ordered
/// ahead of their uploads.
///
/// It also owns the atlas allocator the *glyph* policy packs into — see this
/// module's doc and [`allocator_mut`](Self::allocator_mut).
#[derive(Debug)]
pub struct ImageResidency {
    cache: ImageCache,
    budget: AtlasBudget,
    entries: HashMap<u64, Entry>,
    frame: u64,
    disabled: bool,
    uploads: Vec<ImageUpload>,
    evictions: Vec<AtlasRegion>,
    /// The pending clears' own set index: exactly the rectangles `evictions`
    /// holds, kept so [`release`](Self::release)'s "report a rectangle once"
    /// dedup is a hash lookup rather than a scan of a list whose length grows
    /// with the churn rate.
    eviction_set: HashSet<AtlasRegion>,
    skipped: u64,
    /// Rectangles taken back from a resident image because the packer had no
    /// room for a new one, over this residency's lifetime.
    pressure_evictions: u64,
    /// The same count for the current frame alone, zeroed by
    /// [`begin_frame`](Self::begin_frame) — what a per-frame report reads, since
    /// the lifetime total says nothing about whether the atlas is churning
    /// *now*.
    frame_pressure_evictions: u64,
    minified: u64,
    /// Blob keys whose minification has already been reported, so a source that
    /// is redrawn — or reaped and made resident again — logs once rather than
    /// once per residency.
    reported_minify: HashSet<u64>,
    /// Scratch for the reap's key list, kept so a frame that evicts allocates
    /// nothing.
    reaped: Vec<u64>,
    /// Scratch for the pressure planner's view of the layers — one rectangle
    /// per resident entry, grouped by layer — kept for the same reason: the
    /// frame that has to evict is the one least able to afford an allocation.
    pressure_rects: Vec<PressureRect>,
    /// Scratch for the candidate order over [`Self::pressure_rects`], least
    /// recently seen first. Built and sorted once per refused allocation, never
    /// once per eviction.
    pressure_order: Vec<usize>,
    /// Scratch for each layer's modelled free area, indexed by layer.
    pressure_free_area: Vec<u64>,
    /// Scratch for the distinct layers [`Self::pressure_order`] names, in the
    /// order their stalest candidate appears — the per-layer walk's own visit
    /// order, kept for the same no-allocation-while-evicting reason as the
    /// other pressure scratch.
    pressure_layers: Vec<u32>,
    /// Scratch for the keys an accepted plan releases.
    pressure_plan: Vec<u64>,
}

impl ImageResidency {
    /// A residency over `budget`'s atlas geometry.
    ///
    /// `FRUST_ENGINE_NO_ATLAS` is consulted here, once, rather than per
    /// resolution: the switch is process-global and cached, so re-reading it on
    /// a hot path would buy nothing but a lock. A residency constructed with
    /// the switch set is exactly [`disabled`](Self::disabled).
    #[must_use]
    pub fn new(budget: AtlasBudget) -> Self {
        Self::with_enabled(budget, !config::atlas_disabled())
    }

    /// A residency that refuses every image, allocating no atlas at all.
    ///
    /// What `FRUST_ENGINE_NO_ATLAS` selects, and what a caller wanting the same
    /// effect programmatically constructs. Every [`resolve`](Self::resolve)
    /// answers [`ImageSkip::AtlasDisabled`], so image draws are dropped with a
    /// logged reason rather than painted from an atlas that does not exist.
    #[must_use]
    pub fn disabled(budget: AtlasBudget) -> Self {
        Self::with_enabled(budget, false)
    }

    fn with_enabled(budget: AtlasBudget, enabled: bool) -> Self {
        Self {
            cache: ImageCache::new_with_config(budget.config()),
            budget,
            entries: HashMap::new(),
            frame: 0,
            disabled: !enabled,
            uploads: Vec::new(),
            evictions: Vec::new(),
            eviction_set: HashSet::new(),
            skipped: 0,
            pressure_evictions: 0,
            frame_pressure_evictions: 0,
            minified: 0,
            reported_minify: HashSet::new(),
            reaped: Vec::new(),
            pressure_rects: Vec::new(),
            pressure_order: Vec::new(),
            pressure_free_area: Vec::new(),
            pressure_layers: Vec::new(),
            pressure_plan: Vec::new(),
        }
    }

    /// A residency sized for `caps`' adapter.
    #[must_use]
    pub fn for_caps(caps: &TierCaps) -> Self {
        Self::new(AtlasBudget::for_caps(caps))
    }

    /// The atlas geometry this residency allocates within.
    #[must_use]
    pub fn budget(&self) -> AtlasBudget {
        self.budget
    }

    /// The single atlas allocator, for reading.
    ///
    /// Enough to resolve a handle either side allocated
    /// ([`ImageCache::get`]) or to count the layers created so far, without
    /// the mutable borrow allocation needs.
    #[must_use]
    pub fn allocator(&self) -> &ImageCache {
        &self.cache
    }

    /// The single atlas allocator, for allocating through.
    ///
    /// This is the seam the glyph policy takes its slots from — see this
    /// module's doc for why one cache rather than two is a correctness
    /// requirement. `ImageId`s and rectangles handed out through here and
    /// through [`resolve`](Self::resolve) come from the same packer and the
    /// same id space, so a glyph and an image can never be given the same
    /// handle or overlapping texels.
    ///
    /// **The contract a borrower keeps:** allocate what you like, and
    /// deallocate *only handles you allocated yourself*. This residency's entry
    /// map records the rectangles it allocated and nothing re-checks them;
    /// freeing one of them from outside would leave an entry claiming a
    /// rectangle the packer has since handed to someone else. Nothing here
    /// enforces that, because the type the two sides must share is
    /// `vello_common`'s own and cannot carry an ownership tag — the glyph
    /// policy deallocates strictly inside `glifo`'s eviction, over the handles
    /// in its own entry map.
    #[must_use]
    pub fn allocator_mut(&mut self) -> &mut ImageCache {
        &mut self.cache
    }

    /// Whether this residency refuses every image.
    #[must_use]
    pub fn is_disabled(&self) -> bool {
        self.disabled
    }

    /// How many images are currently resident.
    #[must_use]
    pub fn entry_count(&self) -> usize {
        self.entries.len()
    }

    /// The frame clock entries age against.
    #[must_use]
    pub fn frame(&self) -> u64 {
        self.frame
    }

    /// How many atlas layers have been created so far.
    ///
    /// The array texture must be at least this deep before the frame's uploads
    /// are written.
    ///
    /// Read off the shared allocator rather than accumulated from this
    /// residency's own allocations, so a layer the *glyph* policy created
    /// counts too: the two classes pack into one array, and an array sized to
    /// the image half alone would refuse to hold the glyph half's pages. The
    /// count only ever grows — `MultiAtlasManager` appends layers and never
    /// drops one — so a depth reported on an earlier frame stays valid.
    #[must_use]
    pub fn layers(&self) -> u32 {
        u32::try_from(self.cache.atlas_count()).unwrap_or(u32::MAX)
    }

    /// How many resolutions have been refused over this residency's lifetime.
    ///
    /// Counts only images that could not be uploaded at all — a format or
    /// extent the conversion refuses, a disabled atlas, or an atlas with no
    /// room left *and nothing evictable to make room with*. A source merely
    /// larger than one atlas layer is minified to fit, and one that displaced
    /// another image is counted by
    /// [`pressure_evictions`](Self::pressure_evictions) instead; neither
    /// reaches this counter.
    #[must_use]
    pub fn skipped(&self) -> u64 {
        self.skipped
    }

    /// How many resident images have been evicted to make room for another
    /// over this residency's lifetime.
    ///
    /// The price of never refusing a draw the atlas could hold *something*
    /// for: each one is an image that will re-upload the next frame it is
    /// drawn on. A steady screen leaves this at zero; a scroll through more
    /// distinct images than the atlas has rectangles for grows it once per
    /// displaced image, which is the signal that the budget — not the
    /// mechanism — is what a frame is paying for.
    #[must_use]
    pub fn pressure_evictions(&self) -> u64 {
        self.pressure_evictions
    }

    /// How many resident images the *current* frame has evicted to make room.
    ///
    /// Zeroed by [`begin_frame`](Self::begin_frame), so a per-frame report reads
    /// this rather than differencing
    /// [`pressure_evictions`](Self::pressure_evictions) itself.
    #[must_use]
    pub fn frame_pressure_evictions(&self) -> u64 {
        self.frame_pressure_evictions
    }

    /// How many sources have been downsampled to fit the atlas over this
    /// residency's lifetime.
    ///
    /// Observational, and the counter that makes "an image too large for the
    /// atlas is minified, not dropped" measurable rather than asserted.
    #[must_use]
    pub fn minified(&self) -> u64 {
        self.minified
    }

    /// The regions whose texels must be cleared before the pending uploads.
    ///
    /// Pending rather than per-frame: an entry freed on a frame nobody
    /// serviced is still waiting to be cleared on the next one. Both kinds of
    /// eviction land here — the age-based reap at the head of a frame and a
    /// rectangle taken back under pressure mid-frame — so the consumer's
    /// clear-then-upload contract covers the second without knowing it exists.
    #[must_use]
    pub fn evictions(&self) -> &[AtlasRegion] {
        &self.evictions
    }

    /// The regions whose texels must be written after the pending evictions.
    ///
    /// Pending rather than per-frame: an image made resident on a frame that was
    /// refused is still waiting to be written on the next one.
    #[must_use]
    pub fn uploads(&self) -> &[ImageUpload] {
        &self.uploads
    }

    /// A copy of the pending clear-then-write plan, evictions first.
    ///
    /// A *copy*, not a drain: the plan stays pending until
    /// [`acknowledge_plan`](Self::acknowledge_plan) says it was serviced against
    /// a live atlas, so a frame refused after compiling re-emits the same plan
    /// on the next frame rather than losing it (see the module doc). The upload
    /// pixels are behind an `Arc`, so a re-emitted plan copies handles rather
    /// than texels, and the steady-state plan is empty on both counts.
    ///
    /// The clears are [`evictions`](Self::evictions) less every rectangle a
    /// pending upload covers exactly (see [`uncovered_clears`]): that upload
    /// writes every texel the clear would have zeroed, after it, in the same
    /// plan — so the clear is pure cost, a rectangle's worth of zeroes per
    /// churned image. Dropped from the plan rather than from `evictions`, so
    /// the pending set and its acknowledgement are unchanged; a consumer
    /// forgetting the evicted tenant by rectangle (the renderer's registry)
    /// still needs the full eviction set: [`evictions`](Self::evictions) names
    /// every rectangle to forget, whether or not its clear was skipped.
    #[must_use]
    pub fn plan(&self) -> (Vec<AtlasRegion>, Vec<ImageUpload>) {
        (
            uncovered_clears(&self.evictions, &self.uploads),
            self.uploads.clone(),
        )
    }

    /// Whether any part of the pending plan is still unserviced.
    #[must_use]
    pub fn has_pending_plan(&self) -> bool {
        !self.evictions.is_empty() || !self.uploads.is_empty()
    }

    /// Record that the pending plan reached the atlas array, clearing it.
    ///
    /// Call this only once every region it names has actually been written or
    /// cleared. Calling it on a frame that was refused, or whose writes the
    /// array declined, is exactly the defect the pending plan exists to prevent:
    /// the entry map would go on claiming an image is resident whose texels are
    /// whatever the atlas texture happened to hold.
    pub fn acknowledge_plan(&mut self) {
        self.evictions.clear();
        self.eviction_set.clear();
        self.uploads.clear();
    }

    /// Advance the frame clock and reclaim everything unseen for
    /// [`MAX_UNSEEN_FRAMES`].
    ///
    /// Call once at the head of a frame, before any [`resolve`](Self::resolve).
    /// An unacknowledged plan deliberately survives this: the frame that was to
    /// service it may have been refused, and dropping it here is what would turn
    /// a refused frame into a permanently unwritten atlas region.
    pub fn begin_frame(&mut self) {
        self.frame = self.frame.saturating_add(1);
        self.frame_pressure_evictions = 0;
        self.reap();
    }

    /// The atlas rectangle for `data`'s pixels, allocating and scheduling an
    /// upload on the first frame that asks for it.
    ///
    /// # Errors
    ///
    /// Returns the [`ImageSkip`] naming why the image was refused. Every
    /// refusal happens before any `vello_common` call that could assert on the
    /// same condition, so an image the renderer cannot hold is a skipped draw
    /// rather than a panicked frame (E17).
    pub fn resolve(&mut self, data: &ImageData) -> Result<ResidentImage, ImageSkip> {
        let result = self.resolve_inner(data);
        if result.is_err() {
            self.skipped = self.skipped.saturating_add(1);
        }
        result
    }

    fn resolve_inner(&mut self, data: &ImageData) -> Result<ResidentImage, ImageSkip> {
        if self.disabled {
            return Err(ImageSkip::AtlasDisabled);
        }

        check_supported(data)?;

        let key = data.data.id();
        if let Some(entry) = self.entries.get_mut(&key)
            && entry.natural == [data.width, data.height]
        {
            entry.last_seen = self.frame;
            return Ok(ResidentImage {
                id: entry.id,
                region: entry.region,
                natural: entry.natural,
                padding: u32::from(ATLAS_PADDING),
                may_have_transparency: entry.may_have_transparency,
            });
        }

        // A blob id recorded at a different extent is a caller that rebuilt the
        // metadata around the same buffer. Its old rectangle describes the
        // wrong pixels, so it is released before the new one is taken rather
        // than left to age out holding both — unless *this* frame has already
        // resolved it at the other extent. The plan clears before it uploads,
        // so freeing that rectangle would schedule a clear over texels a draw
        // this same frame already recorded, which is exactly what the pressure
        // path refuses to do. The second extent is skipped instead, on the same
        // terms as a frame whose own working set outgrows the atlas — but this
        // is a same-frame ordering conflict on one blob key, not an atlas
        // capacity shortage, so it is reported as its own reason rather than
        // folded into [`ImageSkip::NoAtlasSpace`].
        if self
            .entries
            .get(&key)
            .is_some_and(|entry| entry.last_seen == self.frame)
        {
            let (width, height) = fit_extent([data.width, data.height], self.budget.atlas_size);
            return Err(ImageSkip::SameFrameExtentConflict { width, height });
        }
        self.release(key);

        // Checked above, so this cannot assert; the conversion is also the one
        // place the straight-alpha premultiply happens, and it happens once per
        // image rather than once per frame.
        let ImageSource::Pixmap(pixels) = ImageSource::from_peniko_image_data(data) else {
            // `from_peniko_image_data` only ever yields a pixmap; an id-backed
            // source would mean a `vello_common` change, which is a skip rather
            // than an unreachable.
            return Err(ImageSkip::UnsupportedFormat(data.format));
        };

        let natural = [data.width, data.height];
        // Minified before the allocation rather than after it: the packer is
        // asked for the rectangle that is actually going to be uploaded, so an
        // over-ceiling source never takes (and then has to give back) space at
        // its declared extent.
        let (width, height) = fit_extent(natural, self.budget.atlas_size);
        let pixels = if [width, height] == natural {
            pixels
        } else {
            self.note_minify(key, natural, [width, height]);
            Arc::new(minify(&pixels, width, height))
        };

        let no_space = ImageSkip::NoAtlasSpace {
            width,
            height,
            atlas_width: self.budget.atlas_size.0,
            atlas_height: self.budget.atlas_size.1,
        };

        let id = match self.cache.allocate(width, height, ATLAS_PADDING) {
            Ok(id) => id,
            // The atlas is full. Take a rectangle back from an image no longer
            // on screen rather than dropping this draw — see
            // [`allocate_under_pressure`](Self::allocate_under_pressure), and
            // the module doc for why a hole is worse than a re-upload.
            Err(_) => self
                .allocate_under_pressure(width, height)
                .ok_or(no_space)?,
        };

        let Some(resource) = self.cache.get(id) else {
            // A successful allocation always populates its slot; refusing here
            // keeps the frame path free of the `expect` the reference renderer
            // takes at this same point.
            return Err(no_space);
        };

        let region = AtlasRegion {
            layer: resource.atlas_id.as_u32(),
            offset: resource.offsets(),
            size: resource.size(),
        };
        let may_have_transparency = pixels.may_have_transparency();

        self.entries.insert(
            key,
            Entry {
                id,
                region,
                natural,
                may_have_transparency,
                last_seen: self.frame,
            },
        );
        self.uploads.push(ImageUpload {
            id,
            region,
            natural,
            pixels,
            may_have_transparency,
        });

        Ok(ResidentImage {
            id,
            region,
            natural,
            padding: u32::from(ATLAS_PADDING),
            may_have_transparency,
        })
    }

    /// Allocate `width` x `height` by taking rectangles back from resident
    /// images, least recently seen first, once the packer has refused.
    ///
    /// Answers the handle that finally fit, or `None` when no bounded set of
    /// candidates would have made room — which is what
    /// [`ImageSkip::NoAtlasSpace`] then reports, with the residency exactly as
    /// it was found.
    ///
    /// **Nothing is released until a plan that fits has been found.** An
    /// eviction that does not produce the allocation buys nothing and costs a
    /// clear plus a re-upload per rectangle, every frame the condition recurs
    /// (see the module doc). So the search runs over a *model* of the layers
    /// and commits only when the model says the request would then be placed:
    ///
    /// 1. **Candidates.** Only an entry last seen on an earlier frame. The
    ///    frame path services [`evictions`](Self::evictions) *before*
    ///    [`uploads`](Self::uploads), so freeing a rectangle this frame has
    ///    already resolved would schedule a clear over texels the same frame is
    ///    about to sample — painting the very hole this eviction exists to
    ///    prevent. A frame whose own working set outgrows the atlas therefore
    ///    still skips, because everything in the atlas belongs to it.
    /// 2. **Order.** `(last_seen, slot, key)`, least recently seen first, built
    ///    and sorted *once* per refused allocation rather than once per
    ///    eviction. The slot index breaks a tie between two entries last drawn
    ///    on the same frame, so which image is displaced is a property of the
    ///    residency rather than of the entry map's iteration order. Because the
    ///    order is by age, an entry drawn on the *immediately preceding* frame
    ///    is reached only after every staler one has already been planned —
    ///    which is what keeps a large image redrawn every frame from thrashing
    ///    a working set that is merely idle. The candidates are then walked one
    ///    *layer* at a time, layers visited in that same staleness order (the
    ///    layer whose stalest candidate sorts earliest goes first).
    /// 3. **Fit.** After each candidate joins the plan, the layer it sat on is
    ///    re-tested: the request must fit inside the freed area that layer now
    ///    models, and must clear every rectangle still occupying it
    ///    ([`layer_admits`]). The first prefix that passes ends the search.
    /// 4. **Budget, per layer.** Each layer's own walk stops after
    ///    [`MAX_PRESSURE_EVICTION_CANDIDATES`] of *that layer's* candidates, or
    ///    once *that layer's* plan has freed [`PRESSURE_EVICTION_AREA_BUDGET`]
    ///    times the requested area — a fresh allowance per layer rather than one
    ///    pool the whole residency shares, so candidates on a layer too
    ///    fragmented to ever admit the request cannot spend the allowance a
    ///    later layer needed to prove it could. A layer that ends on its own
    ///    budget contributes nothing and the walk moves to the next layer.
    /// 5. **Commit.** Only the planned rectangles *on the layer the fit was
    ///    found in* are released — a candidate the walk passed over on another
    ///    layer contributed nothing and is left resident.
    ///
    /// The model is optimistic about occupants it cannot see: the glyph policy
    /// packs through the same allocator (see the module doc) and its pages are
    /// not in this entry map, so [`layer_admits`] tests a subset of the real
    /// obstacles. That direction is the safe one — a placement clearing every
    /// real occupant clears the subset too, so a request eviction could satisfy
    /// on a *given layer* is never refused for that reason. The converse is
    /// possible: the packer may refuse a plan the model approved, which costs
    /// the plan's evictions and no allocation. It is bounded by the same
    /// per-layer budget as the plan itself, and it is the one residual the
    /// planner cannot rule out without knowing where every glyph page sits.
    fn allocate_under_pressure(&mut self, width: u32, height: u32) -> Option<ImageId> {
        // The overwhelmingly common refusal — a frame that filled the atlas
        // with its own draws — costs one pass over the entry map and no model
        // at all.
        if !self
            .entries
            .values()
            .any(|entry| entry.last_seen < self.frame)
        {
            return None;
        }

        // Taken from the residency's own scratch, so the frame least able to
        // afford an allocation does not make one to evict with.
        let mut rects = std::mem::take(&mut self.pressure_rects);
        let mut order = std::mem::take(&mut self.pressure_order);
        let mut free_area = std::mem::take(&mut self.pressure_free_area);
        let mut layers = std::mem::take(&mut self.pressure_layers);
        let mut plan = std::mem::take(&mut self.pressure_plan);

        self.model_layers(&mut rects, &mut order, &mut free_area);
        let fit = plan_pressure_eviction(
            width,
            height,
            self.budget.atlas_size,
            &mut rects,
            &order,
            &mut free_area,
            &mut layers,
        );

        plan.clear();
        if let Some(layer) = fit {
            plan.extend(
                rects
                    .iter()
                    .filter(|rect| rect.freed && rect.layer == layer)
                    .map(|rect| rect.key),
            );
        }

        let allocated = if plan.is_empty() {
            None
        } else {
            for key in &plan {
                self.release(*key);
                self.pressure_evictions = self.pressure_evictions.saturating_add(1);
                self.frame_pressure_evictions = self.frame_pressure_evictions.saturating_add(1);
            }
            self.cache.allocate(width, height, ATLAS_PADDING).ok()
        };

        rects.clear();
        order.clear();
        free_area.clear();
        layers.clear();
        plan.clear();
        self.pressure_rects = rects;
        self.pressure_order = order;
        self.pressure_free_area = free_area;
        self.pressure_layers = layers;
        self.pressure_plan = plan;
        allocated
    }

    /// Fill the planner's scratch with this residency's own view of the atlas.
    ///
    /// `rects` takes one entry per resident image, grouped by layer so a fit
    /// test reads one layer's slice; `order` takes the candidate indices, least
    /// recently seen first across the *whole* residency — [`plan_pressure_eviction`]
    /// is what regroups it per layer for its own bounded walk; `free_area` takes
    /// each layer's free area *as the packer reports it*, which is the one
    /// figure that accounts for the glyph pages this map cannot see.
    fn model_layers(
        &self,
        rects: &mut Vec<PressureRect>,
        order: &mut Vec<usize>,
        free_area: &mut Vec<u64>,
    ) {
        rects.clear();
        rects.extend(self.entries.iter().map(|(key, entry)| {
            let region = padded(entry.region, ATLAS_PADDING);
            PressureRect {
                key: *key,
                layer: region.layer,
                x0: region.offset[0],
                y0: region.offset[1],
                x1: region.offset[0].saturating_add(region.size[0]),
                y1: region.offset[1].saturating_add(region.size[1]),
                area: u64::from(region.size[0]).saturating_mul(u64::from(region.size[1])),
                last_seen: entry.last_seen,
                slot: entry.id.as_u32(),
                freed: false,
            }
        }));
        rects.sort_unstable_by_key(|rect| rect.layer);

        order.clear();
        order.extend(
            rects
                .iter()
                .enumerate()
                .filter_map(|(index, rect)| (rect.last_seen < self.frame).then_some(index)),
        );
        order.sort_unstable_by_key(|index| {
            let rect = &rects[*index];
            (rect.last_seen, rect.slot, rect.key)
        });

        free_area.clear();
        free_area.resize(self.cache.atlas_count(), 0);
        for (id, stats) in self.cache.atlas_manager().atlas_stats() {
            if let Some(free) = free_area.get_mut(id.as_u32() as usize) {
                *free = u64::from(stats.total_area.saturating_sub(stats.allocated_area));
            }
        }
    }

    /// Count and report one source downsampled to fit the atlas.
    ///
    /// Once per blob id, at warning level: a minified image is a silent quality
    /// decision the application may not have intended (a full-resolution photo
    /// where a thumbnail was wanted), so it says so — but a scene redrawing it
    /// every frame must not repeat the message.
    fn note_minify(&mut self, key: u64, natural: [u32; 2], fitted: [u32; 2]) {
        self.minified = self.minified.saturating_add(1);
        if !self.reported_minify.insert(key) {
            return;
        }
        log::warn!(
            "image {}x{} is larger than the {}x{} atlas budget; uploading a {}x{} box-filtered \
             copy instead (reported once per image)",
            natural[0],
            natural[1],
            self.budget.atlas_size.0,
            self.budget.atlas_size.1,
            fitted[0],
            fitted[1],
        );
    }

    /// Deallocate every entry unseen for [`MAX_UNSEEN_FRAMES`], recording each
    /// one's rectangle for clearing.
    ///
    /// The age half of the two bounds; the population half is
    /// [`allocate_under_pressure`](Self::allocate_under_pressure), which runs
    /// only when an allocation has already failed. Keeping them separate is
    /// what makes an idle screen give its texels back promptly whether or not
    /// anything is asking for them.
    fn reap(&mut self) {
        let mut reaped = std::mem::take(&mut self.reaped);
        reaped.clear();
        reaped.extend(self.entries.iter().filter_map(|(key, entry)| {
            (self.frame.saturating_sub(entry.last_seen) > MAX_UNSEEN_FRAMES).then_some(*key)
        }));

        for key in reaped.drain(..) {
            self.release(key);
        }
        self.reaped = reaped;
    }

    /// Drop `key`'s entry, freeing its atlas rectangle and recording it for
    /// clearing.
    ///
    /// The padded rectangle is what is cleared: an allocation reserves its
    /// padding too, so leaving the border behind would leave stale texels a
    /// later allocation could sample through.
    ///
    /// An upload still pending for that rectangle goes with the entry. It
    /// describes pixels no live entry claims any more, and writing them after
    /// the clear would put an evicted image back into a rectangle the packer has
    /// already handed to something else.
    fn release(&mut self, key: u64) {
        let Some(entry) = self.entries.remove(&key) else {
            return;
        };
        // The handle came from this cache's own `allocate`, so the atlas it
        // names exists and the deallocation cannot fail.
        self.cache.deallocate(entry.id);
        self.uploads.retain(|upload| upload.id != entry.id);

        // The plan survives an unacknowledged frame, so the same rectangle can
        // reach here twice before a single clear has been issued for it; one
        // clear is what the second one would write anyway.
        let region = padded(entry.region, ATLAS_PADDING);
        if self.eviction_set.insert(region) {
            self.evictions.push(region);
        }
    }
}

/// The most rectangles one refused allocation may plan to take back from a
/// single atlas layer.
///
/// A bound on the *search*, not only on the damage: a layer whose walk
/// reaches it without finding a fit contributes nothing to the plan, so the
/// cost of one hopeless layer stays bounded rather than eating into the
/// allowance a later, satisfiable layer needs to prove itself. Sixty-four is
/// far past what any satisfiable request has needed — a fit is normally found
/// in one, since the rectangle handed back is usually the size of the one
/// being asked for — while still being a small fraction of what a
/// mobile-tier layer can hold. Applied once per layer a refused allocation
/// considers, never once across every layer together — see
/// [`plan_pressure_eviction`].
const MAX_PRESSURE_EVICTION_CANDIDATES: usize = 64;

/// How much area one refused allocation may plan to free from a single atlas
/// layer, as a multiple of the area it asked for.
///
/// The other half of the bound, and the one that scales with the request: a
/// layer that has already given back four times the texels the request wants
/// and still has nowhere to put them is fighting fragmentation, not scarcity,
/// and freeing more of that same layer will not fix it. Applied per layer,
/// the same way [`MAX_PRESSURE_EVICTION_CANDIDATES`] is.
const PRESSURE_EVICTION_AREA_BUDGET: u64 = 4;

/// One resident rectangle as the pressure planner sees it.
///
/// A flat copy rather than a borrow of the entry map, because the plan is drawn
/// up while the map is still intact and the release that follows needs it
/// mutably. `x1`/`y1` are exclusive, and every field is the *padded* allocation
/// — the rectangle the packer actually holds — rather than the image's own.
#[derive(Debug, Clone, Copy)]
struct PressureRect {
    /// Blob key of the entry holding this rectangle.
    key: u64,
    /// Array layer the rectangle sits in.
    layer: u32,
    /// Left edge, inclusive.
    x0: u32,
    /// Top edge, inclusive.
    y0: u32,
    /// Right edge, exclusive.
    x1: u32,
    /// Bottom edge, exclusive.
    y1: u32,
    /// Texels the rectangle covers.
    area: u64,
    /// The entry's frame stamp, which is what makes it a candidate or not.
    last_seen: u64,
    /// The entry's slot index, which breaks a tie between two entries last
    /// drawn on the same frame.
    slot: u32,
    /// Whether the plan under construction has given this rectangle back.
    freed: bool,
}

/// The layer a bounded prefix of `order` would make room in, or `None`.
///
/// Frees nothing: it marks `rects` and grows `free_area` in the model only, so
/// a request no prefix satisfies leaves the residency untouched. `order` is
/// least-recently-seen-first across the *whole* residency; this regroups it
/// one layer at a time — `layers` scratch, cleared and refilled here, in the
/// order each layer's stalest candidate appears in `order` — and gives each
/// layer its own [`MAX_PRESSURE_EVICTION_CANDIDATES`]/
/// [`PRESSURE_EVICTION_AREA_BUDGET`] allowance rather than pooling one across
/// every layer. Without that split, a layer with many candidates that could
/// never admit the request (too fragmented, or blocked by an occupant no
/// eviction touches) spends the whole budget before a later, satisfiable
/// layer is ever tried — the request is then refused even though a bounded
/// eviction on that later layer would have placed it. See
/// [`ImageResidency::allocate_under_pressure`] for the whole shape and for why
/// the answer is a layer rather than a bool — only the planned rectangles on
/// the fitting layer are worth releasing.
fn plan_pressure_eviction(
    width: u32,
    height: u32,
    atlas_size: (u32, u32),
    rects: &mut [PressureRect],
    order: &[usize],
    free_area: &mut [u64],
    layers: &mut Vec<u32>,
) -> Option<u32> {
    let padding = u32::from(ATLAS_PADDING).saturating_mul(2);
    let request_w = width.saturating_add(padding);
    let request_h = height.saturating_add(padding);
    let request_area = u64::from(request_w).saturating_mul(u64::from(request_h));
    let area_budget = request_area.saturating_mul(PRESSURE_EVICTION_AREA_BUDGET);

    layers.clear();
    for &index in order {
        let Some(rect) = rects.get(index) else {
            continue;
        };
        if !layers.contains(&rect.layer) {
            layers.push(rect.layer);
        }
    }

    for &layer in layers.iter() {
        let mut layer_freed_area = 0_u64;
        let mut step = 0_usize;

        for &index in order {
            let on_this_layer = rects.get(index).is_some_and(|rect| rect.layer == layer);
            if !on_this_layer {
                continue;
            }
            if step >= MAX_PRESSURE_EVICTION_CANDIDATES || layer_freed_area > area_budget {
                break;
            }
            step += 1;

            let area = {
                let Some(rect) = rects.get_mut(index) else {
                    continue;
                };
                rect.freed = true;
                rect.area
            };
            layer_freed_area = layer_freed_area.saturating_add(area);

            let Some(free) = free_area.get_mut(layer as usize) else {
                continue;
            };
            *free = free.saturating_add(area);
            // Area first because it is exact — it comes off the packer's own
            // accounting, so it counts the glyph pages `layer_admits` cannot see —
            // and because it is the cheaper of the two questions.
            if *free >= request_area && layer_admits(rects, layer, request_w, request_h, atlas_size)
            {
                return Some(layer);
            }
        }
    }

    None
}

/// Whether a `width` x `height` rectangle could still be placed in `layer` once
/// every rectangle the plan has marked freed is out of the way.
///
/// A *necessary* condition rather than an exact packing answer. Any placement
/// has to avoid each occupant whole, so it must lie entirely to one side of it:
/// if some occupant leaves less than the requested extent on all four sides,
/// there is nowhere for the rectangle to go and the plan cannot succeed. A plan
/// this refuses is therefore certainly hopeless; one it admits may still be
/// refused by the packer, which is the direction that costs a bounded number of
/// evictions rather than a wrong picture (see
/// [`ImageResidency::allocate_under_pressure`]).
///
/// `rects` is grouped by layer, so this reads one layer's slice rather than the
/// whole residency.
fn layer_admits(
    rects: &[PressureRect],
    layer: u32,
    width: u32,
    height: u32,
    atlas_size: (u32, u32),
) -> bool {
    if width > atlas_size.0 || height > atlas_size.1 {
        return false;
    }

    let start = rects.partition_point(|rect| rect.layer < layer);
    let end = rects.partition_point(|rect| rect.layer <= layer);
    rects[start..end]
        .iter()
        .filter(|rect| !rect.freed)
        .all(|rect| {
            width <= rect.x0
                || width <= atlas_size.0.saturating_sub(rect.x1)
                || height <= rect.y0
                || height <= atlas_size.1.saturating_sub(rect.y1)
        })
}

/// The extent `natural` is stored at inside an `atlas`-sized layer: itself when
/// it already fits, and otherwise the largest rectangle of the same aspect ratio
/// that does.
///
/// The scale is taken as the smaller of the two axis ratios and applied to both,
/// so the two axes of the paint transform stay in step — a per-axis fit would
/// stretch the image. Each axis is floored (never rounding *up* past the layer)
/// and then held at one texel: a source that scales below a whole texel still
/// has to have somewhere to be, and a zero-extent allocation is not a rectangle
/// the packer or the shader can address.
#[must_use]
pub fn fit_extent(natural: [u32; 2], atlas: (u32, u32)) -> (u32, u32) {
    let (width, height) = (natural[0], natural[1]);
    if width <= atlas.0 && height <= atlas.1 {
        return (width, height);
    }

    let scale = (f64::from(atlas.0) / f64::from(width.max(1)))
        .min(f64::from(atlas.1) / f64::from(height.max(1)));
    let axis = |value: u32, ceiling: u32| {
        let scaled = (f64::from(value) * scale).floor();
        // `as` on a non-finite or out-of-range float saturates in Rust, so the
        // clamps below are the whole check rather than a second line of defence.
        (scaled as u32).clamp(1, ceiling.max(1))
    };

    (axis(width, atlas.0), axis(height, atlas.1))
}

/// `source` box-filtered down to `width` x `height`.
///
/// Every output texel is the unweighted mean of the half-open source rectangle
/// it covers, computed on the premultiplied bytes the atlas stores — which is
/// the space the samples are composited in, so averaging there is what keeps a
/// partially transparent source from bleeding its colour outward.
///
/// Minification only: a request at or above the source's own extent copies the
/// texels straight across rather than inventing any, since a box filter has no
/// magnification arm and the caller never asks for one ([`fit_extent`] only ever
/// shrinks).
#[must_use]
pub fn minify(source: &Pixmap, width: u32, height: u32) -> Pixmap {
    let source_w = u32::from(source.width());
    let source_h = u32::from(source.height());
    let width = width.clamp(1, source_w.max(1));
    let height = height.clamp(1, source_h.max(1));

    let texels = source.data();
    let mut out = Vec::with_capacity((width as usize).saturating_mul(height as usize));
    let mut may_have_transparency = false;

    for y in 0..height {
        // Half-open source rows, derived from the output row rather than
        // accumulated, so rounding never leaves a gap or an overlap between
        // consecutive rows.
        let y0 = (u64::from(y) * u64::from(source_h) / u64::from(height)) as u32;
        let y1 = ((u64::from(y) + 1) * u64::from(source_h) / u64::from(height)) as u32;
        let y1 = y1.max(y0.saturating_add(1)).min(source_h);

        for x in 0..width {
            let x0 = (u64::from(x) * u64::from(source_w) / u64::from(width)) as u32;
            let x1 = ((u64::from(x) + 1) * u64::from(source_w) / u64::from(width)) as u32;
            let x1 = x1.max(x0.saturating_add(1)).min(source_w);

            let mut sum = [0_u64; 4];
            let mut count = 0_u64;
            for row in y0..y1 {
                let base = (row as usize).saturating_mul(source_w as usize);
                for column in x0..x1 {
                    let Some(texel) = texels.get(base.saturating_add(column as usize)) else {
                        continue;
                    };
                    sum[0] += u64::from(texel.r);
                    sum[1] += u64::from(texel.g);
                    sum[2] += u64::from(texel.b);
                    sum[3] += u64::from(texel.a);
                    count += 1;
                }
            }

            // A count of zero is unreachable — both ranges are non-empty by
            // construction — and flooring it at one rather than branching keeps
            // the divide-by-zero panic off the frame path (E17) while answering
            // a transparent texel if it ever were reachable, since the sums
            // would then be zero too.
            let divisor = count.max(1);
            // Round to nearest rather than truncating: a uniform source has to
            // come back bit-identical, which truncating an exact integer mean
            // already gives, but a near-uniform one must not drift a level
            // darker.
            let mean = |channel: usize| {
                ((sum[channel] + divisor / 2) / divisor).min(u64::from(u8::MAX)) as u8
            };

            let texel = PremulRgba8 {
                r: mean(0),
                g: mean(1),
                b: mean(2),
                a: mean(3),
            };
            may_have_transparency |= texel.a != u8::MAX;
            out.push(texel);
        }
    }

    // Both extents were clamped into `1..=u16::MAX` above (a source pixmap's own
    // extents are `u16`), so neither conversion can lose information.
    Pixmap::from_parts_with_opacity(
        out,
        width.min(u32::from(u16::MAX)) as u16,
        height.min(u32::from(u16::MAX)) as u16,
        may_have_transparency,
    )
}

/// `axis` held at or below `ceiling`, and never below the one texel an
/// addressable rectangle needs.
fn fit_axis(axis: u32, ceiling: u64) -> u32 {
    axis.min(u32::try_from(ceiling).unwrap_or(u32::MAX)).max(1)
}

/// `region` grown by `padding` on every side, clamped at the layer origin.
fn padded(region: AtlasRegion, padding: u16) -> AtlasRegion {
    let padding = u32::from(padding);
    let pad_x = padding.min(region.offset[0]);
    let pad_y = padding.min(region.offset[1]);

    AtlasRegion {
        layer: region.layer,
        offset: [region.offset[0] - pad_x, region.offset[1] - pad_y],
        size: [
            region.size[0].saturating_add(pad_x + padding),
            region.size[1].saturating_add(pad_y + padding),
        ],
    }
}

/// Refuse an image `vello_common`'s own conversion would assert on.
///
/// The three conditions are exactly the three assertions downstream: the
/// format `unimplemented!()`, the `u16::MAX` dimension check, and
/// `Pixmap::from_parts_with_opacity`'s length equality. Checking them here is
/// what keeps the panic off the frame path.
fn check_supported(data: &ImageData) -> Result<(), ImageSkip> {
    match data.format {
        ImageFormat::Rgba8 | ImageFormat::Bgra8 => {}
        format => return Err(ImageSkip::UnsupportedFormat(format)),
    }

    if data.width == 0 || data.height == 0 {
        return Err(ImageSkip::DegenerateExtent {
            width: data.width,
            height: data.height,
        });
    }

    if data.width > MAX_IMAGE_DIMENSION || data.height > MAX_IMAGE_DIMENSION {
        return Err(ImageSkip::TooLarge {
            width: data.width,
            height: data.height,
            max: MAX_IMAGE_DIMENSION,
        });
    }

    let actual = data.data.data().len();
    let expected = data
        .format
        .size_in_bytes(data.width, data.height)
        .unwrap_or(usize::MAX);
    if actual != expected {
        return Err(ImageSkip::MalformedPixels {
            width: data.width,
            height: data.height,
            expected,
            actual,
        });
    }

    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use peniko::{Blob, ImageAlphaType};

    fn image(width: u32, height: u32) -> ImageData {
        let len = (width as usize) * (height as usize) * 4;
        ImageData {
            data: Blob::new(Arc::new(vec![255_u8; len])),
            format: ImageFormat::Rgba8,
            alpha_type: ImageAlphaType::Alpha,
            width,
            height,
        }
    }

    fn residency() -> ImageResidency {
        ImageResidency::new(AtlasBudget {
            atlas_size: (64, 64),
            max_atlases: 2,
        })
    }

    #[test]
    fn a_mobile_budget_is_never_the_vello_default() {
        let default = AtlasConfig::default();
        for budget in [AtlasBudget::MOBILE, AtlasBudget::DESKTOP] {
            assert_ne!(budget.atlas_size, default.atlas_size);
            assert!(budget.total_bytes() < 4096 * 4096 * ATLAS_FORMAT_BYTES * 8);
        }
        assert_eq!(AtlasBudget::MOBILE.total_bytes(), 16 << 20);
        assert_eq!(AtlasBudget::DESKTOP.total_bytes(), 128 << 20);
    }

    #[test]
    fn a_downlevel_adapter_takes_the_mobile_budget() {
        let webgl2 = TierCaps::fake(DownlevelProfile::WebGl2);
        assert!(is_mobile_tier(&webgl2));

        let mut tiler = TierCaps::fake(DownlevelProfile::Full);
        assert!(!is_mobile_tier(&tiler));
        tiler.transient_saves_memory = true;
        assert!(is_mobile_tier(&tiler));
    }

    fn named(name: &str) -> TierCaps {
        let mut caps = TierCaps::fake(DownlevelProfile::Full);
        caps.adapter_name = name.to_string();
        caps
    }

    #[test]
    fn the_adapter_table_names_the_mobile_and_desktop_families() {
        for name in [
            "Adreno (TM) 618",
            "Adreno (TM) 660",
            "Mali-G78 MP14",
            "Immortalis-G715",
            "PowerVR B-Series BXM-8-256",
            "Samsung Xclipse 920",
            "Maleoon 910",
            "Apple A15 GPU",
            "NVIDIA Tegra X1",
        ] {
            assert_eq!(adapter_family(name), Some(AdapterFamily::Mobile), "{name}");
        }
        for name in [
            "NVIDIA T400",
            "NVIDIA GeForce RTX 4090",
            "Quadro P1000",
            "AMD Radeon RX 7900 XTX",
            "Intel(R) UHD Graphics 770",
        ] {
            assert_eq!(adapter_family(name), Some(AdapterFamily::Desktop), "{name}");
        }
        for name in ["llvmpipe (LLVM 19.1.7, 256 bits)", "fake-full", ""] {
            assert_eq!(adapter_family(name), None, "{name}");
        }
    }

    #[test]
    fn the_tier_table_overrides_a_mobile_driver_that_answers_like_a_desktop() {
        const GIB: u64 = 1 << 30;
        // Adreno 618/620 answer full profile and no transient savings — the
        // desktop answers — and used to take the desktop budget on them.
        let adreno = named("Adreno (TM) 618");
        assert!(!is_mobile_tier(&adreno));

        let small = choose_tier(&adreno, Some(3 * GIB));
        assert_eq!(
            (small.tier, small.rule),
            (AtlasTier::Mobile, TierRule::Table)
        );
        assert_eq!(AtlasBudget::for_choice(small, &adreno), AtlasBudget::MOBILE);

        let unknown = choose_tier(&adreno, None);
        assert_eq!(
            unknown.tier,
            AtlasTier::Mobile,
            "unknown memory earns nothing"
        );

        let large = choose_tier(&adreno, Some(6 * GIB));
        assert_eq!(
            (large.tier, large.rule),
            (AtlasTier::MobileLarge, TierRule::Table)
        );
        assert_eq!(
            AtlasBudget::for_choice(large, &adreno),
            AtlasBudget::MOBILE_LARGE_WIDE
        );
        assert_eq!(
            AtlasBudget::for_choice(choose_tier(&adreno, Some(MOBILE_LARGE_MIN_MEMORY)), &adreno),
            AtlasBudget::MOBILE_LARGE_WIDE,
            "the threshold is inclusive"
        );
        assert_eq!(AtlasBudget::MOBILE_LARGE_WIDE.total_bytes(), 64 << 20);
        assert_eq!(AtlasBudget::MOBILE_LARGE_DEEP.total_bytes(), 64 << 20);

        // A desktop family stays on the desktop budget whatever it answers.
        let mut nvidia = named("NVIDIA T400");
        nvidia.transient_saves_memory = true;
        let desktop = choose_tier(&nvidia, Some(GIB));
        assert_eq!(
            (desktop.tier, desktop.rule),
            (AtlasTier::Desktop, TierRule::Table)
        );
    }

    #[test]
    fn the_large_mobile_shape_is_the_one_the_adapter_can_create() {
        let mut caps = named("Mali-G78 MP14");
        assert_eq!(
            AtlasBudget::mobile_large(&caps),
            Some(AtlasBudget::MOBILE_LARGE_WIDE)
        );

        // An edge short of 2048 takes the deep shape instead.
        caps.max_texture_dimension_2d = 1024;
        assert_eq!(
            AtlasBudget::mobile_large(&caps),
            Some(AtlasBudget::MOBILE_LARGE_DEEP)
        );

        // Neither shape fits: the tier is the plain mobile one, never a large
        // budget clamped below it.
        caps.max_texture_array_layers = 8;
        assert_eq!(AtlasBudget::mobile_large(&caps), None);
        let choice = choose_tier(&caps, Some(16 << 30));
        assert_eq!(choice.tier, AtlasTier::Mobile);
        assert_eq!(AtlasBudget::for_choice(choice, &caps), AtlasBudget::MOBILE);
    }

    #[test]
    fn an_unnamed_adapter_keeps_the_capability_read_and_downlevel_wins_over_the_table() {
        let unnamed = TierCaps::fake(DownlevelProfile::Full);
        let choice = choose_tier(&unnamed, Some(64 << 30));
        assert_eq!(
            (choice.tier, choice.rule),
            (AtlasTier::Desktop, TierRule::Heuristic)
        );

        let mut tiler = unnamed.clone();
        tiler.transient_saves_memory = true;
        let choice = choose_tier(&tiler, Some(64 << 30));
        assert_eq!(
            (choice.tier, choice.rule),
            (AtlasTier::Mobile, TierRule::Heuristic),
            "the heuristic never reaches the large tier"
        );

        let mut gl_nvidia = TierCaps::fake(DownlevelProfile::WebGl2);
        gl_nvidia.adapter_name = "ANGLE (NVIDIA, NVIDIA T400)".to_string();
        let choice = choose_tier(&gl_nvidia, Some(64 << 30));
        assert_eq!(
            (choice.tier, choice.rule),
            (AtlasTier::Mobile, TierRule::Downlevel)
        );
    }

    #[test]
    fn mem_total_is_read_in_bytes_from_meminfo() {
        let meminfo = "MemTotal:        5768520 kB\nMemFree:          123456 kB\n";
        assert_eq!(parse_mem_total(meminfo), Some(5_768_520 * 1024));
        assert_eq!(parse_mem_total("MemFree: 1 kB\n"), None);
        assert_eq!(parse_mem_total("MemTotal: lots\n"), None);
        assert_eq!(parse_mem_total("MemTotal: 1024 MB\n"), None);
    }

    #[test]
    fn a_budget_is_clamped_to_the_adapters_own_ceilings() {
        let mut caps = TierCaps::fake(DownlevelProfile::Full);
        caps.max_texture_dimension_2d = 1024;
        caps.max_texture_array_layers = 2;

        let budget = AtlasBudget::DESKTOP.clamped(&caps);
        assert_eq!(budget.atlas_size, (1024, 1024));
        assert_eq!(budget.max_atlases, 2);

        // The eight-bit `atlas_index` field is a ceiling of its own.
        caps.max_texture_array_layers = 4096;
        let budget = AtlasBudget {
            atlas_size: (256, 256),
            max_atlases: 4096,
        }
        .clamped(&caps);
        assert_eq!(budget.max_atlases, MAX_ATLAS_LAYERS);
    }

    #[test]
    fn the_first_layer_is_created_lazily() {
        let residency = residency();
        assert_eq!(residency.layers(), 0);
        assert_eq!(residency.entry_count(), 0);
    }

    #[test]
    fn one_image_over_many_frames_uploads_exactly_once() {
        let mut residency = residency();
        let data = image(8, 8);
        let mut uploads = 0;

        for _ in 0..60 {
            residency.begin_frame();
            residency.resolve(&data).expect("8x8 fits a 64x64 atlas");
            uploads += residency.uploads().len();
            // Every frame here is a frame that reached the atlas.
            residency.acknowledge_plan();
        }

        assert_eq!(uploads, 1, "residency survives every frame that draws it");
        assert_eq!(residency.entry_count(), 1);
        assert_eq!(residency.layers(), 1);
    }

    #[test]
    fn an_unseen_image_is_reaped_and_its_padded_region_cleared() {
        let mut residency = residency();
        let data = image(8, 8);

        residency.begin_frame();
        let resident = residency.resolve(&data).expect("fits");
        residency.acknowledge_plan();

        for _ in 0..MAX_UNSEEN_FRAMES {
            residency.begin_frame();
            assert!(residency.evictions().is_empty(), "still inside the window");
        }

        residency.begin_frame();
        assert_eq!(residency.entry_count(), 0);
        assert_eq!(
            residency.evictions(),
            &[padded(resident.region, ATLAS_PADDING)]
        );

        // The freed rectangle is available again, and the reallocation
        // schedules a fresh upload.
        residency.resolve(&data).expect("fits");
        assert_eq!(residency.uploads().len(), 1);
    }

    #[test]
    fn an_image_larger_than_the_atlas_is_minified_to_fit_rather_than_skipped() {
        let mut residency = residency();
        residency.begin_frame();

        // Twice the layer's width at a 16:1 aspect ratio: the width sets the
        // scale, and the height follows it rather than being fitted on its own.
        let resident = residency.resolve(&image(128, 8)).expect("minified to fit");

        assert_eq!(resident.region.size, [64, 4]);
        assert_eq!(resident.natural, [128, 8]);
        assert_eq!(resident.minify_scale(), Some((0.5, 0.5)));
        assert_eq!(residency.minified(), 1);
        assert_eq!(residency.skipped(), 0, "a big image is not a skipped one");
        assert_eq!(residency.uploads().len(), 1);
        let upload = &residency.uploads()[0];
        assert_eq!(
            (upload.pixels.width(), upload.pixels.height()),
            (64, 4),
            "the upload carries exactly the region's texels"
        );
    }

    #[test]
    fn an_atlas_with_no_room_left_is_still_a_skip() {
        // Minification fits an image to a LAYER, not to the space left in one:
        // a budget already full refuses the next allocation as it always did.
        let mut residency = ImageResidency::new(AtlasBudget {
            atlas_size: (64, 64),
            max_atlases: 1,
        });
        residency.begin_frame();
        residency
            .resolve(&image(64, 64))
            .expect("fills the one layer");

        let skip = residency
            .resolve(&image(32, 32))
            .expect_err("nothing is left to allocate from");
        assert!(matches!(skip, ImageSkip::NoAtlasSpace { .. }));
        assert_eq!(residency.skipped(), 1);
        assert_eq!(residency.minified(), 0);
    }

    #[test]
    fn a_minified_image_stays_resident_across_frames_at_its_declared_extent() {
        // Residency is keyed on the SOURCE extent, not the resident one: an
        // entry matched against its own minified rectangle would miss every
        // frame and re-upload the image on each of them.
        let mut residency = residency();
        let data = image(128, 8);

        for _ in 0..8 {
            residency.begin_frame();
            let resident = residency.resolve(&data).expect("minified to fit");
            assert_eq!(resident.natural, [128, 8]);
            residency.acknowledge_plan();
        }

        assert_eq!(residency.entry_count(), 1);
        assert_eq!(
            residency.minified(),
            1,
            "downsampled once, not once a frame"
        );
        assert!(
            residency.uploads().is_empty(),
            "no re-upload after the first"
        );
    }

    #[test]
    fn a_fit_extent_preserves_the_aspect_ratio_and_never_grows() {
        let atlas = (64, 64);
        // Already inside the layer: unchanged, both axes.
        assert_eq!(fit_extent([64, 64], atlas), (64, 64));
        assert_eq!(fit_extent([8, 4], atlas), (8, 4));

        // The `adv-huge-image` shape, at this budget's scale.
        assert_eq!(fit_extent([5000, 5000], (2048, 2048)), (2048, 2048));

        // The tighter axis sets the scale for both.
        assert_eq!(fit_extent([128, 8], atlas), (64, 4));
        assert_eq!(fit_extent([8, 128], atlas), (4, 64));
        assert_eq!(fit_extent([1000, 10], atlas), (64, 1));

        // A non-square layer is fitted per axis and still uniformly scaled.
        assert_eq!(fit_extent([400, 400], (100, 50)), (50, 50));
    }

    #[test]
    fn an_extreme_aspect_ratio_still_fits_at_one_texel_rather_than_none() {
        // 10000:1 into a 64-square layer scales the short axis to 0.0064 texels.
        // A zero extent is not a rectangle the packer or the shader can
        // address, so it is held at one.
        let fitted = fit_extent([10_000, 1], (64, 64));
        assert_eq!(fitted, (64, 1));

        let mut residency = residency();
        residency.begin_frame();
        let resident = residency
            .resolve(&image(10_000, 1))
            .expect("a sliver still becomes a rectangle");
        assert_eq!(resident.region.size, [64, 1]);
    }

    #[test]
    fn a_uniform_source_minifies_to_exactly_its_own_colour() {
        // What `adv-huge-image` rests on: a uniform source comes back bit
        // identical whatever the scale, so the case's `Exact` probe is a
        // property of the filter rather than a tolerance.
        let texel = PremulRgba8 {
            r: 255,
            g: 0,
            b: 255,
            a: 255,
        };
        let source = Pixmap::from_parts_with_opacity(vec![texel; 100 * 100], 100, 100, false);

        let small = minify(&source, 7, 3);
        assert_eq!((small.width(), small.height()), (7, 3));
        assert!(small.data().iter().all(|out| *out == texel));
        assert!(
            !small.may_have_transparency(),
            "an opaque source stays opaque"
        );
    }

    #[test]
    fn a_box_filter_averages_the_source_texels_each_output_covers() {
        // A 2x1 source of black and white halves down to one texel, which must
        // be their mean rather than either end.
        let black = PremulRgba8 {
            r: 0,
            g: 0,
            b: 0,
            a: 255,
        };
        let white = PremulRgba8 {
            r: 255,
            g: 255,
            b: 255,
            a: 255,
        };
        let source = Pixmap::from_parts_with_opacity(vec![black, white], 2, 1, false);

        let small = minify(&source, 1, 1);
        assert_eq!((small.width(), small.height()), (1, 1));
        // 255 / 2 rounded to nearest.
        assert_eq!(small.data()[0].r, 128);
        assert_eq!(small.data()[0].a, 255);

        // Transparency is recomputed from the result, not inherited: averaging
        // an opaque and a fully transparent texel produces a partial one.
        let clear = PremulRgba8 {
            r: 0,
            g: 0,
            b: 0,
            a: 0,
        };
        let mixed = Pixmap::from_parts_with_opacity(vec![white, clear], 2, 1, true);
        let small = minify(&mixed, 1, 1);
        assert_eq!(small.data()[0].a, 128);
        assert!(small.may_have_transparency());
    }

    #[test]
    fn minify_never_magnifies() {
        let texel = PremulRgba8 {
            r: 1,
            g: 2,
            b: 3,
            a: 255,
        };
        let source = Pixmap::from_parts_with_opacity(vec![texel; 4], 2, 2, false);

        // A request above the source's own extent is clamped to it rather than
        // inventing texels the box filter has no arm for.
        let same = minify(&source, 8, 8);
        assert_eq!((same.width(), same.height()), (2, 2));
        assert!(same.data().iter().all(|out| *out == texel));
    }

    #[test]
    fn a_malformed_or_degenerate_image_is_refused_before_the_conversion() {
        let mut short = image(4, 4);
        short.data = Blob::new(Arc::new(vec![0_u8; 8]));
        assert!(matches!(
            check_supported(&short),
            Err(ImageSkip::MalformedPixels { .. })
        ));

        assert!(matches!(
            check_supported(&image(0, 4)),
            Err(ImageSkip::DegenerateExtent { .. })
        ));

        let mut huge = image(1, 1);
        huge.width = MAX_IMAGE_DIMENSION + 1;
        assert!(matches!(
            check_supported(&huge),
            Err(ImageSkip::TooLarge { .. })
        ));
    }

    #[test]
    fn two_images_share_a_layer_and_hold_distinct_rectangles() {
        let mut residency = residency();
        residency.begin_frame();

        let first = residency.resolve(&image(8, 8)).expect("fits");
        let second = residency.resolve(&image(16, 16)).expect("fits");

        assert_ne!(first.id, second.id);
        assert_eq!(first.region.layer, second.region.layer);
        assert_ne!(first.region.offset, second.region.offset);
        assert_eq!(residency.uploads().len(), 2);
        assert_eq!(residency.layers(), 1);
    }

    #[test]
    fn a_second_layer_is_created_when_the_first_is_full() {
        let mut residency = residency();
        residency.begin_frame();

        // Two 64x48 images cannot share one 64x64 layer.
        let first = residency.resolve(&image(64, 48)).expect("fits a layer");
        let second = residency.resolve(&image(64, 48)).expect("fits the next");

        assert_eq!(first.region.layer, 0);
        assert_eq!(second.region.layer, 1);
        assert_eq!(residency.layers(), 2);
    }

    #[test]
    fn a_slot_taken_through_the_shared_allocator_is_a_distinct_id_on_the_same_budget() {
        // The allocator half of the glyph/image seam, stated without the glyph
        // policy: whatever else packs into this cache draws from the same id
        // space and the same layer allowance, and its pages are visible in the
        // depth the array is built to.
        let mut residency = residency();
        residency.begin_frame();
        let resident = residency.resolve(&image(64, 48)).expect("fills a layer");

        // Big enough that it cannot share the layer the image took, so it must
        // create the second one rather than report the first.
        let borrowed = residency
            .allocator_mut()
            .allocate(64, 48, ATLAS_PADDING)
            .expect("the second layer is free");

        assert_ne!(
            resident.id, borrowed,
            "one cache never hands the same slot index to two live occupants"
        );
        assert_eq!(
            residency.layers(),
            2,
            "a layer created by the other class still has to be in the array"
        );
        assert_eq!(
            residency
                .allocator()
                .get(resident.id)
                .map(|resource| resource.size()),
            Some(resident.region.size),
            "and neither allocation disturbed the other's rectangle"
        );
    }

    #[test]
    fn a_disabled_residency_refuses_every_image_and_touches_no_atlas() {
        let mut residency = ImageResidency::disabled(AtlasBudget::MOBILE);
        assert!(residency.is_disabled());

        residency.begin_frame();
        let skip = residency
            .resolve(&image(8, 8))
            .expect_err("the kill switch refuses every image");

        assert_eq!(skip, ImageSkip::AtlasDisabled);
        assert_eq!(residency.entry_count(), 0);
        assert_eq!(residency.layers(), 0, "no atlas layer is ever created");
        assert!(residency.uploads().is_empty());
        assert_eq!(residency.skipped(), 1);
    }

    #[test]
    fn an_enabled_residency_is_what_an_unset_kill_switch_produces() {
        // The default test environment leaves `FRUST_ENGINE_NO_ATLAS` unset, so
        // this pins that `new` consults the switch rather than hardcoding
        // either answer; the disabled half is pinned above through the
        // constructor the switch selects.
        assert!(!ImageResidency::new(AtlasBudget::MOBILE).is_disabled());
    }

    #[test]
    fn reading_the_plan_leaves_it_pending_and_acknowledging_it_clears_it() {
        let mut residency = residency();
        residency.begin_frame();
        residency.resolve(&image(8, 8)).expect("fits");

        let (evictions, uploads) = residency.plan();
        assert!(evictions.is_empty());
        assert_eq!(uploads.len(), 1);
        assert!(
            residency.has_pending_plan(),
            "reading the plan is not servicing it"
        );

        residency.acknowledge_plan();
        assert!(!residency.has_pending_plan());
        assert!(residency.uploads().is_empty());
        assert!(residency.evictions().is_empty());
    }

    #[test]
    fn a_clear_is_dropped_only_for_a_rectangle_an_upload_covers_exactly() {
        let region = |layer, x, y, w, h| AtlasRegion {
            layer,
            offset: [x, y],
            size: [w, h],
        };
        let upload = |region| ImageUpload {
            id: ImageId::new(0),
            region,
            natural: region.size,
            pixels: Arc::new(Pixmap::new(
                u16::try_from(region.size[0]).expect("small"),
                u16::try_from(region.size[1]).expect("small"),
            )),
            may_have_transparency: false,
        };

        let exact = region(0, 0, 0, 16, 16);
        let other_layer = region(1, 0, 0, 16, 16);
        let partial = region(0, 16, 0, 16, 16);
        let evictions = [exact, other_layer, partial];
        let uploads = [upload(exact), upload(region(0, 16, 0, 16, 8))];

        assert_eq!(
            uncovered_clears(&evictions, &uploads),
            vec![other_layer, partial],
            "only the exactly covered clear goes, the rest keep their order"
        );
        assert_eq!(uncovered_clears(&evictions, &[]), evictions.to_vec());
        assert!(uncovered_clears(&[], &uploads).is_empty());
    }

    #[test]
    fn the_clear_skip_is_tied_to_zero_padding() {
        // Evictions are reported as `padded(region, ATLAS_PADDING)`, uploads
        // carry the raw allocation; the exact-cover match needs them equal.
        let raw = AtlasRegion {
            layer: 0,
            offset: [4, 8],
            size: [16, 16],
        };
        assert_eq!(padded(raw, ATLAS_PADDING), raw);
    }

    #[test]
    fn a_pressure_eviction_into_the_same_rectangle_plans_no_clear() {
        // One slot: the second image can only take the first one's rectangle.
        let mut residency = ImageResidency::new(AtlasBudget {
            atlas_size: (16, 16),
            max_atlases: 1,
        });
        residency.begin_frame();
        let first = residency.resolve(&image(16, 16)).expect("fits");
        residency.acknowledge_plan();

        residency.begin_frame();
        let second = residency
            .resolve(&image(16, 16))
            .expect("displaces the first");
        assert_eq!(second.region, first.region);
        assert_eq!(
            residency.evictions(),
            &[first.region],
            "the eviction itself is still pending"
        );

        let (clears, uploads) = residency.plan();
        assert!(clears.is_empty(), "but the plan issues no clear for it");
        assert_eq!(uploads.len(), 1);

        residency.acknowledge_plan();
        assert!(!residency.has_pending_plan());
    }

    #[test]
    fn an_unacknowledged_upload_is_re_emitted_until_it_is_serviced() {
        // The invariant the whole pending plan exists for: a frame refused
        // after compiling never wrote these texels, so the entry map claiming
        // the image is resident has to stay answerable by a later frame's plan.
        let mut residency = residency();
        let data = image(8, 8);

        residency.begin_frame();
        let first = residency.resolve(&data).expect("fits");
        let region = residency.uploads()[0].region;

        for _ in 0..4 {
            residency.begin_frame();
            let again = residency.resolve(&data).expect("still resident");
            assert_eq!(again.region, first.region, "residency does not move");
            assert_eq!(
                residency.uploads().len(),
                1,
                "the same upload is re-offered, never duplicated"
            );
            assert_eq!(residency.uploads()[0].region, region);
        }

        residency.acknowledge_plan();
        residency.begin_frame();
        residency.resolve(&data).expect("still resident");
        assert!(
            residency.uploads().is_empty(),
            "a serviced upload is never offered again"
        );
    }

    #[test]
    fn reaping_an_unacknowledged_entry_withdraws_its_upload_with_it() {
        // Otherwise the plan would clear the rectangle and then write the
        // evicted image straight back into it, over whatever the packer handed
        // that space to next.
        let mut residency = residency();
        let data = image(8, 8);

        residency.begin_frame();
        let resident = residency.resolve(&data).expect("fits");
        assert_eq!(residency.uploads().len(), 1);

        for _ in 0..=MAX_UNSEEN_FRAMES {
            residency.begin_frame();
        }

        assert_eq!(residency.entry_count(), 0);
        assert!(
            residency.uploads().is_empty(),
            "the reaped entry's unserviced upload goes with it"
        );
        assert_eq!(
            residency.evictions(),
            &[padded(resident.region, ATLAS_PADDING)]
        );
    }

    #[test]
    fn every_pending_upload_lies_inside_the_atlas_its_layer_count_asks_for() {
        // What keeps a consumer from recording where an image lives and then
        // sampling a rectangle the array refused to write: the plan can only
        // ever name regions the budget and the reported depth already admit.
        let mut residency = residency();
        residency.begin_frame();
        residency.resolve(&image(64, 48)).expect("fills a layer");
        residency.resolve(&image(64, 48)).expect("takes the next");

        let budget = residency.budget();
        let layers = residency.layers();
        assert_eq!(layers, 2);
        for upload in residency.uploads() {
            assert!(
                budget.contains(upload.region, layers),
                "{:?} is outside a {layers}-layer {budget:?}",
                upload.region
            );
        }
    }

    /// A budget holding exactly two 64-square images: one per layer, with no
    /// room to pack a third anywhere, so "the atlas is full" is reachable in
    /// two allocations and needs no fragmentation reasoning.
    fn two_slot_residency() -> ImageResidency {
        ImageResidency::new(AtlasBudget {
            atlas_size: (64, 64),
            max_atlases: 2,
        })
    }

    #[test]
    fn a_working_set_larger_than_the_atlas_re_uploads_rather_than_skipping() {
        // The scrolling-list shape: more distinct images pass the viewport
        // inside one age window than the atlas has rectangles for. Every draw
        // must still resolve — the price is a re-upload, never a hole.
        let mut residency = two_slot_residency();
        let images: Vec<ImageData> = (0..12).map(|_| image(64, 64)).collect();

        for data in &images {
            residency.begin_frame();
            residency
                .resolve(data)
                .expect("a full atlas gives a rectangle back rather than refusing");
            residency.acknowledge_plan();
        }

        assert_eq!(residency.skipped(), 0, "no draw was ever dropped");
        assert_eq!(
            residency.pressure_evictions(),
            10,
            "the ten images past the two slots each displaced one"
        );
        assert_eq!(residency.entry_count(), 2, "and residency stays bounded");
    }

    #[test]
    fn an_image_resolved_this_frame_is_never_the_one_evicted() {
        // The ordering contract's own boundary: the plan clears before it
        // uploads, so a rectangle this frame already resolved cannot be handed
        // back inside it. A frame whose own working set outgrows the atlas is
        // still a skip, and nothing is scheduled for clearing.
        let mut residency = two_slot_residency();
        residency.begin_frame();
        residency.resolve(&image(64, 64)).expect("takes layer 0");
        residency.resolve(&image(64, 64)).expect("takes layer 1");

        let skip = residency
            .resolve(&image(64, 64))
            .expect_err("nothing this frame drew is evictable");

        assert!(matches!(skip, ImageSkip::NoAtlasSpace { .. }));
        assert_eq!(residency.skipped(), 1);
        assert_eq!(residency.pressure_evictions(), 0);
        assert_eq!(residency.entry_count(), 2, "both occupants stayed put");
        assert!(
            residency.evictions().is_empty(),
            "no region a frame is about to sample was scheduled for clearing"
        );
    }

    #[test]
    fn the_least_recently_seen_image_is_the_one_displaced() {
        let mut residency = two_slot_residency();
        let old = image(64, 64);
        let refreshed = image(64, 64);

        residency.begin_frame();
        let refreshed_region = residency.resolve(&refreshed).expect("takes a layer").region;
        residency.begin_frame();
        let old_region = residency.resolve(&old).expect("takes the other").region;
        residency.acknowledge_plan();

        // The older entry is drawn again, which makes the *other* one the least
        // recently seen despite having been made resident second.
        residency.begin_frame();
        residency.resolve(&refreshed).expect("still resident");
        let arrival = residency.resolve(&image(64, 64)).expect("displaces one");

        assert_eq!(residency.pressure_evictions(), 1);
        assert_eq!(
            arrival.region, old_region,
            "the rectangle taken is the least recently seen one"
        );
        assert_eq!(
            residency.resolve(&refreshed).map(|again| again.region),
            Ok(refreshed_region),
            "the image drawn this frame kept its own"
        );
    }

    #[test]
    fn a_pressure_eviction_reports_its_rectangle_once_and_withdraws_its_upload() {
        let mut residency = two_slot_residency();
        let displaced = image(64, 64);

        residency.begin_frame();
        let displaced_region = residency.resolve(&displaced).expect("takes a layer").region;
        residency.resolve(&image(64, 64)).expect("takes the other");
        // Deliberately NOT acknowledged: the displaced image still has an
        // upload pending, which must go with its entry rather than be written
        // into a rectangle its new occupant now owns.
        assert_eq!(residency.uploads().len(), 2);

        residency.begin_frame();
        let arrival = residency
            .resolve(&image(64, 64))
            .expect("displaces the oldest");

        assert_eq!(
            residency.evictions(),
            &[padded(displaced_region, ATLAS_PADDING)],
            "exactly one rectangle, reported exactly once"
        );
        assert_eq!(
            arrival.region, displaced_region,
            "and the freed rectangle is what the new occupant took"
        );
        assert_eq!(
            residency.uploads().len(),
            2,
            "the displaced image's unserviced upload went with its entry — \
             writing it would put an evicted image back over its successor"
        );
    }

    #[test]
    fn an_age_reap_and_a_pressure_eviction_are_counted_separately() {
        // The two bounds answer different questions: one says a screen moved
        // on, the other says the atlas is too small for what is on it.
        let mut residency = two_slot_residency();
        let data = image(64, 64);

        residency.begin_frame();
        residency.resolve(&data).expect("fits");
        for _ in 0..=MAX_UNSEEN_FRAMES {
            residency.begin_frame();
        }

        assert_eq!(residency.entry_count(), 0, "the age reap ran");
        assert_eq!(
            residency.pressure_evictions(),
            0,
            "an age reap is not a pressure eviction"
        );
        assert_eq!(residency.frame_pressure_evictions(), 0);
    }

    #[test]
    fn the_per_frame_eviction_count_is_this_frames_alone() {
        let mut residency = two_slot_residency();
        residency.begin_frame();
        residency.resolve(&image(64, 64)).expect("takes a layer");
        residency.resolve(&image(64, 64)).expect("takes the other");

        residency.begin_frame();
        residency.resolve(&image(64, 64)).expect("displaces one");
        assert_eq!(residency.frame_pressure_evictions(), 1);
        assert_eq!(residency.pressure_evictions(), 1);

        residency.begin_frame();
        assert_eq!(
            residency.frame_pressure_evictions(),
            0,
            "a new frame starts from nothing"
        );
        assert_eq!(
            residency.pressure_evictions(),
            1,
            "while the lifetime total keeps counting"
        );
    }

    /// A budget of exactly four 32-square cells in one layer, so "the atlas is
    /// full" needs no reasoning about how a packer splits free space and a
    /// request that fits *no* arrangement of the holes is one rectangle.
    fn four_slot_residency() -> ImageResidency {
        ImageResidency::new(AtlasBudget {
            atlas_size: (64, 64),
            max_atlases: 1,
        })
    }

    #[test]
    fn a_request_no_candidate_set_can_place_is_refused_before_anything_is_freed() {
        // The storm, at the cache seam. One 32-square is this frame's and three
        // are last frame's; a 48-square fits none of the holes those three
        // would leave while the fourth is resident. Freeing them buys nothing
        // — the draw is skipped either way — and costs three clears plus three
        // re-uploads, every frame the scene repeats. So the refusal has to come
        // before the first rectangle is handed back.
        let mut residency = four_slot_residency();
        let resident: Vec<ImageData> = (0..4).map(|_| image(32, 32)).collect();
        residency.begin_frame();
        for data in &resident {
            residency
                .resolve(data)
                .expect("the four cells take four images");
        }
        residency.acknowledge_plan();

        residency.begin_frame();
        residency.resolve(&resident[0]).expect("still resident");

        let skip = residency
            .resolve(&image(48, 48))
            .expect_err("no arrangement of the three candidates admits a 48-square");

        assert!(matches!(skip, ImageSkip::NoAtlasSpace { .. }));
        assert_eq!(
            residency.pressure_evictions(),
            0,
            "a request that could not have been placed displaced nothing"
        );
        assert_eq!(residency.entry_count(), 4, "the residency is untouched");
        assert!(
            residency.evictions().is_empty(),
            "and nothing was scheduled for clearing on the way to failing"
        );
        assert!(residency.uploads().is_empty());
    }

    #[test]
    fn a_request_larger_than_every_candidate_together_is_refused_on_area_alone() {
        // The cheaper half of the same precheck: freeing all three candidates
        // leaves three quarters of one layer, and the request wants all of it.
        // The area comes off the packer's own accounting, so it counts what the
        // glyph half of the allocator holds as well as this residency's own.
        let mut residency = four_slot_residency();
        let resident: Vec<ImageData> = (0..4).map(|_| image(32, 32)).collect();
        residency.begin_frame();
        for data in &resident {
            residency
                .resolve(data)
                .expect("the four cells take four images");
        }
        residency.acknowledge_plan();

        residency.begin_frame();
        residency.resolve(&resident[0]).expect("still resident");
        residency
            .resolve(&image(64, 64))
            .expect_err("a whole layer is more than the three candidates hold");

        assert_eq!(residency.pressure_evictions(), 0);
        assert_eq!(residency.entry_count(), 4);
        assert!(residency.evictions().is_empty());
    }

    #[test]
    fn a_satisfiable_request_under_pressure_still_evicts_the_minimum() {
        // The other side of the precheck: it must not turn a working eviction
        // into a skip. One 32-square arriving against four resident ones is
        // satisfiable by freeing exactly one, and that is what it costs.
        let mut residency = four_slot_residency();
        let resident: Vec<ImageData> = (0..4).map(|_| image(32, 32)).collect();
        residency.begin_frame();
        for data in &resident {
            residency
                .resolve(data)
                .expect("the four cells take four images");
        }
        residency.acknowledge_plan();

        residency.begin_frame();
        residency.resolve(&image(32, 32)).expect("displaces one");

        assert_eq!(
            residency.pressure_evictions(),
            1,
            "the minimum, not the lot"
        );
        assert_eq!(residency.entry_count(), 4);
        assert_eq!(residency.evictions().len(), 1);
    }

    #[test]
    fn a_blob_redrawn_at_a_new_extent_never_frees_what_this_frame_resolved() {
        // The extent-mismatch release under the pressure path's own rule. The
        // plan clears before it uploads, so giving the rectangle back on a
        // frame whose first draw already resolved it would zero texels that
        // draw is about to sample.
        let square = image(16, 8);
        let transposed = ImageData {
            data: square.data.clone(),
            format: square.format,
            alpha_type: square.alpha_type,
            width: 8,
            height: 16,
        };

        let mut residency = residency();
        residency.begin_frame();
        let first = residency.resolve(&square).expect("takes a rectangle");
        let skip = residency
            .resolve(&transposed)
            .expect_err("the second extent cannot take the first one's rectangle");

        assert!(
            matches!(skip, ImageSkip::SameFrameExtentConflict { .. }),
            "a same-frame ordering conflict, not an atlas-capacity refusal: {skip:?}"
        );
        assert!(
            residency.evictions().is_empty(),
            "no clear over a rectangle this frame already sampled"
        );
        assert_eq!(residency.entry_count(), 1);
        assert_eq!(residency.uploads().len(), 1);

        // On a later frame nothing has sampled it yet, so the release is
        // ordinary again and the new extent takes its place.
        residency.acknowledge_plan();
        residency.begin_frame();
        let second = residency.resolve(&transposed).expect("the new extent fits");
        assert_eq!(second.natural, [8, 16]);
        assert_eq!(
            residency.evictions(),
            &[padded(first.region, ATLAS_PADDING)],
            "the rectangle the old extent held is reported for clearing"
        );
    }

    #[test]
    fn a_layer_admits_only_what_every_occupant_leaves_room_for() {
        // The planner's own necessary condition, in isolation. A placement has
        // to lie entirely to one side of each occupant, so a 32-square in one
        // corner of a 64-square layer leaves nothing a 48-square can occupy —
        // while a 32-square still has three cells to choose from.
        let corner = PressureRect {
            key: 1,
            layer: 0,
            x0: 0,
            y0: 0,
            x1: 32,
            y1: 32,
            area: 1024,
            last_seen: 0,
            slot: 0,
            freed: false,
        };
        let rects = [corner];

        assert!(!layer_admits(&rects, 0, 48, 48, (64, 64)));
        assert!(layer_admits(&rects, 0, 32, 32, (64, 64)));
        assert!(layer_admits(&rects, 0, 32, 64, (64, 64)));
        // A freed rectangle is not an obstacle, and a layer nothing occupies
        // admits anything the layer itself can hold.
        assert!(layer_admits(
            &[PressureRect {
                freed: true,
                ..corner
            }],
            0,
            64,
            64,
            (64, 64)
        ));
        assert!(!layer_admits(&[], 0, 65, 64, (64, 64)));
        // An occupant on another layer says nothing about this one.
        assert!(layer_admits(
            &[PressureRect { layer: 1, ..corner }],
            0,
            64,
            64,
            (64, 64)
        ));
    }

    #[test]
    fn a_plan_that_would_outrun_its_budget_frees_nothing() {
        // The bound is on the *search*, not only on the damage: a walk that
        // reaches the budget without finding a fit answers `None`, and the
        // caller has released nothing by then.
        let mut rects: Vec<PressureRect> = (0..MAX_PRESSURE_EVICTION_CANDIDATES + 8)
            .map(|index| {
                let index = u32::try_from(index).expect("fits");
                PressureRect {
                    key: u64::from(index),
                    layer: 0,
                    x0: index,
                    y0: 0,
                    x1: index + 1,
                    y1: 1,
                    area: 1,
                    last_seen: 0,
                    slot: index,
                    freed: false,
                }
            })
            .collect();
        let order: Vec<usize> = (0..rects.len()).collect();
        let mut free_area = vec![0_u64];
        let mut layers = Vec::new();

        assert_eq!(
            plan_pressure_eviction(
                64,
                64,
                (64, 64),
                &mut rects,
                &order,
                &mut free_area,
                &mut layers
            ),
            None,
            "a walk that cannot fit the request inside its budget plans nothing"
        );
        assert!(
            rects.iter().filter(|rect| rect.freed).count() <= MAX_PRESSURE_EVICTION_CANDIDATES + 1,
            "and it stopped walking rather than marking the whole residency"
        );
    }

    #[test]
    fn a_satisfiable_layer_is_not_starved_by_a_hopeless_ones_candidates() {
        // The multi-layer shape the per-layer budget exists for. Layer 0 is
        // "this frame's" 64x40 anchor plus more than
        // `MAX_PRESSURE_EVICTION_CANDIDATES` one-pixel stale fillers packed
        // into the 64x24 strip left over — every one of them stale (older
        // than the incoming request's frame) and none of them, individually
        // or together, ever able to free a 32-tall rectangle, since the strip
        // they sit in is only 24 pixels tall. Layer 1 is four 32-square
        // quadrants exactly tiling the layer, three "this frame's" and one
        // stale. Layers 2 and 3 are each one "this frame's" 64-square image,
        // filling them outright so the incoming request cannot simply grow
        // into an unused layer.
        //
        // A single pooled budget spends its whole allowance walking layer 0's
        // fillers — none of which can ever satisfy the request — and never
        // reaches layer 1's one satisfiable candidate at all. A per-layer
        // budget exhausts layer 0's own allowance the same way, then gives
        // layer 1 a fresh one and finds the fit immediately.
        let mut residency = ImageResidency::new(AtlasBudget {
            atlas_size: (64, 64),
            max_atlases: 4,
        });

        let anchor = image(64, 40);
        let fillers: Vec<ImageData> = (0..MAX_PRESSURE_EVICTION_CANDIDATES + 6)
            .map(|_| image(1, 1))
            .collect();
        let quadrants: Vec<ImageData> = (0..4).map(|_| image(32, 32)).collect();
        let layer2_filler = image(64, 64);
        let layer3_filler = image(64, 64);

        residency.begin_frame();
        residency
            .resolve(&anchor)
            .expect("the anchor takes layer 0's top");
        for filler in &fillers {
            residency
                .resolve(filler)
                .expect("a one-pixel filler always finds room in the leftover strip");
        }
        for quadrant in &quadrants {
            residency
                .resolve(quadrant)
                .expect("a 32-square quadrant packs layer 1's empty corners");
        }
        residency
            .resolve(&layer2_filler)
            .expect("a whole-layer image grows into a fresh layer 2");
        residency
            .resolve(&layer3_filler)
            .expect("a whole-layer image grows into a fresh layer 3");
        residency.acknowledge_plan();
        assert_eq!(
            residency.layers(),
            4,
            "fixture precondition: all four layers exist"
        );

        // Refresh everything except the fillers and the first quadrant, so
        // only they are stale candidates when the request arrives — the
        // fillers sort ahead of the stale quadrant because they were
        // resolved first and share the same `last_seen`.
        residency.begin_frame();
        residency.resolve(&anchor).expect("still resident");
        residency.resolve(&quadrants[1]).expect("still resident");
        residency.resolve(&quadrants[2]).expect("still resident");
        residency.resolve(&quadrants[3]).expect("still resident");
        residency.resolve(&layer2_filler).expect("still resident");
        residency.resolve(&layer3_filler).expect("still resident");

        let arrival = residency
            .resolve(&image(32, 32))
            .expect("a per-layer budget reaches layer 1's satisfiable candidate");

        assert_eq!(
            residency.pressure_evictions(),
            1,
            "exactly the one quadrant needed, not layer 0's hopeless fillers"
        );
        assert_eq!(residency.evictions().len(), 1);
        assert_eq!(
            residency.evictions()[0].layer,
            arrival.region.layer,
            "the eviction and the arrival land on the same, single layer"
        );
        assert_ne!(
            arrival.region.layer, 0,
            "the winning layer is not the one full of hopeless fillers"
        );
    }

    /// The tier line's own text, which only a `perf-trace` build has.
    #[cfg(feature = "perf-trace")]
    #[test]
    fn the_resolved_tier_line_names_the_tier_and_is_written_once() {
        let mobile = TierCaps::fake(DownlevelProfile::WebGl2);
        let choice = choose_tier(&mobile, None);
        assert_eq!(
            atlas_tier_line(&mobile, choice, AtlasBudget::MOBILE),
            "frust-perf atlas tier=mobile rule=downlevel budget=1024x1024x4 downlevel=WebGl2 \
             transient_saves_memory=false memory_mib=unknown adapter=fake-webgl2"
        );

        let mut adreno = TierCaps::fake(DownlevelProfile::Full);
        adreno.adapter_name = "Adreno (TM) 660".to_string();
        let choice = choose_tier(&adreno, Some(8 << 30));
        assert_eq!(
            atlas_tier_line(&adreno, choice, AtlasBudget::MOBILE_LARGE_WIDE),
            "frust-perf atlas tier=mobile_large rule=table budget=2048x2048x4 downlevel=Full \
             transient_saves_memory=false memory_mib=8192 adapter=Adreno (TM) 660"
        );

        // An adapter the table does not name says which capability answer
        // decided it.
        let mut tiler = TierCaps::fake(DownlevelProfile::Full);
        tiler.transient_saves_memory = true;
        let line = atlas_tier_line(&tiler, choose_tier(&tiler, None), AtlasBudget::MOBILE);
        assert!(line.contains("tier=mobile rule=heuristic"));
        assert!(line.contains("transient_saves_memory=true"));

        let latch = TierLogOnce::new();
        let choice = choose_tier(&mobile, None);
        assert!(
            latch.emit(&mobile, choice, AtlasBudget::MOBILE),
            "the first call writes the line"
        );
        assert!(
            !latch.emit(&mobile, choice, AtlasBudget::MOBILE),
            "and no later one repeats it"
        );
    }

    #[test]
    fn an_override_redistributes_a_tiers_memory_but_never_exceeds_it() {
        let tier = AtlasBudget::DESKTOP;

        // The pathological override the adapter alone would admit: a
        // 16384-square layer is a gibibyte on its own, so clamping the layer
        // count could not have brought it inside the tier's 128 MiB.
        let huge = AtlasBudget {
            atlas_size: (16_384, 16_384),
            max_atlases: tier.max_atlases,
        }
        .within_total_bytes(tier.total_bytes());
        assert!(huge.total_bytes() <= tier.total_bytes());
        assert_eq!(huge.atlas_size.0, huge.atlas_size.1, "aspect preserved");
        assert!(huge.max_atlases >= 1);

        // A modest override is left exactly as asked for.
        let modest = AtlasBudget {
            atlas_size: (1024, 1024),
            max_atlases: tier.max_atlases,
        }
        .within_total_bytes(tier.total_bytes());
        assert_eq!(modest.atlas_size, (1024, 1024));
        assert_eq!(modest.max_atlases, tier.max_atlases);

        // An extreme aspect ratio floors one axis to nothing; it is raised back
        // to a texel and the other axis gives the room up, so the pair still
        // fits and neither axis is zero.
        let sliver = AtlasBudget {
            atlas_size: (65_535, 4),
            max_atlases: 1,
        }
        .within_total_bytes(64);
        assert!(sliver.atlas_size.0 >= 1 && sliver.atlas_size.1 >= 1);
        assert!(sliver.total_bytes() <= 64);
    }
}