src/gpu/graphite/ClipStack.cpp

/*
 * Copyright 2022 Google LLC
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "src/gpu/graphite/ClipStack_graphite.h"

#include "include/core/SkMatrix.h"
#include "include/core/SkShader.h"
#include "include/core/SkStrokeRec.h"
#include "src/base/SkTLazy.h"
#include "src/core/SkMatrixProvider.h"
#include "src/core/SkPathPriv.h"
#include "src/core/SkRRectPriv.h"
#include "src/core/SkRectPriv.h"
#include "src/gpu/graphite/Device.h"
#include "src/gpu/graphite/DrawParams.h"
#include "src/gpu/graphite/geom/BoundsManager.h"
#include "src/gpu/graphite/geom/Geometry.h"

namespace skgpu::graphite {

namespace {

Rect subtract(const Rect& a, const Rect& b, bool exact) {
    SkRect diff;
    if (SkRectPriv::Subtract(a.asSkRect(), b.asSkRect(), &diff) || !exact) {
        // Either A-B is exactly the rectangle stored in diff, or we don't need an exact answer
        // and can settle for the subrect of A excluded from B (which is also 'diff')
        return Rect{diff};
    } else {
        // For our purposes, we want the original A when A-B cannot be exactly represented
        return a;
    }
}

bool oriented_bbox_intersection(const Rect& a, const Transform& aXform,
                                const Rect& b, const Transform& bXform) {
    // NOTE: We intentionally exclude projected bounds for two reasons:
    //   1. We can skip the division by w and worring about clipping to w = 0.
    //   2. W/o the projective case, the separating axes are simpler to compute (see below).
    SkASSERT(aXform.type() != Transform::Type::kProjection &&
             bXform.type() != Transform::Type::kProjection);
    SkV4 quadA[4], quadB[4];

    aXform.mapPoints(a, quadA);
    bXform.mapPoints(b, quadB);

    // There are 4 separating axes, defined by the two normals from quadA and from quadB, but
    // since they were produced by transforming a rectangle by an affine transform, we know the
    // normals are orthoganal to the basis vectors of upper 2x2 of their two transforms.
    auto axesX = skvx::float4(-aXform.matrix().rc(1,0), -aXform.matrix().rc(1,1),
                              -bXform.matrix().rc(1,0), -bXform.matrix().rc(1,1));
    auto axesY = skvx::float4(aXform.matrix().rc(0,0), aXform.matrix().rc(0,1),
                              bXform.matrix().rc(0,0), bXform.matrix().rc(0,1));

    // Projections of the 4 corners of each quadrilateral vs. the 4 axes. For orthonormal
    // transforms, the projections of a quad's corners to its own normal axes should work out
    // to the original dimensions of the rectangle, but this code handles skew and scale factors
    // without branching.
    auto aProj0 = quadA[0].x * axesX + quadA[0].y * axesY;
    auto aProj1 = quadA[1].x * axesX + quadA[1].y * axesY;
    auto aProj2 = quadA[2].x * axesX + quadA[2].y * axesY;
    auto aProj3 = quadA[3].x * axesX + quadA[3].y * axesY;

    auto bProj0 = quadB[0].x * axesX + quadB[0].y * axesY;
    auto bProj1 = quadB[1].x * axesX + quadB[1].y * axesY;
    auto bProj2 = quadB[2].x * axesX + quadB[2].y * axesY;
    auto bProj3 = quadB[3].x * axesX + quadB[3].y * axesY;

    // Minimum and maximum projected values against the 4 axes, for both quadA and quadB, which
    // gives us four pairs of intervals to test for separation.
    auto minA = min(min(aProj0, aProj1), min(aProj2, aProj3));
    auto maxA = max(max(aProj0, aProj1), max(aProj2, aProj3));
    auto minB = min(min(bProj0, bProj1), min(bProj2, bProj3));
    auto maxB = max(max(bProj0, bProj1), max(bProj2, bProj3));

    auto overlaps = (minB <= maxA) & (minA <= maxB);
    return all(overlaps); // any non-overlapping interval would imply no intersection
}

static const Transform kIdentity{SkM44()};

} // anonymous namespace

///////////////////////////////////////////////////////////////////////////////
// ClipStack::TransformedShape

// A flyweight object describing geometry, subject to a local-to-device transform.
// This can be used by SaveRecords, Elements, and draws to determine how two shape operations
// interact with each other, without needing to share a base class, friend each other, or have a
// template for each combination of two types.
struct ClipStack::TransformedShape {
    const Transform& fLocalToDevice;
    const Shape&     fShape;
    const Rect&      fOuterBounds;
    const Rect&      fInnerBounds;

    SkClipOp         fOp;

    // contains() performs a fair amount of work to be as accurate as possible since it can mean
    // greatly simplifying the clip stack. However, in some contexts this isn't worth doing because
    // the actual shape is only an approximation (save records), or there's no current way to take
    // advantage of knowing this shape contains another (draws containing a clip hypothetically
    // could replace their geometry to draw the clip directly, but that isn't implemented now).
    bool fContainsChecksOnlyBounds = false;

    bool intersects(const TransformedShape&) const;
    bool contains(const TransformedShape&) const;
};

bool ClipStack::TransformedShape::intersects(const TransformedShape& o) const {
    if (!fOuterBounds.intersects(o.fOuterBounds)) {
        return false;
    }

    if (fLocalToDevice.type() <= Transform::Type::kRectStaysRect &&
        o.fLocalToDevice.type() <= Transform::Type::kRectStaysRect) {
        // The two shape's coordinate spaces are different but both rect-stays-rect or simpler.
        // This means, though, that their outer bounds approximations are tight to their transormed
        // shape bounds. There's no point to do further tests given that and that we already found
        // that these outer bounds *do* intersect.
        return true;
    } else if (fLocalToDevice == o.fLocalToDevice) {
        // Since the two shape's local coordinate spaces are the same, we can compare shape
        // bounds directly for a more accurate intersection test. We intentionally do not go
        // further and do shape-specific intersection tests since these could have unknown
        // complexity (for paths) and limited utility (e.g. two round rects that are disjoint
        // solely from their corner curves).
        return fShape.bounds().intersects(o.fShape.bounds());
    } else if (fLocalToDevice.type() != Transform::Type::kProjection &&
               o.fLocalToDevice.type() != Transform::Type::kProjection) {
        // The shapes don't share the same coordinate system, and their approximate 'outer'
        // bounds in device space could have substantial outsetting to contain the transformed
        // shape (e.g. 45 degree rotation). Perform a more detailed check on their oriented
        // bounding boxes.
        return oriented_bbox_intersection(fShape.bounds(), fLocalToDevice,
                                          o.fShape.bounds(), o.fLocalToDevice);
    }
    // Else multiple perspective transforms are involved, so assume intersection and allow the
    // rasterizer to handle perspective clipping.
    return true;
}

bool ClipStack::TransformedShape::contains(const TransformedShape& o) const {
    if (fInnerBounds.contains(o.fOuterBounds)) {
        return true;
    }
    // Skip more expensive contains() checks if configured not to, or if the extent of 'o' exceeds
    // this shape's outer bounds. When that happens there must be some part of 'o' that cannot be
    // contained in this shape.
    if (fContainsChecksOnlyBounds || !fOuterBounds.contains(o.fOuterBounds)) {
        return false;
    }

    if (fContainsChecksOnlyBounds) {
        return false; // don't do any more work
    }

    if (fLocalToDevice == o.fLocalToDevice) {
        // Test the shapes directly against each other, with a special check for a rrect+rrect
        // containment (a intersect b == a implies b contains a) and paths (same gen ID, or same
        // path for small paths means they contain each other).
        static constexpr int kMaxPathComparePoints = 16;
        if (fShape.isRRect() && o.fShape.isRRect()) {
            return SkRRectPriv::ConservativeIntersect(fShape.rrect(), o.fShape.rrect())
                    == o.fShape.rrect();
        } else if (fShape.isPath() && o.fShape.isPath()) {
            // TODO: Is this worth doing still if clips only cost as much as a single draw?
            return (fShape.path().getGenerationID() == o.fShape.path().getGenerationID()) ||
                    (fShape.path().countPoints() <= kMaxPathComparePoints &&
                    fShape.path() == o.fShape.path());
        } else {
            return fShape.conservativeContains(o.fShape.bounds());
        }
    } else if (fLocalToDevice.type() <= Transform::Type::kRectStaysRect &&
               o.fLocalToDevice.type() <= Transform::Type::kRectStaysRect) {
        // Optimize the common case where o's bounds can be mapped tightly into this coordinate
        // space and then tested against our shape.
        Rect localBounds = fLocalToDevice.inverseMapRect(
                o.fLocalToDevice.mapRect(o.fShape.bounds()));
        return fShape.conservativeContains(localBounds);
    } else if (fShape.convex()) {
        // Since this shape is convex, if all four corners of o's bounding box are inside it
        // then the entirety of o is also guaranteed to be inside it.
        SkV4 deviceQuad[4];
        o.fLocalToDevice.mapPoints(o.fShape.bounds(), deviceQuad);
        SkV4 localQuad[4];
        fLocalToDevice.inverseMapPoints(deviceQuad, localQuad, 4);
        for (int i = 0; i < 4; ++i) {
            // TODO: Would be nice to make this consistent with how the GPU clips NDC w.
            if (deviceQuad[i].w < SkPathPriv::kW0PlaneDistance ||
                localQuad[i].w < SkPathPriv::kW0PlaneDistance) {
                // Something in O actually projects behind the W = 0 plane and would be clipped
                // to infinity, so it's extremely unlikely that this contains O.
                return false;
            }
            if (!fShape.conservativeContains(skvx::float2::Load(localQuad + i) / localQuad[i].w)) {
                return false;
            }
        }
        return true;
    }

    // Else not an easily comparable pair of shapes so assume this doesn't contain O
    return false;
}

ClipStack::SimplifyResult ClipStack::Simplify(const TransformedShape& a,
                                              const TransformedShape& b) {
    enum class ClipCombo {
        kDD = 0b00,
        kDI = 0b01,
        kID = 0b10,
        kII = 0b11
    };

    switch(static_cast<ClipCombo>(((int) a.fOp << 1) | (int) b.fOp)) {
        case ClipCombo::kII:
            // Intersect (A) + Intersect (B)
            if (!a.intersects(b)) {
                // Regions with non-zero coverage are disjoint, so intersection = empty
                return SimplifyResult::kEmpty;
            } else if (b.contains(a)) {
                // B's full coverage region contains entirety of A, so intersection = A
                return SimplifyResult::kAOnly;
            } else if (a.contains(b)) {
                // A's full coverage region contains entirety of B, so intersection = B
                return SimplifyResult::kBOnly;
            } else {
                // The shapes intersect in some non-trivial manner
                return SimplifyResult::kBoth;
            }
        case ClipCombo::kID:
            // Intersect (A) + Difference (B)
            if (!a.intersects(b)) {
                // A only intersects B's full coverage region, so intersection = A
                return SimplifyResult::kAOnly;
            } else if (b.contains(a)) {
                // B's zero coverage region completely contains A, so intersection = empty
                return SimplifyResult::kEmpty;
            } else {
                // Intersection cannot be simplified. Note that the combination of a intersect
                // and difference op in this order cannot produce kBOnly
                return SimplifyResult::kBoth;
            }
        case ClipCombo::kDI:
            // Difference (A) + Intersect (B) - the mirror of Intersect(A) + Difference(B),
            // but combining is commutative so this is equivalent barring naming.
            if (!b.intersects(a)) {
                // B only intersects A's full coverage region, so intersection = B
                return SimplifyResult::kBOnly;
            } else if (a.contains(b)) {
                // A's zero coverage region completely contains B, so intersection = empty
                return SimplifyResult::kEmpty;
            } else {
                // Cannot be simplified
                return SimplifyResult::kBoth;
            }
        case ClipCombo::kDD:
            // Difference (A) + Difference (B)
            if (a.contains(b)) {
                // A's zero coverage region contains B, so B doesn't remove any extra
                // coverage from their intersection.
                return SimplifyResult::kAOnly;
            } else if (b.contains(a)) {
                // Mirror of the above case, intersection = B instead
                return SimplifyResult::kBOnly;
            } else {
                // Intersection of the two differences cannot be simplified. Note that for
                // this op combination it is not possible to produce kEmpty.
                return SimplifyResult::kBoth;
            }
    }
    SkUNREACHABLE;
}

///////////////////////////////////////////////////////////////////////////////
// ClipStack::Element

ClipStack::RawElement::RawElement(const Rect& deviceBounds,
                                  const Transform& localToDevice,
                                  const Shape& shape,
                                  SkClipOp op)
        : Element{shape, localToDevice, op}
        , fUsageBounds{Rect::InfiniteInverted()}
        , fOrder(DrawOrder::kNoIntersection)
        , fMaxZ(DrawOrder::kClearDepth)
        , fInvalidatedByIndex(-1) {
    // Discard shapes that don't have any area (including when a transform can't be inverted, since
    // it means the two dimensions are collapsed to 0 or 1 dimension in device space).
    if (fShape.isLine() || !localToDevice.valid()) {
        fShape.reset();
    }
    // Make sure the shape is not inverted. An inverted shape is equivalent to a non-inverted shape
    // with the clip op toggled.
    if (fShape.inverted()) {
        fOp = (fOp == SkClipOp::kIntersect) ? SkClipOp::kDifference : SkClipOp::kIntersect;
    }

    fOuterBounds = fLocalToDevice.mapRect(fShape.bounds()).makeIntersect(deviceBounds);
    fInnerBounds = Rect::InfiniteInverted();

    // Apply rect-stays-rect transforms to rects and round rects to reduce the number of unique
    // local coordinate systems that are in play.
    if (!fOuterBounds.isEmptyNegativeOrNaN() &&
        fLocalToDevice.type() <= Transform::Type::kRectStaysRect) {
        if (fShape.isRect()) {
            // The actual geometry can be updated to the device-intersected bounds and we know the
            // inner bounds are equal to the outer.
            fShape.setRect(fOuterBounds);
            fLocalToDevice = kIdentity;
            fInnerBounds = fOuterBounds;
        } else if (fShape.isRRect()) {
            // Can't transform in place and must still check transform result since some very
            // ill-formed scale+translate matrices can cause invalid rrect radii.
            SkRRect xformed;
            if (fShape.rrect().transform(fLocalToDevice, &xformed)) {
                fShape.setRRect(xformed);
                fLocalToDevice = kIdentity;
                // Refresh outer bounds to match the transformed round rect in case
                // SkRRect::transform produces slightly different results from Transform::mapRect.
                fOuterBounds = fShape.bounds().makeIntersect(deviceBounds);
                fInnerBounds = Rect{SkRRectPriv::InnerBounds(xformed)}.makeIntersect(fOuterBounds);
            }
        }
    }

    if (fOuterBounds.isEmptyNegativeOrNaN()) {
        // Either was already an empty shape or a non-empty shape is offscreen, so treat it as such.
        fShape.reset();
        fInnerBounds = Rect::InfiniteInverted();
    }

    // Now that fOp and fShape are canonical, set the shape's fill type to match how it needs to be
    // drawn as a depth-only shape everywhere that is clipped out (intersect is thus inverse-filled)
    fShape.setInverted(fOp == SkClipOp::kIntersect);

    // Post-conditions on inner and outer bounds
    SkASSERT(fShape.isEmpty() || deviceBounds.contains(fOuterBounds));
    this->validate();
}

ClipStack::RawElement::operator ClipStack::TransformedShape() const {
    return {fLocalToDevice, fShape, fOuterBounds, fInnerBounds, fOp};
}

void ClipStack::RawElement::drawClip(Device* device) {
    this->validate();

    // Skip elements that have not affected any draws
    if (!this->hasPendingDraw()) {
        SkASSERT(fUsageBounds.isEmptyNegativeOrNaN());
        return;
    }

    SkASSERT(!fUsageBounds.isEmptyNegativeOrNaN());
    // For clip draws, the usage bounds is the scissor.
    Rect scissor = fUsageBounds.makeRoundOut();
    Rect drawBounds = fOuterBounds.makeIntersect(scissor);
    if (!drawBounds.isEmptyNegativeOrNaN()) {
        // Although we are recording this clip draw after all the draws it affects, 'fOrder' was
        // determined at the first usage, so after sorting by DrawOrder the clip draw will be in the
        // right place. Unlike regular draws that use their own "Z", by writing (1 + max Z this clip
        // affects), it will cause those draws to fail either GREATER and GEQUAL depth tests where
        // they need to be clipped.
        DrawOrder order{fMaxZ.next(), fOrder};
        // An element's clip op is encoded in the shape's fill type. Inverse fills are intersect ops
        // and regular fills are difference ops. This means fShape is already in the right state to
        // draw directly.
        SkASSERT((fOp == SkClipOp::kDifference && !fShape.inverted()) ||
                 (fOp == SkClipOp::kIntersect && fShape.inverted()));
        device->drawClipShape(fLocalToDevice, fShape, Clip{drawBounds, scissor.asSkIRect()}, order);
    }

    // After the clip shape is drawn, reset its state. If the clip element is being popped off the
    // stack or overwritten because a new clip invalidated it, this won't matter. But if the clips
    // were drawn because the Device had to flush pending work while the clip stack was not empty,
    // subsequent draws will still need to be clipped to the elements. In this case, the usage
    // accumulation process will begin again and automatically use the Device's post-flush Z values
    // and BoundsManager state.
    fUsageBounds = Rect::InfiniteInverted();
    fOrder = DrawOrder::kNoIntersection;
    fMaxZ = DrawOrder::kClearDepth;
}

void ClipStack::RawElement::validate() const {
    // If the shape type isn't empty, the outer bounds shouldn't be empty; if the inner bounds are
    // not empty, they must be contained in outer.
    SkASSERT((fShape.isEmpty() || !fOuterBounds.isEmptyNegativeOrNaN()) &&
             (fInnerBounds.isEmptyNegativeOrNaN() || fOuterBounds.contains(fInnerBounds)));
    SkASSERT((fOp == SkClipOp::kDifference && !fShape.inverted()) ||
             (fOp == SkClipOp::kIntersect && fShape.inverted()));
    SkASSERT(!this->hasPendingDraw() || !fUsageBounds.isEmptyNegativeOrNaN());
}

void ClipStack::RawElement::markInvalid(const SaveRecord& current) {
    SkASSERT(!this->isInvalid());
    fInvalidatedByIndex = current.firstActiveElementIndex();
    // NOTE: We don't draw the accumulated clip usage when the element is marked invalid. Some
    // invalidated elements are part of earlier save records so can become re-active after a restore
    // in which case they should continue to accumulate. Invalidated elements that are part of the
    // active save record are removed at the end of the stack modification, which is when they are
    // explicitly drawn.
}

void ClipStack::RawElement::restoreValid(const SaveRecord& current) {
    if (current.firstActiveElementIndex() < fInvalidatedByIndex) {
        fInvalidatedByIndex = -1;
    }
}

bool ClipStack::RawElement::combine(const RawElement& other, const SaveRecord& current) {
    // Don't combine elements that have collected draw usage, since that changes their geometry.
    if (this->hasPendingDraw() || other.hasPendingDraw()) {
        return false;
    }
    // To reduce the number of possibilities, only consider intersect+intersect. Difference and
    // mixed op cases could be analyzed to simplify one of the shapes, but that is a rare
    // occurrence and the math is much more complicated.
    if (other.fOp != SkClipOp::kIntersect || fOp != SkClipOp::kIntersect) {
        return false;
    }

    // At the moment, only rect+rect or rrect+rrect are supported (although rect+rrect is
    // treated as a degenerate case of rrect+rrect).
    bool shapeUpdated = false;
    if (fShape.isRect() && other.fShape.isRect()) {
        if (fLocalToDevice == other.fLocalToDevice) {
            Rect intersection = fShape.rect().makeIntersect(other.fShape.rect());
            // Simplify() should have caught this case
            SkASSERT(!intersection.isEmptyNegativeOrNaN());
            fShape.setRect(intersection);
            shapeUpdated = true;
        }
    } else if ((fShape.isRect() || fShape.isRRect()) &&
               (other.fShape.isRect() || other.fShape.isRRect())) {
        if (fLocalToDevice == other.fLocalToDevice) {
            // Treat rrect+rect intersections as rrect+rrect
            SkRRect a = fShape.isRect() ? SkRRect::MakeRect(fShape.rect().asSkRect())
                                        : fShape.rrect();
            SkRRect b = other.fShape.isRect() ? SkRRect::MakeRect(other.fShape.rect().asSkRect())
                                              : other.fShape.rrect();

            SkRRect joined = SkRRectPriv::ConservativeIntersect(a, b);
            if (!joined.isEmpty()) {
                // Can reduce to a single element
                if (joined.isRect()) {
                    // And with a simplified type
                    fShape.setRect(joined.rect());
                } else {
                    fShape.setRRect(joined);
                }
                shapeUpdated = true;
            }
            // else the intersection isn't representable as a rrect, or doesn't actually intersect.
            // ConservativeIntersect doesn't disambiguate those two cases, and just testing bounding
            // boxes for non-intersection would have already been caught by Simplify(), so
            // just don't combine the two elements and let rasterization resolve the combination.
        }
    }

    if (shapeUpdated) {
        // This logic works under the assumption that both combined elements were intersect.
        SkASSERT(fOp == SkClipOp::kIntersect && other.fOp == SkClipOp::kIntersect);
        fOuterBounds.intersect(other.fOuterBounds);
        fInnerBounds.intersect(other.fInnerBounds);
        // Inner bounds can become empty, but outer bounds should not be able to.
        SkASSERT(!fOuterBounds.isEmptyNegativeOrNaN());
        fShape.setInverted(true); // the setR[R]ect operations reset to non-inverse
        this->validate();
        return true;
    } else {
        return false;
    }
}

void ClipStack::RawElement::updateForElement(RawElement* added, const SaveRecord& current) {
    if (this->isInvalid()) {
        // Already doesn't do anything, so skip this element
        return;
    }

    // 'A' refers to this element, 'B' refers to 'added'.
    switch (Simplify(*this, *added)) {
        case SimplifyResult::kEmpty:
            // Mark both elements as invalid to signal that the clip is fully empty
            this->markInvalid(current);
            added->markInvalid(current);
            break;

        case SimplifyResult::kAOnly:
            // This element already clips more than 'added', so mark 'added' is invalid to skip it
            added->markInvalid(current);
            break;

        case SimplifyResult::kBOnly:
            // 'added' clips more than this element, so mark this as invalid
            this->markInvalid(current);
            break;

        case SimplifyResult::kBoth:
            // Else the bounds checks think we need to keep both, but depending on the combination
            // of the ops and shape kinds, we may be able to do better.
            if (added->combine(*this, current)) {
                // 'added' now fully represents the combination of the two elements
                this->markInvalid(current);
            }
            break;
    }
}

std::pair<bool, CompressedPaintersOrder>
ClipStack::RawElement::updateForDraw(const BoundsManager* boundsManager,
                                     const TransformedShape& draw,
                                     PaintersDepth drawZ) {
    if (this->isInvalid()) {
        // Cannot affect the draw
        return {/*clippedOut=*/false, DrawOrder::kNoIntersection};
    }

    // For this analysis, A refers to the Element and B refers to the draw
    switch(Simplify(*this, draw)) {
        case SimplifyResult::kEmpty:
            // The more detailed per-element checks have determined the draw is clipped out.
            return {/*clippedOut=*/true, DrawOrder::kNoIntersection};

        case SimplifyResult::kBOnly:
            // This element does not affect the draw
            return {/*clippedOut=*/false, DrawOrder::kNoIntersection};

        case SimplifyResult::kAOnly:
            // If this were the only element, we could replace the draw's geometry but that only
            // gives us a win if we know that the clip element would only be used by this draw.
            // For now, just fall through to regular clip handling.
            [[fallthrough]];

        case SimplifyResult::kBoth:
            if (!this->hasPendingDraw()) {
                // No usage yet so we need an order that we will use when drawing to just the depth
                // attachment. It is sufficient to use the next CompressedPaintersOrder after the
                // most recent draw under this clip's outer bounds. It is necessary to use the
                // entire clip's outer bounds because the order has to be determined before the
                // final usage bounds are known and a subsequent draw could require a completely
                // different portion of the clip than this triggering draw.
                //
                // Lazily determining the order has several benefits to computing it when the clip
                // element was first created:
                //  - Elements that are invalidated by nested clips before draws are made do not
                //    waste time in the BoundsManager.
                //  - Elements that never actually modify a draw (e.g. a defensive clip) do not
                //    waste time in the BoundsManager.
                //  - A draw that triggers clip usage on multiple elements will more likely assign
                //    the same order to those elements, meaning their depth-only draws are more
                //    likely to batch in the final DrawPass.
                //
                // However, it does mean that clip elements can have the same order as each other,
                // or as later draws (e.g. after the clip has been popped off the stack). Any
                // overlap between clips or draws is addressed when the clip is drawn by selecting
                // an appropriate DisjointStencilIndex value. Stencil-aside, this order assignment
                // logic, max Z tracking, and the depth test during rasterization are able to
                // resolve everything correctly even if clips have the same order value.
                // See go/clip-stack-order for a detailed analysis of why this works.
                fOrder = boundsManager->getMostRecentDraw(fOuterBounds).next();
                fUsageBounds = draw.fOuterBounds;
                fMaxZ = drawZ;
            } else {
                // Earlier draws have already used this element so we cannot change where the
                // depth-only draw will be sorted to, but we need to ensure we cover the new draw's
                // bounds and use a Z value that will clip out its pixels as appropriate.
                fUsageBounds.join(draw.fOuterBounds);
                if (drawZ > fMaxZ) {
                    fMaxZ = drawZ;
                }
            }

            return {/*clippedOut=*/false, fOrder};
    }

    SkUNREACHABLE;
}

ClipStack::ClipState ClipStack::RawElement::clipType() const {
    // Map from the internal shape kind to the clip state enum
    switch (fShape.type()) {
        case Shape::Type::kEmpty:
            return ClipState::kEmpty;

        case Shape::Type::kRect:
            return fOp == SkClipOp::kIntersect &&
                   fLocalToDevice.type() == Transform::Type::kIdentity
                        ? ClipState::kDeviceRect : ClipState::kComplex;

        case Shape::Type::kRRect:
            return fOp == SkClipOp::kIntersect &&
                   fLocalToDevice.type() == Transform::Type::kIdentity
                        ? ClipState::kDeviceRRect : ClipState::kComplex;

        case Shape::Type::kLine:
            // These types should never become RawElements, but call them kComplex in release builds
            SkASSERT(false);
            [[fallthrough]];

        case Shape::Type::kPath:
            return ClipState::kComplex;
    }
    SkUNREACHABLE;
}

///////////////////////////////////////////////////////////////////////////////
// ClipStack::SaveRecord

ClipStack::SaveRecord::SaveRecord(const Rect& deviceBounds)
        : fInnerBounds(deviceBounds)
        , fOuterBounds(deviceBounds)
        , fShader(nullptr)
        , fStartingElementIndex(0)
        , fOldestValidIndex(0)
        , fDeferredSaveCount(0)
        , fStackOp(SkClipOp::kIntersect)
        , fState(ClipState::kWideOpen)  {}

ClipStack::SaveRecord::SaveRecord(const SaveRecord& prior,
                                  int startingElementIndex)
        : fInnerBounds(prior.fInnerBounds)
        , fOuterBounds(prior.fOuterBounds)
        , fShader(prior.fShader)
        , fStartingElementIndex(startingElementIndex)
        , fOldestValidIndex(prior.fOldestValidIndex)
        , fDeferredSaveCount(0)
        , fStackOp(prior.fStackOp)
        , fState(prior.fState) {
    // If the prior record added an element, this one will insert into the same index
    // (that's okay since we'll remove it when this record is popped off the stack).
    SkASSERT(startingElementIndex >= prior.fStartingElementIndex);
}

ClipStack::ClipState ClipStack::SaveRecord::state() const {
    if (fShader && fState != ClipState::kEmpty) {
        return ClipState::kComplex;
    } else {
        return fState;
    }
}

Rect ClipStack::SaveRecord::scissor(const Rect& deviceBounds, const Rect& drawBounds) const {
    // This should only be called when the clip stack actually has something non-trivial to evaluate
    // It is effectively a reduced version of Simplify() dealing only with device-space bounds and
    // returning the intersection results.
    SkASSERT(this->state() != ClipState::kEmpty && this->state() != ClipState::kWideOpen);
    SkASSERT(deviceBounds.contains(drawBounds)); // This should have already been handled.

    if (fStackOp == SkClipOp::kDifference) {
        // kDifference nominally uses the draw's bounds minus the save record's inner bounds as the
        // scissor. However, if the draw doesn't intersect the clip at all then it doesn't have any
        // visual effect and we can switch to the device bounds as the canonical scissor.
        if (!fOuterBounds.intersects(drawBounds)) {
            return deviceBounds;
        } else {
            // This automatically detects the case where the draw is contained in inner bounds and
            // would be entirely clipped out.
            return subtract(drawBounds, fInnerBounds, /*exact=*/true);
        }
    } else {
        // kIntersect nominally uses the save record's outer bounds as the scissor. However, if the
        // draw is contained entirely within those bounds, it doesn't have any visual effect so
        // switch to using the device bounds as the canonical scissor to minimize state changes.
        if (fOuterBounds.contains(drawBounds)) {
            return deviceBounds;
        } else {
            // This automatically detects the case where the draw does not intersect the clip.
            return fOuterBounds;
        }
    }
}

void ClipStack::SaveRecord::removeElements(RawElement::Stack* elements, Device* device) {
    while (elements->count() > fStartingElementIndex) {
        // Since the element is being deleted now, it won't be in the ClipStack when the Device
        // calls recordDeferredClipDraws(). Record the clip's draw now (if it needs it).
        elements->back().drawClip(device);
        elements->pop_back();
    }
}

void ClipStack::SaveRecord::restoreElements(RawElement::Stack* elements) {
    // Presumably this SaveRecord is the new top of the stack, and so it owns the elements
    // from its starting index to restoreCount - 1. Elements from the old save record have
    // been destroyed already, so their indices would have been >= restoreCount, and any
    // still-present element can be un-invalidated based on that.
    int i = elements->count() - 1;
    for (RawElement& e : elements->ritems()) {
        if (i < fOldestValidIndex) {
            break;
        }
        e.restoreValid(*this);
        --i;
    }
}

void ClipStack::SaveRecord::addShader(sk_sp<SkShader> shader) {
    SkASSERT(shader);
    SkASSERT(this->canBeUpdated());
    if (!fShader) {
        fShader = std::move(shader);
    } else {
        // The total coverage is computed by multiplying the coverage from each element (shape or
        // shader), but since multiplication is associative, we can use kSrcIn blending to make
        // a new shader that represents 'shader' * 'fShader'
        fShader = SkShaders::Blend(SkBlendMode::kSrcIn, std::move(shader), fShader);
    }
}

bool ClipStack::SaveRecord::addElement(RawElement&& toAdd,
                                       RawElement::Stack* elements,
                                       Device* device) {
    // Validity check the element's state first
    toAdd.validate();

    // And we shouldn't be adding an element if we have a deferred save
    SkASSERT(this->canBeUpdated());

    if (fState == ClipState::kEmpty) {
        // The clip is already empty, and we only shrink, so there's no need to record this element.
        return false;
    } else if (toAdd.shape().isEmpty()) {
        // An empty difference op should have been detected earlier, since it's a no-op
        SkASSERT(toAdd.op() == SkClipOp::kIntersect);
        fState = ClipState::kEmpty;
        this->removeElements(elements, device);
        return true;
    }

    // Here we treat the SaveRecord as a "TransformedShape" with the identity transform, and a shape
    // equal to its outer bounds. This lets us get accurate intersection tests against the new
    // element, but we pass true to skip more detailed contains checks because the SaveRecord's
    // shape is potentially very different from its aggregate outer bounds.
    Shape outerSaveBounds{fOuterBounds};
    TransformedShape save{kIdentity, outerSaveBounds, fOuterBounds, fInnerBounds, fStackOp,
                          /*containsChecksOnlyBounds=*/true};

    // In this invocation, 'A' refers to the existing stack's bounds and 'B' refers to the new
    // element.
    switch (Simplify(save, toAdd)) {
        case SimplifyResult::kEmpty:
            // The combination results in an empty clip
            fState = ClipState::kEmpty;
            this->removeElements(elements, device);
            return true;

        case SimplifyResult::kAOnly:
            // The combination would not be any different than the existing clip
            return false;

        case SimplifyResult::kBOnly:
            // The combination would invalidate the entire existing stack and can be replaced with
            // just the new element.
            this->replaceWithElement(std::move(toAdd), elements, device);
            return true;

        case SimplifyResult::kBoth:
            // The new element combines in a complex manner, so update the stack's bounds based on
            // the combination of its and the new element's ops (handled below)
            break;
    }

    if (fState == ClipState::kWideOpen) {
        // When the stack was wide open and the clip effect was kBoth, the "complex" manner is
        // simply to keep the element and update the stack bounds to be the element's intersected
        // with the device.
        this->replaceWithElement(std::move(toAdd), elements, device);
        return true;
    }

    // Some form of actual clip element(s) to combine with.
    if (fStackOp == SkClipOp::kIntersect) {
        if (toAdd.op() == SkClipOp::kIntersect) {
            // Intersect (stack) + Intersect (toAdd)
            //  - Bounds updates is simply the paired intersections of outer and inner.
            fOuterBounds.intersect(toAdd.outerBounds());
            fInnerBounds.intersect(toAdd.innerBounds());
            // Outer should not have become empty, but is allowed to if there's no intersection.
            SkASSERT(!fOuterBounds.isEmptyNegativeOrNaN());
        } else {
            // Intersect (stack) + Difference (toAdd)
            //  - Shrink the stack's outer bounds if the difference op's inner bounds completely
            //    cuts off an edge.
            //  - Shrink the stack's inner bounds to completely exclude the op's outer bounds.
            fOuterBounds = subtract(fOuterBounds, toAdd.innerBounds(), /* exact */ true);
            fInnerBounds = subtract(fInnerBounds, toAdd.outerBounds(), /* exact */ false);
        }
    } else {
        if (toAdd.op() == SkClipOp::kIntersect) {
            // Difference (stack) + Intersect (toAdd)
            //  - Bounds updates are just the mirror of Intersect(stack) + Difference(toAdd)
            Rect oldOuter = fOuterBounds;
            fOuterBounds = subtract(toAdd.outerBounds(), fInnerBounds, /* exact */ true);
            fInnerBounds = subtract(toAdd.innerBounds(), oldOuter,     /* exact */ false);
        } else {
            // Difference (stack) + Difference (toAdd)
            //  - The updated outer bounds is the union of outer bounds and the inner becomes the
            //    largest of the two possible inner bounds
            fOuterBounds.join(toAdd.outerBounds());
            if (toAdd.innerBounds().area() > fInnerBounds.area()) {
                fInnerBounds = toAdd.innerBounds();
            }
        }
    }

    // If we get here, we're keeping the new element and the stack's bounds have been updated.
    // We ought to have caught the cases where the stack bounds resemble an empty or wide open
    // clip, so assert that's the case.
    SkASSERT(!fOuterBounds.isEmptyNegativeOrNaN() &&
             (fInnerBounds.isEmptyNegativeOrNaN() || fOuterBounds.contains(fInnerBounds)));

    return this->appendElement(std::move(toAdd), elements, device);
}

bool ClipStack::SaveRecord::appendElement(RawElement&& toAdd,
                                          RawElement::Stack* elements,
                                          Device* device) {
    // Update past elements to account for the new element
    int i = elements->count() - 1;

    // After the loop, elements between [max(youngestValid, startingIndex)+1, count-1] can be
    // removed from the stack (these are the active elements that have been invalidated by the
    // newest element; since it's the active part of the stack, no restore() can bring them back).
    int youngestValid = fStartingElementIndex - 1;
    // After the loop, elements between [0, oldestValid-1] are all invalid. The value of oldestValid
    // becomes the save record's new fLastValidIndex value.
    int oldestValid = elements->count();
    // After the loop, this is the earliest active element that was invalidated. It may be
    // older in the stack than earliestValid, so cannot be popped off, but can be used to store
    // the new element instead of allocating more.
    RawElement* oldestActiveInvalid = nullptr;
    int oldestActiveInvalidIndex = elements->count();

    for (RawElement& existing : elements->ritems()) {
        if (i < fOldestValidIndex) {
            break;
        }
        // We don't need to pass the actual index that toAdd will be saved to; just the minimum
        // index of this save record, since that will result in the same restoration behavior later.
        existing.updateForElement(&toAdd, *this);

        if (toAdd.isInvalid()) {
            if (existing.isInvalid()) {
                // Both new and old invalid implies the entire clip becomes empty
                fState = ClipState::kEmpty;
                return true;
            } else {
                // The new element doesn't change the clip beyond what the old element already does
                return false;
            }
        } else if (existing.isInvalid()) {
            // The new element cancels out the old element. The new element may have been modified
            // to account for the old element's geometry.
            if (i >= fStartingElementIndex) {
                // Still active, so the invalidated index could be used to store the new element
                oldestActiveInvalid = &existing;
                oldestActiveInvalidIndex = i;
            }
        } else {
            // Keep both new and old elements
            oldestValid = i;
            if (i > youngestValid) {
                youngestValid = i;
            }
        }

        --i;
    }

    // Post-iteration validity check
    SkASSERT(oldestValid == elements->count() ||
             (oldestValid >= fOldestValidIndex && oldestValid < elements->count()));
    SkASSERT(youngestValid == fStartingElementIndex - 1 ||
             (youngestValid >= fStartingElementIndex && youngestValid < elements->count()));
    SkASSERT((oldestActiveInvalid && oldestActiveInvalidIndex >= fStartingElementIndex &&
              oldestActiveInvalidIndex < elements->count()) || !oldestActiveInvalid);

    // Update final state
    SkASSERT(oldestValid >= fOldestValidIndex);
    fOldestValidIndex = std::min(oldestValid, oldestActiveInvalidIndex);
    fState = oldestValid == elements->count() ? toAdd.clipType() : ClipState::kComplex;
    if (fStackOp == SkClipOp::kDifference && toAdd.op() == SkClipOp::kIntersect) {
        // The stack remains in difference mode only as long as all elements are difference
        fStackOp = SkClipOp::kIntersect;
    }

    int targetCount = youngestValid + 1;
    if (!oldestActiveInvalid || oldestActiveInvalidIndex >= targetCount) {
        // toAdd will be stored right after youngestValid
        targetCount++;
        oldestActiveInvalid = nullptr;
    }
    while (elements->count() > targetCount) {
        SkASSERT(oldestActiveInvalid != &elements->back()); // shouldn't delete what we'll reuse
        elements->back().drawClip(device);
        elements->pop_back();
    }
    if (oldestActiveInvalid) {
        oldestActiveInvalid->drawClip(device);
        *oldestActiveInvalid = std::move(toAdd);
    } else if (elements->count() < targetCount) {
        elements->push_back(std::move(toAdd));
    } else {
        elements->back().drawClip(device);
        elements->back() = std::move(toAdd);
    }

    return true;
}

void ClipStack::SaveRecord::replaceWithElement(RawElement&& toAdd,
                                               RawElement::Stack* elements,
                                               Device* device) {
    // The aggregate state of the save record mirrors the element
    fInnerBounds = toAdd.innerBounds();
    fOuterBounds = toAdd.outerBounds();
    fStackOp = toAdd.op();
    fState = toAdd.clipType();

    // All prior active element can be removed from the stack: [startingIndex, count - 1]
    int targetCount = fStartingElementIndex + 1;
    while (elements->count() > targetCount) {
        elements->back().drawClip(device);
        elements->pop_back();
    }
    if (elements->count() < targetCount) {
        elements->push_back(std::move(toAdd));
    } else {
        elements->back().drawClip(device);
        elements->back() = std::move(toAdd);
    }

    SkASSERT(elements->count() == fStartingElementIndex + 1);

    // This invalidates all older elements that are owned by save records lower in the clip stack.
    fOldestValidIndex = fStartingElementIndex;
}

///////////////////////////////////////////////////////////////////////////////
// ClipStack

// NOTE: Based on draw calls in all GMs, SKPs, and SVGs as of 08/20, 98% use a clip stack with
// one Element and up to two SaveRecords, thus the inline size for RawElement::Stack and
// SaveRecord::Stack (this conveniently keeps the size of ClipStack manageable). The max
// encountered element stack depth was 5 and the max save depth was 6. Using an increment of 8 for
// these stacks means that clip management will incur a single allocation for the remaining 2%
// of the draws, with extra head room for more complex clips encountered in the wild.
static constexpr int kElementStackIncrement = 8;
static constexpr int kSaveStackIncrement = 8;

ClipStack::ClipStack(Device* owningDevice)
        : fElements(kElementStackIncrement)
        , fSaves(kSaveStackIncrement)
        , fDevice(owningDevice) {
    // Start with a save record that is wide open
    fSaves.emplace_back(this->deviceBounds());
}

ClipStack::~ClipStack() = default;

void ClipStack::save() {
    SkASSERT(!fSaves.empty());
    fSaves.back().pushSave();
}

void ClipStack::restore() {
    SkASSERT(!fSaves.empty());
    SaveRecord& current = fSaves.back();
    if (current.popSave()) {
        // This was just a deferred save being undone, so the record doesn't need to be removed yet
        return;
    }

    // When we remove a save record, we delete all elements >= its starting index and any masks
    // that were rasterized for it.
    current.removeElements(&fElements, fDevice);

    fSaves.pop_back();
    // Restore any remaining elements that were only invalidated by the now-removed save record.
    fSaves.back().restoreElements(&fElements);
}

Rect ClipStack::deviceBounds() const {
    return Rect::WH(fDevice->width(), fDevice->height());
}

Rect ClipStack::conservativeBounds() const {
    const SaveRecord& current = this->currentSaveRecord();
    if (current.state() == ClipState::kEmpty) {
        return Rect::InfiniteInverted();
    } else if (current.state() == ClipState::kWideOpen) {
        return this->deviceBounds();
    } else {
        if (current.op() == SkClipOp::kDifference) {
            // The outer/inner bounds represent what's cut out, so full bounds remains the device
            // bounds, minus any fully clipped content that spans the device edge.
            return subtract(this->deviceBounds(), current.innerBounds(), /* exact */ true);
        } else {
            SkASSERT(this->deviceBounds().contains(current.outerBounds()));
            return current.outerBounds();
        }
    }
}

ClipStack::SaveRecord& ClipStack::writableSaveRecord(bool* wasDeferred) {
    SaveRecord& current = fSaves.back();
    if (current.canBeUpdated()) {
        // Current record is still open, so it can be modified directly
        *wasDeferred = false;
        return current;
    } else {
        // Must undefer the save to get a new record.
        SkAssertResult(current.popSave());
        *wasDeferred = true;
        return fSaves.emplace_back(current, fElements.count());
    }
}

void ClipStack::clipShader(sk_sp<SkShader> shader) {
    // Shaders can't bring additional coverage
    if (this->currentSaveRecord().state() == ClipState::kEmpty) {
        return;
    }

    bool wasDeferred;
    this->writableSaveRecord(&wasDeferred).addShader(std::move(shader));
    // Geometry elements are not invalidated by updating the clip shader
    // TODO: Integrating clipShader into graphite needs more thought, particularly around how to
    // handle the shader explosion and where to put the effects in the GraphicsPipelineDesc.
    // One idea is to use sample locations and draw the clipShader into the depth buffer.
    // Another is resolve the clip shader into an alpha mask image that is sampled by the draw.
}

void ClipStack::clipShape(const Transform& localToDevice,
                          const Shape& shape,
                          SkClipOp op) {
    if (this->currentSaveRecord().state() == ClipState::kEmpty) {
        return;
    }

    // This will apply the transform if it's shape-type preserving, and clip the element's bounds
    // to the device bounds (NOT the conservative clip bounds, since those are based on the net
    // effect of all elements while device bounds clipping happens implicitly. During addElement,
    // we may still be able to invalidate some older elements).
    // NOTE: Does not try to simplify the shape type by inspecting the SkPath.
    RawElement element{this->deviceBounds(), localToDevice, shape, op};

    // An empty op means do nothing (for difference), or close the save record, so we try and detect
    // that early before doing additional unnecessary save record allocation.
    if (element.shape().isEmpty()) {
        if (element.op() == SkClipOp::kDifference) {
            // If the shape is empty and we're subtracting, this has no effect on the clip
            return;
        }
        // else we will make the clip empty, but we need a new save record to record that change
        // in the clip state; fall through to below and updateForElement() will handle it.
    }

    bool wasDeferred;
    SaveRecord& save = this->writableSaveRecord(&wasDeferred);
    SkDEBUGCODE(int elementCount = fElements.count();)
    if (!save.addElement(std::move(element), &fElements, fDevice)) {
        if (wasDeferred) {
            // We made a new save record, but ended up not adding an element to the stack.
            // So instead of keeping an empty save record around, pop it off and restore the counter
            SkASSERT(elementCount == fElements.count());
            fSaves.pop_back();
            fSaves.back().pushSave();
        }
    }
}

std::pair<Clip, CompressedPaintersOrder> ClipStack::applyClipToDraw(
        const BoundsManager* boundsManager,
        const Transform& localToDevice,
        const Geometry& geometry,
        const SkStrokeRec& style,
        PaintersDepth z) {
    const SaveRecord& cs = this->currentSaveRecord();
    if (cs.state() == ClipState::kEmpty) {
        // We know the draw is clipped out so don't bother computing the base draw bounds.
        return {Clip{Rect::InfiniteInverted(), SkIRect::MakeEmpty()}, DrawOrder::kNoIntersection};
    }
    // Compute draw bounds, clipped only to our device bounds since we need to return that even if
    // the clip stack is known to be wide-open.
    const Rect deviceBounds = this->deviceBounds();

    // When 'style' isn't fill, 'shape' describes the pre-stroke shape so we can't use it to check
    // against clip elements and this will be set to the bounds of the post-stroked shape instead.
    SkTCopyOnFirstWrite<Shape> styledShape;
    if (geometry.isShape()) {
        styledShape.init(geometry.shape());
    } else {
        // The geometry is something special like text or vertices, in which case it's definitely
        // not a shape that could simplify cleanly with the clip stack.
        styledShape.initIfNeeded(geometry.bounds());
    }

    Rect drawBounds; // defined in device space
    if (styledShape->inverted()) {
        // Inverse-filled shapes always fill the entire device (restricted to the clip).
        drawBounds = deviceBounds;
        styledShape.writable()->setRect(drawBounds);
    } else {
        // Regular filled shapes and strokes get larger based on style and transform
        drawBounds = styledShape->bounds();
        if (!style.isHairlineStyle()) {
            float localStyleOutset = style.getInflationRadius();
            drawBounds.outset(localStyleOutset);

            if (!style.isFillStyle()) {
                // While this loses any shape type, the bounds remain local so hopefully tests are
                // fairly accurate.
                styledShape.writable()->setRect(drawBounds);
            }
        }
        drawBounds = localToDevice.mapRect(drawBounds);

        // Hairlines get an extra pixel *after* transforming to device space
        if (style.isHairlineStyle()) {
            drawBounds.outset(0.5f);
            // and the associated transform must be kIdentity since drawBounds has been mapped by
            // localToDevice already.
            styledShape.writable()->setRect(drawBounds);
        }

        // Restrict bounds to the device limits
        drawBounds.intersect(deviceBounds);
    }

    if (drawBounds.isEmptyNegativeOrNaN() || cs.state() == ClipState::kWideOpen) {
        // Either the draw is off screen, so it's clipped out regardless of the state of the
        // SaveRecord, or there are no elements to apply to the draw. In both cases, 'drawBounds'
        // has the correct value, the scissor is the device bounds (ignored if clipped-out), and
        // we can return kNoIntersection for the painter's order.
        return {Clip{drawBounds, deviceBounds.asSkIRect()}, DrawOrder::kNoIntersection};
    }

    // We don't evaluate Simplify() on the SaveRecord and the draw because a reduced version of
    // Simplify is effectively performed in computing the scissor rect.
    // Given that, we can skip iterating over the clip elements when:
    //  - the draw's *scissored* bounds are empty, which happens when the draw was clipped out.
    //  - the draw's *bounds* are contained in our inner bounds, which happens if all we need to
    //    apply to the draw is the computed scissor rect.
    // TODO: The Clip's scissor is defined in terms of integer pixel coords, but if we move to
    // clip plane distances in the vertex shader, it can be defined in terms of the original float
    // coordinates.
    Rect scissor = cs.scissor(deviceBounds, drawBounds).makeRoundOut();
    drawBounds.intersect(scissor);
    if (drawBounds.isEmptyNegativeOrNaN() || cs.innerBounds().contains(drawBounds)) {
        // Like above, in both cases drawBounds holds the right value and can return kNoIntersection
        return {Clip{drawBounds, scissor.asSkIRect()}, DrawOrder::kNoIntersection};
    }

    // If we made it here, the clip stack affects the draw in a complex way so iterate each element.
    // A draw is a transformed shape that "intersects" the clip. We use empty inner bounds because
    // there's currently no way to re-write the draw as the clip's geometry, so there's no need to
    // check if the draw contains the clip (vice versa is still checked and represents an unclipped
    // draw so is very useful to identify).
    TransformedShape draw{style.isHairlineStyle() ? kIdentity : localToDevice,
                          *styledShape,
                          /*outerBounds=*/drawBounds,
                          /*innerBounds=*/Rect::InfiniteInverted(),
                          /*op=*/SkClipOp::kIntersect,
                          /*containsChecksOnlyBounds=*/true};

    CompressedPaintersOrder maxClipOrder = DrawOrder::kNoIntersection;
    int i = fElements.count();
    for (RawElement& e : fElements.ritems()) {
        --i;
        if (i < cs.oldestElementIndex()) {
            // All earlier elements have been invalidated by elements already processed so the draw
            // can't be affected by them and cannot contribute to their usage bounds.
            break;
        }

        auto [clippedOut, order] = e.updateForDraw(boundsManager, draw, z);
        if (clippedOut) {
            drawBounds = Rect::InfiniteInverted();
            break;
        } else {
            maxClipOrder = std::max(order, maxClipOrder);
        }
    }

    return {Clip{drawBounds, scissor.asSkIRect()}, maxClipOrder};
}

void ClipStack::recordDeferredClipDraws() {
    for (auto& e : fElements.items()) {
        // When a Device requires all clip elements to be recorded, we have to iterate all elements,
        // and will draw clip shapes for elements that are still marked as invalid from the clip
        // stack, including those that are older than the current save record's oldest valid index,
        // because they could have accumulated draw usage prior to being invalidated, but weren't
        // flushed when they were invalidated because of an intervening save.
        e.drawClip(fDevice);
    }
}

} // namespace skgpu