src/core/SkRegion.cpp

/*
 * Copyright 2006 The Android Open Source Project
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "include/core/SkRegion.h"

#include "include/private/base/SkMacros.h"
#include "include/private/base/SkMalloc.h"
#include "include/private/base/SkMath.h"
#include "include/private/base/SkTemplates.h"
#include "include/private/base/SkTo.h"
#include "src/base/SkBuffer.h"
#include "src/base/SkSafeMath.h"
#include "src/core/SkRegionPriv.h"

#include <algorithm>
#include <atomic>
#include <cstring>
#include <functional>

using namespace skia_private;

/* Region Layout
 *
 *  TOP
 *
 *  [ Bottom, X-Intervals, [Left, Right]..., X-Sentinel ]
 *  ...
 *
 *  Y-Sentinel
 */

/////////////////////////////////////////////////////////////////////////////////////////////////

#define SkRegion_gEmptyRunHeadPtr   ((SkRegionPriv::RunHead*)-1)
#define SkRegion_gRectRunHeadPtr    nullptr

constexpr int kRunArrayStackCount = 256;

// This is a simple data structure which is like a STArray<N,T,true>, except that:
//   - It does not initialize memory.
//   - It does not distinguish between reserved space and initialized space.
//   - resizeToAtLeast() instead of resize()
//   - Uses sk_realloc_throw()
//   - Can never be made smaller.
// Measurement:  for the `region_union_16` benchmark, this is 6% faster.
class RunArray {
public:
    RunArray() { fPtr = fStack; }
    #ifdef SK_DEBUG
    int count() const { return fCount; }
    #endif
    SkRegionPriv::RunType& operator[](int i) {
        SkASSERT((unsigned)i < (unsigned)fCount);
        return fPtr[i];
    }
    /** Resize the array to a size greater-than-or-equal-to count. */
    void resizeToAtLeast(int count) {
        if (count > fCount) {
            // leave at least 50% extra space for future growth (unless adding would overflow)
            SkSafeMath safe;
            int newCount = safe.addInt(count, count >> 1);
            count = safe ? newCount : SK_MaxS32;
            fMalloc.realloc(count);
            if (fPtr == fStack) {
                memcpy(fMalloc.get(), fStack, fCount * sizeof(SkRegionPriv::RunType));
            }
            fPtr = fMalloc.get();
            fCount = count;
        }
    }
private:
    SkRegionPriv::RunType fStack[kRunArrayStackCount];
    AutoTMalloc<SkRegionPriv::RunType> fMalloc;
    int fCount = kRunArrayStackCount;
    SkRegionPriv::RunType* fPtr;  // non-owning pointer
};

/*  Pass in the beginning with the intervals.
 *  We back up 1 to read the interval-count.
 *  Return the beginning of the next scanline (i.e. the next Y-value)
 */
static SkRegionPriv::RunType* skip_intervals(const SkRegionPriv::RunType runs[]) {
    int intervals = runs[-1];
#ifdef SK_DEBUG
    if (intervals > 0) {
        SkASSERT(runs[0] < runs[1]);
        SkASSERT(runs[1] < SkRegion_kRunTypeSentinel);
    } else {
        SkASSERT(0 == intervals);
        SkASSERT(SkRegion_kRunTypeSentinel == runs[0]);
    }
#endif
    runs += intervals * 2 + 1;
    return const_cast<SkRegionPriv::RunType*>(runs);
}

bool SkRegion::RunsAreARect(const SkRegion::RunType runs[], int count,
                            SkIRect* bounds) {
    assert_sentinel(runs[0], false);    // top
    SkASSERT(count >= kRectRegionRuns);

    if (count == kRectRegionRuns) {
        assert_sentinel(runs[1], false);    // bottom
        SkASSERT(1 == runs[2]);
        assert_sentinel(runs[3], false);    // left
        assert_sentinel(runs[4], false);    // right
        assert_sentinel(runs[5], true);
        assert_sentinel(runs[6], true);

        SkASSERT(runs[0] < runs[1]);    // valid height
        SkASSERT(runs[3] < runs[4]);    // valid width

        bounds->setLTRB(runs[3], runs[0], runs[4], runs[1]);
        return true;
    }
    return false;
}

//////////////////////////////////////////////////////////////////////////

SkRegion::SkRegion() {
    fBounds.setEmpty();
    fRunHead = SkRegion_gEmptyRunHeadPtr;
}

SkRegion::SkRegion(const SkRegion& src) {
    fRunHead = SkRegion_gEmptyRunHeadPtr;   // just need a value that won't trigger sk_free(fRunHead)
    this->setRegion(src);
}

SkRegion::SkRegion(const SkIRect& rect) {
    fRunHead = SkRegion_gEmptyRunHeadPtr;   // just need a value that won't trigger sk_free(fRunHead)
    this->setRect(rect);
}

SkRegion::~SkRegion() {
    this->freeRuns();
}

void SkRegion::freeRuns() {
    if (this->isComplex()) {
        SkASSERT(fRunHead->fRefCnt >= 1);
        if (--fRunHead->fRefCnt == 0) {
            sk_free(fRunHead);
        }
    }
}

void SkRegion::allocateRuns(int count, int ySpanCount, int intervalCount) {
    fRunHead = RunHead::Alloc(count, ySpanCount, intervalCount);
}

void SkRegion::allocateRuns(int count) {
    fRunHead = RunHead::Alloc(count);
}

void SkRegion::allocateRuns(const RunHead& head) {
    fRunHead = RunHead::Alloc(head.fRunCount,
                              head.getYSpanCount(),
                              head.getIntervalCount());
}

SkRegion& SkRegion::operator=(const SkRegion& src) {
    (void)this->setRegion(src);
    return *this;
}

void SkRegion::swap(SkRegion& other) {
    using std::swap;
    swap(fBounds, other.fBounds);
    swap(fRunHead, other.fRunHead);
}

int SkRegion::computeRegionComplexity() const {
  if (this->isEmpty()) {
    return 0;
  } else if (this->isRect()) {
    return 1;
  }
  return fRunHead->getIntervalCount();
}

bool SkRegion::setEmpty() {
    this->freeRuns();
    fBounds.setEmpty();
    fRunHead = SkRegion_gEmptyRunHeadPtr;
    return false;
}

bool SkRegion::setRect(const SkIRect& r) {
    if (r.isEmpty() ||
        SkRegion_kRunTypeSentinel == r.right() ||
        SkRegion_kRunTypeSentinel == r.bottom()) {
        return this->setEmpty();
    }
    this->freeRuns();
    fBounds = r;
    fRunHead = SkRegion_gRectRunHeadPtr;
    return true;
}

bool SkRegion::setRegion(const SkRegion& src) {
    if (this != &src) {
        this->freeRuns();

        fBounds = src.fBounds;
        fRunHead = src.fRunHead;
        if (this->isComplex()) {
            fRunHead->fRefCnt++;
        }
    }
    return fRunHead != SkRegion_gEmptyRunHeadPtr;
}

bool SkRegion::op(const SkIRect& rect, const SkRegion& rgn, Op op) {
    SkRegion tmp(rect);

    return this->op(tmp, rgn, op);
}

bool SkRegion::op(const SkRegion& rgn, const SkIRect& rect, Op op) {
    SkRegion tmp(rect);

    return this->op(rgn, tmp, op);
}

///////////////////////////////////////////////////////////////////////////////

#ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
#include <stdio.h>
char* SkRegion::toString() {
    Iterator iter(*this);
    int count = 0;
    while (!iter.done()) {
        count++;
        iter.next();
    }
    // 4 ints, up to 10 digits each plus sign, 3 commas, '(', ')', SkRegion() and '\0'
    const int max = (count*((11*4)+5))+11+1;
    char* result = (char*)sk_malloc_throw(max);
    if (result == nullptr) {
        return nullptr;
    }
    count = snprintf(result, max, "SkRegion(");
    iter.reset(*this);
    while (!iter.done()) {
        const SkIRect& r = iter.rect();
        count += snprintf(result+count, max - count,
                "(%d,%d,%d,%d)", r.fLeft, r.fTop, r.fRight, r.fBottom);
        iter.next();
    }
    count += snprintf(result+count, max - count, ")");
    return result;
}
#endif

///////////////////////////////////////////////////////////////////////////////

int SkRegion::count_runtype_values(int* itop, int* ibot) const {
    int maxT;

    if (this->isRect()) {
        maxT = 2;
    } else {
        SkASSERT(this->isComplex());
        maxT = fRunHead->getIntervalCount() * 2;
    }
    *itop = fBounds.fTop;
    *ibot = fBounds.fBottom;
    return maxT;
}

static bool isRunCountEmpty(int count) {
    return count <= 2;
}

bool SkRegion::setRuns(RunType runs[], int count) {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));
    SkASSERT(count > 0);

    if (isRunCountEmpty(count)) {
    //  SkDEBUGF("setRuns: empty\n");
        assert_sentinel(runs[count-1], true);
        return this->setEmpty();
    }

    // trim off any empty spans from the top and bottom
    // weird I should need this, perhaps op() could be smarter...
    if (count > kRectRegionRuns) {
        RunType* stop = runs + count;
        assert_sentinel(runs[0], false);    // top
        assert_sentinel(runs[1], false);    // bottom
        // runs[2] is uncomputed intervalCount

        if (runs[3] == SkRegion_kRunTypeSentinel) {  // should be first left...
            runs += 3;  // skip empty initial span
            runs[0] = runs[-2]; // set new top to prev bottom
            assert_sentinel(runs[1], false);    // bot: a sentinal would mean two in a row
            assert_sentinel(runs[2], false);    // intervalcount
            assert_sentinel(runs[3], false);    // left
            assert_sentinel(runs[4], false);    // right
        }

        assert_sentinel(stop[-1], true);
        assert_sentinel(stop[-2], true);

        // now check for a trailing empty span
        if (stop[-5] == SkRegion_kRunTypeSentinel) { // eek, stop[-4] was a bottom with no x-runs
            stop[-4] = SkRegion_kRunTypeSentinel;    // kill empty last span
            stop -= 3;
            assert_sentinel(stop[-1], true);    // last y-sentinel
            assert_sentinel(stop[-2], true);    // last x-sentinel
            assert_sentinel(stop[-3], false);   // last right
            assert_sentinel(stop[-4], false);   // last left
            assert_sentinel(stop[-5], false);   // last interval-count
            assert_sentinel(stop[-6], false);   // last bottom
        }
        count = (int)(stop - runs);
    }

    SkASSERT(count >= kRectRegionRuns);

    if (SkRegion::RunsAreARect(runs, count, &fBounds)) {
        return this->setRect(fBounds);
    }

    //  if we get here, we need to become a complex region

    if (!this->isComplex() || fRunHead->fRunCount != count) {
        this->freeRuns();
        this->allocateRuns(count);
        SkASSERT(this->isComplex());
    }

    // must call this before we can write directly into runs()
    // in case we are sharing the buffer with another region (copy on write)
    fRunHead = fRunHead->ensureWritable();
    memcpy(fRunHead->writable_runs(), runs, count * sizeof(RunType));
    fRunHead->computeRunBounds(&fBounds);

    // Our computed bounds might be too large, so we have to check here.
    if (fBounds.isEmpty()) {
        return this->setEmpty();
    }

    SkDEBUGCODE(SkRegionPriv::Validate(*this));

    return true;
}

void SkRegion::BuildRectRuns(const SkIRect& bounds,
                             RunType runs[kRectRegionRuns]) {
    runs[0] = bounds.fTop;
    runs[1] = bounds.fBottom;
    runs[2] = 1;    // 1 interval for this scanline
    runs[3] = bounds.fLeft;
    runs[4] = bounds.fRight;
    runs[5] = SkRegion_kRunTypeSentinel;
    runs[6] = SkRegion_kRunTypeSentinel;
}

bool SkRegion::contains(int32_t x, int32_t y) const {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));

    if (!fBounds.contains(x, y)) {
        return false;
    }
    if (this->isRect()) {
        return true;
    }
    SkASSERT(this->isComplex());

    const RunType* runs = fRunHead->findScanline(y);

    // Skip the Bottom and IntervalCount
    runs += 2;

    // Just walk this scanline, checking each interval. The X-sentinel will
    // appear as a left-inteval (runs[0]) and should abort the search.
    //
    // We could do a bsearch, using interval-count (runs[1]), but need to time
    // when that would be worthwhile.
    //
    for (;;) {
        if (x < runs[0]) {
            break;
        }
        if (x < runs[1]) {
            return true;
        }
        runs += 2;
    }
    return false;
}

static SkRegionPriv::RunType scanline_bottom(const SkRegionPriv::RunType runs[]) {
    return runs[0];
}

static const SkRegionPriv::RunType* scanline_next(const SkRegionPriv::RunType runs[]) {
    // skip [B N [L R]... S]
    return runs + 2 + runs[1] * 2 + 1;
}

static bool scanline_contains(const SkRegionPriv::RunType runs[],
                              SkRegionPriv::RunType L, SkRegionPriv::RunType R) {
    runs += 2;  // skip Bottom and IntervalCount
    for (;;) {
        if (L < runs[0]) {
            break;
        }
        if (R <= runs[1]) {
            return true;
        }
        runs += 2;
    }
    return false;
}

bool SkRegion::contains(const SkIRect& r) const {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));

    if (!fBounds.contains(r)) {
        return false;
    }
    if (this->isRect()) {
        return true;
    }
    SkASSERT(this->isComplex());

    const RunType* scanline = fRunHead->findScanline(r.fTop);
    for (;;) {
        if (!scanline_contains(scanline, r.fLeft, r.fRight)) {
            return false;
        }
        if (r.fBottom <= scanline_bottom(scanline)) {
            break;
        }
        scanline = scanline_next(scanline);
    }
    return true;
}

bool SkRegion::contains(const SkRegion& rgn) const {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));
    SkDEBUGCODE(SkRegionPriv::Validate(rgn));

    if (this->isEmpty() || rgn.isEmpty() || !fBounds.contains(rgn.fBounds)) {
        return false;
    }
    if (this->isRect()) {
        return true;
    }
    if (rgn.isRect()) {
        return this->contains(rgn.getBounds());
    }

    /*
     *  A contains B is equivalent to
     *  B - A == 0
     */
    return !Oper(rgn, *this, kDifference_Op, nullptr);
}

const SkRegion::RunType* SkRegion::getRuns(RunType tmpStorage[],
                                           int* intervals) const {
    SkASSERT(tmpStorage && intervals);
    const RunType* runs = tmpStorage;

    if (this->isEmpty()) {
        tmpStorage[0] = SkRegion_kRunTypeSentinel;
        *intervals = 0;
    } else if (this->isRect()) {
        BuildRectRuns(fBounds, tmpStorage);
        *intervals = 1;
    } else {
        runs = fRunHead->readonly_runs();
        *intervals = fRunHead->getIntervalCount();
    }
    return runs;
}

///////////////////////////////////////////////////////////////////////////////

static bool scanline_intersects(const SkRegionPriv::RunType runs[],
                                SkRegionPriv::RunType L, SkRegionPriv::RunType R) {
    runs += 2;  // skip Bottom and IntervalCount
    for (;;) {
        if (R <= runs[0]) {
            break;
        }
        if (L < runs[1]) {
            return true;
        }
        runs += 2;
    }
    return false;
}

bool SkRegion::intersects(const SkIRect& r) const {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));

    if (this->isEmpty() || r.isEmpty()) {
        return false;
    }

    SkIRect sect;
    if (!sect.intersect(fBounds, r)) {
        return false;
    }
    if (this->isRect()) {
        return true;
    }
    SkASSERT(this->isComplex());

    const RunType* scanline = fRunHead->findScanline(sect.fTop);
    for (;;) {
        if (scanline_intersects(scanline, sect.fLeft, sect.fRight)) {
            return true;
        }
        if (sect.fBottom <= scanline_bottom(scanline)) {
            break;
        }
        scanline = scanline_next(scanline);
    }
    return false;
}

bool SkRegion::intersects(const SkRegion& rgn) const {
    if (this->isEmpty() || rgn.isEmpty()) {
        return false;
    }

    if (!SkIRect::Intersects(fBounds, rgn.fBounds)) {
        return false;
    }

    bool weAreARect = this->isRect();
    bool theyAreARect = rgn.isRect();

    if (weAreARect && theyAreARect) {
        return true;
    }
    if (weAreARect) {
        return rgn.intersects(this->getBounds());
    }
    if (theyAreARect) {
        return this->intersects(rgn.getBounds());
    }

    // both of us are complex
    return Oper(*this, rgn, kIntersect_Op, nullptr);
}

///////////////////////////////////////////////////////////////////////////////

bool SkRegion::operator==(const SkRegion& b) const {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));
    SkDEBUGCODE(SkRegionPriv::Validate(b));

    if (this == &b) {
        return true;
    }
    if (fBounds != b.fBounds) {
        return false;
    }

    const SkRegion::RunHead* ah = fRunHead;
    const SkRegion::RunHead* bh = b.fRunHead;

    // this catches empties and rects being equal
    if (ah == bh) {
        return true;
    }
    // now we insist that both are complex (but different ptrs)
    if (!this->isComplex() || !b.isComplex()) {
        return false;
    }
    return  ah->fRunCount == bh->fRunCount &&
            !memcmp(ah->readonly_runs(), bh->readonly_runs(),
                    ah->fRunCount * sizeof(SkRegion::RunType));
}

// Return a (new) offset such that when applied (+=) to min and max, we don't overflow/underflow
static int32_t pin_offset_s32(int32_t min, int32_t max, int32_t offset) {
    SkASSERT(min <= max);
    const int32_t lo = -SK_MaxS32-1,
                  hi = +SK_MaxS32;
    if ((int64_t)min + offset < lo) { offset = lo - min; }
    if ((int64_t)max + offset > hi) { offset = hi - max; }
    return offset;
}

void SkRegion::translate(int dx, int dy, SkRegion* dst) const {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));

    if (nullptr == dst) {
        return;
    }
    if (this->isEmpty()) {
        dst->setEmpty();
        return;
    }
    // pin dx and dy so we don't overflow our existing bounds
    dx = pin_offset_s32(fBounds.fLeft, fBounds.fRight, dx);
    dy = pin_offset_s32(fBounds.fTop, fBounds.fBottom, dy);

    if (this->isRect()) {
        dst->setRect(fBounds.makeOffset(dx, dy));
    } else {
        if (this == dst) {
            dst->fRunHead = dst->fRunHead->ensureWritable();
        } else {
            SkRegion    tmp;
            tmp.allocateRuns(*fRunHead);
            SkASSERT(tmp.isComplex());
            tmp.fBounds = fBounds;
            dst->swap(tmp);
        }

        dst->fBounds.offset(dx, dy);

        const RunType*  sruns = fRunHead->readonly_runs();
        RunType*        druns = dst->fRunHead->writable_runs();

        *druns++ = (SkRegion::RunType)(*sruns++ + dy);    // top
        for (;;) {
            int bottom = *sruns++;
            if (bottom == SkRegion_kRunTypeSentinel) {
                break;
            }
            *druns++ = (SkRegion::RunType)(bottom + dy);  // bottom;
            *druns++ = *sruns++;    // copy intervalCount;
            for (;;) {
                int x = *sruns++;
                if (x == SkRegion_kRunTypeSentinel) {
                    break;
                }
                *druns++ = (SkRegion::RunType)(x + dx);
                *druns++ = (SkRegion::RunType)(*sruns++ + dx);
            }
            *druns++ = SkRegion_kRunTypeSentinel;    // x sentinel
        }
        *druns++ = SkRegion_kRunTypeSentinel;    // y sentinel

        SkASSERT(sruns - fRunHead->readonly_runs() == fRunHead->fRunCount);
        SkASSERT(druns - dst->fRunHead->readonly_runs() == dst->fRunHead->fRunCount);
    }

    SkDEBUGCODE(SkRegionPriv::Validate(*this));
}

///////////////////////////////////////////////////////////////////////////////

bool SkRegion::setRects(const SkIRect rects[], int count) {
    if (0 == count) {
        this->setEmpty();
    } else {
        this->setRect(rects[0]);
        for (int i = 1; i < count; i++) {
            this->op(rects[i], kUnion_Op);
        }
    }
    return !this->isEmpty();
}

///////////////////////////////////////////////////////////////////////////////

#if defined _WIN32  // disable warning : local variable used without having been initialized
#pragma warning ( push )
#pragma warning ( disable : 4701 )
#endif

#ifdef SK_DEBUG
static void assert_valid_pair(int left, int rite)
{
    SkASSERT(left == SkRegion_kRunTypeSentinel || left < rite);
}
#else
    #define assert_valid_pair(left, rite)
#endif

struct spanRec {
    const SkRegionPriv::RunType*    fA_runs;
    const SkRegionPriv::RunType*    fB_runs;
    int                         fA_left, fA_rite, fB_left, fB_rite;
    int                         fLeft, fRite, fInside;

    void init(const SkRegionPriv::RunType a_runs[],
              const SkRegionPriv::RunType b_runs[]) {
        fA_left = *a_runs++;
        fA_rite = *a_runs++;
        fB_left = *b_runs++;
        fB_rite = *b_runs++;

        fA_runs = a_runs;
        fB_runs = b_runs;
    }

    bool done() const {
        SkASSERT(fA_left <= SkRegion_kRunTypeSentinel);
        SkASSERT(fB_left <= SkRegion_kRunTypeSentinel);
        return fA_left == SkRegion_kRunTypeSentinel &&
               fB_left == SkRegion_kRunTypeSentinel;
    }

    void next() {
        assert_valid_pair(fA_left, fA_rite);
        assert_valid_pair(fB_left, fB_rite);

        int     inside, left, rite SK_INIT_TO_AVOID_WARNING;
        bool    a_flush = false;
        bool    b_flush = false;

        int a_left = fA_left;
        int a_rite = fA_rite;
        int b_left = fB_left;
        int b_rite = fB_rite;

        if (a_left < b_left) {
            inside = 1;
            left = a_left;
            if (a_rite <= b_left) {   // [...] <...>
                rite = a_rite;
                a_flush = true;
            } else { // [...<..]...> or [...<...>...]
                rite = a_left = b_left;
            }
        } else if (b_left < a_left) {
            inside = 2;
            left = b_left;
            if (b_rite <= a_left) {   // [...] <...>
                rite = b_rite;
                b_flush = true;
            } else {    // [...<..]...> or [...<...>...]
                rite = b_left = a_left;
            }
        } else {    // a_left == b_left
            inside = 3;
            left = a_left;  // or b_left
            if (a_rite <= b_rite) {
                rite = b_left = a_rite;
                a_flush = true;
            }
            if (b_rite <= a_rite) {
                rite = a_left = b_rite;
                b_flush = true;
            }
        }

        if (a_flush) {
            a_left = *fA_runs++;
            a_rite = *fA_runs++;
        }
        if (b_flush) {
            b_left = *fB_runs++;
            b_rite = *fB_runs++;
        }

        SkASSERT(left <= rite);

        // now update our state
        fA_left = a_left;
        fA_rite = a_rite;
        fB_left = b_left;
        fB_rite = b_rite;

        fLeft = left;
        fRite = rite;
        fInside = inside;
    }
};

static int distance_to_sentinel(const SkRegionPriv::RunType* runs) {
    const SkRegionPriv::RunType* ptr = runs;
    while (*ptr != SkRegion_kRunTypeSentinel) { ptr += 2; }
    return ptr - runs;
}

static int operate_on_span(const SkRegionPriv::RunType a_runs[],
                           const SkRegionPriv::RunType b_runs[],
                           RunArray* array, int dstOffset,
                           int min, int max) {
    // This is a worst-case for this span plus two for TWO terminating sentinels.
    array->resizeToAtLeast(
            dstOffset + distance_to_sentinel(a_runs) + distance_to_sentinel(b_runs) + 2);
    SkRegionPriv::RunType* dst = &(*array)[dstOffset]; // get pointer AFTER resizing.

    spanRec rec;
    bool    firstInterval = true;

    rec.init(a_runs, b_runs);

    while (!rec.done()) {
        rec.next();

        int left = rec.fLeft;
        int rite = rec.fRite;

        // add left,rite to our dst buffer (checking for coincidence
        if ((unsigned)(rec.fInside - min) <= (unsigned)(max - min) &&
                left < rite) {    // skip if equal
            if (firstInterval || *(dst - 1) < left) {
                *dst++ = (SkRegionPriv::RunType)(left);
                *dst++ = (SkRegionPriv::RunType)(rite);
                firstInterval = false;
            } else {
                // update the right edge
                *(dst - 1) = (SkRegionPriv::RunType)(rite);
            }
        }
    }
    SkASSERT(dst < &(*array)[array->count() - 1]);
    *dst++ = SkRegion_kRunTypeSentinel;
    return dst - &(*array)[0];
}

#if defined _WIN32
#pragma warning ( pop )
#endif

static const struct {
    uint8_t fMin;
    uint8_t fMax;
} gOpMinMax[] = {
    { 1, 1 },   // Difference
    { 3, 3 },   // Intersection
    { 1, 3 },   // Union
    { 1, 2 }    // XOR
};
// need to ensure that the op enum lines up with our minmax array
static_assert(0 == SkRegion::kDifference_Op, "");
static_assert(1 == SkRegion::kIntersect_Op,  "");
static_assert(2 == SkRegion::kUnion_Op,      "");
static_assert(3 == SkRegion::kXOR_Op,        "");

class RgnOper {
public:
    RgnOper(int top, RunArray* array, SkRegion::Op op)
        : fMin(gOpMinMax[op].fMin)
        , fMax(gOpMinMax[op].fMax)
        , fArray(array)
        , fTop((SkRegionPriv::RunType)top)  // just a first guess, we might update this
        { SkASSERT((unsigned)op <= 3); }

    void addSpan(int bottom, const SkRegionPriv::RunType a_runs[],
                 const SkRegionPriv::RunType b_runs[]) {
        // skip X values and slots for the next Y+intervalCount
        int start = fPrevDst + fPrevLen + 2;
        // start points to beginning of dst interval
        int stop = operate_on_span(a_runs, b_runs, fArray, start, fMin, fMax);
        size_t len = SkToSizeT(stop - start);
        SkASSERT(len >= 1 && (len & 1) == 1);
        SkASSERT(SkRegion_kRunTypeSentinel == (*fArray)[stop - 1]);

        // Assert memcmp won't exceed fArray->count().
        SkASSERT(fArray->count() >= SkToInt(start + len - 1));
        if (fPrevLen == len &&
            (1 == len || !memcmp(&(*fArray)[fPrevDst],
                                 &(*fArray)[start],
                                 (len - 1) * sizeof(SkRegionPriv::RunType)))) {
            // update Y value
            (*fArray)[fPrevDst - 2] = (SkRegionPriv::RunType)bottom;
        } else {    // accept the new span
            if (len == 1 && fPrevLen == 0) {
                fTop = (SkRegionPriv::RunType)bottom; // just update our bottom
            } else {
                (*fArray)[start - 2] = (SkRegionPriv::RunType)bottom;
                (*fArray)[start - 1] = SkToS32(len >> 1);
                fPrevDst = start;
                fPrevLen = len;
            }
        }
    }

    int flush() {
        (*fArray)[fStartDst] = fTop;
        // Previously reserved enough for TWO sentinals.
        SkASSERT(fArray->count() > SkToInt(fPrevDst + fPrevLen));
        (*fArray)[fPrevDst + fPrevLen] = SkRegion_kRunTypeSentinel;
        return (int)(fPrevDst - fStartDst + fPrevLen + 1);
    }

    bool isEmpty() const { return 0 == fPrevLen; }

    uint8_t fMin, fMax;

private:
    RunArray* fArray;
    int fStartDst = 0;
    int fPrevDst = 1;
    size_t fPrevLen = 0;  // will never match a length from operate_on_span
    SkRegionPriv::RunType fTop;
};

// want a unique value to signal that we exited due to quickExit
#define QUICK_EXIT_TRUE_COUNT   (-1)

static int operate(const SkRegionPriv::RunType a_runs[],
                   const SkRegionPriv::RunType b_runs[],
                   RunArray* dst,
                   SkRegion::Op op,
                   bool quickExit) {
    const SkRegionPriv::RunType gEmptyScanline[] = {
        0,  // fake bottom value
        0,  // zero intervals
        SkRegion_kRunTypeSentinel,
        // just need a 2nd value, since spanRec.init() reads 2 values, even
        // though if the first value is the sentinel, it ignores the 2nd value.
        // w/o the 2nd value here, we might read uninitialized memory.
        // This happens when we are using gSentinel, which is pointing at
        // our sentinel value.
        0
    };
    const SkRegionPriv::RunType* const gSentinel = &gEmptyScanline[2];

    int a_top = *a_runs++;
    int a_bot = *a_runs++;
    int b_top = *b_runs++;
    int b_bot = *b_runs++;

    a_runs += 1;    // skip the intervalCount;
    b_runs += 1;    // skip the intervalCount;

    // Now a_runs and b_runs to their intervals (or sentinel)

    assert_sentinel(a_top, false);
    assert_sentinel(a_bot, false);
    assert_sentinel(b_top, false);
    assert_sentinel(b_bot, false);

    RgnOper oper(std::min(a_top, b_top), dst, op);

    int prevBot = SkRegion_kRunTypeSentinel; // so we fail the first test

    while (a_bot < SkRegion_kRunTypeSentinel ||
           b_bot < SkRegion_kRunTypeSentinel) {
        int                         top, bot SK_INIT_TO_AVOID_WARNING;
        const SkRegionPriv::RunType*    run0 = gSentinel;
        const SkRegionPriv::RunType*    run1 = gSentinel;
        bool                        a_flush = false;
        bool                        b_flush = false;

        if (a_top < b_top) {
            top = a_top;
            run0 = a_runs;
            if (a_bot <= b_top) {   // [...] <...>
                bot = a_bot;
                a_flush = true;
            } else {  // [...<..]...> or [...<...>...]
                bot = a_top = b_top;
            }
        } else if (b_top < a_top) {
            top = b_top;
            run1 = b_runs;
            if (b_bot <= a_top) {   // [...] <...>
                bot = b_bot;
                b_flush = true;
            } else {    // [...<..]...> or [...<...>...]
                bot = b_top = a_top;
            }
        } else {    // a_top == b_top
            top = a_top;    // or b_top
            run0 = a_runs;
            run1 = b_runs;
            if (a_bot <= b_bot) {
                bot = b_top = a_bot;
                a_flush = true;
            }
            if (b_bot <= a_bot) {
                bot = a_top = b_bot;
                b_flush = true;
            }
        }

        if (top > prevBot) {
            oper.addSpan(top, gSentinel, gSentinel);
        }
        oper.addSpan(bot, run0, run1);

        if (quickExit && !oper.isEmpty()) {
            return QUICK_EXIT_TRUE_COUNT;
        }

        if (a_flush) {
            a_runs = skip_intervals(a_runs);
            a_top = a_bot;
            a_bot = *a_runs++;
            a_runs += 1;    // skip uninitialized intervalCount
            if (a_bot == SkRegion_kRunTypeSentinel) {
                a_top = a_bot;
            }
        }
        if (b_flush) {
            b_runs = skip_intervals(b_runs);
            b_top = b_bot;
            b_bot = *b_runs++;
            b_runs += 1;    // skip uninitialized intervalCount
            if (b_bot == SkRegion_kRunTypeSentinel) {
                b_top = b_bot;
            }
        }

        prevBot = bot;
    }
    return oper.flush();
}

///////////////////////////////////////////////////////////////////////////////

/*  Given count RunTypes in a complex region, return the worst case number of
    logical intervals that represents (i.e. number of rects that would be
    returned from the iterator).

    We could just return count/2, since there must be at least 2 values per
    interval, but we can first trim off the const overhead of the initial TOP
    value, plus the final BOTTOM + 2 sentinels.
 */
#if 0 // UNUSED
static int count_to_intervals(int count) {
    SkASSERT(count >= 6);   // a single rect is 6 values
    return (count - 4) >> 1;
}
#endif

static bool setEmptyCheck(SkRegion* result) {
    return result ? result->setEmpty() : false;
}

static bool setRectCheck(SkRegion* result, const SkIRect& rect) {
    return result ? result->setRect(rect) : !rect.isEmpty();
}

static bool setRegionCheck(SkRegion* result, const SkRegion& rgn) {
    return result ? result->setRegion(rgn) : !rgn.isEmpty();
}

bool SkRegion::Oper(const SkRegion& rgnaOrig, const SkRegion& rgnbOrig, Op op,
                    SkRegion* result) {
    SkASSERT((unsigned)op < kOpCount);

    if (kReplace_Op == op) {
        return setRegionCheck(result, rgnbOrig);
    }

    // swith to using pointers, so we can swap them as needed
    const SkRegion* rgna = &rgnaOrig;
    const SkRegion* rgnb = &rgnbOrig;
    // after this point, do not refer to rgnaOrig or rgnbOrig!!!

    // collaps difference and reverse-difference into just difference
    if (kReverseDifference_Op == op) {
        using std::swap;
        swap(rgna, rgnb);
        op = kDifference_Op;
    }

    SkIRect bounds;
    bool    a_empty = rgna->isEmpty();
    bool    b_empty = rgnb->isEmpty();
    bool    a_rect = rgna->isRect();
    bool    b_rect = rgnb->isRect();

    switch (op) {
    case kDifference_Op:
        if (a_empty) {
            return setEmptyCheck(result);
        }
        if (b_empty || !SkIRect::Intersects(rgna->fBounds, rgnb->fBounds)) {
            return setRegionCheck(result, *rgna);
        }
        if (b_rect && rgnb->fBounds.containsNoEmptyCheck(rgna->fBounds)) {
            return setEmptyCheck(result);
        }
        break;

    case kIntersect_Op:
        if ((a_empty | b_empty)
                || !bounds.intersect(rgna->fBounds, rgnb->fBounds)) {
            return setEmptyCheck(result);
        }
        if (a_rect & b_rect) {
            return setRectCheck(result, bounds);
        }
        if (a_rect && rgna->fBounds.contains(rgnb->fBounds)) {
            return setRegionCheck(result, *rgnb);
        }
        if (b_rect && rgnb->fBounds.contains(rgna->fBounds)) {
            return setRegionCheck(result, *rgna);
        }
        break;

    case kUnion_Op:
        if (a_empty) {
            return setRegionCheck(result, *rgnb);
        }
        if (b_empty) {
            return setRegionCheck(result, *rgna);
        }
        if (a_rect && rgna->fBounds.contains(rgnb->fBounds)) {
            return setRegionCheck(result, *rgna);
        }
        if (b_rect && rgnb->fBounds.contains(rgna->fBounds)) {
            return setRegionCheck(result, *rgnb);
        }
        break;

    case kXOR_Op:
        if (a_empty) {
            return setRegionCheck(result, *rgnb);
        }
        if (b_empty) {
            return setRegionCheck(result, *rgna);
        }
        break;
    default:
        SkDEBUGFAIL("unknown region op");
        return false;
    }

    RunType tmpA[kRectRegionRuns];
    RunType tmpB[kRectRegionRuns];

    int a_intervals, b_intervals;
    const RunType* a_runs = rgna->getRuns(tmpA, &a_intervals);
    const RunType* b_runs = rgnb->getRuns(tmpB, &b_intervals);

    RunArray array;
    int count = operate(a_runs, b_runs, &array, op, nullptr == result);
    SkASSERT(count <= array.count());

    if (result) {
        SkASSERT(count >= 0);
        return result->setRuns(&array[0], count);
    } else {
        return (QUICK_EXIT_TRUE_COUNT == count) || !isRunCountEmpty(count);
    }
}

bool SkRegion::op(const SkRegion& rgna, const SkRegion& rgnb, Op op) {
    SkDEBUGCODE(SkRegionPriv::Validate(*this));
    return SkRegion::Oper(rgna, rgnb, op, this);
}

///////////////////////////////////////////////////////////////////////////////

size_t SkRegion::writeToMemory(void* storage) const {
    if (nullptr == storage) {
        size_t size = sizeof(int32_t); // -1 (empty), 0 (rect), runCount
        if (!this->isEmpty()) {
            size += sizeof(fBounds);
            if (this->isComplex()) {
                size += 2 * sizeof(int32_t);    // ySpanCount + intervalCount
                size += fRunHead->fRunCount * sizeof(RunType);
            }
        }
        return size;
    }

    SkWBuffer   buffer(storage);

    if (this->isEmpty()) {
        buffer.write32(-1);
    } else {
        bool isRect = this->isRect();

        buffer.write32(isRect ? 0 : fRunHead->fRunCount);
        buffer.write(&fBounds, sizeof(fBounds));

        if (!isRect) {
            buffer.write32(fRunHead->getYSpanCount());
            buffer.write32(fRunHead->getIntervalCount());
            buffer.write(fRunHead->readonly_runs(),
                         fRunHead->fRunCount * sizeof(RunType));
        }
    }
    return buffer.pos();
}

static bool validate_run_count(int ySpanCount, int intervalCount, int runCount) {
    // return 2 + 3 * ySpanCount + 2 * intervalCount;
    if (ySpanCount < 1 || intervalCount < 2) {
        return false;
    }
    SkSafeMath safeMath;
    int sum = 2;
    sum = safeMath.addInt(sum, ySpanCount);
    sum = safeMath.addInt(sum, ySpanCount);
    sum = safeMath.addInt(sum, ySpanCount);
    sum = safeMath.addInt(sum, intervalCount);
    sum = safeMath.addInt(sum, intervalCount);
    return safeMath && sum == runCount;
}

// Validate that a memory sequence is a valid region.
// Try to check all possible errors.
// never read beyond &runs[runCount-1].
static bool validate_run(const int32_t* runs,
                         int runCount,
                         const SkIRect& givenBounds,
                         int32_t ySpanCount,
                         int32_t intervalCount) {
    // Region Layout:
    //    Top ( Bottom Span_Interval_Count ( Left Right )* Sentinel )+ Sentinel
    if (!validate_run_count(SkToInt(ySpanCount), SkToInt(intervalCount), runCount)) {
        return false;
    }
    SkASSERT(runCount >= 7);  // 7==SkRegion::kRectRegionRuns
    // quick safety check:
    if (runs[runCount - 1] != SkRegion_kRunTypeSentinel ||
        runs[runCount - 2] != SkRegion_kRunTypeSentinel) {
        return false;
    }
    const int32_t* const end = runs + runCount;
    SkIRect bounds = {0, 0, 0 ,0};  // calulated bounds
    SkIRect rect = {0, 0, 0, 0};    // current rect
    rect.fTop = *runs++;
    if (rect.fTop == SkRegion_kRunTypeSentinel) {
        return false;  // no rect can contain SkRegion_kRunTypeSentinel
    }
    if (rect.fTop != givenBounds.fTop) {
        return false;  // Must not begin with empty span that does not contribute to bounds.
    }
    do {
        --ySpanCount;
        if (ySpanCount < 0) {
            return false;  // too many yspans
        }
        rect.fBottom = *runs++;
        if (rect.fBottom == SkRegion_kRunTypeSentinel) {
            return false;
        }
        if (rect.fBottom > givenBounds.fBottom) {
            return false;  // Must not end with empty span that does not contribute to bounds.
        }
        if (rect.fBottom <= rect.fTop) {
            return false;  // y-intervals must be ordered; rects must be non-empty.
        }

        int32_t xIntervals = *runs++;
        SkASSERT(runs < end);
        if (xIntervals < 0 || xIntervals > intervalCount || runs + 1 + 2 * xIntervals > end) {
            return false;
        }
        intervalCount -= xIntervals;
        bool firstInterval = true;
        int32_t lastRight = 0;  // check that x-intervals are distinct and ordered.
        while (xIntervals-- > 0) {
            rect.fLeft = *runs++;
            rect.fRight = *runs++;
            if (rect.fLeft == SkRegion_kRunTypeSentinel ||
                rect.fRight == SkRegion_kRunTypeSentinel ||
                rect.fLeft >= rect.fRight ||  // check non-empty rect
                (!firstInterval && rect.fLeft <= lastRight)) {
                return false;
            }
            lastRight = rect.fRight;
            firstInterval = false;
            bounds.join(rect);
        }
        if (*runs++ != SkRegion_kRunTypeSentinel) {
            return false;  // required check sentinal.
        }
        rect.fTop = rect.fBottom;
        SkASSERT(runs < end);
    } while (*runs != SkRegion_kRunTypeSentinel);
    ++runs;
    if (ySpanCount != 0 || intervalCount != 0 || givenBounds != bounds) {
        return false;
    }
    SkASSERT(runs == end);  // if ySpanCount && intervalCount are right, must be correct length.
    return true;
}
size_t SkRegion::readFromMemory(const void* storage, size_t length) {
    SkRBuffer   buffer(storage, length);
    SkRegion    tmp;
    int32_t     count;

    // Serialized Region Format:
    //    Empty:
    //       -1
    //    Simple Rect:
    //       0  LEFT TOP RIGHT BOTTOM
    //    Complex Region:
    //       COUNT LEFT TOP RIGHT BOTTOM Y_SPAN_COUNT TOTAL_INTERVAL_COUNT [RUNS....]
    if (!buffer.readS32(&count) || count < -1) {
        return 0;
    }
    if (count >= 0) {
        if (!buffer.read(&tmp.fBounds, sizeof(tmp.fBounds)) || tmp.fBounds.isEmpty()) {
            return 0;  // Short buffer or bad bounds for non-empty region; report failure.
        }
        if (count == 0) {
            tmp.fRunHead = SkRegion_gRectRunHeadPtr;
        } else {
            int32_t ySpanCount, intervalCount;
            if (!buffer.readS32(&ySpanCount) ||
                !buffer.readS32(&intervalCount) ||
                buffer.available() < count * sizeof(int32_t)) {
                return 0;
            }
            if (!validate_run((const int32_t*)((const char*)storage + buffer.pos()), count,
                              tmp.fBounds, ySpanCount, intervalCount)) {
                return 0;  // invalid runs, don't even allocate
            }
            tmp.allocateRuns(count, ySpanCount, intervalCount);
            SkASSERT(tmp.isComplex());
            SkAssertResult(buffer.read(tmp.fRunHead->writable_runs(), count * sizeof(int32_t)));
        }
    }
    SkASSERT(tmp.isValid());
    SkASSERT(buffer.isValid());
    this->swap(tmp);
    return buffer.pos();
}

///////////////////////////////////////////////////////////////////////////////

bool SkRegion::isValid() const {
    if (this->isEmpty()) {
        return fBounds == SkIRect{0, 0, 0, 0};
    }
    if (fBounds.isEmpty()) {
        return false;
    }
    if (this->isRect()) {
        return true;
    }
    return fRunHead && fRunHead->fRefCnt > 0 &&
           validate_run(fRunHead->readonly_runs(), fRunHead->fRunCount, fBounds,
                        fRunHead->getYSpanCount(), fRunHead->getIntervalCount());
}

#ifdef SK_DEBUG
void SkRegionPriv::Validate(const SkRegion& rgn) { SkASSERT(rgn.isValid()); }

void SkRegion::dump() const {
    if (this->isEmpty()) {
        SkDebugf("  rgn: empty\n");
    } else {
        SkDebugf("  rgn: [%d %d %d %d]", fBounds.fLeft, fBounds.fTop, fBounds.fRight, fBounds.fBottom);
        if (this->isComplex()) {
            const RunType* runs = fRunHead->readonly_runs();
            for (int i = 0; i < fRunHead->fRunCount; i++)
                SkDebugf(" %d", runs[i]);
        }
        SkDebugf("\n");
    }
}

#endif

///////////////////////////////////////////////////////////////////////////////

SkRegion::Iterator::Iterator(const SkRegion& rgn) {
    this->reset(rgn);
}

bool SkRegion::Iterator::rewind() {
    if (fRgn) {
        this->reset(*fRgn);
        return true;
    }
    return false;
}

void SkRegion::Iterator::reset(const SkRegion& rgn) {
    fRgn = &rgn;
    if (rgn.isEmpty()) {
        fDone = true;
    } else {
        fDone = false;
        if (rgn.isRect()) {
            fRect = rgn.fBounds;
            fRuns = nullptr;
        } else {
            fRuns = rgn.fRunHead->readonly_runs();
            fRect.setLTRB(fRuns[3], fRuns[0], fRuns[4], fRuns[1]);
            fRuns += 5;
            // Now fRuns points to the 2nd interval (or x-sentinel)
        }
    }
}

void SkRegion::Iterator::next() {
    if (fDone) {
        return;
    }

    if (fRuns == nullptr) {   // rect case
        fDone = true;
        return;
    }

    const RunType* runs = fRuns;

    if (runs[0] < SkRegion_kRunTypeSentinel) { // valid X value
        fRect.fLeft = runs[0];
        fRect.fRight = runs[1];
        runs += 2;
    } else {    // we're at the end of a line
        runs += 1;
        if (runs[0] < SkRegion_kRunTypeSentinel) { // valid Y value
            int intervals = runs[1];
            if (0 == intervals) {    // empty line
                fRect.fTop = runs[0];
                runs += 3;
            } else {
                fRect.fTop = fRect.fBottom;
            }

            fRect.fBottom = runs[0];
            assert_sentinel(runs[2], false);
            assert_sentinel(runs[3], false);
            fRect.fLeft = runs[2];
            fRect.fRight = runs[3];
            runs += 4;
        } else {    // end of rgn
            fDone = true;
        }
    }
    fRuns = runs;
}

SkRegion::Cliperator::Cliperator(const SkRegion& rgn, const SkIRect& clip)
        : fIter(rgn), fClip(clip), fDone(true) {
    const SkIRect& r = fIter.rect();

    while (!fIter.done()) {
        if (r.fTop >= clip.fBottom) {
            break;
        }
        if (fRect.intersect(clip, r)) {
            fDone = false;
            break;
        }
        fIter.next();
    }
}

void SkRegion::Cliperator::next() {
    if (fDone) {
        return;
    }

    const SkIRect& r = fIter.rect();

    fDone = true;
    fIter.next();
    while (!fIter.done()) {
        if (r.fTop >= fClip.fBottom) {
            break;
        }
        if (fRect.intersect(fClip, r)) {
            fDone = false;
            break;
        }
        fIter.next();
    }
}

///////////////////////////////////////////////////////////////////////////////

SkRegion::Spanerator::Spanerator(const SkRegion& rgn, int y, int left,
                                 int right) {
    SkDEBUGCODE(SkRegionPriv::Validate(rgn));

    const SkIRect& r = rgn.getBounds();

    fDone = true;
    if (!rgn.isEmpty() && y >= r.fTop && y < r.fBottom &&
            right > r.fLeft && left < r.fRight) {
        if (rgn.isRect()) {
            if (left < r.fLeft) {
                left = r.fLeft;
            }
            if (right > r.fRight) {
                right = r.fRight;
            }
            fLeft = left;
            fRight = right;
            fRuns = nullptr;    // means we're a rect, not a rgn
            fDone = false;
        } else {
            const SkRegion::RunType* runs = rgn.fRunHead->findScanline(y);
            runs += 2;  // skip Bottom and IntervalCount
            for (;;) {
                // runs[0..1] is to the right of the span, so we're done
                if (runs[0] >= right) {
                    break;
                }
                // runs[0..1] is to the left of the span, so continue
                if (runs[1] <= left) {
                    runs += 2;
                    continue;
                }
                // runs[0..1] intersects the span
                fRuns = runs;
                fLeft = left;
                fRight = right;
                fDone = false;
                break;
            }
        }
    }
}

bool SkRegion::Spanerator::next(int* left, int* right) {
    if (fDone) {
        return false;
    }

    if (fRuns == nullptr) {   // we're a rect
        fDone = true;   // ok, now we're done
        if (left) {
            *left = fLeft;
        }
        if (right) {
            *right = fRight;
        }
        return true;    // this interval is legal
    }

    const SkRegion::RunType* runs = fRuns;

    if (runs[0] >= fRight) {
        fDone = true;
        return false;
    }

    SkASSERT(runs[1] > fLeft);

    if (left) {
        *left = std::max(fLeft, runs[0]);
    }
    if (right) {
        *right = std::min(fRight, runs[1]);
    }
    fRuns = runs + 2;
    return true;
}

///////////////////////////////////////////////////////////////////////////////////////////////////

static void visit_pairs(int pairCount, int y, const int32_t pairs[],
                        const std::function<void(const SkIRect&)>& visitor) {
    for (int i = 0; i < pairCount; ++i) {
        visitor({ pairs[0], y, pairs[1], y + 1 });
        pairs += 2;
    }
}

void SkRegionPriv::VisitSpans(const SkRegion& rgn,
                              const std::function<void(const SkIRect&)>& visitor) {
    if (rgn.isEmpty()) {
        return;
    }
    if (rgn.isRect()) {
        visitor(rgn.getBounds());
    } else {
        const int32_t* p = rgn.fRunHead->readonly_runs();
        int32_t top = *p++;
        int32_t bot = *p++;
        do {
            int pairCount = *p++;
            if (pairCount == 1) {
                visitor({ p[0], top, p[1], bot });
                p += 2;
            } else if (pairCount > 1) {
                // we have to loop repeated in Y, sending each interval in Y -> X order
                for (int y = top; y < bot; ++y) {
                    visit_pairs(pairCount, y, p, visitor);
                }
                p += pairCount * 2;
            }
            assert_sentinel(*p, true);
            p += 1; // skip sentinel

            // read next bottom or sentinel
            top = bot;
            bot = *p++;
        } while (!SkRegionValueIsSentinel(bot));
    }
}