Vendor changes

vendor/golang.org/x/tools/container/intsets/sparse.go

@@ -21,10 +21,6 @@ package intsets // import "golang.org/x/tools/container/intsets"
 // The space usage would be proportional to Max(), not Len(), and the
 // implementation would be based upon big.Int.
 //
-// TODO(adonovan): experiment with making the root block indirect (nil
-// iff IsEmpty).  This would reduce the memory usage when empty and
-// might simplify the aliasing invariants.
-//
 // TODO(adonovan): opt: make UnionWith and Difference faster.
 // These are the hot-spots for go/pointer.
 
@@ -45,9 +41,10 @@ type Sparse struct {
 	// An uninitialized Sparse represents an empty set.
 	// An empty set may also be represented by
 	//  root.next == root.prev == &root.
-	// In a non-empty set, root.next points to the first block and
-	// root.prev to the last.
-	// root.offset and root.bits are unused.
+	//
+	// The root is always the block with the smallest offset.
+	// It can be empty, but only if it is the only block; in that case, offset is
+	// MaxInt (which is not a valid offset).
 	root block
 }
 
@@ -144,7 +141,6 @@ func (b *block) len() int {
 
 // max returns the maximum element of the block.
 // The block must not be empty.
-//
 func (b *block) max() int {
 	bi := b.offset + bitsPerBlock
 	// Decrement bi by number of high zeros in last.bits.
@@ -161,7 +157,6 @@ func (b *block) max() int {
 // and also removes it if take is set.
 // The block must not be initially empty.
 // NB: may leave the block empty.
-//
 func (b *block) min(take bool) int {
 	for i, w := range b.bits {
 		if w != 0 {
@@ -175,6 +170,26 @@ func (b *block) min(take bool) int {
 	panic("BUG: empty block")
 }
 
+// lowerBound returns the smallest element of the block that is greater than or
+// equal to the element corresponding to the ith bit. If there is no such
+// element, the second return value is false.
+func (b *block) lowerBound(i uint) (int, bool) {
+	w := i / bitsPerWord
+	bit := i % bitsPerWord
+
+	if val := b.bits[w] >> bit; val != 0 {
+		return b.offset + int(i) + ntz(val), true
+	}
+
+	for w++; w < wordsPerBlock; w++ {
+		if val := b.bits[w]; val != 0 {
+			return b.offset + int(w*bitsPerWord) + ntz(val), true
+		}
+	}
+
+	return 0, false
+}
+
 // forEach calls f for each element of block b.
 // f must not mutate b's enclosing Sparse.
 func (b *block) forEach(f func(int)) {
@@ -204,14 +219,20 @@ func offsetAndBitIndex(x int) (int, uint) {
 
 // -- Sparse --------------------------------------------------------------
 
-// start returns the root's next block, which is the root block
-// (if s.IsEmpty()) or the first true block otherwise.
-// start has the side effect of ensuring that s is properly
-// initialized.
-//
-func (s *Sparse) start() *block {
+// none is a shared, empty, sentinel block that indicates the end of a block
+// list.
+var none block
+
+// Dummy type used to generate an implicit panic. This must be defined at the
+// package level; if it is defined inside a function, it prevents the inlining
+// of that function.
+type to_copy_a_sparse_you_must_call_its_Copy_method struct{}
+
+// init ensures s is properly initialized.
+func (s *Sparse) init() {
 	root := &s.root
 	if root.next == nil {
+		root.offset = MaxInt
 		root.next = root
 		root.prev = root
 	} else if root.next.prev != root {
@@ -219,21 +240,45 @@ func (s *Sparse) start() *block {
 		// new Sparse y shares the old linked list, but iteration
 		// on y will never encounter &y.root so it goes into a
 		// loop.  Fail fast before this occurs.
-		panic("A Sparse has been copied without (*Sparse).Copy()")
+		// We don't want to call panic here because it prevents the
+		// inlining of this function.
+		_ = (interface{}(nil)).(to_copy_a_sparse_you_must_call_its_Copy_method)
 	}
+}
 
-	return root.next
+func (s *Sparse) first() *block {
+	s.init()
+	if s.root.offset == MaxInt {
+		return &none
+	}
+	return &s.root
+}
+
+// next returns the next block in the list, or end if b is the last block.
+func (s *Sparse) next(b *block) *block {
+	if b.next == &s.root {
+		return &none
+	}
+	return b.next
+}
+
+// prev returns the previous block in the list, or end if b is the first block.
+func (s *Sparse) prev(b *block) *block {
+	if b.prev == &s.root {
+		return &none
+	}
+	return b.prev
 }
 
 // IsEmpty reports whether the set s is empty.
 func (s *Sparse) IsEmpty() bool {
-	return s.start() == &s.root
+	return s.root.next == nil || s.root.offset == MaxInt
 }
 
 // Len returns the number of elements in the set s.
 func (s *Sparse) Len() int {
 	var l int
-	for b := s.start(); b != &s.root; b = b.next {
+	for b := s.first(); b != &none; b = s.next(b) {
 		l += b.len()
 	}
 	return l
@@ -252,19 +297,34 @@ func (s *Sparse) Min() int {
 	if s.IsEmpty() {
 		return MaxInt
 	}
-	return s.root.next.min(false)
+	return s.root.min(false)
+}
+
+// LowerBound returns the smallest element >= x, or MaxInt if there is no such
+// element.
+func (s *Sparse) LowerBound(x int) int {
+	offset, i := offsetAndBitIndex(x)
+	for b := s.first(); b != &none; b = s.next(b) {
+		if b.offset > offset {
+			return b.min(false)
+		}
+		if b.offset == offset {
+			if y, ok := b.lowerBound(i); ok {
+				return y
+			}
+		}
+	}
+	return MaxInt
 }
 
 // block returns the block that would contain offset,
 // or nil if s contains no such block.
-//
+// Precondition: offset is a multiple of bitsPerBlock.
 func (s *Sparse) block(offset int) *block {
-	b := s.start()
-	for b != &s.root && b.offset <= offset {
+	for b := s.first(); b != &none && b.offset <= offset; b = s.next(b) {
 		if b.offset == offset {
 			return b
 		}
-		b = b.next
 	}
 	return nil
 }
@@ -272,26 +332,49 @@ func (s *Sparse) block(offset int) *block {
 // Insert adds x to the set s, and reports whether the set grew.
 func (s *Sparse) Insert(x int) bool {
 	offset, i := offsetAndBitIndex(x)
-	b := s.start()
-	for b != &s.root && b.offset <= offset {
+
+	b := s.first()
+	for ; b != &none && b.offset <= offset; b = s.next(b) {
 		if b.offset == offset {
 			return b.insert(i)
 		}
-		b = b.next
 	}
 
 	// Insert new block before b.
-	new := &block{offset: offset}
-	new.next = b
-	new.prev = b.prev
-	new.prev.next = new
-	new.next.prev = new
+	new := s.insertBlockBefore(b)
+	new.offset = offset
 	return new.insert(i)
 }
 
-func (s *Sparse) removeBlock(b *block) {
-	b.prev.next = b.next
-	b.next.prev = b.prev
+// removeBlock removes a block and returns the block that followed it (or end if
+// it was the last block).
+func (s *Sparse) removeBlock(b *block) *block {
+	if b != &s.root {
+		b.prev.next = b.next
+		b.next.prev = b.prev
+		if b.next == &s.root {
+			return &none
+		}
+		return b.next
+	}
+
+	first := s.root.next
+	if first == &s.root {
+		// This was the only block.
+		s.Clear()
+		return &none
+	}
+	s.root.offset = first.offset
+	s.root.bits = first.bits
+	if first.next == &s.root {
+		// Single block remaining.
+		s.root.next = &s.root
+		s.root.prev = &s.root
+	} else {
+		s.root.next = first.next
+		first.next.prev = &s.root
+	}
+	return &s.root
 }
 
 // Remove removes x from the set s, and reports whether the set shrank.
@@ -311,8 +394,11 @@ func (s *Sparse) Remove(x int) bool {
 
 // Clear removes all elements from the set s.
 func (s *Sparse) Clear() {
-	s.root.next = &s.root
-	s.root.prev = &s.root
+	s.root = block{
+		offset: MaxInt,
+		next:   &s.root,
+		prev:   &s.root,
+	}
 }
 
 // If set s is non-empty, TakeMin sets *p to the minimum element of
@@ -325,13 +411,12 @@ func (s *Sparse) Clear() {
 // 	for worklist.TakeMin(&x) { use(x) }
 //
 func (s *Sparse) TakeMin(p *int) bool {
-	head := s.start()
-	if head == &s.root {
+	if s.IsEmpty() {
 		return false
 	}
-	*p = head.min(true)
-	if head.empty() {
-		s.removeBlock(head)
+	*p = s.root.min(true)
+	if s.root.empty() {
+		s.removeBlock(&s.root)
 	}
 	return true
 }
@@ -352,7 +437,7 @@ func (s *Sparse) Has(x int) bool {
 // natural control flow with continue/break/return.
 //
 func (s *Sparse) forEach(f func(int)) {
-	for b := s.start(); b != &s.root; b = b.next {
+	for b := s.first(); b != &none; b = s.next(b) {
 		b.forEach(f)
 	}
 }
@@ -363,22 +448,51 @@ func (s *Sparse) Copy(x *Sparse) {
 		return
 	}
 
-	xb := x.start()
-	sb := s.start()
-	for xb != &x.root {
-		if sb == &s.root {
+	xb := x.first()
+	sb := s.first()
+	for xb != &none {
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		sb.offset = xb.offset
 		sb.bits = xb.bits
-		xb = xb.next
-		sb = sb.next
+		xb = x.next(xb)
+		sb = s.next(sb)
 	}
 	s.discardTail(sb)
 }
 
 // insertBlockBefore returns a new block, inserting it before next.
+// If next is the root, the root is replaced. If next is end, the block is
+// inserted at the end.
 func (s *Sparse) insertBlockBefore(next *block) *block {
+	if s.IsEmpty() {
+		if next != &none {
+			panic("BUG: passed block with empty set")
+		}
+		return &s.root
+	}
+
+	if next == &s.root {
+		// Special case: we need to create a new block that will become the root
+		// block.The old root block becomes the second block.
+		second := s.root
+		s.root = block{
+			next: &second,
+		}
+		if second.next == &s.root {
+			s.root.prev = &second
+		} else {
+			s.root.prev = second.prev
+			second.next.prev = &second
+			second.prev = &s.root
+		}
+		return &s.root
+	}
+	if next == &none {
+		// Insert before root.
+		next = &s.root
+	}
 	b := new(block)
 	b.next = next
 	b.prev = next.prev
@@ -389,9 +503,13 @@ func (s *Sparse) insertBlockBefore(next *block) *block {
 
 // discardTail removes block b and all its successors from s.
 func (s *Sparse) discardTail(b *block) {
-	if b != &s.root {
-		b.prev.next = &s.root
-		s.root.prev = b.prev
+	if b != &none {
+		if b == &s.root {
+			s.Clear()
+		} else {
+			b.prev.next = &s.root
+			s.root.prev = b.prev
+		}
 	}
 }
 
@@ -401,16 +519,15 @@ func (s *Sparse) IntersectionWith(x *Sparse) {
 		return
 	}
 
-	xb := x.start()
-	sb := s.start()
-	for xb != &x.root && sb != &s.root {
+	xb := x.first()
+	sb := s.first()
+	for xb != &none && sb != &none {
 		switch {
 		case xb.offset < sb.offset:
-			xb = xb.next
+			xb = x.next(xb)
 
 		case xb.offset > sb.offset:
-			sb = sb.next
-			s.removeBlock(sb.prev)
+			sb = s.removeBlock(sb)
 
 		default:
 			var sum word
@@ -420,12 +537,12 @@ func (s *Sparse) IntersectionWith(x *Sparse) {
 				sum |= r
 			}
 			if sum != 0 {
-				sb = sb.next
+				sb = s.next(sb)
 			} else {
 				// sb will be overwritten or removed
 			}
 
-			xb = xb.next
+			xb = x.next(xb)
 		}
 	}
 
@@ -446,20 +563,20 @@ func (s *Sparse) Intersection(x, y *Sparse) {
 		return
 	}
 
-	xb := x.start()
-	yb := y.start()
-	sb := s.start()
-	for xb != &x.root && yb != &y.root {
+	xb := x.first()
+	yb := y.first()
+	sb := s.first()
+	for xb != &none && yb != &none {
 		switch {
 		case xb.offset < yb.offset:
-			xb = xb.next
+			xb = x.next(xb)
 			continue
 		case xb.offset > yb.offset:
-			yb = yb.next
+			yb = y.next(yb)
 			continue
 		}
 
-		if sb == &s.root {
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		sb.offset = xb.offset
@@ -471,13 +588,13 @@ func (s *Sparse) Intersection(x, y *Sparse) {
 			sum |= r
 		}
 		if sum != 0 {
-			sb = sb.next
+			sb = s.next(sb)
 		} else {
 			// sb will be overwritten or removed
 		}
 
-		xb = xb.next
-		yb = yb.next
+		xb = x.next(xb)
+		yb = y.next(yb)
 	}
 
 	s.discardTail(sb)
@@ -485,22 +602,22 @@ func (s *Sparse) Intersection(x, y *Sparse) {
 
 // Intersects reports whether s ∩ x ≠ ∅.
 func (s *Sparse) Intersects(x *Sparse) bool {
-	sb := s.start()
-	xb := x.start()
-	for sb != &s.root && xb != &x.root {
+	sb := s.first()
+	xb := x.first()
+	for sb != &none && xb != &none {
 		switch {
 		case xb.offset < sb.offset:
-			xb = xb.next
+			xb = x.next(xb)
 		case xb.offset > sb.offset:
-			sb = sb.next
+			sb = s.next(sb)
 		default:
 			for i := range sb.bits {
 				if sb.bits[i]&xb.bits[i] != 0 {
 					return true
 				}
 			}
-			sb = sb.next
-			xb = xb.next
+			sb = s.next(sb)
+			xb = x.next(xb)
 		}
 	}
 	return false
@@ -513,26 +630,26 @@ func (s *Sparse) UnionWith(x *Sparse) bool {
 	}
 
 	var changed bool
-	xb := x.start()
-	sb := s.start()
-	for xb != &x.root {
-		if sb != &s.root && sb.offset == xb.offset {
+	xb := x.first()
+	sb := s.first()
+	for xb != &none {
+		if sb != &none && sb.offset == xb.offset {
 			for i := range xb.bits {
 				if sb.bits[i] != xb.bits[i] {
 					sb.bits[i] |= xb.bits[i]
 					changed = true
 				}
 			}
-			xb = xb.next
-		} else if sb == &s.root || sb.offset > xb.offset {
+			xb = x.next(xb)
+		} else if sb == &none || sb.offset > xb.offset {
 			sb = s.insertBlockBefore(sb)
 			sb.offset = xb.offset
 			sb.bits = xb.bits
 			changed = true
 
-			xb = xb.next
+			xb = x.next(xb)
 		}
-		sb = sb.next
+		sb = s.next(sb)
 	}
 	return changed
 }
@@ -551,33 +668,33 @@ func (s *Sparse) Union(x, y *Sparse) {
 		return
 	}
 
-	xb := x.start()
-	yb := y.start()
-	sb := s.start()
-	for xb != &x.root || yb != &y.root {
-		if sb == &s.root {
+	xb := x.first()
+	yb := y.first()
+	sb := s.first()
+	for xb != &none || yb != &none {
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		switch {
-		case yb == &y.root || (xb != &x.root && xb.offset < yb.offset):
+		case yb == &none || (xb != &none && xb.offset < yb.offset):
 			sb.offset = xb.offset
 			sb.bits = xb.bits
-			xb = xb.next
+			xb = x.next(xb)
 
-		case xb == &x.root || (yb != &y.root && yb.offset < xb.offset):
+		case xb == &none || (yb != &none && yb.offset < xb.offset):
 			sb.offset = yb.offset
 			sb.bits = yb.bits
-			yb = yb.next
+			yb = y.next(yb)
 
 		default:
 			sb.offset = xb.offset
 			for i := range xb.bits {
 				sb.bits[i] = xb.bits[i] | yb.bits[i]
 			}
-			xb = xb.next
-			yb = yb.next
+			xb = x.next(xb)
+			yb = y.next(yb)
 		}
-		sb = sb.next
+		sb = s.next(sb)
 	}
 
 	s.discardTail(sb)
@@ -590,15 +707,15 @@ func (s *Sparse) DifferenceWith(x *Sparse) {
 		return
 	}
 
-	xb := x.start()
-	sb := s.start()
-	for xb != &x.root && sb != &s.root {
+	xb := x.first()
+	sb := s.first()
+	for xb != &none && sb != &none {
 		switch {
 		case xb.offset > sb.offset:
-			sb = sb.next
+			sb = s.next(sb)
 
 		case xb.offset < sb.offset:
-			xb = xb.next
+			xb = x.next(xb)
 
 		default:
 			var sum word
@@ -607,12 +724,12 @@ func (s *Sparse) DifferenceWith(x *Sparse) {
 				sb.bits[i] = r
 				sum |= r
 			}
-			sb = sb.next
-			xb = xb.next
-
 			if sum == 0 {
-				s.removeBlock(sb.prev)
+				sb = s.removeBlock(sb)
+			} else {
+				sb = s.next(sb)
 			}
+			xb = x.next(xb)
 		}
 	}
 }
@@ -633,27 +750,27 @@ func (s *Sparse) Difference(x, y *Sparse) {
 		return
 	}
 
-	xb := x.start()
-	yb := y.start()
-	sb := s.start()
-	for xb != &x.root && yb != &y.root {
+	xb := x.first()
+	yb := y.first()
+	sb := s.first()
+	for xb != &none && yb != &none {
 		if xb.offset > yb.offset {
-			// y has block, x has none
-			yb = yb.next
+			// y has block, x has &none
+			yb = y.next(yb)
 			continue
 		}
 
-		if sb == &s.root {
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		sb.offset = xb.offset
 
 		switch {
 		case xb.offset < yb.offset:
-			// x has block, y has none
+			// x has block, y has &none
 			sb.bits = xb.bits
 
-			sb = sb.next
+			sb = s.next(sb)
 
 		default:
 			// x and y have corresponding blocks
@@ -664,25 +781,25 @@ func (s *Sparse) Difference(x, y *Sparse) {
 				sum |= r
 			}
 			if sum != 0 {
-				sb = sb.next
+				sb = s.next(sb)
 			} else {
 				// sb will be overwritten or removed
 			}
 
-			yb = yb.next
+			yb = y.next(yb)
 		}
-		xb = xb.next
+		xb = x.next(xb)
 	}
 
-	for xb != &x.root {
-		if sb == &s.root {
+	for xb != &none {
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		sb.offset = xb.offset
 		sb.bits = xb.bits
-		sb = sb.next
+		sb = s.next(sb)
 
-		xb = xb.next
+		xb = x.next(xb)
 	}
 
 	s.discardTail(sb)
@@ -695,17 +812,17 @@ func (s *Sparse) SymmetricDifferenceWith(x *Sparse) {
 		return
 	}
 
-	sb := s.start()
-	xb := x.start()
-	for xb != &x.root && sb != &s.root {
+	sb := s.first()
+	xb := x.first()
+	for xb != &none && sb != &none {
 		switch {
 		case sb.offset < xb.offset:
-			sb = sb.next
+			sb = s.next(sb)
 		case xb.offset < sb.offset:
 			nb := s.insertBlockBefore(sb)
 			nb.offset = xb.offset
 			nb.bits = xb.bits
-			xb = xb.next
+			xb = x.next(xb)
 		default:
 			var sum word
 			for i := range sb.bits {
@@ -713,20 +830,21 @@ func (s *Sparse) SymmetricDifferenceWith(x *Sparse) {
 				sb.bits[i] = r
 				sum |= r
 			}
-			sb = sb.next
-			xb = xb.next
 			if sum == 0 {
-				s.removeBlock(sb.prev)
+				sb = s.removeBlock(sb)
+			} else {
+				sb = s.next(sb)
 			}
+			xb = x.next(xb)
 		}
 	}
 
-	for xb != &x.root { // append the tail of x to s
+	for xb != &none { // append the tail of x to s
 		sb = s.insertBlockBefore(sb)
 		sb.offset = xb.offset
 		sb.bits = xb.bits
-		sb = sb.next
-		xb = xb.next
+		sb = s.next(sb)
+		xb = x.next(xb)
 	}
 }
 
@@ -744,24 +862,24 @@ func (s *Sparse) SymmetricDifference(x, y *Sparse) {
 		return
 	}
 
-	sb := s.start()
-	xb := x.start()
-	yb := y.start()
-	for xb != &x.root && yb != &y.root {
-		if sb == &s.root {
+	sb := s.first()
+	xb := x.first()
+	yb := y.first()
+	for xb != &none && yb != &none {
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		switch {
 		case yb.offset < xb.offset:
 			sb.offset = yb.offset
 			sb.bits = yb.bits
-			sb = sb.next
-			yb = yb.next
+			sb = s.next(sb)
+			yb = y.next(yb)
 		case xb.offset < yb.offset:
 			sb.offset = xb.offset
 			sb.bits = xb.bits
-			sb = sb.next
-			xb = xb.next
+			sb = s.next(sb)
+			xb = x.next(xb)
 		default:
 			var sum word
 			for i := range sb.bits {
@@ -771,31 +889,31 @@ func (s *Sparse) SymmetricDifference(x, y *Sparse) {
 			}
 			if sum != 0 {
 				sb.offset = xb.offset
-				sb = sb.next
+				sb = s.next(sb)
 			}
-			xb = xb.next
-			yb = yb.next
+			xb = x.next(xb)
+			yb = y.next(yb)
 		}
 	}
 
-	for xb != &x.root { // append the tail of x to s
-		if sb == &s.root {
+	for xb != &none { // append the tail of x to s
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		sb.offset = xb.offset
 		sb.bits = xb.bits
-		sb = sb.next
-		xb = xb.next
+		sb = s.next(sb)
+		xb = x.next(xb)
 	}
 
-	for yb != &y.root { // append the tail of y to s
-		if sb == &s.root {
+	for yb != &none { // append the tail of y to s
+		if sb == &none {
 			sb = s.insertBlockBefore(sb)
 		}
 		sb.offset = yb.offset
 		sb.bits = yb.bits
-		sb = sb.next
-		yb = yb.next
+		sb = s.next(sb)
+		yb = y.next(yb)
 	}
 
 	s.discardTail(sb)
@@ -807,22 +925,22 @@ func (s *Sparse) SubsetOf(x *Sparse) bool {
 		return true
 	}
 
-	sb := s.start()
-	xb := x.start()
-	for sb != &s.root {
+	sb := s.first()
+	xb := x.first()
+	for sb != &none {
 		switch {
-		case xb == &x.root || xb.offset > sb.offset:
+		case xb == &none || xb.offset > sb.offset:
 			return false
 		case xb.offset < sb.offset:
-			xb = xb.next
+			xb = x.next(xb)
 		default:
 			for i := range sb.bits {
 				if sb.bits[i]&^xb.bits[i] != 0 {
 					return false
 				}
 			}
-			sb = sb.next
-			xb = xb.next
+			sb = s.next(sb)
+			xb = x.next(xb)
 		}
 	}
 	return true
@@ -833,13 +951,13 @@ func (s *Sparse) Equals(t *Sparse) bool {
 	if s == t {
 		return true
 	}
-	sb := s.start()
-	tb := t.start()
+	sb := s.first()
+	tb := t.first()
 	for {
 		switch {
-		case sb == &s.root && tb == &t.root:
+		case sb == &none && tb == &none:
 			return true
-		case sb == &s.root || tb == &t.root:
+		case sb == &none || tb == &none:
 			return false
 		case sb.offset != tb.offset:
 			return false
@@ -847,8 +965,8 @@ func (s *Sparse) Equals(t *Sparse) bool {
 			return false
 		}
 
-		sb = sb.next
-		tb = tb.next
+		sb = s.next(sb)
+		tb = t.next(tb)
 	}
 }
 
@@ -913,7 +1031,7 @@ func (s *Sparse) BitString() string {
 //
 func (s *Sparse) GoString() string {
 	var buf bytes.Buffer
-	for b := s.start(); b != &s.root; b = b.next {
+	for b := s.first(); b != &none; b = s.next(b) {
 		fmt.Fprintf(&buf, "block %p {offset=%d next=%p prev=%p",
 			b, b.offset, b.next, b.prev)
 		for _, w := range b.bits {
@@ -937,13 +1055,18 @@ func (s *Sparse) AppendTo(slice []int) []int {
 
 // check returns an error if the representation invariants of s are violated.
 func (s *Sparse) check() error {
-	if !s.root.empty() {
-		return fmt.Errorf("non-empty root block")
+	s.init()
+	if s.root.empty() {
+		// An empty set must have only the root block with offset MaxInt.
+		if s.root.next != &s.root {
+			return fmt.Errorf("multiple blocks with empty root block")
+		}
+		if s.root.offset != MaxInt {
+			return fmt.Errorf("empty set has offset %d, should be MaxInt", s.root.offset)
+		}
+		return nil
 	}
-	if s.root.offset != 0 {
-		return fmt.Errorf("root block has non-zero offset %d", s.root.offset)
-	}
-	for b := s.start(); b != &s.root; b = b.next {
+	for b := s.first(); ; b = s.next(b) {
 		if b.offset%bitsPerBlock != 0 {
 			return fmt.Errorf("bad offset modulo: %d", b.offset)
 		}
@@ -956,11 +1079,12 @@ func (s *Sparse) check() error {
 		if b.next.prev != b {
 			return fmt.Errorf("bad next.prev link")
 		}
-		if b.prev != &s.root {
-			if b.offset <= b.prev.offset {
-				return fmt.Errorf("bad offset order: b.offset=%d, prev.offset=%d",
-					b.offset, b.prev.offset)
-			}
+		if b.next == &s.root {
+			break
+		}
+		if b.offset >= b.next.offset {
+			return fmt.Errorf("bad offset order: b.offset=%d, b.next.offset=%d",
+				b.offset, b.next.offset)
 		}
 	}
 	return nil