Initial commit

vendor/github.com/cockroachdb/pebble/internal/manifest/level_metadata.go

@@ -0,0 +1,748 @@
+// Copyright 2020 The LevelDB-Go and Pebble Authors. All rights reserved. Use
+// of this source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package manifest
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/cockroachdb/pebble/internal/base"
+	"github.com/cockroachdb/pebble/internal/invariants"
+)
+
+// LevelMetadata contains metadata for all of the files within
+// a level of the LSM.
+type LevelMetadata struct {
+	level     int
+	totalSize uint64
+	// NumVirtual is the number of virtual sstables in the level.
+	NumVirtual uint64
+	// VirtualSize is the size of the virtual sstables in the level.
+	VirtualSize uint64
+	tree        btree
+}
+
+// clone makes a copy of the level metadata, implicitly increasing the ref
+// count of every file contained within lm.
+func (lm *LevelMetadata) clone() LevelMetadata {
+	return LevelMetadata{
+		level:       lm.level,
+		totalSize:   lm.totalSize,
+		NumVirtual:  lm.NumVirtual,
+		VirtualSize: lm.VirtualSize,
+		tree:        lm.tree.Clone(),
+	}
+}
+
+func (lm *LevelMetadata) release() (obsolete []*FileBacking) {
+	return lm.tree.Release()
+}
+
+func makeLevelMetadata(cmp Compare, level int, files []*FileMetadata) LevelMetadata {
+	bcmp := btreeCmpSeqNum
+	if level > 0 {
+		bcmp = btreeCmpSmallestKey(cmp)
+	}
+	var lm LevelMetadata
+	lm.level = level
+	lm.tree, _ = makeBTree(bcmp, files)
+	for _, f := range files {
+		lm.totalSize += f.Size
+		if f.Virtual {
+			lm.NumVirtual++
+			lm.VirtualSize += f.Size
+		}
+	}
+	return lm
+}
+
+func makeBTree(cmp btreeCmp, files []*FileMetadata) (btree, LevelSlice) {
+	var t btree
+	t.cmp = cmp
+	for _, f := range files {
+		t.Insert(f)
+	}
+	return t, newLevelSlice(t.Iter())
+}
+
+func (lm *LevelMetadata) insert(f *FileMetadata) error {
+	if err := lm.tree.Insert(f); err != nil {
+		return err
+	}
+	lm.totalSize += f.Size
+	if f.Virtual {
+		lm.NumVirtual++
+		lm.VirtualSize += f.Size
+	}
+	return nil
+}
+
+func (lm *LevelMetadata) remove(f *FileMetadata) bool {
+	lm.totalSize -= f.Size
+	if f.Virtual {
+		lm.NumVirtual--
+		lm.VirtualSize -= f.Size
+	}
+	return lm.tree.Delete(f)
+}
+
+// Empty indicates whether there are any files in the level.
+func (lm *LevelMetadata) Empty() bool {
+	return lm.tree.Count() == 0
+}
+
+// Len returns the number of files within the level.
+func (lm *LevelMetadata) Len() int {
+	return lm.tree.Count()
+}
+
+// Size returns the cumulative size of all the files within the level.
+func (lm *LevelMetadata) Size() uint64 {
+	return lm.totalSize
+}
+
+// Iter constructs a LevelIterator over the entire level.
+func (lm *LevelMetadata) Iter() LevelIterator {
+	return LevelIterator{iter: lm.tree.Iter()}
+}
+
+// Slice constructs a slice containing the entire level.
+func (lm *LevelMetadata) Slice() LevelSlice {
+	return newLevelSlice(lm.tree.Iter())
+}
+
+// Find finds the provided file in the level if it exists.
+func (lm *LevelMetadata) Find(cmp base.Compare, m *FileMetadata) *LevelFile {
+	iter := lm.Iter()
+	if lm.level != 0 {
+		// If lm holds files for levels >0, we can narrow our search by binary
+		// searching by bounds.
+		o := overlaps(iter, cmp, m.Smallest.UserKey,
+			m.Largest.UserKey, m.Largest.IsExclusiveSentinel())
+		iter = o.Iter()
+	}
+	for f := iter.First(); f != nil; f = iter.Next() {
+		if f == m {
+			lf := iter.Take()
+			return &lf
+		}
+	}
+	return nil
+}
+
+// Annotation lazily calculates and returns the annotation defined by
+// Annotator. The Annotator is used as the key for pre-calculated
+// values, so equal Annotators must be used to avoid duplicate computations
+// and cached annotations. Annotation must not be called concurrently, and in
+// practice this is achieved by requiring callers to hold DB.mu.
+func (lm *LevelMetadata) Annotation(annotator Annotator) interface{} {
+	if lm.Empty() {
+		return annotator.Zero(nil)
+	}
+	v, _ := lm.tree.root.Annotation(annotator)
+	return v
+}
+
+// InvalidateAnnotation clears any cached annotations defined by Annotator. The
+// Annotator is used as the key for pre-calculated values, so equal Annotators
+// must be used to clear the appropriate cached annotation. InvalidateAnnotation
+// must not be called concurrently, and in practice this is achieved by
+// requiring callers to hold DB.mu.
+func (lm *LevelMetadata) InvalidateAnnotation(annotator Annotator) {
+	if lm.Empty() {
+		return
+	}
+	lm.tree.root.InvalidateAnnotation(annotator)
+}
+
+// LevelFile holds a file's metadata along with its position
+// within a level of the LSM.
+type LevelFile struct {
+	*FileMetadata
+	slice LevelSlice
+}
+
+// Slice constructs a LevelSlice containing only this file.
+func (lf LevelFile) Slice() LevelSlice {
+	return lf.slice
+}
+
+// NewLevelSliceSeqSorted constructs a LevelSlice over the provided files,
+// sorted by the L0 sequence number sort order.
+// TODO(jackson): Can we improve this interface or avoid needing to export
+// a slice constructor like this?
+func NewLevelSliceSeqSorted(files []*FileMetadata) LevelSlice {
+	tr, slice := makeBTree(btreeCmpSeqNum, files)
+	tr.Release()
+	slice.verifyInvariants()
+	return slice
+}
+
+// NewLevelSliceKeySorted constructs a LevelSlice over the provided files,
+// sorted by the files smallest keys.
+// TODO(jackson): Can we improve this interface or avoid needing to export
+// a slice constructor like this?
+func NewLevelSliceKeySorted(cmp base.Compare, files []*FileMetadata) LevelSlice {
+	tr, slice := makeBTree(btreeCmpSmallestKey(cmp), files)
+	tr.Release()
+	slice.verifyInvariants()
+	return slice
+}
+
+// NewLevelSliceSpecificOrder constructs a LevelSlice over the provided files,
+// ordering the files by their order in the provided slice. It's used in
+// tests.
+// TODO(jackson): Update tests to avoid requiring this and remove it.
+func NewLevelSliceSpecificOrder(files []*FileMetadata) LevelSlice {
+	tr, slice := makeBTree(btreeCmpSpecificOrder(files), files)
+	tr.Release()
+	slice.verifyInvariants()
+	return slice
+}
+
+// newLevelSlice constructs a new LevelSlice backed by iter.
+func newLevelSlice(iter iterator) LevelSlice {
+	s := LevelSlice{iter: iter}
+	if iter.r != nil {
+		s.length = iter.r.subtreeCount
+	}
+	s.verifyInvariants()
+	return s
+}
+
+// newBoundedLevelSlice constructs a new LevelSlice backed by iter and bounded
+// by the provided start and end bounds. The provided startBound and endBound
+// iterators must be iterators over the same B-Tree. Both start and end bounds
+// are inclusive.
+func newBoundedLevelSlice(iter iterator, startBound, endBound *iterator) LevelSlice {
+	s := LevelSlice{
+		iter:  iter,
+		start: startBound,
+		end:   endBound,
+	}
+	if iter.valid() {
+		s.length = endBound.countLeft() - startBound.countLeft()
+		// NB: The +1 is a consequence of the end bound being inclusive.
+		if endBound.valid() {
+			s.length++
+		}
+		// NB: A slice that's empty due to its bounds may have an endBound
+		// positioned before the startBound due to the inclusive bounds.
+		// TODO(jackson): Consider refactoring the end boundary to be exclusive;
+		// it would simplify some areas (eg, here) and complicate others (eg,
+		// Reslice-ing to grow compactions).
+		if s.length < 0 {
+			s.length = 0
+		}
+	}
+	s.verifyInvariants()
+	return s
+}
+
+// LevelSlice contains a slice of the files within a level of the LSM.
+// A LevelSlice is immutable once created, but may be used to construct a
+// mutable LevelIterator over the slice's files.
+//
+// LevelSlices should be constructed through one of the existing constructors,
+// not manually initialized.
+type LevelSlice struct {
+	iter   iterator
+	length int
+	// start and end form the inclusive bounds of a slice of files within a
+	// level of the LSM. They may be nil if the entire B-Tree backing iter is
+	// accessible.
+	start *iterator
+	end   *iterator
+}
+
+func (ls LevelSlice) verifyInvariants() {
+	if invariants.Enabled {
+		i := ls.Iter()
+		var length int
+		for f := i.First(); f != nil; f = i.Next() {
+			length++
+		}
+		if ls.length != length {
+			panic(fmt.Sprintf("LevelSlice %s has length %d value; actual length is %d", ls, ls.length, length))
+		}
+	}
+}
+
+// Each invokes fn for each element in the slice.
+func (ls LevelSlice) Each(fn func(*FileMetadata)) {
+	iter := ls.Iter()
+	for f := iter.First(); f != nil; f = iter.Next() {
+		fn(f)
+	}
+}
+
+// String implements fmt.Stringer.
+func (ls LevelSlice) String() string {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "%d files: ", ls.length)
+	ls.Each(func(f *FileMetadata) {
+		if buf.Len() > 0 {
+			fmt.Fprintf(&buf, " ")
+		}
+		fmt.Fprint(&buf, f)
+	})
+	return buf.String()
+}
+
+// Empty indicates whether the slice contains any files.
+func (ls *LevelSlice) Empty() bool {
+	return emptyWithBounds(ls.iter, ls.start, ls.end)
+}
+
+// Iter constructs a LevelIterator that iterates over the slice.
+func (ls *LevelSlice) Iter() LevelIterator {
+	return LevelIterator{
+		start: ls.start,
+		end:   ls.end,
+		iter:  ls.iter.clone(),
+	}
+}
+
+// Len returns the number of files in the slice. Its runtime is constant.
+func (ls *LevelSlice) Len() int {
+	return ls.length
+}
+
+// SizeSum sums the size of all files in the slice. Its runtime is linear in
+// the length of the slice.
+func (ls *LevelSlice) SizeSum() uint64 {
+	var sum uint64
+	iter := ls.Iter()
+	for f := iter.First(); f != nil; f = iter.Next() {
+		sum += f.Size
+	}
+	return sum
+}
+
+// NumVirtual returns the number of virtual sstables in the level. Its runtime is
+// linear in the length of the slice.
+func (ls *LevelSlice) NumVirtual() uint64 {
+	var n uint64
+	iter := ls.Iter()
+	for f := iter.First(); f != nil; f = iter.Next() {
+		if f.Virtual {
+			n++
+		}
+	}
+	return n
+}
+
+// VirtualSizeSum returns the sum of the sizes of the virtual sstables in the
+// level.
+func (ls *LevelSlice) VirtualSizeSum() uint64 {
+	var sum uint64
+	iter := ls.Iter()
+	for f := iter.First(); f != nil; f = iter.Next() {
+		if f.Virtual {
+			sum += f.Size
+		}
+	}
+	return sum
+}
+
+// Reslice constructs a new slice backed by the same underlying level, with
+// new start and end positions. Reslice invokes the provided function, passing
+// two LevelIterators: one positioned to i's inclusive start and one
+// positioned to i's inclusive end. The resliceFunc may move either iterator
+// forward or backwards, including beyond the callee's original bounds to
+// capture additional files from the underlying level. Reslice constructs and
+// returns a new LevelSlice with the final bounds of the iterators after
+// calling resliceFunc.
+func (ls LevelSlice) Reslice(resliceFunc func(start, end *LevelIterator)) LevelSlice {
+	if ls.iter.r == nil {
+		return ls
+	}
+	var start, end LevelIterator
+	if ls.start == nil {
+		start.iter = ls.iter.clone()
+		start.iter.first()
+	} else {
+		start.iter = ls.start.clone()
+	}
+	if ls.end == nil {
+		end.iter = ls.iter.clone()
+		end.iter.last()
+	} else {
+		end.iter = ls.end.clone()
+	}
+	resliceFunc(&start, &end)
+	return newBoundedLevelSlice(start.iter.clone(), &start.iter, &end.iter)
+}
+
+// KeyType is used to specify the type of keys we're looking for in
+// LevelIterator positioning operations. Files not containing any keys of the
+// desired type are skipped.
+type KeyType int8
+
+const (
+	// KeyTypePointAndRange denotes a search among the entire keyspace, including
+	// both point keys and range keys. No sstables are skipped.
+	KeyTypePointAndRange KeyType = iota
+	// KeyTypePoint denotes a search among the point keyspace. SSTables with no
+	// point keys will be skipped. Note that the point keyspace includes rangedels.
+	KeyTypePoint
+	// KeyTypeRange denotes a search among the range keyspace. SSTables with no
+	// range keys will be skipped.
+	KeyTypeRange
+)
+
+type keyTypeAnnotator struct{}
+
+var _ Annotator = keyTypeAnnotator{}
+
+func (k keyTypeAnnotator) Zero(dst interface{}) interface{} {
+	var val *KeyType
+	if dst != nil {
+		val = dst.(*KeyType)
+	} else {
+		val = new(KeyType)
+	}
+	*val = KeyTypePoint
+	return val
+}
+
+func (k keyTypeAnnotator) Accumulate(m *FileMetadata, dst interface{}) (interface{}, bool) {
+	v := dst.(*KeyType)
+	switch *v {
+	case KeyTypePoint:
+		if m.HasRangeKeys {
+			*v = KeyTypePointAndRange
+		}
+	case KeyTypePointAndRange:
+		// Do nothing.
+	default:
+		panic("unexpected key type")
+	}
+	return v, true
+}
+
+func (k keyTypeAnnotator) Merge(src interface{}, dst interface{}) interface{} {
+	v := dst.(*KeyType)
+	srcVal := src.(*KeyType)
+	switch *v {
+	case KeyTypePoint:
+		if *srcVal == KeyTypePointAndRange {
+			*v = KeyTypePointAndRange
+		}
+	case KeyTypePointAndRange:
+		// Do nothing.
+	default:
+		panic("unexpected key type")
+	}
+	return v
+}
+
+// LevelIterator iterates over a set of files' metadata. Its zero value is an
+// empty iterator.
+type LevelIterator struct {
+	iter   iterator
+	start  *iterator
+	end    *iterator
+	filter KeyType
+}
+
+func (i LevelIterator) String() string {
+	var buf bytes.Buffer
+	iter := i.iter.clone()
+	iter.first()
+	iter.prev()
+	if i.iter.pos == -1 {
+		fmt.Fprint(&buf, "(<start>)*")
+	}
+	iter.next()
+	for ; iter.valid(); iter.next() {
+		if buf.Len() > 0 {
+			fmt.Fprint(&buf, "   ")
+		}
+
+		if i.start != nil && cmpIter(iter, *i.start) == 0 {
+			fmt.Fprintf(&buf, " [ ")
+		}
+		isCurrentPos := cmpIter(iter, i.iter) == 0
+		if isCurrentPos {
+			fmt.Fprint(&buf, " ( ")
+		}
+		fmt.Fprint(&buf, iter.cur().String())
+		if isCurrentPos {
+			fmt.Fprint(&buf, " )*")
+		}
+		if i.end != nil && cmpIter(iter, *i.end) == 0 {
+			fmt.Fprintf(&buf, " ]")
+		}
+	}
+	if i.iter.n != nil && i.iter.pos >= i.iter.n.count {
+		if buf.Len() > 0 {
+			fmt.Fprint(&buf, "   ")
+		}
+		fmt.Fprint(&buf, "(<end>)*")
+	}
+	return buf.String()
+}
+
+// Clone copies the iterator, returning an independent iterator at the same
+// position.
+func (i *LevelIterator) Clone() LevelIterator {
+	if i.iter.r == nil {
+		return *i
+	}
+	// The start and end iterators are not cloned and are treated as
+	// immutable.
+	return LevelIterator{
+		iter:   i.iter.clone(),
+		start:  i.start,
+		end:    i.end,
+		filter: i.filter,
+	}
+}
+
+// Current returns the item at the current iterator position.
+//
+// Current is deprecated. Callers should instead use the return value of a
+// positioning operation.
+func (i *LevelIterator) Current() *FileMetadata {
+	if !i.iter.valid() ||
+		(i.end != nil && cmpIter(i.iter, *i.end) > 0) ||
+		(i.start != nil && cmpIter(i.iter, *i.start) < 0) {
+		return nil
+	}
+	return i.iter.cur()
+}
+
+func (i *LevelIterator) empty() bool {
+	return emptyWithBounds(i.iter, i.start, i.end)
+}
+
+// Filter clones the iterator and sets the desired KeyType as the key to filter
+// files on.
+func (i *LevelIterator) Filter(keyType KeyType) LevelIterator {
+	l := i.Clone()
+	l.filter = keyType
+	return l
+}
+
+func emptyWithBounds(i iterator, start, end *iterator) bool {
+	// If i.r is nil, the iterator was constructed from an empty btree.
+	// If the end bound is before the start bound, the bounds represent an
+	// empty slice of the B-Tree.
+	return i.r == nil || (start != nil && end != nil && cmpIter(*end, *start) < 0)
+}
+
+// First seeks to the first file in the iterator and returns it.
+func (i *LevelIterator) First() *FileMetadata {
+	if i.empty() {
+		return nil
+	}
+	if i.start != nil {
+		i.iter = i.start.clone()
+	} else {
+		i.iter.first()
+	}
+	if !i.iter.valid() {
+		return nil
+	}
+	return i.skipFilteredForward(i.iter.cur())
+}
+
+// Last seeks to the last file in the iterator and returns it.
+func (i *LevelIterator) Last() *FileMetadata {
+	if i.empty() {
+		return nil
+	}
+	if i.end != nil {
+		i.iter = i.end.clone()
+	} else {
+		i.iter.last()
+	}
+	if !i.iter.valid() {
+		return nil
+	}
+	return i.skipFilteredBackward(i.iter.cur())
+}
+
+// Next advances the iterator to the next file and returns it.
+func (i *LevelIterator) Next() *FileMetadata {
+	if i.iter.r == nil {
+		return nil
+	}
+	if invariants.Enabled && (i.iter.pos >= i.iter.n.count || (i.end != nil && cmpIter(i.iter, *i.end) > 0)) {
+		panic("pebble: cannot next forward-exhausted iterator")
+	}
+	i.iter.next()
+	if !i.iter.valid() {
+		return nil
+	}
+	return i.skipFilteredForward(i.iter.cur())
+}
+
+// Prev moves the iterator the previous file and returns it.
+func (i *LevelIterator) Prev() *FileMetadata {
+	if i.iter.r == nil {
+		return nil
+	}
+	if invariants.Enabled && (i.iter.pos < 0 || (i.start != nil && cmpIter(i.iter, *i.start) < 0)) {
+		panic("pebble: cannot prev backward-exhausted iterator")
+	}
+	i.iter.prev()
+	if !i.iter.valid() {
+		return nil
+	}
+	return i.skipFilteredBackward(i.iter.cur())
+}
+
+// SeekGE seeks to the first file in the iterator's file set with a largest
+// user key greater than or equal to the provided user key. The iterator must
+// have been constructed from L1+, because it requires the underlying files to
+// be sorted by user keys and non-overlapping.
+func (i *LevelIterator) SeekGE(cmp Compare, userKey []byte) *FileMetadata {
+	// TODO(jackson): Assert that i.iter.cmp == btreeCmpSmallestKey.
+	if i.iter.r == nil {
+		return nil
+	}
+	m := i.seek(func(m *FileMetadata) bool {
+		return cmp(m.Largest.UserKey, userKey) >= 0
+	})
+	if i.filter != KeyTypePointAndRange && m != nil {
+		b, ok := m.LargestBound(i.filter)
+		if !ok {
+			m = i.Next()
+		} else if c := cmp(b.UserKey, userKey); c < 0 || c == 0 && b.IsExclusiveSentinel() {
+			// This file does not contain any keys of the type ≥ lower. It
+			// should be filtered, even though it does contain point keys.
+			m = i.Next()
+		}
+	}
+	return i.skipFilteredForward(m)
+}
+
+// SeekLT seeks to the last file in the iterator's file set with a smallest
+// user key less than the provided user key. The iterator must have been
+// constructed from L1+, because it requires the underlying files to be sorted
+// by user keys and non-overlapping.
+func (i *LevelIterator) SeekLT(cmp Compare, userKey []byte) *FileMetadata {
+	// TODO(jackson): Assert that i.iter.cmp == btreeCmpSmallestKey.
+	if i.iter.r == nil {
+		return nil
+	}
+	i.seek(func(m *FileMetadata) bool {
+		return cmp(m.Smallest.UserKey, userKey) >= 0
+	})
+	m := i.Prev()
+	// Although i.Prev() guarantees that the current file contains keys of the
+	// relevant type, it doesn't guarantee that the keys of the relevant type
+	// are < userKey.
+	if i.filter != KeyTypePointAndRange && m != nil {
+		b, ok := m.SmallestBound(i.filter)
+		if !ok {
+			panic("unreachable")
+		}
+		if c := cmp(b.UserKey, userKey); c >= 0 {
+			// This file does not contain any keys of the type ≥ lower. It
+			// should be filtered, even though it does contain point keys.
+			m = i.Prev()
+		}
+	}
+	return i.skipFilteredBackward(m)
+}
+
+// skipFilteredForward takes the file metadata at the iterator's current
+// position, and skips forward if the current key-type filter (i.filter)
+// excludes the file. It skips until it finds an unfiltered file or exhausts the
+// level. If lower is != nil, skipFilteredForward skips any files that do not
+// contain keys with the provided key-type ≥ lower.
+//
+// skipFilteredForward also enforces the upper bound, returning nil if at any
+// point the upper bound is exceeded.
+func (i *LevelIterator) skipFilteredForward(meta *FileMetadata) *FileMetadata {
+	for meta != nil && !meta.ContainsKeyType(i.filter) {
+		i.iter.next()
+		if !i.iter.valid() {
+			meta = nil
+		} else {
+			meta = i.iter.cur()
+		}
+	}
+	if meta != nil && i.end != nil && cmpIter(i.iter, *i.end) > 0 {
+		// Exceeded upper bound.
+		meta = nil
+	}
+	return meta
+}
+
+// skipFilteredBackward takes the file metadata at the iterator's current
+// position, and skips backward if the current key-type filter (i.filter)
+// excludes the file. It skips until it finds an unfiltered file or exhausts the
+// level. If upper is != nil, skipFilteredBackward skips any files that do not
+// contain keys with the provided key-type < upper.
+//
+// skipFilteredBackward also enforces the lower bound, returning nil if at any
+// point the lower bound is exceeded.
+func (i *LevelIterator) skipFilteredBackward(meta *FileMetadata) *FileMetadata {
+	for meta != nil && !meta.ContainsKeyType(i.filter) {
+		i.iter.prev()
+		if !i.iter.valid() {
+			meta = nil
+		} else {
+			meta = i.iter.cur()
+		}
+	}
+	if meta != nil && i.start != nil && cmpIter(i.iter, *i.start) < 0 {
+		// Exceeded lower bound.
+		meta = nil
+	}
+	return meta
+}
+
+func (i *LevelIterator) seek(fn func(*FileMetadata) bool) *FileMetadata {
+	i.iter.seek(fn)
+
+	// i.iter.seek seeked in the unbounded underlying B-Tree. If the iterator
+	// has start or end bounds, we may have exceeded them. Reset to the bounds
+	// if necessary.
+	//
+	// NB: The LevelIterator and LevelSlice semantics require that a bounded
+	// LevelIterator/LevelSlice containing files x0, x1, ..., xn behave
+	// identically to an unbounded LevelIterator/LevelSlice of a B-Tree
+	// containing x0, x1, ..., xn. In other words, any files outside the
+	// LevelIterator's bounds should not influence the iterator's behavior.
+	// When seeking, this means a SeekGE that seeks beyond the end bound,
+	// followed by a Prev should return the last element within bounds.
+	if i.end != nil && cmpIter(i.iter, *i.end) > 0 {
+		i.iter = i.end.clone()
+		// Since seek(fn) positioned beyond i.end, we know there is nothing to
+		// return within bounds.
+		i.iter.next()
+		return nil
+	} else if i.start != nil && cmpIter(i.iter, *i.start) < 0 {
+		i.iter = i.start.clone()
+	}
+	if !i.iter.valid() {
+		return nil
+	}
+	return i.iter.cur()
+}
+
+// Take constructs a LevelFile containing the file at the iterator's current
+// position. Take panics if the iterator is not currently positioned over a
+// file.
+func (i *LevelIterator) Take() LevelFile {
+	m := i.Current()
+	if m == nil {
+		panic("Take called on invalid LevelIterator")
+	}
+	// LevelSlice's start and end fields are immutable and are positioned to
+	// the same position for a LevelFile because they're inclusive, so we can
+	// share one iterator stack between the two bounds.
+	boundsIter := i.iter.clone()
+	s := newBoundedLevelSlice(i.iter.clone(), &boundsIter, &boundsIter)
+	return LevelFile{
+		FileMetadata: m,
+		slice:        s,
+	}
+}