func (f *bitFiler) WriteAt(b []byte, off int64) (n int, err error) {
	off0 := off
	pgI := off >> bfBits
	pgO := int(off & bfMask)
	n = len(b)
	rem := n
	var nc int
	for rem != 0 {
		pg := f.m[pgI]
		if pg == nil {
			pg = &bitPage{}
			if f.parent != nil {
				_, err = f.parent.ReadAt(pg.data[:], off&^bfMask)
				if err != nil && !fileutil.IsEOF(err) {
					return
				}

				err = nil
			}
			f.m[pgI] = pg
		}
		nc = copy(pg.data[pgO:], b)
		pgI++
		pg.dirty = true
		for i := pgO; i < pgO+nc; i++ {
			pg.flags[i>>3] |= bitmask[i&7]
		}
		pgO = 0
		rem -= nc
		b = b[nc:]
		off += int64(nc)
	}
	f.size = mathutil.MaxInt64(f.size, off0+int64(n))
	return
}

// Read returns the next Source ScannerRune.
func (s *Source) Read() (r ScannerRune) {
	for {
		r.Position = s.Position()
		r.Rune, r.Size, r.Err = s.tos.reader.ReadRune()
		if r.Err == nil || !fileutil.IsEOF(r.Err) {
			p := &s.tos.position
			p.Offset += r.Size
			if r.Rune != '\n' {
				p.Column++
			} else {
				p.Line++
				p.Column = 1
			}
			return
		}

		// err == os.EOF, try parent source
		if sp := len(s.stack) - 1; sp >= 0 {
			s.tos = s.stack[sp]
			s.stack = s.stack[:sp]
		} else {
			r.Rune, r.Size = 0, 0
			return
		}
	}
}

func (l *lex) getc() (b byte, err error) {
	switch b = l.current; b {
	case 0:
		break
	case '\n':
		l.line++
		l.column = 0
		fallthrough
	default:
		l.column++
		l.buf = append(l.buf, b)
	}
	l.current = 0
	b, err = l.src.ReadByte()
	if err != nil {
		b = 0
		if !fileutil.IsEOF(err) {
			return
		}
		err = nil
	}
	l.ofs++
	l.current = b
	return
}

func (f *bitFiler) ReadAt(b []byte, off int64) (n int, err error) {
	avail := f.size - off
	pgI := off >> bfBits
	pgO := int(off & bfMask)
	rem := len(b)
	if int64(rem) >= avail {
		rem = int(avail)
		err = io.EOF
	}
	for rem != 0 && avail > 0 {
		pg := f.m[pgI]
		if pg == nil {
			pg = &bitPage{}
			pg.pdata = buffer.CGet(bfSize)
			pg.data = *pg.pdata
			if f.parent != nil {
				_, err = f.parent.ReadAt(pg.data, off&^bfMask)
				if err != nil && !fileutil.IsEOF(err) {
					return
				}

				err = nil
			}
			f.m[pgI] = pg
		}
		nc := copy(b[:mathutil.Min(rem, bfSize)], pg.data[pgO:])
		pgI++
		pgO = 0
		rem -= nc
		n += nc
		b = b[nc:]
		off += int64(nc)
	}
	return
}

func (f *bitFiler) WriteAt(b []byte, off int64) (n int, err error) {
	off0 := off
	pgI := off >> bfBits
	pgO := int(off & bfMask)
	n = len(b)
	rem := n
	var nc int
	for rem != 0 {
		pg := f.m[pgI]
		if pg == nil {
			pg = &bitPage{}
			pg.pdata = buffer.CGet(bfSize)
			pg.data = *pg.pdata
			if f.parent != nil {
				_, err = f.parent.ReadAt(pg.data, off&^bfMask)
				if err != nil && !fileutil.IsEOF(err) {
					return
				}

				err = nil
			}
			f.m[pgI] = pg
		}
		nc = copy(pg.data[pgO:], b)
		pgI++
		pg.dirty = true
		pgO = 0
		rem -= nc
		b = b[nc:]
		off += int64(nc)
	}
	f.size = mathutil.MaxInt64(f.size, off0+int64(n))
	return
}

func readFrom(f Interface, r io.Reader) (n int64, err error) {
	f.Truncate(0)
	p := buffer.Get(copyBufSize)
	b := *p
	defer buffer.Put(p)

	var off int64
	var werr error
	for {
		rn, rerr := r.Read(b)
		if rn != 0 {
			_, werr = f.WriteAt(b[:rn], off)
			n += int64(rn)
			off += int64(rn)
		}
		if rerr != nil {
			if !fileutil.IsEOF(rerr) {
				err = rerr
			}
			break
		}

		if werr != nil {
			err = werr
			break
		}
	}
	return n, err
}

func writeTo(f Interface, w io.Writer) (n int64, err error) {
	p := buffer.Get(copyBufSize)
	b := *p
	defer buffer.Put(p)

	var off int64
	var werr error
	for {
		rn, rerr := f.ReadAt(b, off)
		if rn != 0 {
			_, werr = w.Write(b[:rn])
			n += int64(rn)
			off += int64(rn)
		}
		if rerr != nil {
			if !fileutil.IsEOF(rerr) {
				err = rerr
			}
			break
		}

		if werr != nil {
			err = werr
			break
		}
	}
	return n, err
}

// WriteTo is a helper to copy/persist File's content to w.  If w is also
// an io.WriterAt then WriteTo may attempt to _not_ write any big, for some
// value of big, runs of zeros, i.e. it will attempt to punch holes, where
// possible, in `w` if that happens to be a freshly created or to zero length
// truncated OS file.  'n' reports the number of bytes written to 'w'.
func (f *File) WriteTo(w io.Writer) (n int64, err error) {
	var (
		b      [pgSize]byte
		wn, rn int
		off    int64
		rerr   error
	)

	if wa, ok := w.(io.WriterAt); ok {
		fsize, err := f.Size()
		if err != nil {
			return n, err
		}

		lastPgI := fsize >> pgBits
		for pgI := int64(0); pgI <= lastPgI; pgI++ {
			sz := pgSize
			if pgI == lastPgI {
				sz = int(fsize & pgMask)
			}
			v, err := (*Array)(f).Get(pgI)
			if err != nil {
				return n, err
			}

			pg, _ := v.([]byte)
			if len(pg) != 0 {
				wn, err = wa.WriteAt(pg[:sz], off)
				if err != nil {
					return n, err
				}

				n += int64(wn)
				off += int64(sz)
				if wn != sz {
					return n, io.ErrShortWrite
				}
			}
		}
		return n, err
	}

	var werr error
	for rerr == nil {
		if rn, rerr = f.ReadAt(b[:], off); rn != 0 {
			off += int64(rn)
			if wn, werr = w.Write(b[:rn]); werr != nil {
				return n, werr
			}

			n += int64(wn)
		}
	}
	if !fileutil.IsEOF(rerr) {
		err = rerr
	}
	return
}

// WriteTo is a helper to copy/persist MemFiler's content to w.  If w is also
// an io.WriterAt then WriteTo may attempt to _not_ write any big, for some
// value of big, runs of zeros, i.e. it will attempt to punch holes, where
// possible, in `w` if that happens to be a freshly created or to zero length
// truncated OS file.  'n' reports the number of bytes written to 'w'.
func (f *MemFiler) WriteTo(w io.Writer) (n int64, err error) {
	var (
		b      [pgSize]byte
		wn, rn int
		off    int64
		rerr   error
	)

	if wa, ok := w.(io.WriterAt); ok {
		lastPgI := f.size >> pgBits
		for pgI := int64(0); pgI <= lastPgI; pgI++ {
			sz := pgSize
			if pgI == lastPgI {
				sz = int(f.size & pgMask)
			}
			pg := f.m[pgI]
			if pg != nil {
				wn, err = wa.WriteAt(pg[:sz], off)
				if err != nil {
					return
				}

				n += int64(wn)
				off += int64(sz)
				if wn != sz {
					return n, io.ErrShortWrite
				}
			}
		}
		return
	}

	var werr error
	for rerr == nil {
		if rn, rerr = f.ReadAt(b[:], off); rn != 0 {
			off += int64(rn)
			if wn, werr = w.Write(b[:rn]); werr != nil {
				return n, werr
			}

			n += int64(wn)
		}
	}
	if !fileutil.IsEOF(rerr) {
		err = rerr
	}
	return
}

func (l *lex) getc(c byte) byte {
	if c != 0 {
		l.buf = append(l.buf, c)
	}
	if b, err := l.src.ReadByte(); err == nil {
		l.peek = b
		if b == '\n' {
			l.begin(sc_LINE_START)
			l.line++
			l.column = 1
		} else {
			l.column++
		}
		return b
	} else {
		if fileutil.IsEOF(err) {
			l.peek = 0
			return 0
		}
		panic(err)
	}
}

// Scan scans a pcat formatted text file from src, assuming file name is
// 'name'. Handler 'handler' is called for every Record found in src. If
// handler returns false the scanning process is aborted and Scan returns err
// == nil.  If there is any error detected while scanning then the scanning is
// aborted as well and the error is returned.
func Scan(name string, src io.ByteReader, handler func(*Record) bool) (err error) {
	l := newLex(src)

	defer func() {
		if e := recover(); e != nil {
			x, ok := e.(error)
			if !ok || !fileutil.IsEOF(x) {
				err = fmt.Errorf("%s:%d:%d %s", name, l.line, l.column, e)
			}
		}
	}()

	for {
		var r *Record
		switch r, err = l.scan(); {
		case err != nil:
			panic(err)
		case r == nil || !handler(r):
			return
		}
	}
	panic("unreachable")
}

// ReadFrom is a helper to populate MemFiler's content from r.  'n' reports the
// number of bytes read from 'r'.
func (f *MemFiler) ReadFrom(r io.Reader) (n int64, err error) {
	if err = f.Truncate(0); err != nil {
		return
	}

	var (
		b   [pgSize]byte
		rn  int
		off int64
	)

	var rerr error
	for rerr == nil {
		if rn, rerr = r.Read(b[:]); rn != 0 {
			f.WriteAt(b[:rn], off)
			off += int64(rn)
			n += int64(rn)
		}
	}
	if !fileutil.IsEOF(rerr) {
		err = rerr
	}
	return
}

func TestseekLast(t *testing.T) {
	bt := NewBTree(nil)

	enum, err := bt.seekFirst()
	if !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	bt.Set([]byte("a"), []byte("b"))
	enum, err = bt.seekFirst()
	if err != nil {
		t.Fatal(err)
	}

	err = enum.prev()
	if !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	err = enum.next()
	if !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	k, v, err := enum.current()
	if err != nil {
		t.Fatal(err)
	}

	if string(k) != "a" || string(v) != "b" {
		t.Fatal(k, v)
	}

	bt.Set([]byte("c"), []byte("d"))
	enum, err = bt.seekLast()
	if err != nil {
		t.Fatal(err)
	}

	err = enum.next()
	if !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	k, v, err = enum.current()
	if err != nil {
		t.Fatal(err)
	}

	if string(k) != "c" || string(v) != "d" {
		t.Fatal(k, v)
	}

	err = enum.prev()
	if err != nil {
		t.Fatal(err)
	}

	k, v, err = enum.current()
	if err != nil {
		t.Fatal(err)
	}

	if string(k) != "a" || string(v) != "b" {
		t.Fatal(k, v)
	}
}

func TestbTreePrev(t *testing.T) {
	N := int64(*testN)

	tree := NewBTree(nil)
	enum, _, err := tree.seek(enc8(0))
	if err != nil {
		t.Fatal(err)
	}

	if _, _, err = enum.current(); !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	if err = enum.next(); !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	if err = enum.prev(); !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	// Fill
	for i := int64(1); i <= N; i++ {
		tree.Set(enc8(10*i), enc8(10*i+1))
	}

	var eq bool

	enum, eq, err = tree.seek(enc8(0))
	if err != nil {
		t.Fatal(err)
	}

	if eq {
		t.Fatal(eq)
	}

	// index: 0
	if _, _, err = enum.current(); err != nil {
		t.Fatal(err)
	}

	if err = enum.prev(); !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	enum, eq, err = tree.seek(enc8(N * 10))
	if err != nil {
		t.Fatal(err)
	}

	if !eq {
		t.Fatal(eq)
	}

	// index: N-1
	if _, _, err = enum.current(); err != nil {
		t.Fatal(err)
	}

	if err = enum.prev(); N > 1 && err != nil {
		t.Fatal(err)
	}

	enum, eq, err = tree.seek(enc8(N*10 + 1))
	if err != nil {
		t.Fatal(err)
	}

	if eq {
		t.Fatal(eq)
	}

	// index: N
	if _, _, err = enum.current(); !fileutil.IsEOF(err) {
		t.Fatal(err)
	}

	if err = enum.prev(); err != nil {
		t.Fatal(err)
	}

	enum, _, err = tree.seek(enc8(N * 10))
	if err != nil {
		t.Fatal(err)
	}

	for i := N; i >= 1; i-- {
		expKey, expValue := enc8(10*i), enc8(10*i+1)
		k, v, err := enum.current()
		if err != nil {
			t.Fatal(err)
		}

		if !bytes.Equal(k, expKey) {
			t.Fatalf("%d k|% x| expK|% x| %s\n", i, k, expKey, tree.root.String(tree.store))
		}

		if !bytes.Equal(v, expValue) {
			t.Fatal(i)
//.........这里部分代码省略.........

func TestSeekPrev(t *testing.T) {
	// seeking within 3 keys: 10, 20, 30
	table := []struct {
		k    int
		hit  bool
		keys []int
	}{
		{5, false, []int{10}},
		{10, true, []int{10}},
		{15, false, []int{20, 10}},
		{20, true, []int{20, 10}},
		{25, false, []int{30, 20, 10}},
		{30, true, []int{30, 20, 10}},
		{35, false, []int{}},
	}

	for i, test := range table {
		down := test.keys
		db, err := CreateMem(opts())
		if err != nil {
			t.Fatal(i, err)
		}

		if err := db.Set(n2b(10), n2b(100)); err != nil {
			t.Fatal(i, err)
		}

		if err := db.Set(n2b(20), n2b(200)); err != nil {
			t.Fatal(i, err)
		}

		if err := db.Set(n2b(30), n2b(300)); err != nil {
			t.Fatal(i, err)
		}

		for brokenSerial := 0; brokenSerial < 16; brokenSerial++ {
			en, hit, err := db.Seek(n2b(test.k))
			if err != nil {
				t.Fatal(err)
			}

			if g, e := hit, test.hit; g != e {
				t.Fatal(i, g, e)
			}

			j := 0
			for {
				if brokenSerial&(1<<uint(j)) != 0 {
					if err := db.Set(n2b(20), n2b(200)); err != nil {
						t.Fatal(i, err)
					}
				}

				k, v, err := en.Prev()
				if err != nil {
					if !fileutil.IsEOF(err) {
						t.Fatal(i, err)
					}

					break
				}

				if g, e := len(k), 8; g != e {
					t.Fatal(i, g, e)
				}

				if j >= len(down) {
					t.Fatal(i, j, brokenSerial)
				}

				if g, e := b2n(k), down[j]; g != e {
					t.Fatal(i, j, brokenSerial, g, e)
				}

				if g, e := len(v), 8; g != e {
					t.Fatal(i, g, e)
				}

				if g, e := b2n(v), 10*down[j]; g != e {
					t.Fatal(i, g, e)
				}

				j++

			}

			if g, e := j, len(down); g != e {
				t.Fatal(i, j, g, e)
			}
		}

	}
}

func testFilerReadAtWriteAt(t *testing.T, nf newFunc) {
	f := nf()
	t.Log(f.Name())
	defer func() {
		if err := f.Close(); err != nil {
			t.Error(err)
		}
	}()

	if _, ok := f.(*RollbackFiler); ok {
		if err := f.BeginUpdate(); err != nil {
			t.Fatal(err)
		}

		defer func() {
			if err := f.EndUpdate(); err != nil {
				t.Error(err)
			}
		}()
	}

	const (
		N = 1 << 16
		M = 2e2
	)

	s := make([]byte, N)
	e := make([]byte, N)
	rnd := rand.New(rand.NewSource(42))
	for i := range e {
		s[i] = byte(rnd.Intn(256))
	}
	n2 := 0
	for i := 0; i < M; i++ {
		var from, to int
		for {
			from = rnd.Intn(N)
			to = rnd.Intn(N)
			if from != to {
				break
			}
		}
		if from > to {
			from, to = to, from
		}
		for i := range s[from:to] {
			s[from+i] = byte(rnd.Intn(256))
		}
		copy(e[from:to], s[from:to])
		if to > n2 {
			n2 = to
		}
		n, err := f.WriteAt(s[from:to], int64(from))
		if err != nil {
			t.Error(err)
			return
		}

		if g, e := n, to-from; g != e {
			t.Error(g, e)
			return
		}
	}

	fsz, err := f.Size()
	if err != nil {
		t.Error(err)
		return
	}

	if g, e := fsz, int64(n2); g != e {
		t.Error(g, e)
		return
	}

	b := make([]byte, n2)
	for i := 0; i <= M; i++ {
		from := rnd.Intn(n2)
		to := rnd.Intn(n2)
		if from > to {
			from, to = to, from
		}
		if i == M {
			from, to = 0, n2
		}
		n, err := f.ReadAt(b[from:to], int64(from))
		if err != nil && (!fileutil.IsEOF(err) && n != 0) {
			fsz, err = f.Size()
			if err != nil {
				t.Error(err)
				return
			}

			t.Error(fsz, from, to, err)
			return
		}

		if g, e := n, to-from; g != e {
			t.Error(g, e)
			return
//.........这里部分代码省略.........

// NewL parses a .l source fname from src, returns L or an error if any.
// Currently it is not reentrant and not invokable more than once in an application
// (which is assumed tolerable for a "lex" tool).
// The unoptdfa argument allows to disable optimization of the produced DFA.
// The mode32 parameter is not yet supported and must be false.
func NewL(fname string, src io.RuneReader, unoptdfa, mode32 bool) (l *L, err error) {
	if mode32 {
		return nil, errors.New("lex.NewL: mode32 unsupported yet")
	}

	nodfaopt, bits32 = unoptdfa, mode32
	l = &L{}

	if !hook {
		defer func() {
			if e := recover(); e != nil {
				l = nil
				err = e.(error)
			}
		}()
	}

	// Support \r\n line separators too
	in := []rune{}
loop:
	for {
		r, _, err := src.ReadRune()
		switch {
		case err == nil:
			in = append(in, r)
		case fileutil.IsEOF(err):
			break loop
		default:
			return nil, err
		}
	}
	src = bytes.NewBufferString(strings.Replace(string(in), "\r\n", "\n", -1))

	scanner := lxr.Scanner(fname, src)
	if y := yyParse(newTokenizer(scanner)); y != 0 || len(errors_) != 0 {
		return nil, errors.New(strings.Join(errors_, "\n"))
	}

	computePartialDFAs()
	if len(errors_) != 0 {
		return nil, errors.New(strings.Join(errors_, "\n"))
	}

	computeAllNfa()
	allDfa = allNfa.powerSet()
	for _, irule := range allDfa.acceptRule {
		delete(unreachableRules, irule)
	}
	for irule := range unreachableRules {
		logErr(fmt.Sprintf("%s - pattern `%s` unreachable", rulePos[irule], rules[irule].pattern))
	}
	if len(errors_) != 0 {
		return nil, errors.New(strings.Join(errors_, "\n"))
	}

	l.DefCode = defCode
	l.UserCode = usrCode
	l.StartConditions = iStarts
	l.StartConditionsStates = make(map[int]*lexer.NfaState)
	l.StartConditionsBolStates = make(map[int]*lexer.NfaState)
	for _, edge0 := range allDfa.nfa.in.Consuming {
		switch edge := edge0.(type) {
		default:
			panic(errors.New("internal error"))
		case *lexer.RuneEdge:
			if _, ok := l.StartConditionsStates[int(edge.Rune)]; ok {
				panic(errors.New("internal error"))
			}
			if edge.Rune < 128 {
				l.StartConditionsStates[int(edge.Rune)] = edge.Target()
			} else {
				l.StartConditionsBolStates[int(edge.Rune)-128] = edge.Target()
			}
		case *lexer.RangesEdge:
			for _, rng := range edge.Ranges.R32 {
				for arune := rng.Lo; arune <= rng.Hi; arune += rng.Stride {
					if _, ok := l.StartConditionsStates[int(arune)]; ok {
						panic(errors.New("internal error"))
					}
					if arune < 128 {
						l.StartConditionsStates[int(arune)] = edge.Target()
					} else {
						l.StartConditionsBolStates[int(arune)-128] = edge.Target()
					}
				}
			}
		}

	}
	for _, rule := range rules {
		l.Rules = append(l.Rules, Rule{Conds: rule.conds, Pattern: rule.pattern, RE: rule.re, Action: rule.action, BOL: rule.bol, EOL: rule.eol})
	}
	l.Dfa = allDfa.nfa.nfa[1:]
	l.Accepts = map[*lexer.NfaState]int{}
	for id, state := range allDfa.accept {
//.........这里部分代码省略.........

func noEof(e error) (err error) {
	if !fileutil.IsEOF(e) {
		err = e
	}
	return
}

func TestEnumeratorPrev(t *testing.T) {
	// seeking within 3 keys: 10, 20, 30
	table := []struct {
		k    int
		hit  bool
		keys []int
	}{
		{5, false, []int{10}},
		{10, true, []int{10}},
		{15, false, []int{20, 10}},
		{20, true, []int{20, 10}},
		{25, false, []int{30, 20, 10}},
		{30, true, []int{30, 20, 10}},
		{35, false, []int{}},
	}

	for i, test := range table {
		dn := test.keys
		r := TreeNew(cmp)

		r.Set(10, 100)
		r.Set(20, 200)
		r.Set(30, 300)

		for verChange := 0; verChange < 16; verChange++ {
			en, hit := r.Seek(test.k)

			if g, e := hit, test.hit; g != e {
				t.Fatal(i, g, e)
			}

			j := 0
			for {
				if verChange&(1<<uint(j)) != 0 {
					r.Set(20, 200)
				}

				k, v, err := en.Prev()
				if err != nil {
					if !fileutil.IsEOF(err) {
						t.Fatal(i, err)
					}

					break
				}

				if j >= len(dn) {
					t.Fatal(i, j, verChange)
				}

				if g, e := k, dn[j]; g != e {
					t.Fatal(i, j, verChange, g, e)
				}

				if g, e := v, 10*dn[j]; g != e {
					t.Fatal(i, g, e)
				}

				j++

			}

			if g, e := j, len(dn); g != e {
				t.Fatal(i, j, g, e)
			}
		}

	}
}

func TestRollbackFiler3(t *testing.T) {
	var r *RollbackFiler
	f := NewMemFiler()

	checkpoint := func(sz int64) (err error) {
		return f.Truncate(sz)
	}

	r, err := NewRollbackFiler(f, checkpoint, f)
	if err != nil {
		t.Fatal(err)
	}

	n, err := r.ReadAt([]byte{0}, 0)
	if n != 0 || !fileutil.IsEOF(err) {
		t.Fatal(n, err)
	}

	n, err = r.ReadAt([]byte{0}, 1e6)
	if n != 0 || !fileutil.IsEOF(err) {
		t.Fatal(n, err)
	}

	if err = r.BeginUpdate(); err != nil { // BeginUpdate: 0 -> 1
		t.Fatal(err)
	}

	rng := rand.New(rand.NewSource(42))

	buf := rndBytes(rng, 100)
	if n, err := r.WriteAt(buf, 1e6); n != 100 || err != nil {
		t.Fatal(err)
	}

	buf = make([]byte, 100)
	if n, err := r.ReadAt(buf, 1e6-200); n != 100 || err != nil {
		t.Fatal(err)
	}

	for i, v := range buf {
		if v != 0 {
			t.Fatal(i, v)
		}
	}

	if err := r.Truncate(1e5); err != nil {
		t.Fatal(err)
	}

	if err = r.BeginUpdate(); err != nil { // BeginUpdate: 1 -> 2
		t.Fatal(err)
	}

	if n, err := r.ReadAt(buf, 1e6); n != 0 || err == nil {
		t.Fatal(n, err)
	}

	if err := r.Truncate(2e6); err != nil {
		t.Fatal(err)
	}

	if err = r.BeginUpdate(); err != nil { // BeginUpdate: 2 -> 3
		t.Fatal(err)
	}

	if n, err := r.ReadAt(buf, 1e6); n == 0 || err != nil {
		t.Fatal(n, err)
	}

	for i, v := range buf {
		if v != 0 {
			t.Fatal(i, v)
		}
	}
}