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//
// Copyright (c) SAS Institute Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
package comdoc
import (
"bytes"
"encoding/binary"
"errors"
)
// Read the short sector allocation table
//
// The SSAT works the same way as the SAT, except that the indices correspond
// to positions within the short sector stream instead of the file at large,
// and those sectors are ShortSectorSize in length instead of SectorSize. It's
// also stored differently, using the SAT to chain to each subsequent block
// instead of using the MSAT array, in the same way that any other stream works.
func (r *ComDoc) readShortSAT() error {
count := r.SectorSize / 4
sat := make([]SecID, count*int(r.Header.SSATSectorCount))
position := 0
for sector := r.Header.SSATNextSector; sector >= 0; sector = r.SAT[sector] {
if position >= len(sat) {
return errors.New("ssat has more sectors than indicated")
}
if err := r.readSectorStruct(sector, sat[position:position+count]); err != nil {
return err
}
position += count
}
r.SSAT = sat
return nil
}
// Free the old short-sector allocation table and write a new one
func (r *ComDoc) writeShortSAT() error {
freeSectors(r.SAT, r.Header.SSATNextSector)
perSector := r.SectorSize / 4
freeList := r.makeFreeSectors(len(r.SSAT)/perSector, false)
buf := bytes.NewBuffer(r.sectorBuf)
first := SecIDEndOfChain
previous := first
for i, sector := range freeList {
j := i * perSector
chunk := r.SSAT[j : j+perSector]
buf.Reset()
_ = binary.Write(buf, binary.LittleEndian, chunk)
if err := r.writeSector(sector, buf.Bytes()); err != nil {
return err
}
if previous == SecIDEndOfChain {
first = sector
} else {
r.SAT[previous] = sector
}
previous = sector
}
r.SAT[previous] = SecIDEndOfChain
r.Header.SSATNextSector = first
r.Header.SSATSectorCount = uint32(len(freeList))
return nil
}
// Read a short sector at the given position.
//
// Short sectors work similarly to big sectors except they are smaller, have
// their own allocation table, and they point to a position within the "short
// sector stream" instead of within the file at large. The short sector stream
// is a regular stream whose first block is pointed to by the root storage
// dirent.
func (r *ComDoc) readShortSector(shortSector SecID, buf []byte) (int, error) {
// figure out which big sector holds the short sector
bigSectorIndex := int(shortSector) * r.ShortSectorSize / r.SectorSize
bigSectorID := r.Files[r.rootStorage].NextSector
for i := 0; i < bigSectorIndex; i++ {
bigSectorID = r.SAT[bigSectorID]
}
// translate to a file position
n := r.sectorToOffset(bigSectorID)
n += int64(int(shortSector)*r.ShortSectorSize - bigSectorIndex*r.SectorSize)
return r.File.ReadAt(buf, n)
}
// Write a short sector at the given position. This will allocate new space in
// the short-sector stream if needed.
func (r *ComDoc) writeShortSector(shortSector SecID, content []byte) error {
if len(content) > r.ShortSectorSize {
panic("excessive write")
} else if len(content) < r.ShortSectorSize {
buf := r.sectorBuf
copy(buf, content)
for i := len(content); i < r.ShortSectorSize; i++ {
buf[i] = 0
}
content = buf[:r.ShortSectorSize]
}
// walk the short stream to find the big sector that holds the target small sector
bigSectorIndex := int(shortSector) * r.ShortSectorSize / r.SectorSize
offset := int(shortSector)*r.ShortSectorSize - bigSectorIndex*r.SectorSize
root := &r.Files[r.rootStorage]
bigSectorID := root.NextSector
for ; bigSectorIndex > 0; bigSectorIndex-- {
next := r.SAT[bigSectorID]
if next < 0 {
break
}
bigSectorID = next
}
if bigSectorIndex > 0 {
// extend short sector stream
freeList := r.makeFreeSectors(bigSectorIndex, false)
for _, sector := range freeList {
r.SAT[bigSectorID] = sector
bigSectorID = sector
}
r.SAT[bigSectorID] = SecIDEndOfChain
}
n := r.sectorToOffset(bigSectorID) + int64(offset)
if _, err := r.writer.WriteAt(content, n); err != nil {
return err
}
streamLength := uint32(int(shortSector+1) * r.ShortSectorSize)
if streamLength > root.StreamSize {
root.StreamSize = streamLength
}
return nil
}
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