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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"
"io"
)
// Convert a sector ID to an absolute file position
func (r *ComDoc) sectorToOffset(sector SecID) int64 {
if sector < 0 {
return -1
}
return r.FirstSector + int64(sector)*int64(r.SectorSize)
}
// Read the specified sector
func (r *ComDoc) readSector(sector SecID, buf []byte) (int, error) {
return r.File.ReadAt(buf, r.sectorToOffset(sector))
}
// Read the specified sector into a binary structure. It must be exactly 1
// sector in size already.
func (r *ComDoc) readSectorStruct(sector SecID, v interface{}) error {
n, err := r.readSector(sector, r.sectorBuf)
if err != nil && err != io.EOF {
return err
} else if n < r.SectorSize {
return io.ErrUnexpectedEOF
}
return binary.Read(bytes.NewReader(r.sectorBuf), binary.LittleEndian, v)
}
// Write to a specified sector. Content will be padded with zeroes if it is
// less than a full sector in length.
func (r *ComDoc) writeSector(sector SecID, content []byte) error {
if len(content) > r.SectorSize {
panic("excessive write")
} else if len(content) < r.SectorSize {
buf := r.sectorBuf
copy(buf, content)
for i := len(content); i < r.SectorSize; i++ {
buf[i] = 0
}
content = buf[:r.SectorSize]
}
_, err := r.writer.WriteAt(content, r.sectorToOffset(sector))
return err
}
// Mark a chain of sectors as free
func freeSectors(sat []SecID, sector SecID) {
for {
nextSector := sat[sector]
sat[sector] = SecIDFree
if nextSector < 0 {
break
}
sector = nextSector
}
}
// Return a list of "count" free sectors or short sectors, extending the table if needed
func (r *ComDoc) makeFreeSectors(count int, short bool) []SecID {
if count <= 0 {
return nil
}
freeList := make([]SecID, 0, count)
var sat []SecID
if short {
sat = r.SSAT
} else {
sat = r.SAT
}
// scan for existing free sectors
for i, j := range sat {
if j != SecIDFree {
continue
}
freeList = append(freeList, SecID(i))
count--
if count == 0 {
return freeList
}
}
// extend the sector table
sectorsPerBlock := r.SectorSize / 4
needBlocks := (count + sectorsPerBlock - 1) / sectorsPerBlock
oldCount := len(sat)
newSAT := append(sat, make([]SecID, needBlocks*sectorsPerBlock)...)
for i := oldCount; i < len(newSAT); i++ {
newSAT[i] = SecIDFree
if count > 0 {
freeList = append(freeList, SecID(i))
count--
}
}
if short {
r.SSAT = newSAT
} else {
r.SAT = newSAT
}
return freeList
}
// Read the sector allocation table.
//
// Each index within the table corresponds to a sector within the file itself.
// The value held at that index in the SAT either points to the ID of the
// sector holding the next chunk of data for that stream, or -2 to indicate
// there are no more sectors.
func (r *ComDoc) readSAT() error {
count := r.SectorSize / 4
sat := make([]SecID, count*int(r.Header.SATSectors))
position := 0
for _, sector := range r.MSAT {
if sector < 0 {
continue
}
if position >= len(sat) {
return errors.New("msat has more sectors than indicated")
}
if err := r.readSectorStruct(sector, sat[position:position+count]); err != nil {
return err
}
position += count
}
r.SAT = sat
return nil
}
// Write the new sector allocation table to the listed sector IDs
func (r *ComDoc) writeSAT() error {
satPerSector := r.SectorSize / 4
buf := bytes.NewBuffer(r.sectorBuf)
for i, sector := range r.MSAT {
j := i * satPerSector
buf.Reset()
_ = binary.Write(buf, binary.LittleEndian, r.SAT[j:j+satPerSector])
if err := r.writeSector(sector, buf.Bytes()); err != nil {
return err
}
}
return nil
}
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