forked from sassoftware/relic
/
shortsector.go
142 lines (136 loc) · 4.57 KB
/
shortsector.go
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
//
// 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) 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)
_, err := r.File.ReadAt(buf, n)
return err
}
// 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
}