1
2
3
4
5
6
7
8
9
10
11
12
|
// Package predictor implements the predictor compression/decompression algorithm
// as specified by RFC1978 - PPP Predictor Compression Protocol
package predictor
import (
"io"
)
type context struct {
table [1 << 16]byte
input []byte
hash uint16
|
>
|
1
2
3
4
5
6
7
8
9
10
11
12
13
|
// Package predictor implements the predictor compression/decompression algorithm
// as specified by RFC1978 - PPP Predictor Compression Protocol
package predictor
import (
bits "0dev.org/bits"
"io"
)
type context struct {
table [1 << 16]byte
input []byte
hash uint16
|
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
|
// and decompresses data according to the predictor algorithm
func Decompressor(reader io.Reader) io.Reader {
var ctx context
ctx.input = make([]byte, 0, 8)
return decompressor(func(output []byte) (int, error) {
var (
err error
flags byte
i, rc, total int
)
// Sanity check for space to read into
if len(output) == 0 {
return 0, nil
}
// Check whether we have leftover data in the buffer
if len(ctx.input) > 0 {
rc = copy(output, ctx.input)
// Check whether we still have leftover data in the buffer :)
if rc < len(ctx.input) {
ctx.input = ctx.input[:copy(ctx.input, ctx.input[rc:])]
}
return rc, nil
}
loop:
// Read the flags
return 0, err
}
if rc == 0 {
return total, err
}
ctx.input = ctx.input[:8]
flags = ctx.input[0]
for i = 0; i < 8; i++ {
if flags&(1<<uint(i)) > 0 {
// Guess was right
ctx.input[i] = ctx.table[ctx.hash]
} else {
rc, err = reader.Read(ctx.input[i:(i + 1)])
if err == io.EOF {
break
}
if err != nil {
return total, err
}
if rc == 0 { // treat as EoF
break
}
ctx.hash = (ctx.hash << 4) ^ uint16(ctx.input[i])
}
rc = copy(output, ctx.input[:i])
total += rc
// Place any remaining bytes in the buffer
if rc < i {
ctx.input = ctx.input[:copy(ctx.input, ctx.input[rc:i])]
} else {
// Clear the buffer
ctx.input = ctx.input[:0]
// Advance the output buffer ...
output = output[i:]
// ... and decompress the next block if there is any space left
if len(output) > 0 && err != io.EOF {
goto loop
|
|
|
|
<
|
>
>
|
<
|
>
>
>
>
<
<
<
<
|
|
|
>
>
>
|
>
|
>
|
|
>
>
|
<
<
|
|
|
|
|
>
>
|
>
>
|
>
|
|
|
|
<
|
<
|
|
|
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
|
// and decompresses data according to the predictor algorithm
func Decompressor(reader io.Reader) io.Reader {
var ctx context
ctx.input = make([]byte, 0, 8)
return decompressor(func(output []byte) (int, error) {
var (
err error
flags byte
rc, available, predicted, total int
)
// Sanity check for space to read into
if len(output) == 0 {
return 0, nil
}
// Check whether we have leftover data in the buffer
if len(ctx.input) > 0 {
rc = copy(output, ctx.input)
// Check whether we still have leftover data in the buffer :)
if rc < len(ctx.input) {
ctx.input = ctx.input[:copy(ctx.input, ctx.input[rc:])]
}
return rc, nil
}
// Read the next prediction header
readHeader:
rc, err = reader.Read(ctx.input[:1])
// Fail on error unless it is EOF
if err != nil && err != io.EOF {
return total, err
} else if rc == 0 {
return total, err
}
// Extend the buffer, copy the prediction header
// and calculate the number of subsequent bytes to read
ctx.input = ctx.input[:8]
flags = ctx.input[0]
predicted = int(bits.Hamming(flags))
available = 8 - predicted
// Read the non-predicted bytes and place them in the end of the buffer
rc, err = reader.Read(ctx.input[predicted:])
retryData:
if rc < int(available) && err == nil {
// Retry the read if we have fewer bytes than what the prediction header indicates
var r int
r, err = reader.Read(ctx.input[predicted+rc:])
rc += r
goto retryData
} // Continue on any error, try to decompress and return it along the result
// Walk the buffer, filling in the predicted blanks,
// relocating read bytes and and updating the guess table
for i, a := uint(0), predicted; i < 8; i++ {
if (flags & (1 << i)) > 0 {
// Guess succeeded, fill in from the table
ctx.input[i] = ctx.table[ctx.hash]
rc++
} else {
// Relocate a read byte
ctx.input[i], a = ctx.input[a], a+1
// Guess failed, update the table
ctx.table[ctx.hash] = ctx.input[i]
}
// Update the hash
ctx.hash = (ctx.hash << 4) ^ uint16(ctx.input[i])
}
// rc now contains the precise amount of populated data
ctx.input = ctx.input[:rc]
available = copy(output, ctx.input)
total += available
// Check for remaining bytes that dont fit in the output buffer
if available < rc {
ctx.input = ctx.input[:copy(ctx.input, ctx.input[available:])]
} else {
// Clear the buffer
ctx.input = ctx.input[:0]
output = output[available:]
if len(output) > 0 && err == nil {
goto readHeader
}
}
return total, err
})
}
|