// Package predictor implements the predictor compression/decompression algorithm
// as specified by RFC1978 - PPP Predictor Compression Protocol
package predictor
import (
bits "0dev.org/bits"
iou "0dev.org/ioutil"
"io"
)
// The context struct contains the predictor's algorithm guess table
// and the current value of its input/output hash
type context struct {
table [1 << 16]byte
hash uint16
}
// The following hash code is the heart of the algorithm:
// It builds a sliding hash sum of the previous 3-and-a-bit
// characters which will be used to index the guess table.
// A better hash function would result in additional compression,
// at the expense of time.
func (ctx *context) update(val byte) {
ctx.hash = (ctx.hash << 4) ^ uint16(val)
}
// Returns an io.Writer implementation that wraps the provided io.Writer
// and compresses data according to the predictor algorithm
//
// It can buffer data as the predictor mandates 8-byte blocks with a header.
// A call with no data will force a flush.
func Compressor(writer io.Writer) io.Writer {
var cmp compressor
cmp.target = writer
return iou.SizedWriter(&cmp, 8)
}
type compressor struct {
context
target io.Writer
}
// Note: this method does not implement the full io.Writer's Write() semantics
func (ctx *compressor) Write(data []byte) (int, error) {
var (
blockSize int = 8
datalength int = len(data)
)
if datalength == 0 {
return 0, nil
}
if datalength < blockSize {
blockSize = datalength
}
var buf []byte = make([]byte, 1, blockSize+1)
for block := 0; block < datalength/blockSize; block++ {
for i := 0; i < blockSize; i++ {
var current byte = data[(block*blockSize)+i]
if ctx.table[ctx.hash] == current {
// Guess was right - don't output
buf[0] |= 1 << uint(i)
} else {
// Guess was wrong, output char
ctx.table[ctx.hash] = current
buf = append(buf, current)
}
ctx.update(current)
}
if c, err := ctx.target.Write(buf); err != nil {
return (block * blockSize) + c, err
}
// Reset the flags and buffer for the next iteration
buf, buf[0] = buf[:1], 0
}
return datalength, nil
}
// Returns an io.Reader implementation that wraps the provided io.Reader
// and decompresses data according to the predictor algorithm
func Decompressor(reader io.Reader) io.Reader {
var dcmp decompressor
dcmp.source = reader
dcmp.input = make([]byte, 8)
return iou.SizedReader(&dcmp, 8)
}
type decompressor struct {
context
source io.Reader
input []byte
}
// Note: this method does not implement the full io.Reader's Read() semantics
func (ctx *decompressor) Read(output []byte) (int, error) {
var (
err error
flags, predicted byte
rc, total, copied int
)
// Read the next prediction header
readHeader:
rc, err = ctx.source.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
}
// Copy the prediction header and calculate the number of subsequent bytes to read
flags = ctx.input[0]
predicted = bits.Hamming(flags)
// Read the non-predicted bytes and place them in the end of the buffer
rc, err = ctx.source.Read(ctx.input[predicted:])
retryData:
if rc < int(8-predicted) && err == nil {
// Retry the read if we have fewer bytes than what the prediction header indicates
var r int
r, err = ctx.source.Read(ctx.input[int(predicted)+rc:])
rc += r
goto retryData
} // Continue on any error, try to decompress and return it along the result
// rc now contains the amount of actual bytes in this cycle (usually 8)
rc += int(predicted)
// Walk the buffer, filling in the predicted blanks,
// relocating read bytes and and updating the guess table
for i, a := 0, predicted; i < rc; i++ {
if (flags & (1 << uint(i))) > 0 {
// Guess succeeded, fill in from the table
ctx.input[i] = ctx.table[ctx.hash]
} else {
// Relocate a read byte and advance the read byte index
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.update(ctx.input[i])
}
// Copy the decompressed data to the output and accumulate the count
copied = copy(output, ctx.input[:rc])
total += copied
// Loop for another pass if there is available space in the output
output = output[copied:]
if len(output) > 0 && err == nil {
goto readHeader
}
return total, err
}