// 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
}
type compressor func([]byte) error
func (w compressor) Write(data []byte) (int, error) {
return len(data), w(data)
}
// 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 ctx context
ctx.input = make([]byte, 0, 8)
// Forward declaration as it is required for recursion
var write compressor
write = func(data []byte) error {
var (
blockSize int = 8
bufferLength int = len(ctx.input)
)
// Force a flush if we are called with no data to write
if len(data) == 0 {
// Nothing to flush if the buffer is empty though
if len(ctx.input) == 0 {
return nil
}
// We can't have more than 7 bytes in the buffer so this is safe
data, blockSize, bufferLength = ctx.input, len(ctx.input), 0
}
// Check if there are pending bytes in the buffer
if len(data) < blockSize || bufferLength > 0 {
// If the current buffer + new data can fit into a block
if (len(data) + bufferLength) <= blockSize {
ctx.input = append(ctx.input, data...)
// Flush the block if the buffer fills it
if len(ctx.input) == blockSize {
return write(nil)
}
// ... otherwise just return
return nil
}
// The current buffer + new data overflow the block size
// Complete the block, flush it ...
ctx.input = append(ctx.input, data[:blockSize-bufferLength]...)
if err := write(nil); err != nil {
return err
}
// ... and stage the rest of the data in the buffer
ctx.input = append(ctx.input, data[blockSize-bufferLength:]...)
return nil
}
// TODO allocate this on ctx.buffer ...
var buf []byte = make([]byte, 1, blockSize+1)
for block := 0; block < len(data)/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.hash = (ctx.hash << 4) ^ uint16(current)
}
if _, err := writer.Write(buf); err != nil {
return err
}
// Reset the flags and buffer for the next iteration
buf, buf[0] = buf[:1], 0
}
if remaining := len(data) % blockSize; remaining > 0 {
ctx.input = ctx.input[:remaining]
copy(ctx.input, data[len(data)-remaining:])
} else {
ctx.input = ctx.input[:0]
}
return nil
}
return write
}
// A function type alias so that it can have methods attached to it
type decompressor func([]byte) (int, error)
// Required to implement io.Reader
func (r decompressor) Read(output []byte) (int, error) {
return r(output)
}
// Returns an io.Reader implementation that wraps the provided io.Reader
// and decompresses data according to the predictor algorithm
func Decompressor(wrapped 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
readCount 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 {
readCount = copy(output, ctx.input)
ctx.input = ctx.input[readCount:]
return readCount, nil
}
// This is single-iteration only but it is fine according to io.Reader's contract ?!
// TODO - read all bytes from a block based on the hamming weight of the flag
// and just shuffle them for predictions instead of bite-sized reads ;)
// Read the flags
ctx.input = ctx.input[:1]
readCount, err = wrapped.Read(ctx.input)
if readCount == 0 || err != nil {
return readCount, err
}
flags = ctx.input[0]
ctx.input = ctx.input[:8]
var i uint = 0
for ; i < 8; i++ {
if flags&(1<<i) > 0 {
// Guess was right
ctx.input[i] = ctx.table[ctx.hash]
} else {
readCount, err = wrapped.Read(ctx.input[i:(i + 1)])
if err == io.EOF {
break
}
if err != nil {
return readCount, err
}
if readCount == 0 { // treat as EoF
break
}
ctx.table[ctx.hash] = ctx.input[i]
}
ctx.hash = (ctx.hash << 4) ^ uint16(ctx.input[i])
}
readCount = copy(output, ctx.input[:i])
// Place any remaining bytes in the buffer
if uint(readCount) < i {
ctx.input = ctx.input[readCount:i]
} else {
ctx.input = ctx.input[:0]
}
return readCount, nil
})
}