// Package predictor implements the predictor compression/decompression algorithm
// as specified by RFC1978 - PPP Predictor Compression Protocol
package predictor
import (
"io"
)
type context struct {
table [65536]byte
hash uint16
input []byte
}
// Returns a closure over the provided writer that compresses data when called.
//
// 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) func([]byte) error {
var ctx context
ctx.input = make([]byte, 8)
// Forward declaration as it is required for recursion
var write func(data []byte) error
write = func(data []byte) error {
var (
err error
blockSize int = 8
bufferLength int = len(ctx.input)
)
// Force a flush if we are called with no data to write
if len(data) == 0 {
// We can't have more than 7 bytes in the buffer so this is safe
blockSize = len(ctx.input)
goto write
}
// Check if there are pending bytes in the buffer
if bufferLength > 0 && bufferLength < 8 {
// Check whether we have enough bytes for a complete block
if len(data) > 8-bufferLength {
// Fill the buffer ...
ctx.input = append(ctx.input, data[:8-bufferLength]...)
// ... and recurse, calling ourselves with the full buffer
err = write(ctx.input)
if err != nil {
return err
}
// Clear the buffer
ctx.input = ctx.input[:0]
// Handle remaining bytes, if any
var remaining []byte = data[8-bufferLength:]
if len(remaining) > 0 {
// Recurse, calling ourselves with the rest of the bytes
err = write(data[8-bufferLength:])
if err != nil {
return err
}
}
} else {
// Add the insufficient data to the buffer and return
ctx.input = append(ctx.input, data...)
return nil
}
}
write:
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)
}
_, err = writer.Write(buf)
if err != nil {
return err
}
// Reset the flags and buffer for the next iteration
buf[0] ^= buf[0]
buf = buf[:1]
}
return nil
}
return write
}