// 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

				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)

// 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 {
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
			readCount int
			err                      error
			flags                    byte
			rc, available, predicted 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)
			rc = copy(output, ctx.input)

			// Check whether we still have leftover data in the buffer :)
			if readCount < len(ctx.input) {
				ctx.input = ctx.input[:copy(ctx.input, ctx.input[readCount:])]
			if rc < len(ctx.input) {
				ctx.input = ctx.input[:copy(ctx.input, ctx.input[rc:])]
			}
			return readCount, nil
			return rc, 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
		readCount, err = wrapped.Read(ctx.input[:1])
		if readCount == 0 || err != nil {
			return readCount, err
		// Read the next prediction header
		rc, err = reader.Read(ctx.input[:1])
		// Fail on error unless it is EOF
		if err != nil && err != io.EOF {
			return 0, err
		} else if rc == 0 {
			return 0, 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 i uint = 0
		for ; i < 8; i++ {
			if flags&(1<<i) > 0 {
				// Guess was right
			var rc int
			rc, err = reader.Read(ctx.input[predicted+rc:])
			rc += rc
			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 {
				readCount, err = wrapped.Read(ctx.input[i:(i + 1)])

				// Relocate a read byte
				if err == io.EOF {
					break
				}

				ctx.input[i], a = ctx.input[a], a+1
				if err != nil {
					return readCount, err
				}

				// Guess failed, update the table
				if readCount == 0 { // treat as EoF
					break
				}

				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]
		readCount = copy(output, ctx.input[:i])
		available = copy(output, ctx.input)

		// Place any remaining bytes in the buffer
		if uint(readCount) < i {
			ctx.input = ctx.input[readCount:i]
		// 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]
		}

		return readCount, nil
		return available, err
	})
}