// 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 ctx context
	ctx.input = make([]byte, 0, 8)

	// Forward declaration as it is required for recursion
	var write iou.WriterFunc

	write = func(data []byte) (int, error) {
		var (
			blockSize    int = 8
			bufferLength int = len(ctx.input)
			datalength   int = len(data)
		)

		// Force a flush if we are called with no data to write
		if datalength == 0 {
			// Nothing to flush if the buffer is empty though
			if len(ctx.input) == 0 {
				return 0, nil
			}
			// We can't have more than 7 bytes in the buffer so this is safe
			data, datalength = ctx.input, len(ctx.input)
			blockSize, bufferLength = datalength, 0
		}

		// Check if there are pending bytes in the buffer
		if datalength < blockSize || bufferLength > 0 {

			// If the current buffer + new data can fit into a block
			if (datalength + 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 datalength, nil
			}

			// The current buffer + new data overflow the block size
			// Complete the block, flush it ...
			ctx.input = append(ctx.input, data[:blockSize-bufferLength]...)
			if c, err := write(nil); err != nil {
				return c, err
			}
			// ... and stage the rest of the data in the buffer
			ctx.input = append(ctx.input, data[blockSize-bufferLength:]...)
			return datalength, nil
		}

		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 := writer.Write(buf); err != nil {
				return (block * blockSize) + c, err
			}

			// Reset the flags and buffer for the next iteration
			buf, buf[0] = buf[:1], 0
		}

		if remaining := datalength % blockSize; remaining > 0 {
			ctx.input = ctx.input[:remaining]
			copy(ctx.input, data[datalength-remaining:])
		} else {
			ctx.input = ctx.input[:0]
		}

		return datalength, nil
	}

	return write
}

// Returns an io.Reader implementation that wraps the provided io.Reader
// and decompresses data according to the predictor algorithm