// 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
	buffer [1 << 3]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 = ctx.buffer[:0]

	// Forward declaration as it is required for recursion
	var write compressor

	write = func(data []byte) error {
		var (
			blockSize    int = 8

			}

			// 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.buffer[:remaining]
			copy(ctx.input, data[len(data)-remaining:])
		} else {
			ctx.input = ctx.buffer[: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 = ctx.buffer[:0]

	return decompressor(func(output []byte) (int, error) {
		var (
			err       error
			flags     byte
			readCount int
		)

		}

		// 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.buffer[:1]
		readCount, err = wrapped.Read(ctx.input)
		if readCount == 0 || err != nil {
			return readCount, err
		}

		flags = ctx.input[0]
		ctx.input = ctx.buffer[: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 = 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.buffer[:0]
		}

		return readCount, nil
	})
}