File src/0dev.org/encoding/mtf/mtf.go artifact f2ad12a795 part of check-in 783d0b7f51

// Package mtf provides a move-to-front encoder and decoder implementations
package mtf

import (
	"io"
)

// A static table with the initial condition for the mtf algorithm
var initial [256]byte = [...]byte{
	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
	16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
	32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
	48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
	64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
	80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95,
	96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111,
	112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127,
	128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143,
	144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159,
	160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175,
	176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191,
	192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207,
	208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223,
	224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239,
	240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255,
}

type context struct {
	table [256]byte
}

// Returns an MTF encoder over the provided io.Writer
func Encoder(writer io.Writer) io.Writer {
	var enc encoder
	enc.table = initial
	enc.target = writer
	return &enc
}

type encoder struct {
	context
	target io.Writer
}

func (c *encoder) Write(data []byte) (int, error) {
	var (
		dataLength int    = len(data)
		buffer     []byte = make([]byte, dataLength)
	)

	// io.Write must not modify the passed data in any way
	// TODO - check sync.Pool or a local free-list for amortizing buffers
	// TODO - use a buffer with a fixed max size to avoid OOM conditions

	// Loop over the input data
	for index, value := range data {

		// Shortcut for sequential, equal values
		if c.table[0] == value {
			buffer[index] = 0
			continue
		}

		// Loop over the MTF table
		for j := byte(1); j != 0; j++ {
			if c.table[j] == value {
				// Output the value
				buffer[index] = j

				// Shift the table
				copy(c.table[1:], c.table[:j])

				// Restore the value in front and break
				c.table[0] = value
				break
			}
		}
	}

	return c.target.Write(buffer)
}

// Returns an MTF decoder over the provided io.Writer
func Decoder(reader io.Reader) io.Reader {
	var dec decoder
	dec.table = initial
	dec.source = reader
	return &dec
}

type decoder struct {
	context
	source io.Reader
}

func (c *decoder) Read(output []byte) (int, error) {
	var (
		count    int
		err      error
		position byte
	)

	// Read from the source and decode in place
	count, err = c.source.Read(output)
	for i := 0; i < count; i++ {
		position = output[i]

		// Shortcut for sequential, equal values
		if position == 0 {
			output[i] = c.table[0]
			continue
		}

		// Output the value
		output[i] = c.table[position]

		// Shift the table and restore the value in front
		copy(c.table[1:], c.table[:position])
		c.table[0] = output[i]
	}

	return count, err
}