ecc.go

package words

import (
	"encoding/binary"
	"errors"
	"hash/crc32"
	"hash/crc64"

	"github.com/howeyc/crc16"
)

// ECC is used for anything that calculates an error-correcting code
type ECC interface {
	// AddECC calculates an error-correcting code for the input
	// returns an output with the code appended
	AddECC([]byte) []byte

	// CheckECC verifies if the ECC is proper on the input and returns
	// the data with the code removed, or an error
	CheckECC([]byte) ([]byte, error)
}

var errInputTooShort = errors.New("input too short, no checksum present")
var errChecksumDoesntMatch = errors.New("checksum does not match")

// NoECC is a no-op placeholder, kind of useless... except for tests
type NoECC struct{}

var _ ECC = NoECC{}

func (_ NoECC) AddECC(input []byte) []byte            { return input }
func (_ NoECC) CheckECC(input []byte) ([]byte, error) { return input, nil }

// CRC16 does the ieee crc16 polynomial check
type CRC16 struct {
	Poly  uint16
	table *crc16.Table
}

var _ ECC = (*CRC16)(nil)

const crc16ByteCount = 2

func NewIBMCRC16() *CRC16 {
	return &CRC16{Poly: crc16.IBM}
}

func NewSCSICRC16() *CRC16 {
	return &CRC16{Poly: crc16.SCSI}
}

func NewCCITTCRC16() *CRC16 {
	return &CRC16{Poly: crc16.CCITT}
}

func (c *CRC16) AddECC(input []byte) []byte {
	table := c.getTable()

	// get crc and convert to some bytes...
	crc := crc16.Checksum(input, table)
	check := make([]byte, crc16ByteCount)
	binary.BigEndian.PutUint16(check, crc)

	// append it to the input
	output := append(input, check...)
	return output
}

func (c *CRC16) CheckECC(input []byte) ([]byte, error) {
	table := c.getTable()

	if len(input) <= crc16ByteCount {
		return nil, errInputTooShort
	}
	cut := len(input) - crc16ByteCount
	data, check := input[:cut], input[cut:]
	crc := binary.BigEndian.Uint16(check)
	calc := crc16.Checksum(data, table)
	if crc != calc {
		return nil, errChecksumDoesntMatch
	}
	return data, nil
}

func (c *CRC16) getTable() *crc16.Table {
	if c.table != nil {
		return c.table
	}
	if c.Poly == 0 {
		c.Poly = crc16.IBM
	}
	c.table = crc16.MakeTable(c.Poly)
	return c.table
}

// CRC32 does the ieee crc32 polynomial check
type CRC32 struct {
	Poly  uint32
	table *crc32.Table
}

var _ ECC = (*CRC32)(nil)

func NewIEEECRC32() *CRC32 {
	return &CRC32{Poly: crc32.IEEE}
}

func NewCastagnoliCRC32() *CRC32 {
	return &CRC32{Poly: crc32.Castagnoli}
}

func NewKoopmanCRC32() *CRC32 {
	return &CRC32{Poly: crc32.Koopman}
}

func (c *CRC32) AddECC(input []byte) []byte {
	table := c.getTable()

	// get crc and convert to some bytes...
	crc := crc32.Checksum(input, table)
	check := make([]byte, crc32.Size)
	binary.BigEndian.PutUint32(check, crc)

	// append it to the input
	output := append(input, check...)
	return output
}

func (c *CRC32) CheckECC(input []byte) ([]byte, error) {
	table := c.getTable()

	if len(input) <= crc32.Size {
		return nil, errInputTooShort
	}
	cut := len(input) - crc32.Size
	data, check := input[:cut], input[cut:]
	crc := binary.BigEndian.Uint32(check)
	calc := crc32.Checksum(data, table)
	if crc != calc {
		return nil, errChecksumDoesntMatch
	}
	return data, nil
}

func (c *CRC32) getTable() *crc32.Table {
	if c.table == nil {
		if c.Poly == 0 {
			c.Poly = crc32.IEEE
		}
		c.table = crc32.MakeTable(c.Poly)
	}
	return c.table
}

// CRC64 does the ieee crc64 polynomial check
type CRC64 struct {
	Poly  uint64
	table *crc64.Table
}

var _ ECC = (*CRC64)(nil)

func NewISOCRC64() *CRC64 {
	return &CRC64{Poly: crc64.ISO}
}

func NewECMACRC64() *CRC64 {
	return &CRC64{Poly: crc64.ECMA}
}

func (c *CRC64) AddECC(input []byte) []byte {
	table := c.getTable()

	// get crc and convert to some bytes...
	crc := crc64.Checksum(input, table)
	check := make([]byte, crc64.Size)
	binary.BigEndian.PutUint64(check, crc)

	// append it to the input
	output := append(input, check...)
	return output
}

func (c *CRC64) CheckECC(input []byte) ([]byte, error) {
	table := c.getTable()

	if len(input) <= crc64.Size {
		return nil, errInputTooShort
	}
	cut := len(input) - crc64.Size
	data, check := input[:cut], input[cut:]
	crc := binary.BigEndian.Uint64(check)
	calc := crc64.Checksum(data, table)
	if crc != calc {
		return nil, errChecksumDoesntMatch
	}
	return data, nil
}

func (c *CRC64) getTable() *crc64.Table {
	if c.table == nil {
		if c.Poly == 0 {
			c.Poly = crc64.ISO
		}
		c.table = crc64.MakeTable(c.Poly)
	}
	return c.table
}