Consistent overhead byte stuffing

Consistent overhead byte stuffing is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

The Consistent Overhead Byte Stuffing (COBS) algorithm is used to demarcate a byte stream into frames. Examples can be found in its Wikipedia article.

Task

To encode, instances of the byte 0x00 in the unencoded stream are substituted with the number of bytes until the next instance of 0x00 (we'll call this a milestone). If 0x00 does not appear within the next 254 bytes after a milestone the algorithm must emit an extra milestone at the 255th byte. Of the encoded output, the first byte is the first milestone, and the final milestone will be the number of bytes until the final byte - 0x00.

Bonus tasks

•   Decode an encoded stream, including error-check functionality.
•   Support a non-zero marker

C

Encoder:

#include <stddef.h>
#include <stdint.h>
#include <assert.h>

/** COBS encode data to buffer
	@param data Pointer to input data to encode
	@param length Number of bytes to encode
	@param buffer Pointer to encoded output buffer
	@return Encoded buffer length in bytes
	@note Does not output delimiter byte
*/
size_t cobsEncode(const void *data, size_t length, uint8_t *buffer)
{
	assert(data && buffer);

	uint8_t *encode = buffer; // Encoded byte pointer
	uint8_t *codep = encode++; // Output code pointer
	uint8_t code = 1; // Code value

	for (const uint8_t *byte = (const uint8_t *)data; length--; ++byte)
	{
		if (*byte) // Byte not zero, write it
			*encode++ = *byte, ++code;

		if (!*byte || code == 0xff) // Input is zero or block completed, restart
		{
			*codep = code, code = 1, codep = encode;
			if (!*byte || length)
				++encode;
		}
	}
	*codep = code; // Write final code value

	return (size_t)(encode - buffer);
}

Decoder:

/** COBS decode data from buffer
	@param buffer Pointer to encoded input bytes
	@param length Number of bytes to decode
	@param data Pointer to decoded output data
	@return Number of bytes successfully decoded
	@note Stops decoding if delimiter byte is found
*/
size_t cobsDecode(const uint8_t *buffer, size_t length, void *data)
{
	assert(buffer && data);

	const uint8_t *byte = buffer; // Encoded input byte pointer
	uint8_t *decode = (uint8_t *)data; // Decoded output byte pointer

	for (uint8_t code = 0xff, block = 0; byte < buffer + length; --block)
	{
		if (block) // Decode block byte
			*decode++ = *byte++;
		else
		{
			if (code != 0xff) // Encoded zero, write it
				*decode++ = 0;
			block = code = *byte++; // Next block length
			if (!code) // Delimiter code found
				break;
		}
	}

	return (size_t)(decode - (uint8_t *)data);
}

Insitux

(function COBS unencoded
  (let chunk   (take-until [0] unencoded)
       length  (len chunk)
       encoded (append (first 254 chunk) (or %1 []))
       more?   (< length (len unencoded)))
  (if (or (and (= length 254) more?) (> length 254))
    (recur (skip 254 unencoded) encoded)
    (if more?
      (recur (skip (inc length) unencoded) encoded)
      (append 0 (flat-map @(.. vec (inc (len %))) encoded)))))

Unit tests:

(for [a b] [
    [[0x00] [0x01 0x01 0x00]]
    [[0x00 0x00] [0x01 0x01 0x01 0x00]]
    [[0x00 0x11 0x00] [0x01 0x02 0x11 0x01 0x00]]
    [[0x11 0x22 0x00 0x33] [0x03 0x11 0x22 0x02 0x33 0x00]]
    [[0x11 0x22 0x33 0x44] [0x05 0x11 0x22 0x33 0x44 0x00]]
    [[0x11 0x00 0x00 0x00] [0x02 0x11 0x01 0x01 0x01 0x00]]
    [(range 1 255) (.. vec 0xFF (range 1 255) 0x00)]
    [(range 255) (.. vec 0x01 0xFF (range 1 255) 0x00)]
    [(range 1 256) (.. vec 0xFF (range 1 255) 0x02 0xFF 0x00)]
    [(.. vec (range 2 256) 0x00) (.. vec 0xFF (range 2 256) 0x01 0x01 0x00)]
    [(.. vec (range 3 256) 0x00 0x01) (.. vec 0xFE (range 3 256) 0x02 0x01 0x00)]
  ]
  (assert (= (COBS a) b)))

Julia

From Wren code for encode and tests, from Phix code for decode.

module COBSPackets

export COBSencode, COBSdecode

"""
    COBSencode(data)

Return result of encoding `inputdata` into COBS packet format.
"""
function COBSencode(inputdata, marker = 0x00)
    output = [0xff]
    codeindex, code = 1, 1
    addlastcode = true
    for byte in inputdata
        if byte != marker
            push!(output, byte)
            code += 1
        end
        addlastcode = true
        if byte == marker || code == 255
            code == 255 && (addlastcode = false)
            output[codeindex] = code
            code = 1
            push!(output, 0xff)
            codeindex = length(output)
        end
    end
    if addlastcode
        output[codeindex] = code
        push!(output, marker)
    else
        output[codeindex] = marker
    end
    return output
end

"""
    COBSdecode(buffer)

    Return result of decoding `buffer` from COBS encoded format.
"""
function COBSdecode(buffer::AbstractVector, marker = 0x00)
    decoded = UInt8[]
    bdx, len = 1, length(buffer)
    while bdx < len
        code = buffer[bdx]
        bdx += 1
        for i in 1:code-1
            push!(decoded, buffer[bdx])
            bdx += 1
            bdx >= len && break
        end
        code < 0xff && bdx < len && push!(decoded, marker)
    end
    return decoded
end

end # module

using .COBSPackets

const tests = [
    [0x00],
    [0x00, 0x00],
    [0x00, 0x11, 0x00],
    [0x11, 0x22, 0x00, 0x33],
    [0x11, 0x22, 0x33, 0x44],
    [0x11, 0x00, 0x00, 0x00],
    collect(0x01:0xfe),
    collect(0x00:0xfe),
    collect(0x01:0xff),
    [collect(0x02:0xff); 0x00],
    [collect(0x03:0xff); 0x00; 0x01],
]

function ab(arr)
    hx = join(map(c -> string(c, base = 16, pad = 2), arr), " ")
    return length(hx) < 20 ? hx : hx[begin:begin+10] * " ... " * hx[end-10:end]
end

for t in tests
    println(ab(t), " -> ", ab(COBSencode(t, 3)), " -> ", ab(COBSdecode(COBSencode(t, 3), 3)))
end

Output:

00 -> 01 01 00 -> 00
00 00 -> 01 01 01 00 -> 00 00
00 11 00 -> 01 02 11 01 00 -> 00 11 00
11 22 00 33 -> 03 11 22 02 33 00 -> 11 22 00 33
11 22 33 44 -> 05 11 22 33 44 00 -> 11 22 33 44
11 00 00 00 -> 02 11 01 01 01 00 -> 11 00 00 00
01 02 03 04 ... fb fc fd fe -> ff 01 02 03 ... fc fd fe 00 -> 01 02 03 04 ... fb fc fd fe
00 01 02 03 ... fb fc fd fe -> 01 ff 01 02 ... fc fd fe 00 -> 00 01 02 03 ... fb fc fd fe
01 02 03 04 ... fc fd fe ff -> ff 01 02 03 ... fe 02 ff 00 -> 01 02 03 04 ... fc fd fe ff
02 03 04 05 ... fd fe ff 00 -> ff 02 03 04 ... ff 01 01 00 -> 02 03 04 05 ... fd fe ff 00
03 04 05 06 ... fe ff 00 01 -> fe 03 04 05 ... ff 02 01 00 -> 03 04 05 06 ... fe ff 00 01

Phix

with javascript_semantics
function cobs_encode(sequence data)
    sequence buffer = {-1}
    integer cdx = 1, code = 1, addlast = true
    for b in data do
        if b then
            buffer &= b
            code += 1
        end if
        addlast = true
        if b=0 or code=0xFF then
            addlast = code!=0xFF
            buffer[cdx] = code
            code = 1
            buffer &= -1
            cdx = length(buffer)
        end if
    end for
    buffer[cdx] = iff(addlast?code:0)
    if addlast then buffer &= 0 end if
    return buffer
end function

function cobs_decode(sequence buffer)
    sequence data = {}
    integer bdx = 1, l = length(buffer)
    while bdx<l do
        integer code = buffer[bdx]
        bdx += 1
        for i=1 to code-1 do
            data &= buffer[bdx]
            bdx += 1
        end for
        if code<0xFF and bdx<l then
            data &= 0
        end if
    end while
    return data
end function

constant tests = {{{0x00}, {0x01, 0x01, 0x00}},
                  {{0x00, 0x00}, {0x01, 0x01, 0x01, 0x00}},
                  {{0x00, 0x11, 0x00}, {0x01, 0x02, 0x11, 0x01, 0x00}},
                  {{0x11, 0x22, 0x00, 0x33}, {0x03, 0x11, 0x22, 0x02, 0x33, 0x00}},
                  {{0x11, 0x22, 0x33, 0x44}, {0x05, 0x11, 0x22, 0x33, 0x44, 0x00}},
                  {{0x11, 0x00, 0x00, 0x00}, {0x02, 0x11, 0x01, 0x01, 0x01, 0x00}},
                  {tagset(0xFE,0x01), 0xFF & tagset(0xFE,0x01) & 0x00},
                  {tagset(0xFE,0x00), {0x01, 0xFF} & tagset(0xFE,0x01) & 0x00},
                  {tagset(0xFE,0x00), {0x01, 0xFF} & tagset(0xFE,0x01) & 0x00},
                  {tagset(0xFF,0x01), 0xFF & tagset(0xFE,0x01) & {0x02, 0xFF, 0x00}},
                  {tagset(0xFF,0x02) & {0x00}, 0xFF & tagset(0xFF,0x02) & {0x01, 0x01, 0x00}},
                  {tagset(0xFF,0x03) & {0x00, 0x01}, 0xFE & tagset(0xFF,0x03) & {0x02, 0x01, 0x00}}}

function ab(sequence s, integer m,f,l)
    s = iff(length(s)<m?join(s,fmt:="%02x")
                       :join(s[1..f],fmt:="%02x")
            & " ... " & join(s[l..$],fmt:="%02x"))
    return s
end function

for t in tests do
    sequence {orig,expected} = t,
             actual = cobs_encode(orig),
             decoded = cobs_decode(actual)
    bool eOK = (actual==expected),
         dOK = (decoded==orig)
    printf(1,"%-23s -> %-33s encode:%t, decode:%t\n",
        {ab(orig,5,3,253),ab(actual,7,4,254),eOK,dOK})
end for

Output:

00                      -> 01 01 00                          encode:true, decode:true
00 00                   -> 01 01 01 00                       encode:true, decode:true
00 11 00                -> 01 02 11 01 00                    encode:true, decode:true
11 22 00 33             -> 03 11 22 02 33 00                 encode:true, decode:true
11 22 33 44             -> 05 11 22 33 44 00                 encode:true, decode:true
11 00 00 00             -> 02 11 01 01 01 00                 encode:true, decode:true
01 02 03 ... FD FE      -> FF 01 02 03 ... FD FE 00          encode:true, decode:true
00 01 02 ... FC FD FE   -> 01 FF 01 02 ... FC FD FE 00       encode:true, decode:true
00 01 02 ... FC FD FE   -> 01 FF 01 02 ... FC FD FE 00       encode:true, decode:true
01 02 03 ... FD FE FF   -> FF 01 02 03 ... FD FE 02 FF 00    encode:true, decode:true
02 03 04 ... FE FF 00   -> FF 02 03 04 ... FE FF 01 01 00    encode:true, decode:true
03 04 05 ... FF 00 01   -> FE 03 04 05 ... FF 02 01 00       encode:true, decode:true

Wren

Library: Wren-fmt

This only supports using a zero marker (see talk page).

import "./fmt" for Fmt

class COBS {
    static isByte(i) { (i is Num) && i.isInteger && i >= 0 && i < 256 }

    static encode(data) {
        if (!((data is List) && data.count > 0 && data.all { |i| isByte(i) })) {
            Fiber.abort("data must be a non-empty list of bytes.")
        }
        var enc = [-1]
        var ins = 0
        var code = 1
        var addLastCode = true
        for (byte in data) {
            if (byte != 0) {
                enc.add(byte)
                code = code + 1
            }
            addLastCode = true
            if (byte == 0|| code == 255) {
                if (code == 255) addLastCode = false
                enc[ins] = code
                code = 1
                ins = enc.count
                enc.add(-1)
            }
        }
        if (addLastCode) {
            enc[ins] = code
            enc.add(0)
        } else {
            enc[ins] = 0
        }
        return enc
    }

    static decode(enc) {
        if (!((enc is List) && enc.count > 0 && enc[-1] == 0 &&
            enc.all { |i| isByte(i) })) {
            Fiber.abort("encoded data must be a non-empty list of zero-terminated bytes.")
        }
        var dec = []
        var code = 255
        var block = 0
        for (byte in enc[0..-2]) {
            if (block != 0) {
                dec.add(byte)
            } else {
                if (code != 255) dec.add(0)
                code = byte
                block = code
                if (code == 0) break
            }
            block = block - 1
        }
        return dec
    }
}

var examples = [
    [0x00],
    [0x00, 0x00],
    [0x00, 0x11, 0x00],
    [0x11, 0x22, 0x00, 0x33],
    [0x11, 0x22, 0x33, 0x44],
    [0x11, 0x00, 0x00, 0x00],
    (0x01..0xfe).toList,
    (0x00..0xfe).toList,
    (0x01..0xff).toList,
    (0x02..0xff).toList + [0x00],
    (0x03..0xff).toList + [0x00, 0x01]
]

var encoded = []
System.print("COBS encoding (hex):")
for (example in examples) {
    var res = COBS.encode(example)
    encoded.add(res)
    if (example.count < 5) {
        Fmt.print("$-33s -> $02X", Fmt.swrite("$02X", example), res)
    } else {
        var e = Fmt.va("Xz", 2, example, 0, " ", "", "", 5, "...")
        var r = Fmt.va("Xz", 2, res, 0, " ", "", "", 5, "...")
        Fmt.print("$s -> $s", e, r)
    }
}
System.print("\nCOBS decoding (hex):")
for (enc in encoded) {
    var res = COBS.decode(enc)
    if (enc.count < 7) {
        Fmt.print("$-33s -> $02X", Fmt.swrite("$02X", enc), res)
    } else {
        var e = Fmt.va("Xz", 2, enc, 0, " ", "", "", 5, "...")
        var r = Fmt.va("Xz", 2, res, 0, " ", "", "", 5, "...")
        Fmt.print("$s -> $s", e, r)
    }
}

Output:

COBS encoding (hex):
00                                -> 01 01 00
00 00                             -> 01 01 01 00
00 11 00                          -> 01 02 11 01 00
11 22 00 33                       -> 03 11 22 02 33 00
11 22 33 44                       -> 05 11 22 33 44 00
11 00 00 00                       -> 02 11 01 01 01 00
01 02 03 04 05 ... FA FB FC FD FE -> FF 01 02 03 04 ... FB FC FD FE 00
00 01 02 03 04 ... FA FB FC FD FE -> 01 FF 01 02 03 ... FB FC FD FE 00
01 02 03 04 05 ... FB FC FD FE FF -> FF 01 02 03 04 ... FD FE 02 FF 00
02 03 04 05 06 ... FC FD FE FF 00 -> FF 02 03 04 05 ... FE FF 01 01 00
03 04 05 06 07 ... FD FE FF 00 01 -> FE 03 04 05 06 ... FE FF 02 01 00

COBS decoding (hex):
01 01 00                          -> 00
01 01 01 00                       -> 00 00
01 02 11 01 00                    -> 00 11 00
03 11 22 02 33 00                 -> 11 22 00 33
05 11 22 33 44 00                 -> 11 22 33 44
02 11 01 01 01 00                 -> 11 00 00 00
FF 01 02 03 04 ... FB FC FD FE 00 -> 01 02 03 04 05 ... FA FB FC FD FE
01 FF 01 02 03 ... FB FC FD FE 00 -> 00 01 02 03 04 ... FA FB FC FD FE
FF 01 02 03 04 ... FD FE 02 FF 00 -> 01 02 03 04 05 ... FB FC FD FE FF
FF 02 03 04 05 ... FE FF 01 01 00 -> 02 03 04 05 06 ... FC FD FE FF 00
FE 03 04 05 06 ... FE FF 02 01 00 -> 03 04 05 06 07 ... FD FE FF 00 01