encode_decode_len

Andy Piper edited this page Dec 2, 2013 · 1 revision
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Encoding and decoding the message lengths

The variable length encoding scheme uses a single byte for messages up to 127 bytes long. Longer messages are handled as follows. Seven bits of each byte encode the Remaining Length data, and the eighth bit indicates any following bytes in the representation. Each byte encodes 128 values and a "continuation bit". For example, the number 64 decimal is encoded as a single byte, decimal value 64, hex 0x40. The number 321 decimal (= 65 + 2*128) is encoded as two bytes, least significant first. The first byte 65+128 = 193. Note that the top bit is set to indicate at least one following byte. The second byte is 2.

The protocol specification has an example of how to do the length encoding/decoding but in some languages that can be a difficult function to implement.

Here's a working example in REXX.

mqtt_length_encode: procedure

   x = arg(1)
   remlen = ""
   do while(x > 0)

      digit = x // 128
      x = x % 128

      if x > 0 then digit = digit + 128
      remlen = remlen || d2x(digit) 

   end

   return x2c(remlen)
`</code>`((d2x(decimal) converts a decimal value to hex.))

`<code>`
mqtt_length_decode: procedure

   remlen = arg(1)
   digits = c2x(remlen)

   multiplier = 1
   value = 0
   done = 0
   offset = 2
   do i = 1 to length(digits) by 2

      digit = x2d(substr(digits,i,2))
      if digit > 128 then digit = digit - 128
      else done = 1
      value = value + (digit * multiplier)
      if done = 1 then leave
      multiplier = multiplier * 128
      offset = offset + 1
   end
   ret = value offset
   return ret
`</code>`((x2d(hex) converts a hex value to decimal.))