Michael Cotherman wrote:
> I am a noob to converting pointers in C++ to arrays in
> python, although the first time I see it done, I will
> have no problem. Can you help converting the below
> (what I think is the 'decoder' section) to python?
OK I'll bite. Your code is copying from a src buffer to a dst buffer. The src
buffer has a count byte followed by count-1 bytes of data. If the count is less
that 0xFF, a zero byte has to be inserted into the dst. So the decode loop gets
a count, copies that many bytes to the dst buffer, then optionally appends a 0.
Python doesn't have a direct equivalent to this sort of manipulation of memory
pointers and raw memory buffers. They are commonly replaced by strings or
lists. My function takes a string as an input argument, builds the output as
string fragments in a list, then builds a string to return.
Note that Python strings include an implicit length so there is no need to pass
and return a length argument.
I included a test case with the data from your previous email. Your data is
evidently hex-encoded as well as COBS encoded - in other words your strings are
hex values where each two chars represent a single byte. I have converted to
and from byte strings to feed this to the decoder.
Kent
PS Please send your replies to the whole list, not to me personally. Thanks!
def unstuff(src):
# src is a COBS compressed string
current = 0 # index into src
dst = [] # a list that will build the result
while current < len(src):
# Get the count and convert it to an integer
count = ord(src[current])
# Append count-1 chars from src to dst
dst.append(src[current+1:current+count])
# Do we need to add a zero byte?
if count < 0xFF:
dst.append('\x00')
# Bump the counter and continue
if count>0:
current += count
else:
current += 1
# dst is a list of string fragments; this converts it to a single string
return ''.join(dst)
def hexToString(hexStr):
''' Convert a string of two-digit hex values to a string of bytes with
those values '''
return ''.join([chr(int(hexStr[i:i+2], 16)) for i in range(0, len(hexStr),
2)])
def stringToHex(src):
''' Convert a byte string to a string of two-digit hex values '''
return ''.join([ '%02x' % ord(s) for s in src ])
if __name__ == '__main__':
data = '0002860104DB203F0100'
print data
data = hexToString(data)
print
newData = unstuff(data)
print stringToHex(newData)
> UINT CCobsPackets::UnStuffData(unsigned char *src,
> unsigned char *dst, UINT length)
> {
> unsigned char *dstStart = dst;
> unsigned char *end = src + length;
>
> while (src < end)
> {
> int i, code = *src++;
>
> for (i=1; i<code; i++)
> {
> *dst++ = *src++;
> }
>
> if (code < 0xFF)
> {
> *dst++ = 0;
> }
> }
>
> return (UINT)(dst - dstStart);
> }
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