def ecc_encode(msg, pad = False):
""" encode message 'msg' which must be of length MAX_LENGTH:
1. pad msg if 'pad' is True
2. Map each 4bit word of 'msg' to one of the base vectors.
3. Transpose and repeat message twice.
The encoding attempts to map nicely to current physical low
power devices which generally support packet handling and
where packet lengths are generally limited to 255 bytes (TI:
CC1101 and CC1120 etc, Freescale RF MCU, HopeRF RFM69W,
Silicon Labs RF MCU etc).
"""
if pad:
msg = pad(msg)
length = MAX_LENGTH
assert len(msg) == length
div = length / 8
enc_msg_high = []
enc_msg_low = []
for c in msg:
enc_msg_high.append(BASE_VECTORS[c >> 4])
enc_msg_low.append(BASE_VECTORS[c % 16])
## transpose
tr_msg_high = bit_transpose(enc_msg_high)
tr_msg_low = bit_transpose(enc_msg_low)
## duplicate
out = []
for i in range(8 * 2):
_from = i * div
_to = (i+1) * div
out.extend(tr_msg_high[_from:_to])
out.extend(tr_msg_high[_from:_to])
out.extend(tr_msg_low[_from:_to])
out.extend(tr_msg_low[_from:_to])
return s_to_a(out)
def ecc_encode(msg, pad=False):
""" encode message 'msg' which must be of length MAX_LENGTH:
1. pad msg if 'pad' is True
2. Map each 4bit word of 'msg' to one of the base vectors.
3. Transpose and repeat message twice.
The encoding attempts to map nicely to current physical low
power devices which generally support packet handling and
where packet lengths are generally limited to 255 bytes (TI:
CC1101 and CC1120 etc, Freescale RF MCU, HopeRF RFM69W,
Silicon Labs RF MCU etc).
"""
if pad:
msg = pad(msg)
length = MAX_LENGTH
assert len(msg) == length
div = length / 8
enc_msg_high = []
enc_msg_low = []
for c in msg:
enc_msg_high.append(BASE_VECTORS[c >> 4])
enc_msg_low.append(BASE_VECTORS[c % 16])
## transpose
tr_msg_high = bit_transpose(enc_msg_high)
tr_msg_low = bit_transpose(enc_msg_low)
## duplicate
out = []
for i in range(8 * 2):
_from = i * div
_to = (i + 1) * div
out.extend(tr_msg_high[_from:_to])
out.extend(tr_msg_high[_from:_to])
out.extend(tr_msg_low[_from:_to])
out.extend(tr_msg_low[_from:_to])
return s_to_a(out)
def test():
import random
msg = pad(s_to_a('Hello QuietCasting'))
assert msg[0] == 18
assert len(msg) == MAX_LENGTH
tr_msg = bit_transpose(msg)
assert reverse_bit_transpose(tr_msg) == msg
print 'Encoding ...'
enc = ecc_encode(msg)
print enc
print 'Corrupting message (random) ...'
rec = []
for c in enc:
e = 0
for i in range(8):
r = random.randint(0, 999)
if r < 5:
e |= 1
e <<= 1
rec.append(c ^ e)
total = 0
for (i, j) in zip(rec, enc):
if i != j:
total += 1
print total, 'corrupted characters detected ...'
print rec
print 'Decoding ...'
dec = ecc_decode(rec)
print dec
if dec[1] == msg:
print 'Decoded succesfully!'
else:
print 'Errors detected'
print dec
print msg
print 'Corrupting message (burst) ...'
rec = array.array('B', [c ^ 255 for c in enc[:24]]) + enc[24:]
total = 0
for (i, j) in zip(rec, enc):
if i != j:
total += 1
print total, 'corrupted characters detected ...'
print rec
print 'Decoding ...'
dec = ecc_decode(rec)
print dec
if dec[1] == msg:
print 'Decoded succesfully!'
else:
print 'Errors detected'
print dec
print msg
def test():
import random
msg = pad(s_to_a('Hello QuietCasting'))
assert msg[0] == 18
assert len(msg) == MAX_LENGTH
tr_msg = bit_transpose(msg)
assert reverse_bit_transpose(tr_msg) == msg
print 'Encoding ...'
enc = ecc_encode(msg)
print enc
print 'Corrupting message (random) ...'
rec = []
for c in enc:
e = 0
for i in range(8):
r = random.randint(0,999)
if r < 5:
e |= 1
e <<= 1
rec.append(c ^ e)
total = 0
for (i,j) in zip(rec, enc):
if i!=j:
total += 1
print total, 'corrupted characters detected ...'
print rec
print 'Decoding ...'
dec = ecc_decode(rec)
print dec
if dec[1] == msg:
print 'Decoded succesfully!'
else:
print 'Errors detected'
print dec
print msg
print 'Corrupting message (burst) ...'
rec = array.array('B', [c^255 for c in enc[:24]]) + enc[24:]
total = 0
for (i,j) in zip(rec, enc):
if i!=j:
total += 1
print total, 'corrupted characters detected ...'
print rec
print 'Decoding ...'
dec = ecc_decode(rec)
print dec
if dec[1] == msg:
print 'Decoded succesfully!'
else:
print 'Errors detected'
print dec
print msg