UPC: Difference between revisions
Added a Python implementation
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(7 0 6 4 6 6 7 4 3 0 3 0)
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</pre>
=={{header|Python}}==
<lang python>"""UPC-A barcode reader. Requires Python =>3.6"""
import itertools
import re
RE_BARCODE = re.compile(
r"^(?P<s_quiet> +)" # quiet zone
r"(?P<s_guard># #)" # start guard
r"(?P<left>[ #]{42})" # left digits
r"(?P<m_guard> # # )" # middle guard
r"(?P<right>[ #]{42})" # right digits
r"(?P<e_guard># #)" # end guard
r"(?P<e_quiet> +)$" # quiet zone
)
LEFT_DIGITS = {
(0, 0, 0, 1, 1, 0, 1): 0,
(0, 0, 1, 1, 0, 0, 1): 1,
(0, 0, 1, 0, 0, 1, 1): 2,
(0, 1, 1, 1, 1, 0, 1): 3,
(0, 1, 0, 0, 0, 1, 1): 4,
(0, 1, 1, 0, 0, 0, 1): 5,
(0, 1, 0, 1, 1, 1, 1): 6,
(0, 1, 1, 1, 0, 1, 1): 7,
(0, 1, 1, 0, 1, 1, 1): 8,
(0, 0, 0, 1, 0, 1, 1): 9,
}
RIGHT_DIGITS = {
(1, 1, 1, 0, 0, 1, 0): 0,
(1, 1, 0, 0, 1, 1, 0): 1,
(1, 1, 0, 1, 1, 0, 0): 2,
(1, 0, 0, 0, 0, 1, 0): 3,
(1, 0, 1, 1, 1, 0, 0): 4,
(1, 0, 0, 1, 1, 1, 0): 5,
(1, 0, 1, 0, 0, 0, 0): 6,
(1, 0, 0, 0, 1, 0, 0): 7,
(1, 0, 0, 1, 0, 0, 0): 8,
(1, 1, 1, 0, 1, 0, 0): 9,
}
MODULES = {
" ": 0,
"#": 1,
}
DIGITS_PER_SIDE = 6
MODULES_PER_DIGIT = 7
class ParityError(Exception):
"""Exception raised when a parity error is found."""
class ChecksumError(Exception):
"""Exception raised when check digit does not match."""
def group(iterable, n):
"""Chunk the iterable into groups of size ``n``."""
args = [iter(iterable)] * n
return tuple(itertools.zip_longest(*args))
def parse(barcode):
"""Return the 12 digits represented by the given barcode. Raises a
ParityError if any digit fails the parity check."""
match = RE_BARCODE.match(barcode)
# Translate bars and spaces to 1s and 0s so we can do arithmetic
# with them. Group "modules" into chunks of 7 as we go.
left = group((MODULES[c] for c in match.group("left")), MODULES_PER_DIGIT)
right = group((MODULES[c] for c in match.group("right")), MODULES_PER_DIGIT)
# Parity check
left, right = check_parity(left, right)
# Lookup digits
return tuple(
itertools.chain(
(LEFT_DIGITS[d] for d in left),
(RIGHT_DIGITS[d] for d in right),
)
)
def check_parity(left, right):
"""Check left and right parity. Flip left and right if the barcode
was scanned upside down."""
# When reading from left to right, each digit on the left should
# have odd parity, and each digit on the right should have even
# parity.
left_parity = sum(sum(d) % 2 for d in left)
right_parity = sum(sum(d) % 2 for d in right)
# Use left and right parity to check if the barcode was scanned
# upside down. Flip it if it was.
if left_parity == 0 and right_parity == DIGITS_PER_SIDE:
_left = tuple(tuple(reversed(d)) for d in reversed(right))
right = tuple(tuple(reversed(d)) for d in reversed(left))
left = _left
elif left_parity != DIGITS_PER_SIDE or right_parity != 0:
# Error condition. Mixed parity.
error = tuple(
itertools.chain(
(LEFT_DIGITS.get(d, "_") for d in left),
(RIGHT_DIGITS.get(d, "_") for d in right),
)
)
raise ParityError(" ".join(str(d) for d in error))
return left, right
def checksum(digits):
"""Return the check digit for the given digits. Raises a
ChecksumError if the check digit does not match."""
odds = (digits[i] for i in range(0, 11, 2))
evens = (digits[i] for i in range(1, 10, 2))
check_digit = (sum(odds) * 3 + sum(evens)) % 10
if check_digit != 0:
check_digit = 10 - check_digit
if digits[-1] != check_digit:
raise ChecksumError(str(check_digit))
return check_digit
def main():
barcodes = [
" # # # ## # ## # ## ### ## ### ## #### # # # ## ## # # ## ## ### # ## ## ### # # # ",
" # # # ## ## # #### # # ## # ## # ## # # # ### # ### ## ## ### # # ### ### # # # ",
" # # # # # ### # # # # # # # # # # ## # ## # ## # ## # # #### ### ## # # ",
" # # ## ## ## ## # # # # ### # ## ## # # # ## ## # ### ## ## # # #### ## # # # ",
" # # ### ## # ## ## ### ## # ## # # ## # # ### # ## ## # # ### # ## ## # # # ",
" # # # # ## ## # # # # ## ## # # # # # #### # ## # #### #### # # ## # #### # # ",
" # # # ## ## # # ## ## # ### ## ## # # # # # # # # ### # # ### # # # # # ",
" # # # # ## ## # # ## ## ### # # # # # ### ## ## ### ## ### ### ## # ## ### ## # # ",
" # # ### ## ## # # #### # ## # #### # #### # # # # # ### # # ### # # # ### # # # ",
" # # # #### ## # #### # # ## ## ### #### # # # # ### # ### ### # # ### # # # ### # # ",
" # # # #### ## # #### # # ## ## ### #### # # # # ### # ### ### # # ### ## ## # ### # # ",
]
for barcode in barcodes:
try:
digits = parse(barcode)
except ParityError as err:
print(f"{err} parity error!")
continue
try:
check_digit = checksum(digits)
except ChecksumError as err:
print(f"{' '.join(str(d) for d in digits)} checksum error! ({err})")
continue
print(f"{' '.join(str(d) for d in digits)}")
if __name__ == "__main__":
main()
</lang>
{{out}}
<pre>
$ python barcode.py
9 2 4 7 7 3 2 7 1 0 1 9
4 0 3 9 4 4 4 4 1 0 5 0
8 3 4 9 9 9 6 7 6 7 0 6
9 3 9 8 2 5 1 5 8 8 1 1
7 4 8 1 5 9 9 2 3 9 2 _ parity error!
3 1 6 3 1 3 7 1 8 7 1 7
2 1 4 5 7 5 8 7 5 6 0 8
8 1 8 7 7 8 8 4 1 8 1 3
7 0 6 4 6 6 7 4 3 0 3 0
6 5 3 4 8 3 5 4 0 4 3 5
6 5 3 4 8 3 5 4 0 4 2 5 checksum error! (8)
</pre>
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