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safer isbn normalization

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This commit is contained in:
Mouse Reeve 2020-10-30 12:57:31 -07:00
parent 3fc1f46897
commit 3ca50a7573
1 changed files with 12 additions and 4 deletions
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12
bookwyrm/models/book.py
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@ -1,4 +1,6 @@
''' database schema for books and shelves '''
import re
from django.db import models
from django.utils import timezone
from django.utils.http import http_date
@ -186,15 +188,18 @@ class Edition(Book):
def isbn_10_to_13(isbn_10):
''' convert an isbn 10 into an isbn 13 '''
isbn_10 = isbn_10.replace('-', '')
isbn_10 = re.sub(r'[^0-9X]', '', isbn_10)
# drop the last character of the isbn 10 number (the original checkdigit)
converted = isbn_10[:9]
# add "978" to the front
converted = '978' + converted
# add a check digit to the end
# multiply the odd digits by 1 and the even digits by 3 and sum them
try:
checksum = sum(int(i) for i in converted[::2]) + \
sum(int(i) * 3 for i in converted[1::2])
except ValueError:
return None
# add the checksum mod 10 to the end
checkdigit = checksum % 10
if checkdigit != 0:
@ -207,13 +212,16 @@ def isbn_13_to_10(isbn_13):
if isbn_13[:3] != '978':
return None
isbn_13 = isbn_13.replace('-', '')
isbn_13 = re.sub(r'[^0-9X]', '', isbn_13)
# remove '978' and old checkdigit
converted = isbn_13[3:-1]
# calculate checkdigit
# multiple each digit by 10,9,8.. successively and sum them
try:
checksum = sum(int(d) * (10 - idx) for (idx, d) in enumerate(converted))
except ValueError:
return None
checkdigit = checksum % 11
checkdigit = 11 - checkdigit
if checkdigit == 10: