add isbn check

bookwyrm/book_search.py

@@ -12,6 +12,9 @@ from bookwyrm.settings import MEDIA_FULL_URL
 
 # pylint: disable=arguments-differ
 def search(query, min_confidence=0, filters=None, return_first=False):
+    # check if query is isbn
+    if isbn_check(query):
+        query = query.replace('-','')
     """search your local database"""
     filters = filters or []
     if not query:
@@ -133,6 +136,32 @@ def search_title_author(query, min_confidence, *filters, return_first=False):
     return list_results
 
 
+def isbn_check(query):
+    n = query.replace('-','')
+    if len(n) == 13:
+        # Multiply every other digit by  3
+        # Add these numbers and the other digits
+        product = (sum(int(ch) for ch in n[::2]) 
+                + sum(int(ch) * 3 for ch in n[1::2]))
+        if product % 10 == 0:
+            return True
+    elif len(n) == 10:
+        if n[0:8].isdigit() and (n[9].isdigit() or n[9].lower() == "x"):
+            product = 0
+            # Iterate through code_string
+            for i in range(9):
+                # for each character, multiply by a different decreasing number: 10 - x
+                product = product + int(n[i]) * (10 - i)
+            # Handle last character
+            if n[9].lower() == "x":
+                product += 10
+            else:
+                product += int(n[9])
+            if product % 11 == 0:
+                return True
+    return False
+
+
 @dataclass
 class SearchResult:
     """standardized search result object"""