Bm: Check edge conditions like empty strings for the search text

src/bm.py

@@ -58,12 +58,14 @@ class Bm(object):
         Return the index of the *first* occurrence of the pattern in the
         text, or '-1' if the pattern is not in the text.
         """
+        PATTERN_LENGTH = len(self.pattern)
+        if not self.pattern or not self.text or len(self.text) < PATTERN_LENGTH:
+            return -1
 
         # The search works on one character at a time in the text. The current
         # location is the "head" (like a read-head on a disk)
         # It starts at the last character in the pattern, and gets moved in the
         # loop until we reach the end of the text
-        PATTERN_LENGTH = len(self.pattern)
         head = PATTERN_LENGTH - 1
         while head < len(self.text):
             # When the last character of the pattern matches the current head

test/test_bm.py

@@ -14,6 +14,10 @@ from src.bm import Bm
 
     # Other useful test cases:
     ("ユニコード", "コード", 2),
+    ("", "", -1),
+    ("a", "", -1),
+    ("", "a", -1),
+    ("t", "too long", -1),
 ])
 def test_bm(text, pattern, expected):
     bm = Bm(text, pattern)