python · tim-one · Feb 28, 2021 · Oct 17, 2020 · Oct 21, 2020 · Oct 21, 2020
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
@@ -5,6 +5,7 @@
 import unittest, string, sys, struct
 from test import support
 from collections import UserList
+import random
 
 class Sequence:
     def __init__(self, seq='wxyz'): self.seq = seq
@@ -317,6 +318,45 @@ def test_rindex(self):
         else:
             self.checkraises(TypeError, 'hello', 'rindex', 42)
 
+    def test_find_periodic_pattern(self):
+        """Cover the special path for periodic patterns."""
+        def reference_find(p, s):
+            m = len(p)
+            for i in range(len(s)):
+                if s[i:i+m] == p:
+                    return i
+            return -1
+
+        rr = random.randrange
+        choices = random.choices
+        for _ in range(1000):
+            p0 = ''.join(choices('abcde', k=rr(10))) * rr(10, 20)
+            p = p0[:len(p0) - rr(10)] # pop off some characters
+            left = ''.join(choices('abcdef', k=rr(200)))
+            right = ''.join(choices('abcdef', k=rr(200)))
+            text = left + p + right
+            with self.subTest(p=p, text=text):
+                self.checkequal(reference_find(p, text),
+                                text, 'find', p)
+
+    def test_find_shift_table_overflow(self):
+        """When the table of 16-bit shifts overflows."""
+        N = 2**16 + 100 # Overflow the 16-bit shift table
+
+        # first check the periodic case
+        # here, the shift for 'b' is N.
+        pattern1 = 'a' * N + 'b' + 'a' * N
+        text1 = 'babbaa' * N + pattern1
+        self.checkequal(len(text1)-len(pattern1),
+                        text1, 'find', pattern1)
+
+        # now check the non-periodic case
+        # here, the shift for 'd' is 3*(N+1)
+        pattern2 = 'ddd' + 'abc' * N + "eee"
+        text2 = pattern2[:-1] + "ddeede" * 2 * N + pattern2 + "de" * N
+        self.checkequal(len(text2) - N*len("de") - len(pattern2),
+                        text2, 'find', pattern2)
+
     def test_lower(self):
         self.checkequal('hello', 'HeLLo', 'lower')
         self.checkequal('hello', 'hello', 'lower')

diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-10-23-23-17-23.bpo-41972.kbAwg4.rst b/Misc/NEWS.d/next/Core and Builtins/2020-10-23-23-17-23.bpo-41972.kbAwg4.rst
@@ -0,0 +1 @@
+Substring search functions such as ``str1 in str2`` and ``str2.find(str1)`` now sometimes use the "Two-Way" string comparison algorithm to avoid quadratic behavior on long strings.