diff --git a/contrib/bdiff-torture.py b/contrib/bdiff-torture.py
new file mode 100644
--- /dev/null
+++ b/contrib/bdiff-torture.py
@@ -0,0 +1,98 @@
+# Randomized torture test generation for bdiff
+
+from __future__ import absolute_import, print_function
+import random, sys
+from mercurial import (
+    bdiff,
+    mpatch,
+)
+
+def reducetest(a, b):
+    tries = 0
+    reductions = 0
+    print("reducing...")
+    while tries < 1000:
+        a2 = "\n".join(l for l in a.splitlines()
+                       if random.randint(0, 100) > 0) + "\n"
+        b2 = "\n".join(l for l in b.splitlines()
+                       if random.randint(0, 100) > 0) + "\n"
+        if a2 == a and b2 == b:
+            continue
+        if a2 == b2:
+            continue
+        tries += 1
+
+        try:
+            test1(a, b)
+        except Exception as inst:
+            reductions += 1
+            tries = 0
+            a = a2
+            b = b2
+
+    print("reduced:", reductions, len(a) + len(b),
+          repr(a), repr(b))
+    try:
+        test1(a, b)
+    except Exception as inst:
+        print("failed:", inst)
+
+    sys.exit(0)
+
+def test1(a, b):
+    d = bdiff.bdiff(a, b)
+    if not d:
+        raise ValueError("empty")
+    c = mpatch.patches(a, [d])
+    if c != b:
+        raise ValueError("bad")
+
+def testwrap(a, b):
+    try:
+        test1(a, b)
+        return
+    except Exception as inst:
+        pass
+    print("exception:", inst)
+    reducetest(a, b)
+
+def test(a, b):
+    testwrap(a, b)
+    testwrap(b, a)
+
+def rndtest(size, noise):
+    a = []
+    src = "                aaaaaaaabbbbccd"
+    for x in xrange(size):
+        a.append(src[random.randint(0, len(src) - 1)])
+
+    while True:
+        b = [c for c in a if random.randint(0, 99) > noise]
+        b2 = []
+        for c in b:
+            b2.append(c)
+            while random.randint(0, 99) < noise:
+                b2.append(src[random.randint(0, len(src) - 1)])
+        if b2 != a:
+            break
+
+    a = "\n".join(a) + "\n"
+    b = "\n".join(b2) + "\n"
+
+    test(a, b)
+
+maxvol = 10000
+startsize = 2
+while True:
+    size = startsize
+    count = 0
+    while size < maxvol:
+        print(size)
+        volume = 0
+        while volume < maxvol:
+            rndtest(size, 2)
+            volume += size
+            count += 2
+        size *= 2
+    maxvol *= 4
+    startsize *= 4
diff --git a/mercurial/bdiff.c b/mercurial/bdiff.c
--- a/mercurial/bdiff.c
+++ b/mercurial/bdiff.c
@@ -265,6 +265,8 @@ static int diff(struct line *a, int an, 
 
 		if (curr->a2 == next->a1 || curr->b2 == next->b1)
 			while (curr->a2 < an && curr->b2 < bn
+			       && next->a1 < next->a2
+			       && next->b1 < next->b2
 			       && !cmp(a + curr->a2, b + curr->b2)) {
 				curr->a2++;
 				next->a1++;