Issue #1772851. Alters long.__hash__ from being *almost* completely
predictable to being completely predictable. The value of hash(n)
is unchanged for any n that's small enough to be representable as an
int, and also unchanged for the vast majority of long integers n of
reasonable size.
diff --git a/Objects/longobject.c b/Objects/longobject.c
index abb69b0..f14f298 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -1937,10 +1937,18 @@
i = -(i);
}
#define LONG_BIT_SHIFT (8*sizeof(long) - SHIFT)
+ /* The following loop produces a C long x such that (unsigned long)x
+ is congruent to the absolute value of v modulo ULONG_MAX. The
+ resulting x is nonzero if and only if v is. */
while (--i >= 0) {
/* Force a native long #-bits (32 or 64) circular shift */
x = ((x << SHIFT) & ~MASK) | ((x >> LONG_BIT_SHIFT) & MASK);
x += v->ob_digit[i];
+ /* If the addition above overflowed (thinking of x as
+ unsigned), we compensate by incrementing. This preserves
+ the value modulo ULONG_MAX. */
+ if ((unsigned long)x < v->ob_digit[i])
+ x++;
}
#undef LONG_BIT_SHIFT
x = x * sign;