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Patch by Tim Peters to improve the range checks for range() and 

xrange(), especially for platforms where int and long are different
sizes (so sys.maxint isn't actually the theoretical limit for the
length of a list, but the largest C int is -- sys.maxint is the
largest Python int, which is actually a C long).
This commit is contained in:
Guido van Rossum 1999-02-23 16:11:01 +00:00
parent 717d1fdf2a
commit 124eff0225
1 changed files with 48 additions and 30 deletions
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78
Python/bltinmodule.c
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@ -1395,13 +1395,47 @@ With two arguments, equivalent to x**y. With three arguments,\n\
equivalent to (x**y) % z, but may be more efficient (e.g. for longs).";
/* Return number of items in range/xrange (lo, hi, step). step > 0
* required. Return a value < 0 if & only if the true value is too
* large to fit in a signed long.
*/
static long
get_len_of_range(lo, hi, step)
long lo;
long hi;
long step; /* must be > 0 */
{
/* -------------------------------------------------------------
If lo >= hi, the range is empty.
Else if n values are in the range, the last one is
lo + (n-1)*step, which must be <= hi-1. Rearranging,
n <= (hi - lo - 1)/step + 1, so taking the floor of the RHS gives
the proper value. Since lo < hi in this case, hi-lo-1 >= 0, so
the RHS is non-negative and so truncation is the same as the
floor. Letting M be the largest positive long, the worst case
for the RHS numerator is hi=M, lo=-M-1, and then
hi-lo-1 = M-(-M-1)-1 = 2*M. Therefore unsigned long has enough
precision to compute the RHS exactly.
---------------------------------------------------------------*/
long n = 0;
if (lo < hi) {
unsigned long uhi = (unsigned long)hi;
unsigned long ulo = (unsigned long)lo;
unsigned long diff = uhi - ulo - 1;
n = (long)(diff / (unsigned long)step + 1);
}
return n;
}
static PyObject *
builtin_range(self, args)
PyObject *self;
PyObject *args;
{
long ilow = 0, ihigh = 0, istep = 1;
long bign;
int i, n;
PyObject *v;
if (PyTuple_Size(args) <= 1) {
@ -1420,32 +1454,14 @@ builtin_range(self, args)
PyErr_SetString(PyExc_ValueError, "zero step for range()");
return NULL;
}
/* A bit convoluted because this might overflow; due to Tim Peters */
if (istep > 0) {
if (ihigh <= ilow)
n = 0;
else {
unsigned long hi = (unsigned long)ihigh;
unsigned long lo = (unsigned long)ilow;
unsigned long diff = hi - lo - 1;
n = (long)(diff / istep + 1);
}
}
else {
/* But any errors in this branch are my own --Guido */
if (ihigh >= ilow)
n = 0;
else {
/* Swap lo and hi; use abs(istep) */
unsigned long hi = (unsigned long)ilow;
unsigned long lo = (unsigned long)ihigh;
unsigned long diff = hi - lo - 1;
n = (long)(diff / (-istep) + 1);
}
}
if (n < 0) {
if (istep > 0)
bign = get_len_of_range(ilow, ihigh, istep);
else
bign = get_len_of_range(ihigh, ilow, -istep);
n = (int)bign;
if (bign < 0 || (long)n != bign) {
PyErr_SetString(PyExc_OverflowError,
"range() has more than sys.maxint items");
"range() has too many items");
return NULL;
}
v = PyList_New(n);
@ -1497,13 +1513,15 @@ builtin_xrange(self, args)
PyErr_SetString(PyExc_ValueError, "zero step for xrange()");
return NULL;
}
/* XXX ought to check overflow of subtraction */
if (istep > 0)
n = (ihigh - ilow + istep - 1) / istep;
n = get_len_of_range(ilow, ihigh, istep);
else
n = (ihigh - ilow + istep + 1) / istep;
if (n < 0)
n = 0;
n = get_len_of_range(ihigh, ilow, -istep);
if (n < 0) {
PyErr_SetString(PyExc_OverflowError,
"xrange() has more than sys.maxint items");
return NULL;
}
return PyRange_New(ilow, n, istep, 1);
}