Add declarations for longintrepr.h

Cython/Includes/cpython/longintrepr.pxd

+# Internals of the "long" type (Python 2) or "int" type (Python 3).
+# This is not part of Python's published API.
+
+cdef extern from "longintrepr.h":
+    # Add explicit cast to avoid compiler warnings
+    cdef _PyLong_New "(PyObject*)_PyLong_New"(Py_ssize_t s)
+
+    ctypedef unsigned int digit
+    ctypedef int sdigit  # Python >= 2.7 only
+
+    ctypedef struct PyLongObject:
+        digit* ob_digit
+
+    cdef long PyLong_SHIFT
+    cdef digit PyLong_BASE
+    cdef digit PyLong_MASK

tests/run/longintrepr.pyx

+# mode: run
+
+# Test longintrepr declarations by implementing a simple function
+
+from cpython.longintrepr cimport *
+cimport cython
+
+cdef extern from *:
+    Py_ssize_t* Py_SIZE_PTR "&Py_SIZE"(object)
+
+
+@cython.cdivision(True)
+def lshift(long a, unsigned long n):
+    """
+    Return a * 2^n as Python long.
+
+    >>> print(lshift(3, 1))
+    6
+    >>> print(lshift(-3, 1))
+    -6
+    >>> print(lshift(1, 30))
+    1073741824
+    >>> print(lshift(-12345, 115))
+    -512791237748899576593671817473776680960
+    >>> print(-12345 << 115)
+    -512791237748899576593671817473776680960
+    >>> [i for i in range(100) if (65535 << i) != lshift(65535, i)]
+    []
+    >>> print(lshift(0, 12345))
+    0
+    >>> print(lshift(2**62, 0))   # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    ...
+    OverflowError...
+    """
+    if not a:
+        return _PyLong_New(0)
+    cdef unsigned long apos = a if a > 0 else -a
+    if (apos >> 1) >= <unsigned long>PyLong_BASE:
+        raise OverflowError
+
+    cdef unsigned long index = n // PyLong_SHIFT
+    cdef unsigned long shift = n % PyLong_SHIFT
+
+    cdef digit d = apos
+    cdef digit low = (d << shift) & PyLong_MASK
+    cdef digit high = (d >> (PyLong_SHIFT - shift))
+
+    if high == 0:
+        ret = _PyLong_New(index + 1)
+        (<PyLongObject*>ret).ob_digit[index] = low
+    else:
+        ret = _PyLong_New(index + 2)
+        (<PyLongObject*>ret).ob_digit[index] = low
+        (<PyLongObject*>ret).ob_digit[index + 1] = high
+
+    while index >= 1:
+        index -= 1
+        (<PyLongObject*>ret).ob_digit[index] = 0
+
+    if a < 0:
+        Py_SIZE_PTR(ret)[0] *= -1
+
+    return ret