pythongh-102471: Add PyLong import and export API

Co-authored-by: Sergey B Kirpichev <[email protected]>

Doc/c-api/long.rst

@@ -540,10 +540,163 @@ distinguished from a number.  Use :c:func:`PyErr_Occurred` to disambiguate.
    Exactly what values are considered compact is an implementation detail
    and is subject to change.
 
+   .. versionadded:: 3.12
+
+
 .. c:function:: Py_ssize_t PyUnstable_Long_CompactValue(const PyLongObject* op)
 
    If *op* is compact, as determined by :c:func:`PyUnstable_Long_IsCompact`,
    return its value.
 
    Otherwise, the return value is undefined.
 
+   .. versionadded:: 3.12
+
+
+Export API
+^^^^^^^^^^
+
+.. versionadded:: 3.14
+
+.. c:type:: Py_digit
+
+   A single unsigned digit in the range [``0``; ``PyLong_BASE - 1``].
+
+   It is usually used in an *array of digits*, such as the
+   :c:member:`PyLong_DigitArray.digits` array.
+
+   Its size depend on the :c:macro:`!PYLONG_BITS_IN_DIGIT` macro:
+   see the ``configure`` :option:`--enable-big-digits` option.
+
+   See :c:member:`PyLong_LAYOUT.bits_per_digit` for the number of bits per
+   digit and :c:member:`PyLong_LAYOUT.digit_size` for the size of a digit (in
+   bytes).
+
+
+.. c:struct:: PyLong_LAYOUT
+
+   Layout of an array of digits, used by Python :class:`int` object.
+
+   See also :attr:`sys.int_info` which exposes similar information to Python.
+
+   .. c:member:: uint8_t bits_per_digit
+
+      Bits per digit.
+
+   .. c:member:: uint8_t digit_size
+
+      Digit size in bytes.
+
+   .. c:member:: int8_t word_endian
+
+      Word endian:
+
+      - ``1`` for most significant word first (big endian)
+      - ``-1`` for least significant first (little endian)
+
+   .. c:member:: int8_t array_endian
+
+      Array endian:
+
+      - ``1`` for most significant byte first (big endian)
+      - ``-1`` for least significant first (little endian)
+
+
+.. c:struct:: PyLong_DigitArray
+
+   A Python :class:`int` object exported as an array of digits.
+
+   See :c:struct:`PyLong_LAYOUT` for the :c:member:`digits` layout.
+
+   .. c:member:: PyObject *obj
+
+      Strong reference to the Python :class:`int` object.
+
+   .. c:member:: int negative
+
+      1 if the number is negative, 0 otherwise.
+
+   .. c:member:: Py_ssize_t ndigits
+
+      Number of digits in :c:member:`digits` array.
+
+   .. c:member:: const Py_digit *digits
+
+      Read-only array of unsigned digits.
+
+
+.. c:function:: int PyLong_AsDigitArray(PyObject *obj, PyLong_DigitArray *array)
+
+   Export a Python :class:`int` object as an array of digits.
+
+   On success, set *\*array* and return 0.
+   On error, set an exception and return -1.
+
+   This function always succeeds if *obj* is a Python :class:`int` object or a
+   subclass.
+
+   :c:func:`PyLong_FreeDigitArray` must be called once done with using
+   *export*.
+
+
+.. c:function:: void PyLong_FreeDigitArray(PyLong_DigitArray *array)
+
+   Release the export *array* created by :c:func:`PyLong_AsDigitArray`.
+
+
+PyLongWriter API
+^^^^^^^^^^^^^^^^
+
+The :c:type:`PyLongWriter` API can be used to import an integer.
+
+.. versionadded:: 3.14
+
+.. c:struct:: PyLongWriter
+
+   A Python :class:`int` writer instance.
+
+   The instance must be destroyed by :c:func:`PyLongWriter_Finish`.
+
+
+.. c:function:: PyLongWriter* PyLongWriter_Create(int negative, Py_ssize_t ndigits, Py_digit **digits)
+
+   Create a :c:type:`PyLongWriter`.
+
+   On success, set *\*digits* and return a writer.
+   On error, set an exception and return ``NULL``.
+
+   *negative* is ``1`` if the number is negative, or ``0`` otherwise.
+
+   *ndigits* is the number of digits in the *digits* array. It must be
+   positive.
+
+   The caller must initialize the array of digits *digits* and then call
+   :c:func:`PyLongWriter_Finish` to get a Python :class:`int`. Digits must be
+   in the range [``0``; ``PyLong_BASE - 1``]. Unused digits must be set to
+   ``0``.
+
+   See :c:struct:`PyLong_LAYOUT` for the layout of an array of digits.
+
+
+.. c:function:: PyObject* PyLongWriter_Finish(PyLongWriter *writer)
+
+   Finish a :c:type:`PyLongWriter` created by :c:func:`PyLongWriter_Create`.
+
+   On success, return a Python :class:`int` object.
+   On error, set an exception and return ``NULL``.
+
+
+Example creating an integer from an array of digits::
+
+    PyObject *
+    long_import(int negative, Py_ssize_t ndigits, Py_digit *digits)
+    {
+        Py_digit *writer_digits;
+        PyLongWriter *writer = PyLongWriter_Create(negative, ndigits,
+                                                   &writer_digits);
+        if (writer == NULL) {
+            return NULL;
+        }
+        memcpy(writer_digits, digits, ndigits * sizeof(digit));
+        return PyLongWriter_Finish(writer);
+    }

Doc/conf.py

@@ -141,6 +141,8 @@
     ('c:type', 'size_t'),
     ('c:type', 'ssize_t'),
     ('c:type', 'time_t'),
+    ('c:type', 'int8_t'),
+    ('c:type', 'uint8_t'),
     ('c:type', 'uint64_t'),
     ('c:type', 'uintmax_t'),
     ('c:type', 'uintptr_t'),

Doc/using/configure.rst

@@ -129,7 +129,8 @@ General Options
 
    Define the ``PYLONG_BITS_IN_DIGIT`` to ``15`` or ``30``.
 
-   See :data:`sys.int_info.bits_per_digit <sys.int_info>`.
+   See :data:`sys.int_info.bits_per_digit <sys.int_info>` and the
+   :c:type:`Py_digit` type.
 
 .. option:: --with-suffix=SUFFIX
 

Doc/whatsnew/3.14.rst

@@ -405,6 +405,16 @@ New Features
 
   (Contributed by Victor Stinner in :gh:`119182`.)
 
+* Add a new import and export API for Python :class:`int` objects:
+
+  * :c:func:`PyLong_AsDigitArray`;
+  * :c:func:`PyLong_FreeDigitArray`;
+  * :c:func:`PyLongWriter_Create`;
+  * :c:func:`PyLongWriter_Finish`;
+  * :c:struct:`PyLong_LAYOUT`.
+
+  (Contributed by Victor Stinner in :gh:`102471`.)
+
 Porting to Python 3.14
 ----------------------
 

Include/cpython/longintrepr.h

@@ -58,8 +58,10 @@ typedef long stwodigits; /* signed variant of twodigits */
 #else
 #error "PYLONG_BITS_IN_DIGIT should be 15 or 30"
 #endif
-#define PyLong_BASE     ((digit)1 << PyLong_SHIFT)
-#define PyLong_MASK     ((digit)(PyLong_BASE - 1))
+#define PyLong_BASE     ((Py_digit)1 << PyLong_SHIFT)
+#define PyLong_MASK     ((Py_digit)(PyLong_BASE - 1))
+
+typedef digit Py_digit;
 
 /* Long integer representation.
 
@@ -139,6 +141,52 @@ _PyLong_CompactValue(const PyLongObject *op)
 #define PyUnstable_Long_CompactValue _PyLong_CompactValue
 
 
+/* --- Import/Export API -------------------------------------------------- */
+
+typedef struct PyLongLayout {
+    // Bits per digit
+    uint8_t bits_per_digit;
+
+    // Digit size in bytes
+    uint8_t digit_size;
+
+    // Word endian:
+    // * 1 for most significant word first (big endian)
+    // * -1 for least significant first (little endian)
+    int8_t word_endian;
+
+    // Array endian:
+    // * 1 for most significant byte first (big endian)
+    // * -1 for least significant first (little endian)
+    int8_t array_endian;
+} PyLongLayout;
+
+PyAPI_DATA(const PyLongLayout) PyLong_LAYOUT;
+
+typedef struct PyLong_DigitArray {
+    PyObject *obj;
+    int negative;
+    Py_ssize_t ndigits;
+    const Py_digit *digits;
+} PyLong_DigitArray;
+
+PyAPI_FUNC(int) PyLong_AsDigitArray(
+    PyObject *obj,
+    PyLong_DigitArray *array);
+PyAPI_FUNC(void) PyLong_FreeDigitArray(
+    PyLong_DigitArray *array);
+
+
+/* --- PyLongWriter API --------------------------------------------------- */
+
+typedef struct PyLongWriter PyLongWriter;
+
+PyAPI_FUNC(PyLongWriter*) PyLongWriter_Create(
+    int negative,
+    Py_ssize_t ndigits,
+    Py_digit **digits);
+PyAPI_FUNC(PyObject*) PyLongWriter_Finish(PyLongWriter *writer);
+
 #ifdef __cplusplus
 }
 #endif

Lib/test/test_capi/test_long.py

@@ -631,6 +631,72 @@ def test_long_getsign(self):
 
         # CRASHES getsign(NULL)
 
+    def test_long_layout(self):
+        # Test PyLong_LAYOUT
+        int_info = sys.int_info
+        layout = _testcapi.get_pylong_layout()
+        expected = {
+            'array_endian': -1,
+            'bits_per_digit': int_info.bits_per_digit,
+            'digit_size': int_info.sizeof_digit,
+            'word_endian': -1 if sys.byteorder == 'little' else 1,
+        }
+        self.assertEqual(layout, expected)
+
+    def test_long_export(self):
+        # Test PyLong_Export()
+        layout = _testcapi.get_pylong_layout()
+        base = 2 ** layout['bits_per_digit']
+
+        pylong_export = _testcapi.pylong_export
+        self.assertEqual(pylong_export(0), (0, [0]))
+        self.assertEqual(pylong_export(123), (0, [123]))
+        self.assertEqual(pylong_export(-123), (1, [123]))
+        self.assertEqual(pylong_export(base**2 * 3 + base * 2 + 1),
+                         (0, [1, 2, 3]))
+
+        with self.assertRaises(TypeError):
+            pylong_export(1.0)
+        with self.assertRaises(TypeError):
+            pylong_export(0+1j)
+        with self.assertRaises(TypeError):
+            pylong_export("abc")
+
+    def test_longwriter_create(self):
+        # Test PyLong_Import()
+        layout = _testcapi.get_pylong_layout()
+        base = 2 ** layout['bits_per_digit']
+
+        pylongwriter_create = _testcapi.pylongwriter_create
+        self.assertEqual(pylongwriter_create(0, []), 0)
+        self.assertEqual(pylongwriter_create(0, [0]), 0)
+        self.assertEqual(pylongwriter_create(0, [123]), 123)
+        self.assertEqual(pylongwriter_create(1, [123]), -123)
+        self.assertEqual(pylongwriter_create(1, [1, 2]),
+                         -(base * 2 + 1))
+        self.assertEqual(pylongwriter_create(0, [1, 2, 3]),
+                         base**2 * 3 + base * 2 + 1)
+        max_digit = base - 1
+        self.assertEqual(pylongwriter_create(0, [max_digit, max_digit, max_digit]),
+                         base**2 * max_digit + base * max_digit + max_digit)
+
+        # normalize
+        self.assertEqual(pylongwriter_create(0, [123, 0, 0]), 123)
+
+        # test singletons + normalize
+        for num in (-2, 0, 1, 5, 42, 100):
+            self.assertIs(pylongwriter_create(bool(num < 0), [abs(num), 0]),
+                          num)
+
+        # round trip: Python int -> export -> Python int
+        pylong_export = _testcapi.pylong_export
+        numbers = [*range(0, 10), 12345, 0xdeadbeef, 2**100, 2**100-1]
+        numbers.extend(-num for num in list(numbers))
+        for num in numbers:
+            with self.subTest(num=num):
+                export = pylong_export(num)
+                self.assertEqual(pylongwriter_create(*export), num, export)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C API/2024-07-03-17-26-53.gh-issue-102471.XpmKYk.rst

@@ -0,0 +1,9 @@
+Add a new import and export API for Python :class:`int` objects:
+
+* :c:func:`PyLong_AsDigitArray`;
+* :c:func:`PyLong_FreeDigitArray`;
+* :c:func:`PyLongWriter_Create`;
+* :c:func:`PyLongWriter_Finish`;
+* :c:struct:`PyLong_LAYOUT`.
+
+Patch by Victor Stinner.