bpo-41324 Add a minimal decimal capsule API (#21519)
diff --git a/Doc/c-api/decimal.rst b/Doc/c-api/decimal.rst
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+.. sectionauthor:: Stefan Krah
+
+.. highlight:: c
+
+
+Decimal capsule API
+===================
+
+Capsule API functions can be used in the same manner as regular library
+functions, provided that the API has been initialized.
+
+
+Initialize
+----------
+
+Typically, a C extension module that uses the decimal API will do these
+steps in its init function:
+
+.. code-block::
+
+ #include "pydecimal.h"
+
+ static int decimal_initialized = 0;
+ if (!decimal_initialized) {
+ if (import_decimal() < 0) {
+ return NULL;
+ }
+
+ decimal_initialized = 1;
+ }
+
+
+Type checking, predicates, accessors
+------------------------------------
+
+.. c:function:: int PyDec_TypeCheck(const PyObject *dec)
+
+ Return 1 if ``dec`` is a Decimal, 0 otherwise. This function does not set
+ any exceptions.
+
+
+.. c:function:: int PyDec_IsSpecial(const PyObject *dec)
+
+ Return 1 if ``dec`` is ``NaN``, ``sNaN`` or ``Infinity``, 0 otherwise.
+
+ Set TypeError and return -1 if ``dec`` is not a Decimal. It is guaranteed that
+ this is the only failure mode, so if ``dec`` has already been type-checked, no
+ errors can occur and the function can be treated as a simple predicate.
+
+
+.. c:function:: int PyDec_IsNaN(const PyObject *dec)
+
+ Return 1 if ``dec`` is ``NaN`` or ``sNaN``, 0 otherwise.
+
+ Set TypeError and return -1 if ``dec`` is not a Decimal. It is guaranteed that
+ this is the only failure mode, so if ``dec`` has already been type-checked, no
+ errors can occur and the function can be treated as a simple predicate.
+
+
+.. c:function:: int PyDec_IsInfinite(const PyObject *dec)
+
+ Return 1 if ``dec`` is ``Infinity``, 0 otherwise.
+
+ Set TypeError and return -1 if ``dec`` is not a Decimal. It is guaranteed that
+ this is the only failure mode, so if ``dec`` has already been type-checked, no
+ errors can occur and the function can be treated as a simple predicate.
+
+
+.. c:function:: int64_t PyDec_GetDigits(const PyObject *dec)
+
+ Return the number of digits in the coefficient. For ``Infinity``, the
+ number of digits is always zero. Typically, the same applies to ``NaN``
+ and ``sNaN``, but both of these can have a payload that is equivalent to
+ a coefficient. Therefore, ``NaNs`` can have a nonzero return value.
+
+ Set TypeError and return -1 if ``dec`` is not a Decimal. It is guaranteed that
+ this is the only failure mode, so if ``dec`` has already been type-checked, no
+ errors can occur and the function can be treated as a simple accessor.
+
+
+Exact conversions between decimals and primitive C types
+--------------------------------------------------------
+
+This API supports conversions for decimals with a coefficient up to 38 digits.
+
+Data structures
+~~~~~~~~~~~~~~~
+
+The conversion functions use the following status codes and data structures:
+
+.. code-block::
+
+ /* status cases for getting a triple */
+ enum mpd_triple_class {
+ MPD_TRIPLE_NORMAL,
+ MPD_TRIPLE_INF,
+ MPD_TRIPLE_QNAN,
+ MPD_TRIPLE_SNAN,
+ MPD_TRIPLE_ERROR,
+ };
+
+ typedef struct {
+ enum mpd_triple_class tag;
+ uint8_t sign;
+ uint64_t hi;
+ uint64_t lo;
+ int64_t exp;
+ } mpd_uint128_triple_t;
+
+The status cases are explained below. ``sign`` is 0 for positive and 1 for negative.
+``((uint128_t)hi << 64) + lo`` is the coefficient, ``exp`` is the exponent.
+
+The data structure is called "triple" because the decimal triple (sign, coeff, exp)
+is an established term and (``hi``, ``lo``) represents a single ``uint128_t`` coefficient.
+
+
+Functions
+~~~~~~~~~
+
+.. c:function:: mpd_uint128_triple_t PyDec_AsUint128Triple(const PyObject *dec)
+
+ Convert a decimal to a triple. As above, it is guaranteed that the only
+ Python failure mode is a TypeError, checks can be omitted if the type is
+ known.
+
+ For simplicity, the usage of the function and all special cases are
+ explained in code form and comments:
+
+.. code-block::
+
+ triple = PyDec_AsUint128Triple(dec);
+ switch (triple.tag) {
+ case MPD_TRIPLE_QNAN:
+ /*
+ * Success: handle a quiet NaN.
+ * 1) triple.sign is 0 or 1.
+ * 2) triple.exp is always 0.
+ * 3) If triple.hi or triple.lo are nonzero, the NaN has a payload.
+ */
+ break;
+
+ case MPD_TRIPLE_SNAN:
+ /*
+ * Success: handle a signaling NaN.
+ * 1) triple.sign is 0 or 1.
+ * 2) triple.exp is always 0.
+ * 3) If triple.hi or triple.lo are nonzero, the sNaN has a payload.
+ */
+ break;
+
+ case MPD_TRIPLE_INF:
+ /*
+ * Success: handle Infinity.
+ * 1) triple.sign is 0 or 1.
+ * 2) triple.exp is always 0.
+ * 3) triple.hi and triple.lo are always zero.
+ */
+ break;
+
+ case MPD_TRIPLE_NORMAL:
+ /* Success: handle a finite value. */
+ break;
+
+ case MPD_TRIPLE_ERROR:
+ /* TypeError check: can be omitted if the type of dec is known. */
+ if (PyErr_Occurred()) {
+ return NULL;
+ }
+
+ /* Too large for conversion. PyDec_AsUint128Triple() does not set an
+ exception so applications can choose themselves. Typically this
+ would be a ValueError. */
+ PyErr_SetString(PyExc_ValueError,
+ "value out of bounds for a uint128 triple");
+ return NULL;
+ }
+
+.. c:function:: PyObject *PyDec_FromUint128Triple(const mpd_uint128_triple_t *triple)
+
+ Create a decimal from a triple. The following rules must be observed for
+ initializing the triple:
+
+ 1) ``triple.sign`` must always be 0 (for positive) or 1 (for negative).
+
+ 2) ``MPD_TRIPLE_QNAN``: ``triple.exp`` must be 0. If ``triple.hi`` or ``triple.lo``
+ are nonzero, create a ``NaN`` with a payload.
+
+ 3) ``MPD_TRIPLE_SNAN``: ``triple.exp`` must be 0. If ``triple.hi`` or ``triple.lo``
+ are nonzero, create an ``sNaN`` with a payload.
+
+ 4) ``MPD_TRIPLE_INF``: ``triple.exp``, ``triple.hi`` and ``triple.lo`` must be zero.
+
+ 5) ``MPD_TRIPLE_NORMAL``: ``MPD_MIN_ETINY + 38 < triple.exp < MPD_MAX_EMAX - 38``.
+ ``triple.hi`` and ``triple.lo`` can be chosen freely.
+
+ 6) ``MPD_TRIPLE_ERROR``: It is always an error to set this tag.
+
+
+ If one of the above conditions is not met, the function returns ``NaN`` if
+ the ``InvalidOperation`` trap is not set in the thread local context. Otherwise,
+ it sets the ``InvalidOperation`` exception and returns NULL.
+
+ Additionally, though extremely unlikely give the small allocation sizes,
+ the function can set ``MemoryError`` and return ``NULL``.
+
+
+Advanced API
+------------
+
+This API enables the use of ``libmpdec`` functions. Since Python is compiled with
+hidden symbols, the API requires an external libmpdec and the ``mpdecimal.h``
+header.
+
+
+Functions
+~~~~~~~~~
+
+.. c:function:: PyObject *PyDec_Alloc(void)
+
+ Return a new decimal that can be used in the ``result`` position of ``libmpdec``
+ functions.
+
+.. c:function:: mpd_t *PyDec_Get(PyObject *v)
+
+ Get a pointer to the internal ``mpd_t`` of the decimal. Decimals are immutable,
+ so this function must only be used on a new Decimal that has been created by
+ PyDec_Alloc().
+
+.. c:function:: const mpd_t *PyDec_GetConst(const PyObject *v)
+
+ Get a pointer to the constant internal ``mpd_t`` of the decimal.