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- # /* Copyright (C) 2002
- # * Housemarque Oy
- # * http://www.housemarque.com
- # *
- # * Distributed under the Boost Software License, Version 1.0. (See
- # * accompanying file LICENSE_1_0.txt or copy at
- # * http://www.boost.org/LICENSE_1_0.txt)
- # */
- #
- # /* Revised by Paul Mensonides (2002) */
- #
- # /* See http://www.boost.org for most recent version. */
- #
- # /* This example shows how BOOST_PP_WHILE() can be used for implementing macros. */
- #
- # include <stdio.h>
- #
- # include <boost/preprocessor/arithmetic/add.hpp>
- # include <boost/preprocessor/arithmetic/sub.hpp>
- # include <boost/preprocessor/comparison/less_equal.hpp>
- # include <boost/preprocessor/control/while.hpp>
- # include <boost/preprocessor/list/adt.hpp>
- # include <boost/preprocessor/tuple/elem.hpp>
- #
- # /* First consider the following C implementation of Fibonacci. */
- typedef struct linear_fib_state {
- int a0, a1, n;
- } linear_fib_state;
- static int linear_fib_c(linear_fib_state p) {
- return p.n;
- }
- static linear_fib_state linear_fib_f(linear_fib_state p) {
- linear_fib_state r = { p.a1, p.a0 + p.a1, p.n - 1 };
- return r;
- }
- static int linear_fib(int n) {
- linear_fib_state p = { 0, 1, n };
- while (linear_fib_c(p)) {
- p = linear_fib_f(p);
- }
- return p.a0;
- }
- # /* Then consider the following preprocessor implementation of Fibonacci. */
- #
- # define LINEAR_FIB(n) LINEAR_FIB_D(1, n)
- # /* Since the macro is implemented using BOOST_PP_WHILE, the actual
- # * implementation takes a depth as a parameters so that it can be called
- # * inside a BOOST_PP_WHILE. The above easy-to-use version simply uses 1
- # * as the depth and cannot be called inside a BOOST_PP_WHILE.
- # */
- #
- # define LINEAR_FIB_D(d, n) \
- BOOST_PP_TUPLE_ELEM(3, 0, BOOST_PP_WHILE_ ## d(LINEAR_FIB_C, LINEAR_FIB_F, (0, 1, n)))
- # /* ^^^^ ^^^^^ ^^ ^^ ^^^^^^^
- # * #1 #2 #3 #3 #4
- # *
- # * 1) The state is a 3-element tuple. After the iteration is finished, the first
- # * element of the tuple is the result.
- # *
- # * 2) The WHILE primitive is "invoked" directly. BOOST_PP_WHILE(D, ...)
- # * can't be used because it would not be expanded by the preprocessor.
- # *
- # * 3) ???_C is the condition and ???_F is the iteration macro.
- # */
- #
- # define LINEAR_FIB_C(d, p) \
- /* p.n */ BOOST_PP_TUPLE_ELEM(3, 2, p) \
- /**/
- #
- # define LINEAR_FIB_F(d, p) \
- ( \
- /* p.a1 */ BOOST_PP_TUPLE_ELEM(3, 1, p), \
- /* p.a0 + p.a1 */ BOOST_PP_ADD_D(d, BOOST_PP_TUPLE_ELEM(3, 0, p), BOOST_PP_TUPLE_ELEM(3, 1, p)), \
- /* ^^ ^ \
- * BOOST_PP_ADD() uses BOOST_PP_WHILE(). Therefore we \
- * pass the recursion depth explicitly to BOOST_PP_ADD_D(). \
- */ \
- /* p.n - 1 */ BOOST_PP_DEC(BOOST_PP_TUPLE_ELEM(3, 2, p)) \
- ) \
- /**/
- int main() {
- printf("linear_fib(10) = %d\n", linear_fib(10));
- printf("LINEAR_FIB(10) = %d\n", LINEAR_FIB(10));
- return 0;
- }
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