/*
 * Copyright 2012-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// @author: Andrei Alexandrescu

#pragma once

/**
 * Necessarily evil preprocessor-related amenities.
 */

// MSVC's preprocessor is a pain, so we have to
// forcefully expand the VA args in some places.
#define FB_VA_GLUE(a, b) a b

/**
 * FB_ONE_OR_NONE(hello, world) expands to hello and
 * FB_ONE_OR_NONE(hello) expands to nothing. This macro is used to
 * insert or eliminate text based on the presence of another argument.
 */
#define FB_ONE_OR_NONE(a, ...) FB_VA_GLUE(FB_THIRD, (a, ## __VA_ARGS__, a))
#define FB_THIRD(a, b, ...) __VA_ARGS__

/**
 * Helper macro that extracts the first argument out of a list of any
 * number of arguments.
 */
#define FB_ARG_1(a, ...) a

/**
 * Helper macro that extracts the second argument out of a list of any
 * number of arguments. If only one argument is given, it returns
 * that.
 */
#ifdef _MSC_VER
// GCC refuses to expand this correctly if this macro itself was
// called with FB_VA_GLUE :(
#define FB_ARG_2_OR_1(...) \
  FB_VA_GLUE(FB_ARG_2_OR_1_IMPL, (__VA_ARGS__, __VA_ARGS__))
#else
#define FB_ARG_2_OR_1(...) FB_ARG_2_OR_1_IMPL(__VA_ARGS__, __VA_ARGS__)
#endif
// Support macro for the above
#define FB_ARG_2_OR_1_IMPL(a, b, ...) b

/**
 * Helper macro that provides a way to pass argument with commas in it to
 * some other macro whose syntax doesn't allow using extra parentheses.
 * Example:
 *
 *   #define MACRO(type, name) type name
 *   MACRO(FB_SINGLE_ARG(std::pair<size_t, size_t>), x);
 *
 */
#define FB_SINGLE_ARG(...) __VA_ARGS__

/**
 * Helper macro that just ignores its parameters.
 */
#define FOLLY_IGNORE(...)

/**
 * Helper macro that just ignores its parameters and inserts a semicolon.
 */
#define FOLLY_SEMICOLON(...) ;

/**
 * FB_ANONYMOUS_VARIABLE(str) introduces an identifier starting with
 * str and ending with a number that varies with the line.
 */
#ifndef FB_ANONYMOUS_VARIABLE
#define FB_CONCATENATE_IMPL(s1, s2) s1##s2
#define FB_CONCATENATE(s1, s2) FB_CONCATENATE_IMPL(s1, s2)
#ifdef __COUNTER__
#define FB_ANONYMOUS_VARIABLE(str) FB_CONCATENATE(str, __COUNTER__)
#else
#define FB_ANONYMOUS_VARIABLE(str) FB_CONCATENATE(str, __LINE__)
#endif
#endif

/**
 * Use FB_STRINGIZE(x) when you'd want to do what #x does inside
 * another macro expansion.
 */
#define FB_STRINGIZE(x) #x

#define FOLLY_PP_DETAIL_NARGS_1(dummy, _7, _6, _5, _4, _3, _2, _1, _0, ...) _0
#define FOLLY_PP_DETAIL_NARGS(...) \
  FOLLY_PP_DETAIL_NARGS_1(dummy, ##__VA_ARGS__, 7, 6, 5, 4, 3, 2, 1, 0)

#define FOLLY_PP_DETAIL_FOR_EACH_REC_0(fn, ...)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_1(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_0(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_2(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_1(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_3(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_2(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_4(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_3(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_5(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_4(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_6(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_5(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_REC_7(fn, a, ...) \
  fn(a) FOLLY_PP_DETAIL_FOR_EACH_REC_6(fn, __VA_ARGS__)

#define FOLLY_PP_DETAIL_FOR_EACH_2(fn, n, ...) \
  FOLLY_PP_DETAIL_FOR_EACH_REC_##n(fn, __VA_ARGS__)
#define FOLLY_PP_DETAIL_FOR_EACH_1(fn, n, ...) \
  FOLLY_PP_DETAIL_FOR_EACH_2(fn, n, __VA_ARGS__)

/**
 *  FOLLY_PP_FOR_EACH
 *
 *  Used to invoke a preprocessor macro, the name of which is passed as the
 *  first argument, once for each subsequent variadic argument.
 *
 *  At present, supports [0, 8) arguments.
 *
 *  This input:
 *
 *    #define DOIT(a) go_do_it(a);
 *    FOLLY_PP_FOR_EACH(DOIT, 3, 5, 7)
 *    #undef DOIT
 *
 *  Expands to this output (with whitespace adjusted for clarity):
 *
 *    go_do_it(3);
 *    go_do_it(5);
 *    go_do_it(7);
 */
#define FOLLY_PP_FOR_EACH(fn, ...) \
  FOLLY_PP_DETAIL_FOR_EACH_1(      \
      fn, FOLLY_PP_DETAIL_NARGS(__VA_ARGS__), __VA_ARGS__)
