////
Copyright 2017 Peter Dimov

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
////

[#algorithm]
# Algorithms, <boost/mp11/algorithm.hpp>
:toc:
:toc-title:
:idprefix:

## mp_assign<L1, L2>

    template<class L1, class L2> using mp_assign = /*...*/;

`mp_assign<L1<T1...>, L2<T2...>>` is an alias for `L1<T2...>`. That is, it replaces the elements of `L1` with those of `L2`.

.Using mp_assign with mp_list and std::tuple
```
using L1 = std::tuple<long>;
using L2 = mp_list<int, float>;

using R1 = mp_assign<L1, L2>; // std::tuple<int, float>
```

.Using mp_assign with mp_list and std::pair
```
using L1 = std::pair<long, char>;
using L2 = mp_list<int, float>;

using R1 = mp_assign<L1, L2>; // std::pair<int, float>
```

## mp_clear<L>

    template<class L> using mp_clear = mp_assign<L, mp_list<>>;

`mp_clear<L<T...>>` is an alias for `L<>`, that is, it removes the elements of `L`.

.Using mp_clear with std::tuple
```
using L1 = std::tuple<int, float>;
using R1 = mp_clear<L1>; // std::tuple<>
```

## mp_transform<F, L...>

    template<template<class...> class F, class... L> using mp_transform = /*...*/;

`mp_transform<F, L1<T1...>, L2<T2...>, ..., Ln<Tn...>>` applies `F` to each successive tuple of elements and returns `L1<F<T1, T2, ..., Tn>...>`.

.Using mp_transform to produce a list of pointers from a list of pointees
```
template<class T> using add_pointer_t =
    typename std::add_pointer<T>::type;  // std::add_pointer_t in C++14

using L1 = std::tuple<void, int, float>;
using R1 = mp_transform<add_pointer_t, L1>; // std::tuple<void*, int*, float*>
```

.Using mp_transform to compare the contents of two lists of types
```
using L1 = std::tuple<void, int, float>;
using L2 = mp_list<void, int, float>;

using R1 = mp_all<mp_transform<std::is_same, L1, L2>>; // mp_true
```

.Using mp_transform to compare the contents of two lists of integral constants
```
template<class T1, class T2> using eq = mp_bool<T1::value == T2::value>;

using L1 = std::tuple<mp_int<1>, mp_int<2>, mp_int<3>>;
using L2 = mp_list<mp_size_t<1>, mp_size_t<2>, mp_size_t<3>>;

using R1 = mp_all<mp_transform<eq, L1, L2>>; // mp_true
```

.mp_transform on one list
[cols="<.^4m,4*^.^1m",options="header",width=85%]
|===
^|List
4+|Contents
5+|
|*L1*|A0|A1|...|An
5+|
|*mp_transform<F, L1>*|F<A0>|F<A1>|...|F<An>
|===

.mp_transform on two lists
[cols="<.^4m,4*^.^1m",options="header",width=85%]
|===
^|List
4+|Contents
5+|
|*L1*|A0|A1|...|An
5+|
|*L2*|B0|B1|...|Bn
5+|
|*mp_transform<F, L1, L2>*|F<A0,B0>|F<A1,B1>|...|F<An,Bn>
|===

## mp_transform_q<Q, L...>

    template<class Q, class... L> using mp_transform_q =
        mp_transform<Q::template fn, L...>;

As `mp_transform`, but takes a quoted metafunction.

.Using mp_transform_q to count the occurences of `void` in a list
```
using L1 = std::tuple<void, int, float, void, int>;

using R1 = mp_apply<mp_plus,
    mp_transform_q<mp_bind_front<std::is_same, void>, L1>>; // mp_int\<2>
```

## mp_transform_if<P, F, L...>

    template<template<class...> class P, template<class...> class F, class L...>
        using mp_transform_if = /*...*/;

`mp_transform_if<P, F, L1, L2, ..., Ln>` replaces the elements of the list `L1` for which `mp_to_bool<P<T1, T2, ..., Tn>>` is `mp_true` with
`F<T1, T2, ..., Tn>`, and returns the result, where `Ti` are the corresponding elements of `Li`.

.Using mp_transform_if to replace the occurences of 'void' in a list with the corresponding elements of a second list
```
using L1 = std::tuple<void, int, float, void, int>;
using L2 = std::tuple<char[1], char[2], char[3], char[4], char[5]>;

template<class T1, class T2> using first_is_void = std::is_same<T1, void>;
template<class T1, class T2> using second = T2;

using R1 = mp_transform_if<first_is_void, second, L1, L2>;
  // std::tuple<char[1], int, float, char[4], int>
```

## mp_transform_if_q<Qp, Qf, L...>

    template<class Qp, class Qf, class... L> using mp_transform_if_q =
        mp_transform_if<Qp::template fn, Qf::template fn, L...>;

As `mp_transform_if`, but takes quoted metafunctions.

.Using mp_transform_if_q to replace the occurences of 'void' in a list with the corresponding elements of a second list
```
using L1 = std::tuple<void, int, float, void, int>;
using L2 = std::tuple<char[1], char[2], char[3], char[4], char[5]>;

using R1 = mp_transform_if_q<mp_bind<std::is_same, _1, void>, _2, L1, L2>;
  // std::tuple<char[1], int, float, char[4], int>
```

## mp_fill<L, V>

    template<class L, class V> using mp_fill = /*...*/;

`mp_fill<L<T...>, V>` returns `L<V, V, ..., V>`, with the result having the same size as the input.

.Using mp_fill with std::tuple
```
using L1 = std::tuple<void, int, float>;
using R1 = mp_fill<L1, double>; // std::tuple<double, double, double>
```

.Using mp_fill with std::pair
```
using L1 = std::pair<int, float>;
using R1 = mp_fill<L1, void>; // std::pair<void, void>
```

## mp_count<L, V>

    template<class L, class V> using mp_count = /*...*/;

`mp_count<L, V>` returns `mp_size_t<N>`, where `N` is the number of elements of `L` same as `V`.

## mp_count_if<L, P>

    template<class L, template<class...> class P> using mp_count_if = /*...*/;

`mp_count_if<L, P>` returns `mp_size_t<N>`, where `N` is the number of elements `T` of `L` for which `mp_to_bool<P<T>>` is `mp_true`.

## mp_contains<L, V>

    template<class L, class V> using mp_contains = mp_to_bool<mp_count<L, V>>;

`mp_contains<L, V>` is `mp_true` when `L` contains an element `V`, `mp_false` otherwise.

## mp_repeat_c<L, N>

    template<class L, std::size_t N> using mp_repeat_c = /*...*/;

`mp_repeat_c<L, N>` returns a list of the same type as `L` that consists of `N` concatenated copies of `L`.

## mp_repeat<L, N>

    template<class L, class N> using mp_repeat = /*...*/;

Same as `mp_repeat_c` but with a type argument `N`. The number of copies is `N::value` and must be nonnegative.

## mp_product<F, L...>

    template<template<class...> class F, class... L> using mp_product = /*...*/;

`mp_product<F, L1<T1...>, L2<T2...>, ..., Ln<Tn...>>` evaluates `F<U1, U2, ..., Un>` for values `Ui` taken from
the Cartesian product of the lists, as if the elements `Ui` are formed by `n` nested loops, each traversing `Li`.
It returns a list of type `L1` containing the results of the application of `F`.

.mp_product on two lists
[cols="<.^4m,4*^.^1m",options="header",width=85%]
|===
^|List
4+|Contents
5+|
|*L1*|A0|A1|...|An
5+|
|*L2*|B0|B1|...|Bm
5+|
|*mp_product<F, L1, L2>*
|F<A0,B0>
F<A0,B1>
...
F<A0,Bm>
|F<A1,B0>
F<A1,B1>
...
F<A1,Bm>
|...
|F<An,B0>
F<An,B1>
...
F<An,Bm>
|===

## mp_product_q<Q, L...>

    template<class Q, class... L> using mp_product_q = mp_product<Q::template fn, L...>;

As `mp_product`, but takes a quoted metafunction.

## mp_drop_c<L, N>

    template<class L, std::size_t N> using mp_drop_c = /*...*/;

`mp_drop_c<L, N>` removes the first `N` elements of `L` and returns the result.

## mp_drop<L, N>

    template<class L, class N> using mp_drop = /*...*/;

Same as `mp_drop_c`, but with a type argument `N`. `N::value` must be a nonnegative number.

## mp_iota_c<N>

    template<std::size_t N> using mp_iota_c = /*...*/;

`mp_iota_c<N>` is an alias for `mp_list<mp_size_t<0>, mp_size_t<1>, ..., mp_size_t<N-1>>`.

## mp_iota<N>

    template<class N> using mp_iota = /*...*/;

Same as `mp_iota_c`, but with a type argument `N`. `N::value` must be a nonnegative number. Returns
`mp_list<std::integral_constant<T, 0>, std::integral_constant<T, 1>, ..., std::integral_constant<T, N::value-1>>`
where `T` is the type of `N::value`.

## mp_at_c<L, I>

    template<class L, std::size_t I> using mp_at_c = /*...*/;

`mp_at_c<L, I>` returns the `I`-th element of `L`, zero-based.

## mp_at<L, I>

    template<class L, class I> using mp_at = /*...*/;

Same as `mp_at_c`, but with a type argument `I`. `I::value` must be a nonnegative number.

## mp_take_c<L, N>

    template<class L, std::size_t N> using mp_take_c = /*...*/;

`mp_take_c<L, N>` returns a list of the same type as `L` containing the first `N` elements of `L`.

## mp_take<L, N>

    template<class L, class N> using mp_take = /*...*/;

Same as `mp_take_c`, but with a type argument `N`. `N::value` must be a nonnegative number.

## mp_insert_c<L, I, T...>

    template<class L, std::size_t I, class... T> using mp_insert_c =
        mp_append<mp_take_c<L, I>, mp_push_front<mp_drop_c<L, I>, T...>>;

Inserts the elements `T...` into the list `L` at position `I` (a zero-based index).

## mp_insert<L, I, T...>

    template<class L, class I, class... T> using mp_insert =
        mp_append<mp_take<L, I>, mp_push_front<mp_drop<L, I>, T...>>;

Same as `mp_insert_c`, but with a type argument `I`.

## mp_erase_c<L, I, J>

    template<class L, std::size_t I, std::size_t J> using mp_erase_c =
        mp_append<mp_take_c<L, I>, mp_drop_c<L, J>>;

Removes from the list `L` the elements with indices from `I` (inclusive) to `J` (exclusive).

## mp_erase<L, I, J>

    template<class L, class I, class J> using mp_erase =
        mp_append<mp_take<L, I>, mp_drop<L, J>>;

Same as `mp_erase_c`, but with a type arguments `I` and `J`.

## mp_replace<L, V, W>

    template<class L, class V, class W> using mp_replace = /*...*/;

Replaces all `V` elements of `L` with `W` and returns the result.

## mp_replace_if<L, P, W>

    template<class L, template<class...> class P, class W> using mp_replace_if = /*...*/;

Replaces all `T` elements of `L` for which `mp_to_bool<P<T>>` is `mp_true` with `W` and returns the result.

## mp_replace_at_c<L, I, W>

    template<class L, std::size_t I, class W> using mp_replace_at_c = /*...*/;

Replaces the element of `L` at zero-based index `I` with `W` and returns the result.

## mp_replace_at<L, I, W>

    template<class L, class I, class W> using mp_replace_at = /*...*/;

Same as `mp_replace_at_c`, but with a type argument `I`. `I::value` must be a nonnegative number.

## mp_copy_if<L, P>

    template<class L, template<class...> class P> using mp_copy_if = /*...*/;

Copies the elements `T` of `L` for which `mp_to_bool<P<T>>` is `mp_true` to a new list of the same type and returns it.

## mp_remove<L, V>

    template<class L, class V> using mp_remove = /*...*/;

Removes all `V` elements of `L` and returns the result.

## mp_remove_if<L, P>

    template<class L, template<class...> class P> using mp_remove_if = /*...*/;

Removes all elements `T` of `L` for which `mp_to_bool<P<T>>` is `mp_true` and returns the result.

## mp_partition<L, P>

    template<class L, template<class...> class P> using mp_partition = /*...*/;

`mp_partition<L<T...>, P>` partitions `L` into two lists `L<U1...>` and `L<U2...>` such that `mp_to_bool<P<T>>` is `mp_true`
for the elements of `L<U1...>` and `mp_false` for the elements of `L<U2...>`. Returns `L<L<U1...>, L<U2...>>`.

## mp_sort<L, P>

    template<class L, template<class...> class P> using mp_sort = /*...*/;

`mp_sort<L, P>` sorts the list `L` according to the strict weak ordering `mp_to_bool<P<T, U>>`.

.Using mp_sort to sort a list of std::ratio values
----
#include <ratio>

using L1 = mp_list<std::ratio<1,2>, std::ratio<1,4>>;
using R1 = mp_sort<L1, std::ratio_less>; // mp_list<ratio<1,4>, ratio<1,2>>
----

## mp_find<L, V>

    template<class L, class V> using mp_find = /*...*/;

`mp_find<L, V>` returns the index at which the type `V` is located in the list `L`. It's an alias for `mp_size_t<I>`,
where `I` is the zero-based index of the first occurence of `V` in `L`. If `L` does not contain `V`, `mp_find<L, V>`
is `mp_size<L>`.

## mp_find_if<L, P>

    template<class L, template<class...> class P> using mp_find_if = /*...*/;

`mp_find_f<L, P>` is an alias for `mp_size_t<I>`, where `I` is the zero-based index of the first element `T` in `L` for which
`mp_to_bool<P<T>>` is `mp_true`. If there is no such element, `mp_find_if<L, P>` is `mp_size<L>`.

## mp_reverse<L>

    template<class L> using mp_reverse = /*...*/;

`mp_reverse<L<T1, T2, ..., Tn>>` is `L<Tn, ..., T2, T1>`.

## mp_fold<L, V, F>

    template<class L, class V, template<class...> class F> using mp_fold = /*...*/;

`mp_fold<L<T1, T2, ..., Tn>, V, F>` is `F< F< F< F<V, T1>, T2>, ...>, Tn>`, or `V`, if `L` is empty.

## mp_reverse_fold<L, V, F>

    template<class L, class V, template<class...> class F> using mp_reverse_fold =
        /*...*/;

`mp_reverse_fold<L<T1, T2, ..., Tn>, V, F>` is `F<T1, F<T2, F<..., F<Tn, V>>>>`, or `V`, if `L` is empty.

## mp_unique<L>

    template<class L> using mp_unique = /*...*/;

`mp_unique<L>` returns a list of the same type as `L` with the duplicate elements removed.

## mp_all_of<L, P>

    template<class L, template<class...> class P> using mp_all_of =
        mp_bool< mp_count_if<L, P>::value == mp_size<L>::value >;

`mp_all_of<L, P>` is `mp_true` when `P` holds for all elements of `L`, `mp_false` otherwise. When `L` is empty, the result is `mp_true`.

## mp_none_of<L, P>

    template<class L, template<class...> class P> using mp_none_of =
        mp_bool< mp_count_if<L, P>::value == 0 >;

`mp_none_of<L, P>` is `mp_true` when `P` holds for no element of `L`, `mp_false` otherwise. When `L` is empty, the result is `mp_true`.

## mp_any_of<L, P>

    template<class L, template<class...> class P> using mp_any_of =
        mp_bool< mp_count_if<L, P>::value != 0 >;

`mp_any_of<L, P>` is `mp_true` when `P` holds for at least one element of `L`, `mp_false` otherwise. When `L` is empty, the result is `mp_false`.

## mp_for_each<L>(f)

    template<class L, class F> constexpr F mp_for_each(F&& f);

`mp_for_each<L>(f)` calls `f` with `T()` for each element `T` of the list `L`, in order.

Returns `std::forward<F>(f)`.

.Using mp_for_each and a C++14 lambda to print a tuple
```
template<class... T> void print( std::tuple<T...> const & tp )
{
    std::size_t const N = sizeof...(T);

    mp_for_each<mp_iota_c<N>>( []( auto I ){

        // I is mp_size_t<0>, mp_size_t<1>, ..., mp_size_t<N-1>

        std::cout << std::get<I>(tp) << std::endl;

    });
}
```

## mp_with_index<N>(i, f)

    template<std::size_t N, class F>
      constexpr auto mp_with_index( std::size_t i, F && f )
        -> decltype(std::declval<F>()(std::declval<mp_size_t<0>>()));

`mp_with_index<N>(i, f)` calls `f` with `mp_size_t<i>()` and returns the result. `i` must be less than `N`.
Only `constexpr` on C++14 and higher.

    template<class N, class F>
      constexpr auto mp_with_index( std::size_t i, F && f )
        -> decltype(std::declval<F>()(std::declval<mp_size_t<0>>()));

Returns `mp_with_index<N::value>(i, f)`.

.Using mp_with_index and a C++14 lambda to print the active element of a variant
```
template<class... T> void print( std::variant<T...> const& v )
{
    mp_with_index<sizeof...(T)>( v.index(), [&]( auto I ) {

        // I is mp_size_t<v.index()> here

        std::cout << std::get<I>( v ) << std::endl;

    });
}
```