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Functional.hpp
/*
Copyright 2016 Statoil ASA.
This file is part of the Open Porous Media project (OPM).
OPM is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
OPM is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with OPM. If not, see <http://www.gnu.org/licenses/>.
*/
#ifndef OPM_FUNCTIONAL_HPP
#define OPM_FUNCTIONAL_HPP
#include <algorithm>
#include <iterator>
#include <vector>
#include <numeric>
#include <functional>
namespace Opm {
namespace fun {
/*
* The Utility/Functional library provides convenient high level
* functionality and higher order functions inspiried by functional
* languages (in particular Haskell) and modern C++. The goal is to provide
* lightweight features that reduce boilerplate and make code more
* declarative.
*/
/*
* map :: (a -> b) -> [a] -> [b]
*
* maps the elements [a] of the passed container C to [b], by using the
* passed function f :: a -> b. Works like map in haskell, lisp, python etc.
*
* C can be any foreach-compatible container (that supports .begin,
* .end), but will always return a vector.
*
* F can be any Callable, that is both function pointer,
* operator()-providing class or std::function, including lambdas. F is
* typically passed by reference. F must be unary of type A (which must
* match what C::const_iterator::operator* returns) and have return
* type B (by value).
*
* In short, this function deal with vector allocation, resizing and
* population based on some function f.
*
* fun::map( f, vec ) is equivalent to:
* vector dst;
* for( auto& x : vec ) dst.push_back( f( x ) );
* return dst;
*
* The behaviour is undefined if F has any side effects.
*
* --
*
* int plus1( int x ) { return x + 1; }
* base_vec = { 0, 1, 2, 3, 4 };
* vec = fun::map( &plus1, base_vec );
*
* vec => { 1, 2, 3, 4, 5 }
*
* --
*
* int mul2 = []( int x ) { return x * 2; };
* base_vec = { 0, 1, 2, 3, 4 };
* vec = fun::map( mul2, base_vec );
*
* vec => { 0, 2, 4, 6, 8 };
*
*/
template <typename F, typename C>
auto map(F&& f, C&& c)
{
using Val = std::remove_cv_t<std::remove_reference_t<
decltype(std::invoke(std::forward<F>(f), *std::begin(std::forward<C>(c))))>>;
std::vector<Val> ret{};
ret.reserve(std::size(std::forward<C>(c)));
std::transform(std::begin(c), std::end(c), std::back_inserter(ret), std::forward<F>(f));
return ret;
}
/*
* concat :: [[a]] -> [a]
*
* A primitive concat taking a vector of vectors, flattened into a
* single 1 dimensional vector. Moves all the elements so no unecessary
* copies are done.
*
* vec = { { 1 }, { 2, 2 }, { 3, 3, 3 } }
* cvec = concat( vec ) => { 1, 2, 2, 3, 3, 3 }
*/
template< typename A >
std::vector< A > concat( std::vector< std::vector< A > >&& src ) {
const auto size = std::accumulate( src.begin(), src.end(), 0,
[]( std::size_t acc, const std::vector< A >& x ) {
return acc + x.size();
}
);
std::vector< A > dst;
dst.reserve( size );
for( auto& x : src )
std::move( x.begin(), x.end(), std::back_inserter( dst ) );
return dst;
}
/*
* iota :: int -> [int]
* iota :: (int,int) -> [int]
*
* iota (ι) is borrowed from the APL programming language. This particular
* implementation behaves as a generator-like constant-space consecutive
* sequence of integers [m,n). Written to feel similar to std::iota, but as
* a producer instead of straight-up writer. This is similar to python2.7s
* xrange(), python3s range() and haskell's [0..(n-1)]. Some examples
* follow.
*
* Notes:
* * iota defaults to [0,n)
* * iota uses 0 indexing to feel more familiar to C++'s zero indexing.
* * iota can start at negative indices, but will always count upwards.
* * iota::const_iterator does not support operator-- (which would allow
* support for reverse iterators). This can be implemented if need arises.
* * iota is meant to play nice with the rest of fun and to be able to
* replace mundane for loops when the loops only purpose is to create the
* sequence of elements. iota can feel more declarative and work better
* with functions.
* * iota adds value semantics to things that in C++ normally relies on
* variable mutations. iota is meant to make it less painful to write
* immutable and declarative code.
* * as with all iterators, iota( n, m ) behaviour is undefined if m < n
* * unlike python's range, iota doesn't support steps (only increments).
* this is by design to keep this simple and minimal, as well as the name
* iota being somewhat unsuitable for stepping ranges. If the need for
* this arises it will be a separate function.
*
* fun::iota( 5 ) => [ 0, 1, 2, 3, 4 ]
* fun::iota( 3 ) => [ 0, 1, 2 ]
* fun::iota( 1, 6 ) => [ 1, 2, 3, 4, 5 ]
*
* --
*
* std::vector< int > vec ( 5, 0 );
* std::iota( vec.begin(), vec.end(), 0 );
* vec => [ 0, 1, 2, 3, 4 ]
*
* fun::iota i( 5 );
* std::vector vec( i.begin(), i.end() );
* vec => [ 0, 1, 2, 3, 4 ]
*
* --
*
* int plus( int x ) { return x + 1; }
* auto vec = fun::map( &plus, fun::iota( 5 ) );
* vec => [ 1, 2, 3, 4, 5 ]
*
* is equivalent to
*
* int plus( int x ) { return x + 1; }
* std::vector< int > vec;
* for( int i = 0; i < 5; ++i )
* vec.push_back( plus( i ) );
* vec => [ 1, 2, 3, 4, 5 ]
*
* --
*
* While not the primary intended use case, this enables foreach loop
* syntax over intervals:
*
* for( auto i : fun::iota( 5 ) )
* std::cout << i << " ";
*
* => 0 1 2 3 4
*
* for( auto i : fun::iota( 1, 6 ) )
* std::cout << i << " ";
*
* => 1 2 3 4 5
*
*/
class iota {
public:
explicit iota( int end );
iota( int begin, int end );
class const_iterator {
public:
using difference_type = int;
using value_type = int;
using pointer = int*;
using reference = int&;
using iterator_category = std::forward_iterator_tag;
const_iterator() = default;
int operator*() const;
const_iterator& operator++();
const_iterator operator++( int );
bool operator==( const const_iterator& rhs ) const;
bool operator!=( const const_iterator& rhs ) const;
private:
explicit const_iterator( int );
int value{};
friend class iota;
};
size_t size() const;
const_iterator begin() const;
const_iterator end() const;
private:
int first;
int last;
};
}
}
#endif //OPM_FUNCTIONAL_HPP
Opm::fun::iota::const_iterator
Definition Functional.hpp:202
Opm
This class implements a small container which holds the transmissibility mulitpliers for all the face...
Definition Exceptions.hpp:30