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// Copyright (C) 2016-2018 T. Zachary Laine
//
// 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)
//[ lazy_vector
// Defining this allows the assignment below of an expression to a double
// without writing any specific code to do so.
#include <boost/yap/expression.hpp>
#include <algorithm>
#include <cassert>
#include <iostream>
#include <vector>
template<boost::yap::expr_kind Kind, typename Tuple>
struct lazy_vector_expr;
// This transform turns a terminal of std::vector<double> into a terminal
// containing the nth double in that vector. Think of it as turning our
// expression of vectors into an expression of scalars.
struct take_nth
{
boost::yap::terminal<lazy_vector_expr, double> operator()(
boost::yap::terminal<lazy_vector_expr, std::vector<double>> const &
expr);
std::size_t n;
};
// A custom expression template that defines lazy + and - operators that
// produce expressions, and an eager [] operator that returns the nth element
// of the expression.
//[ lazy_vector_decl
template<boost::yap::expr_kind Kind, typename Tuple>
struct lazy_vector_expr
{
static const boost::yap::expr_kind kind = Kind;
Tuple elements;
// Note that this does not return an expression; it is greedily evaluated.
auto operator[](std::size_t n) const;
};
BOOST_YAP_USER_BINARY_OPERATOR(plus, lazy_vector_expr, lazy_vector_expr)
BOOST_YAP_USER_BINARY_OPERATOR(minus, lazy_vector_expr, lazy_vector_expr)
//]
template<boost::yap::expr_kind Kind, typename Tuple>
auto lazy_vector_expr<Kind, Tuple>::operator[](std::size_t n) const
{
return boost::yap::evaluate(boost::yap::transform(*this, take_nth{n}));
}
boost::yap::terminal<lazy_vector_expr, double> take_nth::operator()(
boost::yap::terminal<lazy_vector_expr, std::vector<double>> const & expr)
{
double x = boost::yap::value(expr)[n];
// This move is something of a hack. The move indicates that the terminal
// should keep the value of x (since, being an rvalue, it may be a
// temporary), rather than a reference to x. See the "How Expression
// Operands Are Treated" section of the tutorial for details.
return boost::yap::make_terminal<lazy_vector_expr, double>(std::move(x));
}
// In order to define the += operator with the semantics we want, it's
// convenient to derive a terminal type from a terminal instantiation of
// lazy_vector_expr. note that we could have written a template
// specialization here instead -- either one would work. That would of course
// have required more typing.
struct lazy_vector : lazy_vector_expr<
boost::yap::expr_kind::terminal,
boost::hana::tuple<std::vector<double>>>
{
lazy_vector() {}
explicit lazy_vector(std::vector<double> && vec)
{
elements = boost::hana::tuple<std::vector<double>>(std::move(vec));
}
template<boost::yap::expr_kind Kind, typename Tuple>
lazy_vector & operator+=(lazy_vector_expr<Kind, Tuple> const & rhs)
{
std::vector<double> & this_vec = boost::yap::value(*this);
for (int i = 0, size = (int)this_vec.size(); i < size; ++i) {
this_vec[i] += rhs[i];
}
return *this;
}
};
lazy_vector v1{std::vector<double>(4, 1.0)};
lazy_vector v2{std::vector<double>(4, 2.0)};
lazy_vector v3{std::vector<double>(4, 3.0)};
double get_d1_with_yap()
{
double retval = (v2 + v3)[2];
return retval;
}
double get_d1_by_hand()
{
std::vector<double> & v2_ref = boost::yap::value(v2);
std::vector<double> & v3_ref = boost::yap::value(v3);
double retval = v2_ref[2] + v3_ref[2];
return retval;
}
void update_v1_with_yap() { v1 += v2 - v3; }
void update_v1_by_hand()
{
std::vector<double> & v1_ref = boost::yap::value(v1);
std::vector<double> & v2_ref = boost::yap::value(v2);
std::vector<double> & v3_ref = boost::yap::value(v3);
for (int i = 0, size = (int)v1_ref.size(); i < size; ++i) {
v1_ref[i] += v2_ref[i] - v3_ref[i];
}
}
int main()
{
double d1_1 = get_d1_with_yap();
std::cout << d1_1 << "\n";
double d1_2 = get_d1_by_hand();
std::cout << d1_2 << "\n";
update_v1_with_yap();
std::cout << '{' << v1[0] << ',' << v1[1] << ',' << v1[2] << ',' << v1[3]
<< '}' << "\n";
boost::yap::value(v1) = std::vector<double>(4, 1.0);
update_v1_by_hand();
std::cout << '{' << v1[0] << ',' << v1[1] << ',' << v1[2] << ',' << v1[3]
<< '}' << "\n";
// This expression is disallowed because it does not conform to the
// implicit grammar. operator+= is only defined on terminals, not
// arbitrary expressions.
// (v2 + v3) += v1;
return 0;
}
//]
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