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/*!
@file
Forward declares `boost::hana::lexicographical_compare`.
@copyright Louis Dionne 2013-2017
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_LEXICOGRAPHICAL_COMPARE_HPP
#define BOOST_HANA_FWD_LEXICOGRAPHICAL_COMPARE_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Short-circuiting lexicographical comparison of two `Iterable`s with
//! an optional custom predicate, by default `hana::less`.
//! @ingroup group-Iterable
//!
//! Given two `Iterable`s `xs` and `ys` and a binary predicate `pred`,
//! `lexicographical_compare` returns whether `xs` is to be considered
//! less than `ys` in a lexicographical ordering. Specifically, let's
//! denote the linearizations of `xs` and `ys` by `[x1, x2, ...]` and
//! `[y1, y2, ...]`, respectively. If the first couple satisfying the
//! predicate is of the form `xi, yi`, `lexicographical_compare` returns
//! true. Otherwise, if the first couple to satisfy the predicate is of
//! the form `yi, xi`, `lexicographical_compare` returns false. If no
//! such couple can be found, `lexicographical_compare` returns whether
//! `xs` has fewer elements than `ys`.
//!
//! @note
//! This algorithm will short-circuit as soon as it can determine that one
//! sequence is lexicographically less than the other. Hence, it can be
//! used to compare infinite sequences. However, for the procedure to
//! terminate on infinite sequences, the predicate has to be satisfied
//! at a finite index.
//!
//!
//! Signature
//! ---------
//! Given two `Iterable`s `It1(T)` and `It2(T)` and a predicate
//! \f$ pred : T \times T \to Bool \f$ (where `Bool` is some `Logical`),
//! `lexicographical_compare` has the following signatures. For the
//! variant with a provided predicate,
//! \f[
//! \mathtt{lexicographical\_compare}
//! : It1(T) \times It2(T) \times (T \times T \to Bool) \to Bool
//! \f]
//!
//! for the variant without a custom predicate, `T` is required to be
//! `Orderable`. The signature is then
//! \f[
//! \mathtt{lexicographical\_compare} : It1(T) \times It2(T) \to Bool
//! \f]
//!
//! @param xs, ys
//! Two `Iterable`s to compare lexicographically.
//!
//! @param pred
//! A binary function called as `pred(x, y)` and `pred(y, x)`, where `x`
//! and `y` are elements of `xs` and `ys`, respectively. `pred` must
//! return a `Logical` representing whether its first argument is to be
//! considered as less than its second argument. Also note that `pred`
//! must define a total ordering as defined by the `Orderable` concept.
//! When `pred` is not provided, it defaults to `less`.
//!
//!
//! Example
//! -------
//! @include example/lexicographical_compare.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto lexicographical_compare = [](auto const& xs, auto const& ys, auto const& pred = hana::less) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct lexicographical_compare_impl : lexicographical_compare_impl<T, when<true>> { };
struct lexicographical_compare_t {
template <typename Xs, typename Ys>
constexpr auto operator()(Xs const& xs, Ys const& ys) const;
template <typename Xs, typename Ys, typename Pred>
constexpr auto operator()(Xs const& xs, Ys const& ys, Pred const& pred) const;
};
constexpr lexicographical_compare_t lexicographical_compare{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_LEXICOGRAPHICAL_COMPARE_HPP
|
