branch.hpp

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// Copyright (C) 2020-2024 Jonathan Müller and lexy contributors
// SPDX-License-Identifier: BSL-1.0
 
#ifndef LEXY_DSL_BRANCH_HPP_INCLUDED
#define LEXY_DSL_BRANCH_HPP_INCLUDED
 
#include <lexy/dsl/base.hpp>
#include <lexy/dsl/sequence.hpp>
 
namespace lexyd
{
template <typename Condition, typename... R>
struct _br : _copy_base<Condition>
{
    static_assert(sizeof...(R) >= 0);
 
    template <typename NextParser>
    using _pc = lexy::parser_for<_seq_impl<R...>, NextParser>;
 
    // We parse Condition and then seq<R...>.
    // Condition's try_parse() checks the branch condition, which is what we want.
    template <typename Reader>
    using bp = lexy::continuation_branch_parser<Condition, Reader, _pc>;
 
    template <typename NextParser>
    using p = lexy::parser_for<_seq_impl<Condition, R...>, NextParser>;
};
 
//=== operator>> ===//
template <typename Condition, typename Then>
constexpr auto operator>>(Condition, Then)
{
    LEXY_REQUIRE_BRANCH_RULE(Condition, "Left-hand-side of >>");
    return _br<Condition, Then>{};
}
template <typename Condition, typename... R>
constexpr auto operator>>(Condition, _seq<R...>)
{
    LEXY_REQUIRE_BRANCH_RULE(Condition, "Left-hand-side of >>");
    return _br<Condition, R...>{};
}
template <typename Condition, typename C, typename... R>
constexpr auto operator>>(Condition, _br<C, R...>)
{
    LEXY_REQUIRE_BRANCH_RULE(Condition, "Left-hand-side of >>");
    return _br<Condition, C, R...>{};
}
 
// Prevent nested branches in `_br`'s condition.
template <typename C, typename... R, typename Then>
constexpr auto operator>>(_br<C, R...>, Then)
{
    return C{} >> _seq<R..., Then>{};
}
template <typename C, typename... R, typename... S>
constexpr auto operator>>(_br<C, R...>, _seq<S...>)
{
    return C{} >> _seq<R..., S...>{};
}
 
// Disambiguation.
template <typename C1, typename... R, typename C2, typename... S>
constexpr auto operator>>(_br<C1, R...>, _br<C2, S...>)
{
    return _br<C1, R..., C2, S...>{};
}
 
//=== operator+ ===//
// If we add something on the left to a branch, we loose the branchy-ness.
template <typename Rule, typename Condition, typename... R>
constexpr auto operator+(Rule rule, _br<Condition, R...>)
{
    return rule + _seq<Condition, R...>{};
}
// Disambiguation.
template <typename... R, typename Condition, typename... S>
constexpr auto operator+(_seq<R...>, _br<Condition, S...>)
{
    return _seq<R...>{} + _seq<Condition, S...>{};
}
 
// If we add something on the right to a branch, we extend the then.
template <typename Condition, typename... R, typename Rule>
constexpr auto operator+(_br<Condition, R...>, Rule)
{
    return _br<Condition, R..., Rule>{};
}
// Disambiguation.
template <typename Condition, typename... R, typename... S>
constexpr auto operator+(_br<Condition, R...>, _seq<S...>)
{
    return _br<Condition, R..., S...>{};
}
 
// If we add two branches, we use the condition of the first one and treat the second as sequence.
template <typename C1, typename... R, typename C2, typename... S>
constexpr auto operator+(_br<C1, R...>, _br<C2, S...>)
{
    return _br<C1, R..., C2, S...>{};
}
 
template <typename Condition, typename Then>
constexpr auto _maybe_branch(Condition condition, Then then)
{
    if constexpr (lexy::is_branch_rule<Condition>)
        return condition >> then;
    else
        return condition + then;
}
} // namespace lexyd
 
namespace lexyd
{
struct _else : unconditional_branch_base
{
    template <typename NextParser>
    using p = NextParser;
 
    template <typename Reader>
    using bp = lexy::unconditional_branch_parser<_else, Reader>;
};
struct _else_dsl
{
    template <typename R>
    friend constexpr auto operator>>(_else_dsl, R rule)
    {
        return _else{} >> rule;
    }
    template <typename... R>
    friend constexpr auto operator>>(_else_dsl, _seq<R...> rule)
    {
        return _else{} >> rule;
    }
    template <typename C, typename... R>
    friend constexpr auto operator>>(_else_dsl, _br<C, R...> rule)
    {
        return _else{} >> rule;
    }
};
 
inline constexpr auto else_ = _else_dsl{};
} // namespace lexyd
 
#endif // LEXY_DSL_BRANCH_HPP_INCLUDED