c++ - 使用Boost::Spirit从中缀到前缀的n元 bool 语法转换？

我需要使用Boost::Spirit将下面的中缀表示法转换为n元前缀表示法，但是我无法基于https://stackoverflow.com/a/8707598/1816477等人的答案进行构建。

这就是我要解析的内容:

not (xyz='a' or xyz='b' or xyz='c') and abc='s' xor (pqr ='v' and xyz='d')

而这种LISP风格的格式就是我要提供的输出格式(不要介意缩进):

(xor (and (= pqr 'v') (= xyz 'd'))
     (and (= abc 's')
          (not (or (= xyz 'a')
                   (= xyz 'b')
                   (= xyz 'c')))))

因此，我尝试解析的术语由前缀(not <expression>)和中缀表达式(<expression> and <expression> and ...等)组成，即:赋值，否定和n元AND，OR，XOR等，表示运算符优先级(或
我没有做的是正确的语法。我认为我能够完成输出到表示已解析的 bool(boolean) 表达式的合适boost::variant的操作。我正在考虑这样的输出结构:

struct prefixExpr;
struct infixExpr;

typedef boost::variant<
    std::string,    // identifiers, values etc.
    boost::recursive_wrapper<prefixExpr>,   // e.g. negation
    boost::recursive_wrapper<infixExpr>     // assignment, and, or, xor etc.
> expression;

struct prefixExpr {
    std::string op;    // currently only "not"
    expression expr;
};
BOOST_FUSION_ADAPT_STRUCT(prefixExpr, op, expr)

struct infixExpr {
    std::string op;    // "and", "or", "xor", "="
    std::vector<expression> exprs;
};
BOOST_FUSION_ADAPT_STRUCT(infixExpr, op, exprs)

为了能够解析上述表达式并将其转换为前缀表示法，我需要做什么？

我正在使用boost 1.67.0(在撰写本文时为最新)和Visual Studio 15.7.3(也是在撰写本文时为最新)。

最佳答案

该代码不是完美的，但应该易于理解:

#include <boost/variant.hpp>
#include <boost/spirit/home/x3.hpp>
#include <vector>
#include <string>
#include <iostream>


struct id : std::string {};
struct value : std::string {};
struct nary_expr;

using expr = boost::variant<
    id, value,
    boost::recursive_wrapper<nary_expr>
>;


struct nary_expr
{
    std::string op;
    std::vector<expr> exprs;
};


namespace x3 = boost::spirit::x3;

auto compose_nary_expr = [](auto& ctx)
{
    //auto&& [left, tail] = x3::_attr(ctx);
    auto&& left = boost::fusion::at_c<0>(x3::_attr(ctx));
    auto&& tail = boost::fusion::at_c<1>(x3::_attr(ctx));

    if (tail.size() == 0) {
        x3::_val(ctx) = left;
        return;
    }

    // left associativity
    auto op = boost::fusion::at_c<0>(tail[0]);
    std::vector<expr> exprs = { left, boost::fusion::at_c<1>(tail[0]) };
    for (std::size_t i = 1; i < tail.size(); ++i) {
        // same priority but different operator
        auto&& next_op = boost::fusion::at_c<0>(tail[i]);
        if (op != next_op) {
            exprs = std::vector<expr>{ nary_expr{ op, std::move(exprs) } };
            op = next_op;
        }
        exprs.push_back(boost::fusion::at_c<1>(tail[i]));
    }
    x3::_val(ctx) = nary_expr{ op, std::move(exprs) };
};

x3::rule<class prec4_expr_rule, expr> const prec4_expr("prec4_expr");
x3::rule<class prec3_expr_rule, expr> const prec3_expr("prec3_expr");
x3::rule<class prec2_expr_rule, expr> const prec2_expr("prec2_expr");
x3::rule<class prec1_expr_rule, expr> const prec1_expr("prec1_expr");
x3::rule<class prec0_expr_rule, expr> const prec0_expr("prec0_expr");

auto const prec4_expr_def = prec4_expr = (
    prec3_expr
    >> *(   (x3::string("or") > prec3_expr)
        )
    )[compose_nary_expr];

auto const prec3_expr_def = prec3_expr = (
    prec2_expr
    >> *(   (x3::string("xor") > prec2_expr)
        )
    )[compose_nary_expr];

auto const prec2_expr_def = prec2_expr = (
    prec1_expr
    >> *(   (x3::string("and") > prec1_expr)
        )
    )[compose_nary_expr];


auto compose_binary_expr = [](auto& ctx)
{
    auto&& rhs = boost::fusion::at_c<0>(x3::_attr(ctx));
    auto&& tail = boost::fusion::at_c<1>(x3::_attr(ctx));
    if (tail.size() > 0) {
        auto&& op = boost::fusion::at_c<0>(tail[0]);
        auto&& lhs = boost::fusion::at_c<1>(tail[0]);
        x3::_val(ctx) = nary_expr{ op, { rhs, lhs } };
    }
    else {
        x3::_val(ctx) = rhs;
    }
};


// should use optional, but something wrong with spirit
auto const prec1_expr_def = prec1_expr = (
    prec0_expr >> *(x3::string("=") > prec0_expr)
    )[compose_binary_expr];



x3::rule<class not_expr_rule, expr> const not_expr("not_expr");

auto compose_unary_expr = [](auto& ctx)
{
    //auto&& [op, expr] = x3::_attr(ctx);
    auto&& op = boost::fusion::at_c<0>(x3::_attr(ctx));
    auto&& expr = boost::fusion::at_c<1>(x3::_attr(ctx));
    x3::_val(ctx) = nary_expr{ op, { expr } };
};

auto const not_expr_def = not_expr = (x3::string("not") > prec0_expr)[compose_unary_expr];
auto const id_term = x3::rule<class id_r, id>{} = x3::lexeme[x3::alpha >> *x3::alnum];
auto const value_term = x3::rule<class value_r, value>{} = x3::lexeme["'" > +~x3::char_('\'') >> "'"];

auto const prec0_expr_def =
      value_term
    | ( '(' > prec4_expr >> ')' )
    | not_expr
    | id_term
    ;


BOOST_SPIRIT_DEFINE(
    prec0_expr
  , prec1_expr
  , prec2_expr
  , prec3_expr
  , prec4_expr
  , not_expr
);


struct indent
{
    std::size_t cur;
};

indent operator+(indent lhs, std::size_t rhs)
{
    return { lhs.cur + rhs };
}

std::ostream& operator<<(std::ostream& os, indent const& v)
{
    for (unsigned i = 0; i < v.cur; ++i) os << ' ';
    return os;
}

struct is_simple
{
    template <typename T>
    bool operator()(T const&) const
    {
        return std::is_same<T, id>::value || std::is_same<T, value>::value;
    }
};

struct printer
{
    indent indent_;

    void operator()(id const& v)
    {
        std::cout << v;
    }

    void operator()(value const& v)
    {
        std::cout << '\'' << v << '\'';
    }

    void operator()(nary_expr const& v)
    {
        std::cout << '(' << v.op << ' ';
        printer p{ indent_ + 2 + v.op.size() };
        boost::apply_visitor(p, v.exprs[0]);
        for (std::size_t i = 1; i < v.exprs.size(); ++i) {
            if (boost::apply_visitor(is_simple{}, v.exprs[i])) {
                std::cout << ' ';
            }
            else {
                std::cout << '\n' << p.indent_;
            }
            boost::apply_visitor(p, v.exprs[i]);
        }
        std::cout << ')';
    }
};

int main()
{
    std::string s = "not (xyz='a' or xyz='b' or xyz='c') and abc='s' xor (pqr ='v' and xyz='d')";
    expr expr;
    auto iter = s.cbegin();
    if (phrase_parse(iter, s.cend(), prec4_expr_def, x3::space, expr) && iter == s.cend()) {
        boost::apply_visitor(printer{}, expr);
    }

    return 0;
}

它打印:

(xor (and (not (or (= xyz 'a')
                   (= xyz 'b')
                   (= xyz 'c')))
          (= abc 's'))
     (and (= pqr 'v')
          (= xyz 'd')))

关于c++ - 使用Boost::Spirit从中缀到前缀的n元 bool 语法转换？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/50843249/