Your example
I don’t see how your sample requires any of this. Just reorder your branches, then realize that the short branch is just a special case of the qualified case with n=1: Live On Coliru¹ (or using X3 version if you prefer).
The generic case
Now, having mentioned x3, it has the capacity to make your live much easier!
Here’s what I think you wanted, in the general case:
namespace parser {
template <typename... Parsers>
struct longest_parser : x3::parser_base {
longest_parser(Parsers... sub) : _alternatives {sub...} { }
template <typename It, typename Ctx, typename Other, typename Attr>
bool parse(It& f, It l, Ctx& ctx, Other const& other, Attr& attr) const {
auto const saved = f;
//// To exclude pre-skip from length comparisons, do pre-skip here:
// x3::skip_over(f, l, ctx);
auto seq = std::make_index_sequence<sizeof...(Parsers)>();
auto best = select_best(f, l, ctx, seq);
//std::cout << "Longest match at index #" << best << "\n";
bool ok = dispatch(f, l, ctx, other, attr, best, seq);
if (!ok)
f = saved;
return ok;
}
private:
template <typename It, typename Ctx, typename P>
size_t length_of(It f, It l, Ctx ctx, P const& p) const {
boost::iterator_range<It> matched;
return x3::raw[p].parse(f, l, ctx, x3::unused, matched)? boost::size(matched) : 0;
}
template <typename It, typename Ctx, size_t... I>
size_t select_best(It f, It l, Ctx& ctx, std::index_sequence<I...>) const {
std::array<size_t, sizeof...(I)> lengths { length_of(f, l, ctx, std::get<I>(_alternatives))... };
return std::distance(lengths.begin(), std::max_element(lengths.begin(), lengths.end()));
}
template <typename It, typename Ctx, typename Other, typename Attr, size_t... I>
bool dispatch(It& f, It l, Ctx& ctx, Other const& other, Attr& attr, size_t targetIdx, std::index_sequence<I...>) const {
//return (real_parse<I>(f, l, ctx, other, attr, targetIdx) || ...);
std::array<bool, sizeof...(I)> b = { real_parse<I>(f, l, ctx, other, attr, targetIdx)... };
return std::accumulate(b.begin(), b.end(), false, std::logical_or<bool>());
}
template <size_t Idx, typename It, typename Ctx, typename Other, typename Attr>
bool real_parse(It& f, It l, Ctx& ctx, Other const& other, Attr& attr, size_t targetIdx) const {
if (targetIdx != Idx)
return false;
return std::get<Idx>(_alternatives).parse(f, l, ctx, other, attr);
}
std::tuple<Parsers...> _alternatives;
};
template <typename... Ps>
longest_parser<Ps...> longest(Ps... p) { return {x3::as_parser(p)...}; }
}
Note the fold expression you could use in dispatch if your compiler supports it (Coliru does, edit it to see!).
Note also the subtle choice regarding skippable (probably whitespace); if it’s not significant for the length comparisons, uncomment the pre-skip.
Live Demo
Live On Coliru
#include <boost/spirit/home/x3.hpp>
#include <type_traits>
#include <iostream>
#include <numeric>
namespace x3 = boost::spirit::x3;
namespace std {
template <typename T> // just for easy debug printing; hack
static std::ostream& operator<<(std::ostream& os, std::vector<T> const& v) {
for (auto& el : v) std::cout << '[' << el << ']';
return os;
}
}
using string_vec = std::vector<std::string>;
using result_type = boost::variant<std::string, double, string_vec>;
template <typename Parser>
void parse(const std::string message, const std::string &input, const std::string &rule, const Parser &parser) {
auto iter = input.begin(), end = input.end();
std::cout << "-------------------------\n";
std::cout << message << "\n";
std::cout << "Rule: " << rule << "\n";
std::cout << "Parsing: '" << input << "'\n";
result_type parsed_result;
bool result = phrase_parse(iter, end, parser, x3::space, parsed_result);
if (result) {
std::cout << "Parsed " << parsed_result << "\n";
} else {
std::cout << "Parser failed\n";
}
if (iter != end)
std::cout << "EOI not reached. Unparsed: '" << std::string(iter, end) << "'\n";
}
namespace parser {
template <typename... Parsers>
struct longest_parser : x3::parser_base {
longest_parser(Parsers... sub) : _alternatives {sub...} { }
template <typename It, typename Ctx, typename Other, typename Attr>
bool parse(It& f, It l, Ctx& ctx, Other const& other, Attr& attr) const {
auto const saved = f;
//// To exclude pre-skip from length comparisons, do pre-skip here:
// x3::skip_over(f, l, ctx);
auto seq = std::make_index_sequence<sizeof...(Parsers)>();
auto best = select_best(f, l, ctx, seq);
//std::cout << "Longest match at index #" << best << "\n";
bool ok = dispatch(f, l, ctx, other, attr, best, seq);
if (!ok)
f = saved;
return ok;
}
private:
template <typename It, typename Ctx, typename P>
size_t length_of(It f, It l, Ctx ctx, P const& p) const {
boost::iterator_range<It> matched;
return x3::raw[p].parse(f, l, ctx, x3::unused, matched)? boost::size(matched) : 0;
}
template <typename It, typename Ctx, size_t... I>
size_t select_best(It f, It l, Ctx& ctx, std::index_sequence<I...>) const {
std::array<size_t, sizeof...(I)> lengths { length_of(f, l, ctx, std::get<I>(_alternatives))... };
return std::distance(lengths.begin(), std::max_element(lengths.begin(), lengths.end()));
}
template <typename It, typename Ctx, typename Other, typename Attr, size_t... I>
bool dispatch(It& f, It l, Ctx& ctx, Other const& other, Attr& attr, size_t targetIdx, std::index_sequence<I...>) const {
//return (real_parse<I>(f, l, ctx, other, attr, targetIdx) || ...);
std::array<bool, sizeof...(I)> b = { real_parse<I>(f, l, ctx, other, attr, targetIdx)... };
return std::accumulate(b.begin(), b.end(), false, std::logical_or<bool>());
}
template <size_t Idx, typename It, typename Ctx, typename Other, typename Attr>
bool real_parse(It& f, It l, Ctx& ctx, Other const& other, Attr& attr, size_t targetIdx) const {
if (targetIdx != Idx)
return false;
return std::get<Idx>(_alternatives).parse(f, l, ctx, other, attr);
}
std::tuple<Parsers...> _alternatives;
};
template <typename... Ps>
longest_parser<Ps...> longest(Ps... p) { return {x3::as_parser(p)...}; }
}
int main() {
auto id = x3::rule<void, std::string> {} = x3::lexeme [ x3::char_("a-zA-Z_") >> *x3::char_("a-zA-Z0-9_") ];
auto qualified = x3::rule<void, string_vec> {} = id % "::";
#define TEST_CASE(label, input, rule) parse(label, input, #rule, rule)
TEST_CASE("unqualified" , "willy" , parser::longest(id, x3::int_, x3::double_));
TEST_CASE("unqualified with whitespace", " willy \t" , parser::longest(id, x3::int_, x3::double_));
TEST_CASE("integral or number" , "123.78::anton::lutz" , parser::longest(id, x3::int_, x3::double_));
TEST_CASE("qualified" , "willy anton::lutz" , parser::longest(id, x3::int_, x3::double_));
TEST_CASE("qualified with whitespace" , "willy \tanton::lutz" , parser::longest(id, x3::int_, x3::double_));
TEST_CASE("unqualified" , "willy" , parser::longest(id, x3::int_, x3::double_, qualified));
TEST_CASE("unqualified with whitespace", " willy \t" , parser::longest(id, x3::int_, x3::double_, qualified));
TEST_CASE("integral or number" , "123.78::anton::lutz" , parser::longest(id, x3::int_, x3::double_, qualified));
TEST_CASE("qualified" , "willy::anton::lutz" , parser::longest(id, x3::int_, x3::double_, qualified));
TEST_CASE("qualified with whitespace" , "willy ::\tanton::lutz", parser::longest(id, x3::int_, x3::double_, qualified));
TEST_CASE("unqualified" , "willy" , parser::longest(x3::int_, x3::double_, qualified));
TEST_CASE("unqualified with whitespace", " willy \t" , parser::longest(x3::int_, x3::double_, qualified));
TEST_CASE("integral or number" , "123.78::anton::lutz" , parser::longest(x3::int_, x3::double_, qualified));
TEST_CASE("qualified" , "willy::anton::lutz" , parser::longest(x3::int_, x3::double_, qualified));
TEST_CASE("qualified with whitespace" , "willy ::\tanton::lutz", parser::longest(x3::int_, x3::double_, qualified));
}
Prints
-------------------------
unqualified
Rule: parser::longest(id, x3::int_, x3::double_)
Parsing: 'willy'
Parsed willy
-------------------------
unqualified with whitespace
Rule: parser::longest(id, x3::int_, x3::double_)
Parsing: ' willy '
Parsed willy
-------------------------
integral or number
Rule: parser::longest(id, x3::int_, x3::double_)
Parsing: '123.78::anton::lutz'
Parsed 123.78
EOI not reached. Unparsed: '::anton::lutz'
-------------------------
qualified
Rule: parser::longest(id, x3::int_, x3::double_)
Parsing: 'willy anton::lutz'
Parsed willy
EOI not reached. Unparsed: 'anton::lutz'
-------------------------
qualified with whitespace
Rule: parser::longest(id, x3::int_, x3::double_)
Parsing: 'willy anton::lutz'
Parsed willy
EOI not reached. Unparsed: 'anton::lutz'
-------------------------
unqualified
Rule: parser::longest(id, x3::int_, x3::double_, qualified)
Parsing: 'willy'
Parsed willy
-------------------------
unqualified with whitespace
Rule: parser::longest(id, x3::int_, x3::double_, qualified)
Parsing: ' willy '
Parsed willy
-------------------------
integral or number
Rule: parser::longest(id, x3::int_, x3::double_, qualified)
Parsing: '123.78::anton::lutz'
Parsed 123.78
EOI not reached. Unparsed: '::anton::lutz'
-------------------------
qualified
Rule: parser::longest(id, x3::int_, x3::double_, qualified)
Parsing: 'willy::anton::lutz'
Parsed [willy][anton][lutz]
-------------------------
qualified with whitespace
Rule: parser::longest(id, x3::int_, x3::double_, qualified)
Parsing: 'willy :: anton::lutz'
Parsed [willy][anton][lutz]
-------------------------
unqualified
Rule: parser::longest(x3::int_, x3::double_, qualified)
Parsing: 'willy'
Parsed [willy]
-------------------------
unqualified with whitespace
Rule: parser::longest(x3::int_, x3::double_, qualified)
Parsing: ' willy '
Parsed [willy]
-------------------------
integral or number
Rule: parser::longest(x3::int_, x3::double_, qualified)
Parsing: '123.78::anton::lutz'
Parsed 123.78
EOI not reached. Unparsed: '::anton::lutz'
-------------------------
qualified
Rule: parser::longest(x3::int_, x3::double_, qualified)
Parsing: 'willy::anton::lutz'
Parsed [willy][anton][lutz]
-------------------------
qualified with whitespace
Rule: parser::longest(x3::int_, x3::double_, qualified)
Parsing: 'willy :: anton::lutz'
Parsed [willy][anton][lutz]
Note the different results depending on the parser expressions in the alternatives.