Enable filtering by upkeep in SUF

Supports both numeric values and the special values loyal, free, full.

Closes #3333

src/units/filter.cpp

@@ -433,6 +433,21 @@ void unit_filter_compound::fill(vconfig cfg)
 				return gender == args.u.gender();
 			}
 		);
+
+		create_attribute(literal["upkeep"],
+			[](const config::attribute_value& c) -> unit::upkeep_t
+			{
+				try {
+					return c.apply_visitor(unit::upkeep_parser_visitor());
+				} catch(std::invalid_argument&) {
+					return unit::upkeep_full();
+				}
+			},
+			[](unit::upkeep_t upkeep, const unit_filter_args& args)
+			{
+				return args.u.upkeep() == boost::apply_visitor(unit::upkeep_value_visitor(args.u), upkeep);
+			}
+		);
 
 		create_attribute(literal["side"],
 			[](const config::attribute_value& c)

src/units/unit.cpp

@@ -2635,16 +2635,10 @@ void unit::parse_upkeep(const config::attribute_value& upkeep)
 		return;
 	}
 
-	// TODO: create abetter way to check whether it is actually an int.
-	int upkeep_int = upkeep.to_int(-99);
-	if(upkeep_int != -99) {
-		upkeep_ = upkeep_int;
-	} else if(upkeep == upkeep_loyal::type() || upkeep == "free") {
-		upkeep_ = upkeep_loyal();
-	} else if(upkeep == upkeep_full::type()) {
-		upkeep_ = upkeep_full();
-	} else {
-		WRN_UT << "Found invalid upkeep=\"" << upkeep <<  "\" in a unit" << std::endl;
+	try {
+		upkeep_ = upkeep.apply_visitor(upkeep_parser_visitor());
+	} catch(std::invalid_argument& e) {
+		WRN_UT << "Found invalid upkeep=\"" << e.what() <<  "\" in a unit" << std::endl;
 		upkeep_ = upkeep_full();
 	}
 }

src/units/unit.hpp

@@ -1132,6 +1132,41 @@ class unit
 	};
 
 	using upkeep_t = boost::variant<upkeep_full, upkeep_loyal, int>;
+
+	/** Visitor helper class to parse the upkeep value from a config. */
+	class upkeep_parser_visitor : public boost::static_visitor<upkeep_t>
+	{
+	public:
+		template<typename N>
+		std::enable_if_t<std::is_arithmetic<N>::value, upkeep_t>
+		operator()(N n) const
+		{
+			if(n == 0) return upkeep_loyal();
+			if(n < 0) throw std::invalid_argument(std::to_string(n));
+			return n;
+		}
+
+		template<typename B>
+		std::enable_if_t<std::is_convertible<B, bool>::value && !std::is_arithmetic<B>::value, upkeep_t>
+		operator()(B b) const
+		{
+			throw std::invalid_argument(b.str());
+		}
+
+		upkeep_t operator()(boost::blank) const
+		{
+			return upkeep_full();
+		}
+
+		upkeep_t operator()(const std::string& s) const
+		{
+			if(s == "loyal" || s == "free")
+				return upkeep_loyal();
+			if(s == "full")
+				return upkeep_full();
+			throw std::invalid_argument(s);
+		}
+	};
 
 	/**
 	 * Gets the raw variant controlling the upkeep value.