Unit: added function to get current and max XP difference

See the function documentation for a explanation of its uses.

src/actions/advancement.cpp

@@ -287,7 +287,7 @@ unit_ptr get_advanced_unit(const unit &u, const std::string& advance_to)
 			" to: " + advance_to);
 	}
 	unit_ptr new_unit(new unit(u));
-	new_unit->set_experience(new_unit->experience() - new_unit->max_experience());
+	new_unit->set_experience(new_unit->experience_differential());
 	new_unit->advance_to(*new_type);
 	new_unit->heal_fully();
 	new_unit->set_state(unit::STATE_POISONED, false);
@@ -305,7 +305,7 @@ unit_ptr get_advanced_unit(const unit &u, const std::string& advance_to)
 unit_ptr get_amla_unit(const unit &u, const config &mod_option)
 {
 	unit_ptr amla_unit(new unit(u));
-	amla_unit->set_experience(amla_unit->experience() - amla_unit->max_experience());
+	amla_unit->set_experience(amla_unit->experience_differential());
 	amla_unit->add_modification("advancement", mod_option);
 	return amla_unit;
 }

src/ai/default/attack.cpp

@@ -180,7 +180,7 @@ void attack_analysis::analyze(const gamemap& map, unit_map& units,
 		double advance_prob = 0.0;
 		// The reward for advancing a unit is to get a 'negative' loss of that unit
 		if (!up->advances_to().empty()) {
-			int xp_for_advance = up->max_experience() - up->experience();
+			int xp_for_advance = up->experience_differential();
 
 			// See bug #6272... in some cases, unit already has got enough xp to advance,
 			// but hasn't (bug elsewhere?).  Can cause divide by zero.
@@ -235,7 +235,7 @@ void attack_analysis::analyze(const gamemap& map, unit_map& units,
 	}
 
 	if (!defend_it->advances_to().empty() &&
-		def_avg_experience >= defend_it->max_experience() - defend_it->experience()) {
+		def_avg_experience >= defend_it->experience_differential()) {
 		// It's likely to advance: only if we can kill with first blow.
 		chance_to_kill = first_chance_kill;
 		// Negative average damage (it will advance).

src/ai/simulated_actions.cpp

@@ -249,7 +249,7 @@ void helper_advance_unit(const map_location& loc){
 			ERR_AI_SIM_ACTIONS << "Simulating advancing to unknown unit type: " << advance_unit_typename;
 			assert(false && "simulating to unknown unit type");
 		}
-		advanced_unit->set_experience(advanced_unit->experience() - advanced_unit->max_experience());
+		advanced_unit->set_experience(advanced_unit->experience_differential());
 		advanced_unit->advance_to(*advanced_type);
 		advanced_unit->heal_fully();
 		advanced_unit->set_state(unit::STATE_POISONED, false);
@@ -257,7 +257,7 @@ void helper_advance_unit(const map_location& loc){
 		advanced_unit->set_state(unit::STATE_PETRIFIED, false);
 	}else{
 		const config &mod_option = mod_options[advance_choice-options.size()];
-		advanced_unit->set_experience(advanced_unit->experience()-advanced_unit->max_experience());
+		advanced_unit->set_experience(advanced_unit->experience_differential());
 		advanced_unit->add_modification("advancement", mod_option);
 	}
 

src/units/unit.cpp

@@ -1095,9 +1095,9 @@ color_t unit::xp_color() const
 	const color_t far_amla_color     {139,0,237,0};
 	const color_t amla_color         {170,0,255,0};
 
-	const bool near_advance = max_experience() - experience() <= game_config::kill_experience;
-	const bool mid_advance  = max_experience() - experience() <= game_config::kill_experience*2;
-	const bool far_advance  = max_experience() - experience() <= game_config::kill_experience*3;
+	const bool near_advance = static_cast<int>(experience_differential()) <= game_config::kill_experience;
+	const bool mid_advance  = static_cast<int>(experience_differential()) <= game_config::kill_experience*2;
+	const bool far_advance  = static_cast<int>(experience_differential()) <= game_config::kill_experience*3;
 
 	color_t color = normal_color;
 

src/units/unit.hpp

@@ -409,6 +409,18 @@ public:
 		return max_experience_;
 	}
 
+	/**
+	 * Absolute difference between current and max experience points.
+	 *
+	 * This function can serve two purposes:
+	 * - If current XP < max XP, the result is the amount of XP needed to advance.
+	 * - If current XP > max XP, the result is how much XP the unit will retain once they advance.
+	 */
+	unsigned int experience_differential() const
+	{
+		return std::abs(experience_ - max_experience_);
+	}
+
 	/** Sets the current experience point amount. */
 	void set_experience(int xp)
 	{