Codechange: Each town cache the number of each type of industry

This allows for a quick FindTownForIndustry without having to iterate over all industries.

src/cachecheck.cpp

@@ -47,6 +47,7 @@ void CheckCaches()
 
 	RebuildTownCaches();
 	RebuildSubsidisedSourceAndDestinationCache();
+	RebuildTownIndustryCounts();
 
 	uint i = 0;
 	for (Town *t : Town::Iterate()) {

src/industry.h

@@ -343,4 +343,6 @@ enum IndustryDirectoryInvalidateWindowData {
 
 void TrimIndustryAcceptedProduced(Industry *ind);
 
+void RebuildTownIndustryCounts();
+
 #endif /* INDUSTRY_H */

src/industry_cmd.cpp

@@ -201,6 +201,8 @@ Industry::~Industry()
 	for (Station *st : this->stations_near) {
 		st->RemoveIndustryToDeliver(this);
 	}
+
+	this->town->cache.industry_counts[this->type]--;
 }
 
 /**
@@ -1428,17 +1430,15 @@ static CheckNewIndustryProc * const _check_new_industry_procs[CHECK_END] = {
  * @pre \c *t != nullptr
  * @post \c *t points to a town on success, and \c nullptr on failure.
  */
-static CommandCost FindTownForIndustry(TileIndex tile, int type, Town **t)
+static CommandCost FindTownForIndustry(TileIndex tile, IndustryType type, Town **t)
 {
 	*t = ClosestTownFromTile(tile, UINT_MAX);
 
 	if (_settings_game.economy.multiple_industry_per_town) return CommandCost();
 
-	for (const Industry *i : Industry::Iterate()) {
-		if (i->type == (uint8_t)type && i->town == *t) {
-			*t = nullptr;
-			return_cmd_error(STR_ERROR_ONLY_ONE_ALLOWED_PER_TOWN);
-		}
+	if ((*t)->cache.industry_counts[type] != 0) {
+		*t = nullptr;
+		return_cmd_error(STR_ERROR_ONLY_ONE_ALLOWED_PER_TOWN);
 	}
 
 	return CommandCost();
@@ -1812,6 +1812,7 @@ static void DoCreateNewIndustry(Industry *i, TileIndex tile, IndustryType type,
 	}
 
 	i->town = t;
+	t->cache.industry_counts[i->type]++;
 	i->owner = OWNER_NONE;
 
 	uint16_t r = Random();
@@ -3235,3 +3236,14 @@ void TrimIndustryAcceptedProduced(Industry *ind)
 	ind->produced.erase(itp.base(), std::end(ind->produced));
 	ind->produced.shrink_to_fit();
 }
+
+void RebuildTownIndustryCounts()
+{
+	for (Town *t : Town::Iterate()) {
+		std::ranges::fill(t->cache.industry_counts, 0);
+	}
+
+	for (const Industry *ind : Industry::Iterate()) {
+		ind->town->cache.industry_counts[ind->type]++;
+	}
+}

src/saveload/afterload.cpp

@@ -311,6 +311,8 @@ static void InitializeWindowsAndCaches()
 
 	/* Rebuild the smallmap list of owners. */
 	BuildOwnerLegend();
+
+	RebuildTownIndustryCounts();
 }
 
 typedef void (CDECL *SignalHandlerPointer)(int);

src/town.h

@@ -16,6 +16,7 @@
 #include "subsidy_type.h"
 #include "newgrf_storage.h"
 #include "cargotype.h"
+#include "industry_type.h"
 
 template <typename T>
 struct BuildingCounts {
@@ -46,6 +47,7 @@ struct TownCache {
 	PartOfSubsidy part_of_subsidy;            ///< Is this town a source/destination of a subsidy?
 	std::array<uint32_t, HZB_END> squared_town_zone_radius; ///< UpdateTownRadius updates this given the house count
 	BuildingCounts<uint16_t> building_counts;   ///< The number of each type of building in the town
+	uint16_t industry_counts[NUM_INDUSTRYTYPES]; ///< The number of each type of industry in the town
 
 	auto operator<=>(const TownCache &) const = default;
 };

src/town_cmd.cpp

@@ -2059,6 +2059,7 @@ static void DoCreateTown(Town *t, TileIndex tile, uint32_t townnameparts, TownSi
 	UpdateTownGrowthRate(t);
 	UpdateTownMaxPass(t);
 	UpdateAirportsNoise();
+	std::ranges::fill(t->cache.industry_counts, 0);
 }
 
 /**