Only recalculate seen cache when necessary.

src/lightmap.cpp

@@ -80,14 +80,14 @@ void map::add_light_from_items( const tripoint &p, std::list<item>::iterator beg
 }
 
 // TODO: Consider making this just clear the cache and dynamically fill it in as trans() is called
-void map::build_transparency_cache( const int zlev )
+bool map::build_transparency_cache( const int zlev )
 {
     auto &map_cache = get_cache( zlev );
     auto &transparency_cache = map_cache.transparency_cache;
     auto &outside_cache = map_cache.outside_cache;
 
     if( !map_cache.transparency_cache_dirty ) {
-        return;
+        return false;
     }
 
     // Default to just barely not transparent.
@@ -167,6 +167,7 @@ void map::build_transparency_cache( const int zlev )
         }
     }
     map_cache.transparency_cache_dirty = false;
+    return true;
 }
 
 void map::apply_character_light( player &p )

src/map.cpp

@@ -7781,11 +7781,11 @@ void map::build_obstacle_cache( const tripoint &start, const tripoint &end,
     }
 }
 
-void map::build_floor_cache( const int zlev )
+bool map::build_floor_cache( const int zlev )
 {
     auto &ch = get_cache( zlev );
     if( !ch.floor_cache_dirty ) {
-        return;
+        return false;
     }
 
     auto &floor_cache = ch.floor_cache;
@@ -7811,6 +7811,7 @@ void map::build_floor_cache( const int zlev )
     }
 
     ch.floor_cache_dirty = false;
+    return zlevels;
 }
 
 void map::build_floor_caches()
@@ -7826,10 +7827,11 @@ void map::build_map_cache( const int zlev, bool skip_lightmap )
 {
     const int minz = zlevels ? -OVERMAP_DEPTH : zlev;
     const int maxz = zlevels ? OVERMAP_HEIGHT : zlev;
+    bool seen_cache_dirty = false;
     for( int z = minz; z <= maxz; z++ ) {
         build_outside_cache( z );
-        build_transparency_cache( z );
-        build_floor_cache( z );
+        seen_cache_dirty |= build_transparency_cache( z );
+        seen_cache_dirty |= build_floor_cache( z );
     }
 
     tripoint start( 0, 0, minz );
@@ -7876,10 +7878,14 @@ void map::build_map_cache( const int zlev, bool skip_lightmap )
     const tripoint &p = g->u.pos();
     if( ( has_furn( p ) && !furn( p ).obj().transparent ) || !ter( p ).obj().transparent ) {
         get_cache( p.z ).transparency_cache[p.x][p.y] = LIGHT_TRANSPARENCY_CLEAR;
-        set_transparency_cache_dirty( p.z );
     }
 
-    build_seen_cache( g->u.pos(), zlev );
+    // Initial value is illegal player position.
+    static tripoint player_prev_pos = tripoint_zero;
+    if( seen_cache_dirty || player_prev_pos != p ) {
+        build_seen_cache( g->u.pos(), zlev );
+        player_prev_pos = p;
+    }
     if( !skip_lightmap ) {
         generate_lightmap( zlev );
     }

src/map.h

@@ -1463,11 +1463,15 @@ class map
         void draw_connections( const oter_id &terrain_type, mapgendata &dat, const time_point &when,
                                const float density );
 
-        void build_transparency_cache( int zlev );
+        // Builds a transparency cache and returns true if the cache was invalidated.
+        // Used to determine if seen cache should be rebuilt.
+        bool build_transparency_cache( int zlev );
         void build_sunlight_cache( int zlev );
     public:
         void build_outside_cache( int zlev );
-        void build_floor_cache( int zlev );
+        // Builds a floor cache and returns true if the cache was invalidated.
+        // Used to determine if seen cache should be rebuilt.
+        bool build_floor_cache( int zlev );
         // We want this visible in `game`, because we want it built earlier in the turn than the rest
         void build_floor_caches();