Make many checks for monster (non)existence on squares use monster_at().

Not all are changed yet, as there are several index checks still needed
for debugging purposes.

Also, make many checks for player/monster (non)existence use actor_at().

21 files changed, 67 insertions, 71 deletions

diff --git a/crawl-ref/source/abyss.cc b/crawl-ref/source/abyss.cc
index c7d96b3b6d..db0a527fec 100644
--- a/crawl-ref/source/abyss.cc
+++ b/crawl-ref/source/abyss.cc
@@ -496,7 +496,7 @@ void area_shift(void)
         move_item_stack_to_grid(*ri, newpos);
 
         // Move monster.
-        if (mgrd(*ri) != NON_MONSTER)
+        if (monster_at(*ri))
         {
             menv[mgrd(*ri)].moveto(newpos);
             mgrd(newpos) = mgrd(*ri);
@@ -569,7 +569,7 @@ void abyss_teleport( bool new_area )
 
             if ((grd(newspot) == DNGN_FLOOR
                     || grd(newspot) == DNGN_SHALLOW_WATER)
-                && mgrd(newspot) == NON_MONSTER
+                && !monster_at(newspot)
                 && env.cgrid(newspot) == EMPTY_CLOUD)
             {
                 found = true;
diff --git a/crawl-ref/source/arena.cc b/crawl-ref/source/arena.cc
index 3dbcfa9c28..d6ae03e1ef 100644
--- a/crawl-ref/source/arena.cc
+++ b/crawl-ref/source/arena.cc
@@ -783,7 +783,7 @@ namespace arena
                                         true);
 
             if (idx == -1 && fac.active_members == 0
-                && mgrd(pos) != NON_MONSTER)
+                && monster_at(pos))
             {
                 // We have no members left, so to prevent the round
                 // from ending attempt to displace whatever is in
diff --git a/crawl-ref/source/debug.cc b/crawl-ref/source/debug.cc
index 867697a8fc..fee098066b 100644
--- a/crawl-ref/source/debug.cc
+++ b/crawl-ref/source/debug.cc
@@ -2607,7 +2607,7 @@ void debug_stethoscope(int mon)
                  env.cloud[ env.cgrid(stethpos) ].decay );
         }
 
-        if (mgrd(stethpos) == NON_MONSTER)
+        if (!monster_at(stethpos))
         {
             mprf(MSGCH_DIAGNOSTICS, "item grid = %d", igrd(stethpos) );
             return;
@@ -6305,7 +6305,7 @@ static void _dump_player(FILE *file)
     }
     fprintf(file, EOL);
 
-    if (in_bounds(you.pos()) && mgrd(you.pos()) != NON_MONSTER)
+    if (in_bounds(you.pos()) && monster_at(you.pos()))
     {
         fprintf(file, "Standing on same square as: ");
         const unsigned short midx = mgrd(you.pos());
diff --git a/crawl-ref/source/dungeon.cc b/crawl-ref/source/dungeon.cc
index 17d488a8e9..0236a8b39a 100644
--- a/crawl-ref/source/dungeon.cc
+++ b/crawl-ref/source/dungeon.cc
@@ -2966,7 +2966,7 @@ static void _place_specific_feature(dungeon_feature_type feat)
 
     do
         c = random_in_bounds();
-    while (grd(c) != DNGN_FLOOR || mgrd(c) != NON_MONSTER);
+    while (grd(c) != DNGN_FLOOR || monster_at(c));
 
     grd(c) = feat;
 }
@@ -3832,9 +3832,9 @@ static void _special_room(int level_number, spec_room &sr,
 }                               // end special_room()
 
 // Used for placement of rivers/lakes.
-static bool _may_overwrite_pos(const int i, const int j)
+static bool _may_overwrite_pos(coord_def c)
 {
-    const dungeon_feature_type grid = grd[i][j];
+    const dungeon_feature_type grid = grd(c);
 
     // Don't overwrite any stairs or branch entrances.
     if (grid >= DNGN_ENTER_SHOP && grid <= DNGN_EXIT_PORTAL_VAULT
@@ -3845,7 +3845,7 @@ static bool _may_overwrite_pos(const int i, const int j)
 
     // Don't overwrite feature if there's a monster or item there.
     // Otherwise, items/monsters might end up stuck in deep water.
-    return (mgrd[i][j] == NON_MONSTER && igrd[i][j] == NON_ITEM);
+    return (!monster_at(c) && igrd(c) == NON_ITEM);
 }
 
 static void _build_rooms(const dgn_region_list &excluded,
@@ -4779,7 +4779,7 @@ int dgn_place_monster(mons_spec &mspec,
         mg.mname     = mspec.monname;
 
         coord_def place(where);
-        if (!force_pos && mgrd(place) != NON_MONSTER
+        if (!force_pos && monster_at(place)
             && (mg.cls < NUM_MONSTERS || mg.cls == RANDOM_MONSTER))
         {
             const monster_type habitat_target =
@@ -5573,7 +5573,7 @@ static void _place_altar()
         for (int i = px - 2; i <= px + 2; i++)
             for (int j = py - 2; j <= py + 2; j++)
             {
-                if (mgrd[i][j] != NON_MONSTER)
+                if (monster_at(coord_def(i, j)))
                     mon_there = true;
             }
 
@@ -7574,7 +7574,7 @@ static void _build_river( dungeon_feature_type river_type ) //mv
                 // Note that vaults might have been created in this area!
                 // So we'll avoid the silliness of orcs/royal jelly on
                 // lava and deep water grids. -- bwr
-                if (!one_chance_in(200) && _may_overwrite_pos(i, j))
+                if (!one_chance_in(200) && _may_overwrite_pos(coord_def(i, j)))
                 {
                     if (width == 2 && river_type == DNGN_DEEP_WATER
                         && coinflip())
@@ -7638,7 +7638,7 @@ static void _build_lake(dungeon_feature_type lake_type) //mv
                 // Note that vaults might have been created in this area!
                 // So we'll avoid the silliness of monsters and items
                 // on lava and deep water grids. -- bwr
-                if (!one_chance_in(200) && _may_overwrite_pos(i,j))
+                if (!one_chance_in(200) && _may_overwrite_pos(coord_def(i, j)))
                 {
                     grd[i][j] = lake_type;
 
diff --git a/crawl-ref/source/losparam.cc b/crawl-ref/source/losparam.cc
index 27f32b73c2..5417e9d62e 100644
--- a/crawl-ref/source/losparam.cc
+++ b/crawl-ref/source/losparam.cc
@@ -10,11 +10,11 @@
 #include "cloud.h"
 #include "externs.h"
 #include "los.h"
+#include "mon-util.h"
 #include "terrain.h"
 
 opacity_type opacity_default::operator()(const coord_def& p) const
 {
-    int m;
     dungeon_feature_type f = env.grid(p);
     if (feat_is_opaque(f))
         return OPC_OPAQUE;
@@ -22,7 +22,7 @@ opacity_type opacity_default::operator()(const coord_def& p) const
         return OPC_HALF;
     else if (f == DNGN_TREES)
         return OPC_HALF;
-    else if ((m = mgrd(p)) != NON_MONSTER && menv[m].type == MONS_BUSH)
+    else if (monster_at(p) && monster_at(p)->type == MONS_BUSH)
         return OPC_HALF;
     else
         return OPC_CLEAR;
@@ -40,7 +40,6 @@ opacity_type opacity_fullyopaque::operator()(const coord_def& p) const
 // XXX: Are trees, bushes solid?
 opacity_type opacity_solid::operator()(const coord_def& p) const
 {
-    int m;
     dungeon_feature_type f = env.grid(p);
     if (feat_is_solid(f))
         return OPC_OPAQUE;
@@ -48,7 +47,7 @@ opacity_type opacity_solid::operator()(const coord_def& p) const
         return OPC_HALF;
     else if (f == DNGN_TREES)
         return OPC_HALF;
-    else if ((m = mgrd(p)) != NON_MONSTER && menv[m].type == MONS_BUSH)
+    else if (monster_at(p) && monster_at(p)->type == MONS_BUSH)
         return OPC_HALF;
     else
         return OPC_CLEAR;
diff --git a/crawl-ref/source/maps.cc b/crawl-ref/source/maps.cc
index 2c7060f31b..dbbabd1dab 100644
--- a/crawl-ref/source/maps.cc
+++ b/crawl-ref/source/maps.cc
@@ -21,8 +21,9 @@
 #include "enum.h"
 #include "files.h"
 #include "message.h"
-#include "monplace.h"
 #include "mapdef.h"
+#include "mon-util.h"
+#include "monplace.h"
 #include "random.h"
 #include "state.h"
 #include "terrain.h"
@@ -228,8 +229,8 @@ static bool _safe_vault_place(const map_def &map,
         if (!_may_overwrite_feature(dfeat, water_ok))
             return (false);
 
-        // Don't overwrite items or monsters, either!
-        if (igrd(cp) != NON_ITEM || mgrd(cp) != NON_MONSTER)
+        // Don't overwrite monsters or items, either!
+        if (monster_at(cp) || igrd(cp) != NON_ITEM)
             return (false);
     }
 
diff --git a/crawl-ref/source/mon-abil.cc b/crawl-ref/source/mon-abil.cc
index 094e64abef..1742d00860 100644
--- a/crawl-ref/source/mon-abil.cc
+++ b/crawl-ref/source/mon-abil.cc
@@ -501,7 +501,7 @@ static bool _siren_movement_effect(const monsters *monster)
 
                 if (swapping)
                 {
-                    if (mgrd(oldpos) != NON_MONSTER)
+                    if (monster_at(oldpos))
                     {
                         mprf("Something prevents you from swapping places "
                              "with %s.",
diff --git a/crawl-ref/source/mon-act.cc b/crawl-ref/source/mon-act.cc
index 2ad17e09a2..ffa34a4804 100644
--- a/crawl-ref/source/mon-act.cc
+++ b/crawl-ref/source/mon-act.cc
@@ -534,7 +534,7 @@ static void _handle_movement(monsters *monster)
     // First, check whether the monster is smart enough to even consider
     // this.
     if ((newpos == you.pos()
-           || mgrd(newpos) != NON_MONSTER && monster->foe == mgrd(newpos))
+           || monster_at(newpos) && monster->foe == mgrd(newpos))
         && mons_intel(monster) >= I_ANIMAL
         && coinflip()
         && !mons_is_confused(monster) && !mons_is_caught(monster)
diff --git a/crawl-ref/source/mon-cast.cc b/crawl-ref/source/mon-cast.cc
index a9535cea2e..0c78d3e7ef 100644
--- a/crawl-ref/source/mon-cast.cc
+++ b/crawl-ref/source/mon-cast.cc
@@ -1896,7 +1896,7 @@ void mons_cast(monsters *monster, bolt &pbolt, spell_type spell_cast,
         for (adjacent_iterator ai(monster->pos()); ai; ++ai)
         {
             // we can blink away the crowd, but only our allies
-            if (mgrd(*ai) != NON_MONSTER
+            if (monster_at(*ai)
                 && monster_at(*ai)->attitude != monster->attitude)
                 sumcount++;
             if (grd(*ai) != DNGN_FLOOR && grd(*ai) > DNGN_MAX_NONREACH
@@ -1915,13 +1915,13 @@ void mons_cast(monsters *monster, bolt &pbolt, spell_type spell_cast,
         sumcount = 0;
         for (adjacent_iterator ai(monster->pos()); ai; ++ai)
         {
-            if (mgrd(*ai) != NON_MONSTER && monster_at(*ai) != monster)
+            if (monster_at(*ai) && monster_at(*ai) != monster)
             {
                 monster_at(*ai)->blink();
-                if (mgrd(*ai) != NON_MONSTER)
+                if (monster_at(*ai))
                 {
                     monster_at(*ai)->teleport(true);
-                    if (mgrd(*ai) != NON_MONSTER)
+                    if (monster_at(*ai))
                         continue;
                 }
             }
diff --git a/crawl-ref/source/monplace.cc b/crawl-ref/source/monplace.cc
index 0bfa4d2897..77b31695f5 100644
--- a/crawl-ref/source/monplace.cc
+++ b/crawl-ref/source/monplace.cc
@@ -1047,7 +1047,7 @@ static int _place_monster_aux(const mgen_data &mg,
             return (-1);
     }
 
-    ASSERT(mgrd(fpos) == NON_MONSTER);
+    ASSERT(!monster_at(fpos));
 
     if (crawl_state.arena
         && arena_veto_place_monster(mg, first_band_member, fpos))
diff --git a/crawl-ref/source/monster.cc b/crawl-ref/source/monster.cc
index 300c44a4ae..4fb0f265ba 100644
--- a/crawl-ref/source/monster.cc
+++ b/crawl-ref/source/monster.cc
@@ -3898,7 +3898,7 @@ static bool _prepare_del_ench(monsters* mon, const mon_enchant &me)
 
     int midx = mon->mindex();
 
-    if (mgrd(mon->pos()) == NON_MONSTER)
+    if (!monster_at(mon->pos()))
         mgrd(mon->pos()) = midx;
 
     if (mon->pos() != you.pos() && midx == mgrd(mon->pos()))
@@ -3935,7 +3935,7 @@ static bool _prepare_del_ench(monsters* mon, const mon_enchant &me)
     int       okay_squares = 0;
 
     for (adjacent_iterator ai; ai; ++ai)
-        if (mgrd(*ai) == NON_MONSTER && *ai != you.pos()
+        if (!actor_at(*ai)
             && monster_can_submerge(mon, grd(*ai))
             && one_chance_in(++okay_squares))
         {
@@ -3957,7 +3957,7 @@ static bool _prepare_del_ench(monsters* mon, const mon_enchant &me)
     // Try to move to an adjacent square where it would be happy.
     for (adjacent_iterator ai; ai; ++ai)
     {
-        if (mgrd(*ai) == NON_MONSTER
+        if (!monster_at(*ai)
             && monster_habitable_grid(mon, grd(*ai))
             && !find_trap(*ai))
         {
diff --git a/crawl-ref/source/monstuff.cc b/crawl-ref/source/monstuff.cc
index ff2eddd821..661e7c62d6 100644
--- a/crawl-ref/source/monstuff.cc
+++ b/crawl-ref/source/monstuff.cc
@@ -2611,7 +2611,7 @@ bool swap_places(monsters *monster, const coord_def &loc)
     ASSERT(map_bounds(loc));
     ASSERT(_habitat_okay(monster, grd(loc)));
 
-    if (mgrd(loc) != NON_MONSTER)
+    if (monster_at(loc))
     {
         mpr("Something prevents you from swapping places.");
         return (false);
@@ -2657,7 +2657,7 @@ bool swap_check(monsters *monster, coord_def &loc, bool quiet)
         int num_found = 0;
 
         for (adjacent_iterator ai; ai; ++ai)
-            if (mgrd(*ai) == NON_MONSTER && _habitat_okay( monster, grd(*ai))
+            if (!monster_at(*ai) && _habitat_okay(monster, grd(*ai))
                 && one_chance_in(++num_found))
             {
                 loc = *ai;
diff --git a/crawl-ref/source/player.cc b/crawl-ref/source/player.cc
index 40c39454aa..254c1c17ac 100644
--- a/crawl-ref/source/player.cc
+++ b/crawl-ref/source/player.cc
@@ -120,10 +120,10 @@ bool move_player_to_grid( const coord_def& p, bool stepped, bool allow_shift,
     ASSERT( you.can_pass_through_feat( new_grid ) );
 
     // Better not be an unsubmerged monster either.
-    ASSERT(swapping && mgrd(p) != NON_MONSTER
-           || !swapping && (mgrd(p) == NON_MONSTER
-                            || menv[ mgrd(p) ].submerged()
-                            || feawn_passthrough(&menv[mgrd(p)])));
+    ASSERT(swapping && monster_at(p)
+           || !swapping && (!monster_at(p)
+                            || monster_at(p)->submerged()
+                            || feawn_passthrough(monster_at(p))));
 
     // Don't prompt if force is true.
     if (!force)
diff --git a/crawl-ref/source/spl-cast.cc b/crawl-ref/source/spl-cast.cc
index 6b4c21bd08..bc2b9a6bb6 100644
--- a/crawl-ref/source/spl-cast.cc
+++ b/crawl-ref/source/spl-cast.cc
@@ -1037,7 +1037,7 @@ static bool _spell_is_uncastable(spell_type spell)
 static void _try_monster_cast(spell_type spell, int powc,
                               dist &spd, bolt &beam)
 {
-    if (mgrd(you.pos()) != NON_MONSTER)
+    if (monster_at(you.pos()))
     {
         mpr("Couldn't try casting monster spell because you're "
             "on top of a monster.");
@@ -1074,7 +1074,7 @@ static void _try_monster_cast(spell_type spell, int powc,
 
     if (!spd.isTarget)
         mon->foe = MHITNOT;
-    else if (mgrd(spd.target) == NON_MONSTER)
+    else if (!monster_at(spd.target))
     {
         if (spd.isMe)
             mon->foe = MHITYOU;
diff --git a/crawl-ref/source/tags.cc b/crawl-ref/source/tags.cc
index a11c8676eb..45471455d9 100644
--- a/crawl-ref/source/tags.cc
+++ b/crawl-ref/source/tags.cc
@@ -2277,7 +2277,7 @@ static void tag_read_level_monsters(reader &th, char minorVersion)
             }
             int midx = mgrd(m.pos());
             if (midx != NON_MONSTER)
-                mprf(MSGCH_ERROR, "(%d,%d) for %s already occupied by %s",
+                mprf(MSGCH_ERROR, "(%d, %d) for %s already occupied by %s",
                      m.pos().x, m.pos().y,
                      m.name(DESC_PLAIN, true).c_str(),
                      menv[midx].name(DESC_PLAIN, true).c_str());
diff --git a/crawl-ref/source/terrain.cc b/crawl-ref/source/terrain.cc
index 8a834dac7b..e7f9458985 100644
--- a/crawl-ref/source/terrain.cc
+++ b/crawl-ref/source/terrain.cc
@@ -958,9 +958,9 @@ bool swap_features(const coord_def &pos1, const coord_def &pos2,
     mgrd(pos1) = m2;
     mgrd(pos2) = m1;
 
-    if (mgrd(pos1) != NON_MONSTER)
+    if (monster_at(pos1))
         menv[mgrd(pos1)].position = pos1;
-    if (mgrd(pos2) != NON_MONSTER)
+    if (monster_at(pos2))
         menv[mgrd(pos2)].position = pos2;
 
     // Swap clouds.
@@ -1099,8 +1099,8 @@ bool fall_into_a_pool( const coord_def& entry, bool allow_shift,
         else
         {
             // back out the way we came in, if possible
-            if (grid_distance( you.pos(), entry ) == 1
-                && mgrd(entry) == NON_MONSTER)
+            if (grid_distance(you.pos(), entry) == 1
+                && !monster_at(entry))
             {
                 escape = true;
                 empty = entry;
diff --git a/crawl-ref/source/tilepick.cc b/crawl-ref/source/tilepick.cc
index 579d704cf1..47caa84e56 100644
--- a/crawl-ref/source/tilepick.cc
+++ b/crawl-ref/source/tilepick.cc
@@ -2406,9 +2406,9 @@ int tileidx_feature(dungeon_feature_type feat, int gx, int gy)
         else if (player_in_branch(BRANCH_SHOALS))
             t = TILE_SHOALS_SHALLOW_WATER;
 
-        if (mgrd[gx][gy] != NON_MONSTER)
+        monsters *mon = monster_at(coord_def(gx, gy));
+        if (mon)
         {
-            monsters *mon = &menv[mgrd[gx][gy]];
             // Add disturbance to tile.
             if (mon->submerged())
                 t += tile_dngn_count(t);
@@ -4764,12 +4764,12 @@ void tile_finish_dngn(unsigned int *tileb, int cx, int cy)
                 bool print_blood = true;
                 if (inside_halo(gc))
                 {
-                    if (see_cell(gc) && mgrd(gc) != NON_MONSTER)
+                    monsters *mon = monster_at(gc);
+                    if (see_cell(gc) && mon)
                     {
-                        monsters* m = &menv[mgrd(gc)];
-                        if (!mons_class_flag(m->type, M_NO_EXP_GAIN)
-                             && (!mons_is_mimic(m->type)
-                                 || testbits(m->flags, MF_KNOWN_MIMIC)))
+                        if (!mons_class_flag(mon->type, M_NO_EXP_GAIN)
+                             && (!mons_is_mimic(mon->type)
+                                 || testbits(mon->flags, MF_KNOWN_MIMIC)))
                         {
                             tileb[count+1] |= TILE_FLAG_HALO;
                             print_blood = false;
diff --git a/crawl-ref/source/tilereg.cc b/crawl-ref/source/tilereg.cc
index cff4edb5ae..f54d178452 100644
--- a/crawl-ref/source/tilereg.cc
+++ b/crawl-ref/source/tilereg.cc
@@ -1382,14 +1382,12 @@ bool DungeonRegion::update_tip_text(std::string& tip)
 
         if (!cell_is_solid(m_cursor[CURSOR_MOUSE]))
         {
-            int mon_num = mgrd(m_cursor[CURSOR_MOUSE]);
-            if (mon_num == NON_MONSTER || mons_friendly(&menv[mon_num]))
-            {
+            const monsters *mon = monster_at(m_cursor[CURSOR_MOUSE]));
+            if (!mon || mons_friendly(mon))
                 tip = "[L-Click] Move\n";
-            }
-            else if (mon_num != NON_MONSTER)
+            else if (mon)
             {
-                tip = menv[mon_num].name(DESC_CAP_A);
+                tip = mon->name(DESC_CAP_A);
                 tip += "\n[L-Click] Attack\n";
             }
         }
diff --git a/crawl-ref/source/tilesdl.cc b/crawl-ref/source/tilesdl.cc
index ce00292b20..20cd825e36 100644
--- a/crawl-ref/source/tilesdl.cc
+++ b/crawl-ref/source/tilesdl.cc
@@ -1330,19 +1330,17 @@ void TilesFramework::update_minimap(int gx, int gy, map_feature f)
 
     coord_def gc(gx, gy);
 
-    if (you.pos() == gc && you.on_current_level)
-    {
+    if (gc == you.pos() && you.on_current_level)
         f = MF_PLAYER;
-    }
-    else if (f == MF_MONS_HOSTILE && mgrd[gx][gy] != NON_MONSTER)
+    else if (monster_at(gc) && f == MF_MONS_HOSTILE)
     {
-        const int grid = mgrd[gx][gy];
-        if (mons_friendly_real(&menv[grid]))
+        const monsters *mon = monster_at(gc);
+        if (mons_friendly_real(mon))
             f = MF_MONS_FRIENDLY;
-        else if (mons_class_flag(menv[grid].type, M_NO_EXP_GAIN))
-            f = MF_MONS_NO_EXP;
-        else if (mons_neutral(&menv[grid]))
+        else if (mons_neutral(mon))
             f = MF_MONS_NEUTRAL;
+        else if (mons_class_flag(mon->type, M_NO_EXP_GAIN))
+            f = MF_MONS_NO_EXP;
     }
     else if (f == MF_FLOOR || f == MF_MAP_FLOOR || f == MF_WATER)
     {
diff --git a/crawl-ref/source/tutorial.cc b/crawl-ref/source/tutorial.cc
index d011841572..e1607229cc 100644
--- a/crawl-ref/source/tutorial.cc
+++ b/crawl-ref/source/tutorial.cc
@@ -1407,7 +1407,7 @@ void tutorial_first_item(const item_def &item)
         // corpse is first seen.
         if (!Options.tut_just_triggered
             && item.base_type == OBJ_CORPSES
-            && monster_at(item.pos) == NULL)
+            && !monster_at(item.pos))
         {
             learned_something_new(TUT_SEEN_CARRION, item.pos);
         }
@@ -2135,7 +2135,7 @@ void learned_something_new(tutorial_event_type seen_what, coord_def gc)
 
     case TUT_STAIR_BRAND:
         // Monster or player standing on stairs.
-        if (monster_at(gc) || you.pos() == gc)
+        if (actor_at(gc))
             DELAY_EVENT;
 
         viewwindow(true, false);
@@ -2152,7 +2152,7 @@ void learned_something_new(tutorial_event_type seen_what, coord_def gc)
 
     case TUT_HEAP_BRAND:
         // Monster or player standing on heap.
-        if (monster_at(gc) || you.pos() == gc)
+        if (actor_at(gc))
             DELAY_EVENT;
 
         viewwindow(true, false);
@@ -2174,7 +2174,7 @@ void learned_something_new(tutorial_event_type seen_what, coord_def gc)
         return;
 #else
         // Monster or player standing on trap.
-        if (monster_at(gc) || (you.pos() == gc))
+        if (actor_at(gc))
             DELAY_EVENT;
 
         viewwindow(true, false);
@@ -4460,7 +4460,7 @@ static bool _water_is_disturbed(int x, int y)
     const coord_def c(x,y);
     const monsters *mon = monster_at(c);
 
-    if (mon == NULL || grd(c) != DNGN_SHALLOW_WATER || !see_cell(c))
+    if (!mon || grd(c) != DNGN_SHALLOW_WATER || !see_cell(c))
         return (false);
 
     return (!mon->visible_to(&you) && !mons_flies(mon));
diff --git a/crawl-ref/source/view.cc b/crawl-ref/source/view.cc
index 4b4ad25e4d..8e89b2639b 100644
--- a/crawl-ref/source/view.cc
+++ b/crawl-ref/source/view.cc
@@ -745,7 +745,7 @@ void monster_grid(bool do_updates)
                      "improperly placed.  Updating mgrd.",
                      monster->name(DESC_PLAIN, true).c_str(), s,
                      monster->pos().x, monster->pos().y);
-                ASSERT(mgrd(monster->pos()) == NON_MONSTER);
+                ASSERT(!monster_at(monster->pos()));
                 mgrd(monster->pos()) = s;
             }