Add `POS_MOD` macro and use for all positive modulos

This macro is to make it so it won't be error-prone to write the
semi-confusing `(a % b + b) % b` statement, and you can just use an easy
macro instead.

Currently, the only places a positive modulo is needed is when switching
tilesets, enemies, and warp directions in the editor, as well as when
getting a tile in the tower, since towers just repeat themselves
vertically. Towers used this weird while-loop to sort of emulate a
modulo, which isn't half-bad, but is unnecessary, and I don't think any
compiler would recognize it as a modulo. (And if it's not optimized to a
proper modulo... what happens if the number being moduloed is really,
really big?)

desktop_version/src/Editor.cpp

@@ -4175,7 +4175,7 @@ void editorclass::switch_tileset(const bool reversed)
     }
 
     const int modulus = SDL_arraysize(tilesets);
-    tiles = (tiles % modulus + modulus) % modulus;
+    tiles = POS_MOD(tiles, modulus);
     cl.setroomtileset(levx, levy, tiles);
 
     clamp_tilecol(levx, levy, false);
@@ -4270,7 +4270,7 @@ void editorclass::switch_enemy(const bool reversed)
     }
 
     const int modulus = 10;
-    enemy = (enemy % modulus + modulus) % modulus;
+    enemy = POS_MOD(enemy, modulus);
     cl.setroomenemytype(levx, levy, enemy);
 
     note = "Enemy Type Changed";
@@ -4293,7 +4293,7 @@ void editorclass::switch_warpdir(const bool reversed)
         ++warpdir;
     }
 
-    warpdir = (warpdir % modulus + modulus) % modulus;
+    warpdir = POS_MOD(warpdir, modulus);
     cl.setroomwarpdir(levx, levy, warpdir);
 
     switch (warpdir)

desktop_version/src/Tower.cpp

@@ -5,6 +5,7 @@
 
 #include "Constants.h"
 #include "MakeAndPlay.h"
+#include "UtilityClass.h"
 
 towerclass::towerclass(void)
 {
@@ -24,8 +25,7 @@ int towerclass::backat(int xp, int yp, int yoff)
 
     if (xp >= 0 && xp < 40)
     {
-        while (yp < 0) yp += 120;
-        while (yp >= 120) yp -= 120;
+        yp = POS_MOD(yp, 120);
         return back[TILE_IDX(xp, yp)];
     }
     return 0;
@@ -41,8 +41,7 @@ int towerclass::at(int xp, int yp, int yoff)
     {
         yp = (yp*8 + yoff) / 8;
 
-        while (yp < 0) yp += 700;
-        while (yp >= 700) yp -= 700;
+        yp = POS_MOD(yp, 700);
         if (xp >= 0 && xp < 40)
         {
             return contents[TILE_IDX(xp, yp)];
@@ -63,8 +62,7 @@ int towerclass::miniat(int xp, int yp, int yoff)
 {
     yp = (yp*8 + yoff) / 8;
 
-    while (yp < 0) yp += 100;
-    while (yp >= 100) yp -= 100;
+    yp = POS_MOD(yp, 100);
     if (xp >= 0 && xp < 40)
     {
         return minitower[TILE_IDX(xp, yp)];

desktop_version/src/UtilityClass.h

@@ -88,6 +88,11 @@ void _VVV_between(
     } \
     while (false)
 
+/* Positive modulo, because C/C++'s modulo operator sucks and is negative given
+ * negative divisors.
+ * WARNING! This double- and triple- evaluates. */
+#define POS_MOD(a, b) (((a) % (b) + (b)) % (b))
+
 //helperClass
 class UtilityClass
 {