games/doom: fix desyncing demo (thanks qwx)

the code used P_Random()-P_Random() in some places which has
undefined evaluation order resulting in the wrong pseudo random
numbers being returned causing demo playback to desync.

this change adds P_Random2() function which returns the right
delta-random number and uses it in place of P_Random()-P_Random()
expression.

sys/src/games/doom/m_random.c

@@ -60,6 +60,12 @@ int P_Random (void)
     return rndtable[prndindex];
 }
 
+int P_Random2 (void)
+{
+    int tmp = P_Random();
+    return tmp - P_Random();
+}
+
 int M_Random (void)
 {
     rndindex = (rndindex+1)&0xff;

sys/src/games/doom/m_random.h

@@ -34,6 +34,7 @@ int M_Random (void);
 
 // As M_Random, but used only by the play simulation.
 int P_Random (void);
+int P_Random2 (void);	/* P_Raomdom() - P_Random() */
 
 // Fix randoms for demos.
 void M_ClearRandom (void);

sys/src/games/doom/p_enemy.c

@@ -789,7 +789,7 @@ void A_FaceTarget (void *_actor, void*)
 				    actor->target->y);
     
     if (actor->target->flags & MF_SHADOW)
-	actor->angle += (P_Random()-P_Random())<<21;
+	actor->angle += P_Random2()<<21;
 }
 
 
@@ -811,7 +811,7 @@ void A_PosAttack (void *_actor, void*)
     slope = P_AimLineAttack (actor, angle, MISSILERANGE);
 
     S_StartSound (actor, sfx_pistol);
-    angle += (P_Random()-P_Random())<<20;
+    angle += P_Random2()<<20;
     damage = ((P_Random()%5)+1)*3;
     P_LineAttack (actor, angle, MISSILERANGE, slope, damage);
 }
@@ -835,7 +835,7 @@ void A_SPosAttack (void *_actor, void*)
 
     for (i=0 ; i<3 ; i++)
     {
-	angle = bangle + ((P_Random()-P_Random())<<20);
+	angle = bangle + (P_Random2()<<20);
 	damage = ((P_Random()%5)+1)*3;
 	P_LineAttack (actor, angle, MISSILERANGE, slope, damage);
     }
@@ -857,7 +857,7 @@ void A_CPosAttack (void *_actor, void*)
     bangle = actor->angle;
     slope = P_AimLineAttack (actor, bangle, MISSILERANGE);
 
-    angle = bangle + ((P_Random()-P_Random())<<20);
+    angle = bangle + (P_Random2()<<20);
     damage = ((P_Random()%5)+1)*3;
     P_LineAttack (actor, angle, MISSILERANGE, slope, damage);
 }
@@ -1908,7 +1908,7 @@ void A_BrainExplode (void *_mo, void*)
     int		z;
     mobj_t*	th;
 	
-    x = mo->x + (P_Random () - P_Random ())*2048;
+    x = mo->x + P_Random2()*2048;
     y = mo->y;
     z = 128 + P_Random()*2*FRACUNIT;
     th = P_SpawnMobj (x,y,z, MT_ROCKET);

sys/src/games/doom/p_map.c

@@ -1302,8 +1302,8 @@ boolean PIT_ChangeSector (mobj_t*	thing)
 			  thing->y,
 			  thing->z + thing->height/2, MT_BLOOD);
 	
-	mo->momx = (P_Random() - P_Random ())<<12;
+	mo->momx = P_Random2()<<12;
-	mo->momy = (P_Random() - P_Random ())<<12;
+	mo->momy = P_Random2()<<12;
     }
 
     // keep checking (crush other things)	

sys/src/games/doom/p_mobj.c

@@ -818,7 +818,7 @@ P_SpawnPuff
 {
     mobj_t*	th;
 	
-    z += ((P_Random()-P_Random())<<10);
+    z += P_Random2()<<10;
 
     th = P_SpawnMobj (x,y,z, MT_PUFF);
     th->momz = FRACUNIT;
@@ -846,7 +846,7 @@ P_SpawnBlood
 {
     mobj_t*	th;
 	
-    z += ((P_Random()-P_Random())<<10);
+    z += P_Random2()<<10;
     th = P_SpawnMobj (x,y,z, MT_BLOOD);
     th->momz = FRACUNIT*2;
     th->tics -= P_Random()&3;
@@ -909,7 +909,7 @@ P_SpawnMissile
 
     // fuzzy player
     if (dest->flags & MF_SHADOW)
-	an += (P_Random()-P_Random())<<20;	
+	an += P_Random2()<<20;	
 
     th->angle = an;
     an >>= ANGLETOFINESHIFT;

sys/src/games/doom/p_pspr.c

@@ -471,7 +471,7 @@ A_Punch(void *_player, void* /*psp*/)
 	damage *= 10;
 
     angle = player->mo->angle;
-    angle += (P_Random()-P_Random())<<18;
+    angle += P_Random2()<<18;
     slope = P_AimLineAttack (player->mo, angle, MELEERANGE);
     P_LineAttack (player->mo, angle, MELEERANGE, slope, damage);
 
@@ -500,7 +500,7 @@ A_Saw(void *_player, void * /*psp*/)
 
     damage = 2*(P_Random ()%10+1);
     angle = player->mo->angle;
-    angle += (P_Random()-P_Random())<<18;
+    angle += P_Random2()<<18;
     
     // use meleerange + 1 se the puff doesn't skip the flash
     slope = P_AimLineAttack (player->mo, angle, MELEERANGE+1);
@@ -622,7 +622,7 @@ P_GunShot
     angle = mo->angle;
 
     if (!accurate)
-	angle += (P_Random()-P_Random())<<18;
+	angle += P_Random2()<<18;
 
     P_LineAttack (mo, angle, MISSILERANGE, bulletslope, damage);
 }
@@ -703,11 +703,11 @@ A_FireShotgun2(void *_player, void* /*psp*/)
     {
 	damage = 5*(P_Random ()%3+1);
 	angle = player->mo->angle;
-	angle += (P_Random()-P_Random())<<19;
+	angle += P_Random2()<<19;
 	P_LineAttack (player->mo,
 		      angle,
 		      MISSILERANGE,
-		      bulletslope + ((P_Random()-P_Random())<<5), damage);
+		      bulletslope + (P_Random2()<<5), damage);
     }
 }