Almost a new feature: For the final level range the enemies spawn in waves. In this level, after beating the wolfmen two more waves of other enemies come in. And all without any more memory in the level data!
The key is in the previously used SPAWN_SPOT_SPAWN_COUNT. We use the upper 4 bits to contain the wave the spawn spots is part of. This limits us to 15 enemies per spawn spot which we were never even close to.
We add a new counter named NUMBER_DELAYED_SPAWN_SPOTS_ALIVE. This contains the number of spawn spots that are currently not active (future waves). SPAWN_SPOT_LEVEL holds the wave number (in the upper for bits, that's why we add 16)
The ProcessSpawnSpots routine is enhanced thusly:
lda NUMBER_ENEMIES_ALIVE ora NUMBER_SPAWN_SPOTS_ALIVE bne .NoDelayedSpawnSpots lda NUMBER_DELAYED_SPAWN_SPOTS_ALIVE beq .NoDelayedSpawnSpots ;undelay them now lda SPAWN_SPOT_LEVEL clc adc #16 sta SPAWN_SPOT_LEVEL ;check all spots ldx #0 - lda SPAWN_SPOT_ACTIVE,x beq + lda SPAWN_SPOT_SPAWN_COUNT,x and #$f0 cmp SPAWN_SPOT_LEVEL bne + ;undelay now lda SPAWN_SPOT_SPAWN_COUNT,x and #$0f sta SPAWN_SPOT_SPAWN_COUNT,x dec NUMBER_DELAYED_SPAWN_SPOTS_ALIVE inc NUMBER_SPAWN_SPOTS_ALIVE + inx cpx #SPAWN_SPOT_COUNT bne - .NoDelayedSpawnSpots
Obviously in the spawn spot update loop we need to skip any spawn spots where the upper four bits are set:
lda SPAWN_SPOT_SPAWN_COUNT,x and #$f0 bne .NextSpawnSpot
Also, during level buildup we need to increment the correct counter depending on the upper four bits again:
;count iny lda (ZEROPAGE_POINTER_1),y sta SPAWN_SPOT_SPAWN_COUNT,x ;upper 4 bits set? then it's a delayed spawn spot! and #$f0 bne + inc NUMBER_SPAWN_SPOTS_ALIVE jmp ++ + inc NUMBER_DELAYED_SPAWN_SPOTS_ALIVE ++
Have fun surviving!