Castlevania Tricks

var FrameCounter = byte at RAM[0x1A],
    CurrentStage = byte at RAM[0x28],
    SpecialItemCounter = byte at RAM[0x7B],
    CurrentSpecialWeapon = byte at RAM[0x15B],
    MultiplierSpawnCounter = byte at RAM[0x79],
    SpecialWeaponOnScreen = byte at RAM[0x72],
    WhipLength012 = RAM[0x70],
    NumberOfHearts = RAM[0x71],
    CurrentMultiplier = RAM[0x64],
    CurrentRandomBonusId = RAM[0x6F]

function SpawnItem_WhenEnemyDies: # Is at $1E09A:
  var ai_type = this->ObjectAItype
  
  if((ai_type < 32 AND ai_type ≠ 11) OR (ai_type ≥ 40))
  {
    if((FrameCounter MOD 16) = 2)
      return this->SpawnSpecialItem();
  }
  
  if(ai_type = 10 OR ai_type = 19)
  {
    return this->BecomeBonusItem(4);  # unconditionally a large heart
  }

  var default_item ≔ 0; # random bonus
  
  if(ai_type = 3 OR ai_type = 8 OR ai_type = 12 OR ai_type = 14 OR ai_type = 18)
  {
    default_item ≔ 4; # a large heart instead of random bonus
    # (but it can still become a multiplier instead of large heart)
  }
  
  if( (FrameCounter MOD 8) ≠ 0)
    ai_type ≔ 0x30 # just disappears after a while, no bonus!
  else
    ai_type ≔ 0x31 # will become a bonus item.
  
  this->BecomeBonusItem(default_item, ai_type);

function GetCurrentLevel:
  return floor((CurrentStage - 1) / 3)

function SpawnSpecialItem:
  var special_table ≔ Array(0..5, 0..3)
  {
    {10,15,11,0},  # level 0(boss:bat):          rosary, watch, firebomb, <none>
    {11,10,0,9},   # level 1(boss:medusa):       firebomb, rosary, <none>, boomerang
    {10,10,14,0},  # level 2(boss:mummies):      rosary, rosary, amphora, <none>
    {10,14,13,0},  # level 3(boss:frankenstein): rosary, amphora, axe, <none>
    {0,15,14,8},   # level 4(boss:death):        <none>, watch, amphora, dagger
    {9,15,10,0}    # level 5(boss:dracula):      boomerang, watch, rosary, <none>
  }

  # Note: SpecialItemCounter is a global variable that is never ever referenced
  # anywhere else but in this function. It is a 2-bit counter.

  do {
    var special_item ≔ special_table[ GetCurrentLevel ] [ (++SpecialItemCounter) MOD 4 ];
  } while(special_item = CurrentWeapon);

  this->BecomeBonusItem (special_item)

function BecomeBonusItem(bonustype, ai_type = 0x31):
  # This function does not really change the item yet.
  # It only assigns the AI to be used by this object.
  this->AItype     ≔ ai_type
  this->BonusType  ≔ bonustype

function BecomeActor(actorid):
  # This function creates the actual object from the given id.
  # implementation is omitted here, it is not relevant.

var TranslateBonusIdToActorId = Array(0..15)
  {0x0F,0x0E,0x05,0x0C,0x42,0x10,0x23,0x00,0x17,0x43,0x0A,0x44,0x2F,0x18,0x45,0x46};

function RunObjectAI_30:
  if(--this->ObjectMultiPurposeCounter) return;
  this->AItype ≔ 0x32;

function RunObjectAI_32:
  this->StopExisting();

function RunObjectAI_2F:
  # Common AI code for bonus actors. Omitted here.

function RunObjectAI_31:
  if(--this->ObjectMultiPurposeCounter) return;
  this->AItype ≔ 0x2F;

  switch(this->BonusType MOD 16)
  {
    default:
      if(SpecialWeaponOnScreen OR (CurrentWeapon = this->BonusType))
        return this->BecomeTinyRandomBonusA();
      SpecialWeaponOnScreen ≔ 0xFF;
      # fallthrough:
    case 5: # beef
    case 6: # red sphere
    case 7: # unknown (ghost?)
      # Keep this decision of "bonus type", and become the actual object.
      return this->BecomeActor(TranslateBonusIdToActorId[this->BonusType]);

    case 1: # bonus sac
      this->BecomeActor(0x0E); # become a sac
      this->PaletteIndex ≔ this->BonusType SHR 4;
      return;
    
    case 12: # multiplier
      # Try becoming a multiplier
      
      MultiplierSpawnCounter ≔ 0;
      var tmp ≔ CurrentMultiplier;
      if(tmp < 2)
      {
        this->BonusType ≔ 12;
        tmp ≔ tmp + BonusTypeToActorTypeTransTable[this->BonusType];
        return this->BecomeActor(tmp);
      }
      # passthru

    case 0: # small heart or a sac of some value
      return this->BecomeCompletelyRandomBonus();
  }

function BecomeCompletelyRandomBonus:
  # It is unclear where MultiplierSpawnCounter is incremented.
  # It happens at $E65D, but the circumstances in which that
  # code is executed are not clear. It seems to occur whenever
  # a special weapon delivers damage, though. In the case of
  # the fire bomb, for enemies that are not immediately killed
  # from damage, it can mean that the counter is incremented
  # by ~50 times at once.
  if(MultiplierSpawnCounter ≥ 10)
  {
    # Try becoming a multiplier
    
    MultiplierSpawnCounter = 0;
    var tmp ≔ CurrentMultiplier;
    if(tmp < 2)
    {
      this->BonusType ≔ 12;
      tmp ≔ tmp + BonusTypeToActorTypeTransTable[this->BonusType];
      return this->BecomeActor(tmp);
    }
  }
  if(NOT SpecialWeaponOnScreen)
  {
    if(NumberOfHearts < 8)
    {
      if(NumberOfHearts ≥ 4 AND WhipLength012 < 1)
        return this->BecomeWhipUpgrade();
    }
    else if(WhipLength012 < 2)
      return this->BecomeWhipUpgrade();
  }
  return this->BecomeTinyRandomBonusA();

function BecomeTinyRandomBonusA:
  this->BonusType ≔ CurrentRandomBonusId MOD 2; # 0 = heart, 1 = sac
  return this->BecomeActor(TranslateBonusIdToActorId[this->BonusType]);

function BecomeWhipUpgrade:
  SpecialWeaponOnScreen ≔ 0xFF;
  this->BonusType ≔ 3; # whip upgrade
  return this->BecomeActor(TranslateBonusIdToActorId[this->BonusType]);

# This code is executed non-stop in the main-loop of the program.
# Each time NMI resumes, it returns to this loop.
# It is responsible for permutating the CurrentRandomBonusId variable.
# If you want to influence the outcome of this loop before the next NMI,
# you need to influence the number of CPU cycles spent in the previous NMI.
function MainLoop:
  var RandomTable ≔ Array(0..15)
  {
    0x33,0xBB,0x3F,0x80,
    0x2E,0xA9,0x61,0x87,
    0xAD,0xC3,0xB2,0xC8,
    0x7C,0x25,0x48,0x7A
  };
  loop_forever:
  {
    var tmp ≔ (CurrentRandomBonusId OR FrameCounter) AND 15;
    CurrentRandomBonusId ≔ RandomTable[tmp];
  }

Adding own damage into enemy damage

Bumping from enemies to reach upper platforms