XFR Pokerus

/**
Infection chance is 3 in 65536
1-7 have 14% chance (7 a bit less)
9-F less than 0.5%

Initial duration for strain i is 1+i%4

Consequence: the following strains are illegal:
0x, 13, 14, 24, 42, 43, 44, 53, 54, 64,
8x, 93, 94, A4, C2, C3, C4, D3, D4, E4

The infected Pokémon is chosen randomly in the party.
You can't infect an egg from a wild battle.
An infected/cured Pokémon can't get a different strain.
Strain 0x will always transform into 10 regardless of duration

xfr
**/

typedef enum {
 DexNumber=0x05,
 PokerusByte=0x97,
 IsEgg=0x4C,
 Happiness=0x09,
 ...
} Stat;

typedef struct {
 PartyPkm *target;
 Stat stat;
 uint value;
} PartyPkmStat;

PartyPkm *GetPartyPkm(void *PartyStart, uint position)
{
 return (PartyPkm*)(PartyStart+8+position*220);
}

// @0201B054
void GeneratePokerus(void *PartyStart)
{
 PartyPkmStat s;
 uint i, party_count, pkrs_bitmask, dex, isEgg;

 party_count = (uint)*(PartyStart+4);
 pkrs_bitmask = 0;

 for(i=0; i<=party_count; i++)
 {
  s.target = GetPartyPkm(PartyStart, i);
  s.stat = PokerusByte;
  GetPokemonStat(&s);

  if(s.value) pkrs_bitmask |= 1<<i;
 }

 // 3 in 0x10000 chance
 if((MTRand()>>16) & 0x3FFF == 0)
 {
  do
  {
   i = (MTRand()*party_count)>>32;
   s.target = GetPartyPkm(PartyStart, i);

   s.stat = DexNumber;
   GetPokemonStat(&s);
   dex = s.value;

   s.stat = IsEgg;
   GetPokemonStat(&s);
   isEgg = s.value;
  }
  while(!dex || isEgg);

  // Do nothing if it already has it
  if(pkrs_bitmask & (1<<i)) return;

  do
  {
   // Generate strain and duration
   i = MTRand()>>24;
  }
  while(!(i&7));

  if(i>>4) i &= 7;
  i = ((i | i<<4)&0xF3)+1; // duration (1-4 days)

  s.stat = PokerusByte;
  s.value = i;
  SetPokemonStat(&s);
 }
}

// @0201B11C
void DecayPokerus(void *PartyStart, uint8 days)
{
 PartyPkmStat s;
 uint i, party_count, dex, pkr;

 party_count = (uint)*(PartyStart+4);
 for(i=0; i<=party_count; i++)
 {
  s.target = GetPartyPkm(PartyStart, i);

  s.stat = DexNumber;
  GetPokemonStat(&s);
  dex = s.value;

  s.stat = PokerusByte;
  GetPokemonStat(&s);
  pkr = s.value & 0xFF;

  if(!dex || !(pkr & 0xF)) continue;
  else if(!(pkr>>4)) pkr = 0x10;
  else if((pkr & 0xF)<days && days<=4) pkr -= days;
  else pkr &= 0xF0;

  s.value = pkr;
  SetPokemonStat(&s);
 }
}