spawn formula

every spawn cycle (every randInt(200,799) ticks when a player is within
    17 blocks of the spawner),
loop 4 times {
        if there are 6 or more of the mob type in the 17x9x17 area around the spawner,
           skip this spawn cycle
        calculate spawnerX = X coordinate of center of spawner - 0.5
        calculate spawnerY = Y coordinate of center of spawner - 0.5
        calculate spawnerZ = Z coordinate of center of spawner - 0.5
        calculate x coordinate of mob = spawnerX + (randDouble() - randDouble()) * 4,
            randDouble being a random number between 0 and 1
        calculate y coordinate of mob = spawnerY + a random integer between -1 and 1
        calculate z coordinate of mob = spawnerZ + (randDouble() - randDouble()) * 4,
            randDouble being a random number between 0 and 1
        if all of the conditions specific to the mob type are met, spawn the mob {
          chicken, cow, mooshroom, pig, sheep, wolf:
                block below spawning block is grass
                spawning block has a light level of 9 or higher

          ocelot:
                pass a 2/3 random test
                block below spawning block is grass or leaves
                on layer 63 or higher

          creeper, enderman, skeleton, spider, cave spider, zombie:
                light level 7 or less

          blaze:
                light level 11 or less

          silverfish:
                light level 11 or less, or on Stone blocks
                no players within 5 blocks

          slime:
                difficulty is not set to peaceful or slime size is small
                either:
                        spawn block y coordinate is less than 40
                        the chunk containing the spawn block is a slime chunk (1 in 10 chance)
                        pass a 1 in 10 random test
                or:
                        swamp biome, layer 51 through 69, light level 7 or less

          ghast:
                pass a 1 in 20 random test

          squid:
                must not collide with any other entities
                must be in layer 46-62

          all except squid and slimes:
                must not collide with any blocks or other entities
                must not collide with water or lava
        }
}
if all 4 mobs failed to spawn, repeat on the next tick