Minecraft mob spawning algorithm

Minecraft spawning works like this:
Every spawn cycle (1 tick or 1/20 second by default for hostile mobs) the game makes a list of all the chunks around each player, up to a radius of seven chunks. Each chunk will only be counted once, even if that chunk is around several players.

Once this list is complete a mob limit is calculated. The limit is higher the more chunks are in the list. If the limit is lower than the current amount of a mob type in the world then the cycle will be cancelled.

If the limit is not yet reached a random block from each chunk in the list will be calculated and this chunk will be used as the centre point for a pack of mobs to spawn around. In order to continue, this block must be water for water mobs (only squid so far) and air for any other type of mob.

If this block is suitable then a maximum of 12 random blocks are chosen from up to 20 blocks away, but always at the same height as the centre block.

Lastly, The individual blocks are checked that they can physically fit the mob, so they need to be air, have air above and an opaque block below. The type of mob is chosen based upon the matching conditions (biome, height, floor block type, etc.) and the final mob is chosen randomly from all mobs that are eligible to spawn there. All other mobs in the pack will be the same type.