[ENet-discuss] Best practises for synchronizing states

Philip Bennefall philip at blastbay.com
Tue Nov 23 19:00:40 PST 2010

Hi all,

This is a fairly lengthy email, so I apologize in advance.

I am relatively new to ENet. I have integrated it in my game engine and have successfully written a game which is working perfectly. It is one of those finger twitching action games where speed is of the utmost importance and delay is the most evil thing in the world. The way I do it is to keep sending the entire state for a player to the other party unreliably. The other client then works out the events that "must" have occured by looking at the differences between the last known state and the new one. This is far from a perfect solution as it neither scales well nor provides for very readable code. Here's a summary of how I'm handling things:

The game is a shooter where you run a round on a 1d grid firing rocks at your oponent's palace. When a rock hits a square on the other side it'll quickly begin piercing its way through until that section of the wall crumbles. During that time, the other player may use their hammer to defend the section.

I wrote an internal layout description for myself before I started coding, which I then followed to ensure proper handling of all possible situations.

---Start of Internal Description---
The way in which this game communicates over the network is fairly simple. The two players have a direct connection established between one another, and they send out their player state roughly 30 times a second. This player state contains the current x position, the current throwing position which is -1 if nothing is being thrown, and then a list of all the squares on that player's side. For each square, two values are stored. The first is the current resistance which is 20 on max, and 0 if the square is destroyed. The second one is crumbling speed which is 0 if the square is not currently crumbling, and a value in milliseconds otherwise. This value is used to measure if two or more rocks have been thrown on the same square.

When a new state is received from the remote player, we have to analyse this information in order to determine what changes that have occured and if we need to take any action.

If a new x position is received, we simply move the player's local variable to this square and play a footstep sound.

If the new throwing position is different from our old one, we can take a few actions depending on its value:

1. If the new one is greater than -1 and the old one is -1, the remote player threw a rock at us. we play the throwing sound in the appropriate location, but take no other action.

2. If the new one is -1 and the old one is greater than -1, we can assume that the remote player hit us and start the crumbling sequence for that square on our end. Naturally if our square is already crumbled, the remote player will not get any hit notifications from us. 

3. If they both are greater than -1, we can assume that the remote player hit us and so we activate that sequence as above. We can also assume that the remote player made a new throwing attempt in another location and that we didn't see the -1 state that came prior to it probably because of lag, and so we play another throwing sound in the new location.

After this, we go on to scan the game board and do the following for each square:

If the new resistance is greater than the old one, the remote player hammered and so we play that sound.

If the new resistance is lower than the old one, we can take four actions.

1. If the new resistance is 0 and the old resistance is greater than 0, the square just broke so we play the appropriate sound and forget about it.

2. If the old crumbling speed is the same as the new one, this was a normal crumbling step and so we play the appropriate sound.

3. If the old crumbling speed is 0 and the new one is greater than 0, this was a new hit so we react accordingly.

4. If the old one is greater than the new one and the new one is greater than 0, this was a new hit on the same square.
---End of Internal Description---

I realize that this is a lot of very game specific information, and that's exactly my point. This all seems like a very dirty hack to me, and I was wondering if any of you can suggest a better and more scalable way of doing this while still keeping latency to an absolute minimum?

Thanks in advance for any help!

Kind regards,

Philip Bennefall
P.S. I should perhaps mention that this is a game for the blind; entirely based on sound, which is why sounds are mentioned all over the place rather than graphics.
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