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Applied a simpler version of Alayan's change to selecting the

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a random item: the computed 'random' number is now sent through
a PRNG once to avoid long sequences of picking the same item.
This commit is contained in:
hiker
2018-08-01 08:51:44 +10:00
parent 9b7f63b4ca
commit 46706fb109
1 changed files with 56 additions and 51 deletions
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107
src/items/powerup.cpp
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@@ -510,58 +510,63 @@ void Powerup::hitBonusBox(const ItemState &item_state)
powerup_manager->setBallCollectTicks(0);
World *world = World::getWorld();
// Check if two bouncing balls are collected less than getRubberBallTimer()
//seconds apart. If yes, then call getRandomPowerup again. If no, then break.
for (int i = 0; i < 20; i++)
{
// Determine a 'random' number based on time, index of the item,
// and position of the kart. The idea is that this process is
// randomly enough to get the right distribution of the powerups,
// does not involve additional network communication to keep
// client and server in sync, and is not exploitable:
// While it is not possible for a client to determine the item
// (the server will always finally determine which item a player
// receives), we need to make sure that people cannot modify the
// sources and display the item that will be collected next
// at a box - otherwise the player could chose the 'best' box.
// Using synchronised pseudo-random-generators would not prevent
// cheating, since the a cheater could determine the next random
// number that will be used. If we use the server to always
// send the information to the clients, we need to add a delay
// before items can be used.
// So instead we determine a random number that is based on:
// (1) The item id
// (2) The time
// (3) The position of the kart
// Using (1) means that not all boxes at a certain time for a kart
// will give the same box. Using (2) means that the item will
// change over time - even if the next item is displayed, it
// will mean a cheater has to wait, and because of the frequency
// of the time component it will also be difficult to get the
// item at the right time. Using (3) adds another cheat-prevention
// layer: even if a cheater is waiting for the right sequence
// of items, if he is overtaken the sequence will change.
//
// In order to increase the probability of correct client prediction
// in networking (where there might be 1 or 2 frames difference
// between client and server when collecting an item), the time
// is divided by 10, meaning even if there is one frame difference,
// the client will still have a 90% chance to correctly predict the
// item. We multiply the item with a 'large' (more or less random)
// number to spread the random values across the (typically 200)
// weights used in the PowerupManager - same for the position.
int random_number = item_state.getItemId()*31
+ world->getTicksSinceStart() / 10 + position*23;
new_powerup =
powerup_manager->getRandomPowerup(position, &n, random_number);
if (new_powerup != PowerupManager::POWERUP_RUBBERBALL ||
(world->getTicksSinceStart() - powerup_manager->getBallCollectTicks())
> RubberBall::getTicksBetweenRubberBalls())
break;
}
if(new_powerup == PowerupManager::POWERUP_RUBBERBALL)
powerup_manager->setBallCollectTicks(world->getTicksSinceStart());
// Determine a 'random' number based on time, index of the item,
// and position of the kart. The idea is that this process is
// randomly enough to get the right distribution of the powerups,
// does not involve additional network communication to keep
// client and server in sync, and is not exploitable:
// While it is not possible for a client to determine the item
// (the server will always finally determine which item a player
// receives), we need to make sure that people cannot modify the
// sources and display the item that will be collected next
// at a box - otherwise the player could chose the 'best' box.
// Using synchronised pseudo-random-generators would not prevent
// cheating, since the a cheater could determine the next random
// number that will be used. If we use the server to always
// send the information to the clients, we need to add a delay
// before items can be used.
// So instead we determine a random number that is based on:
// (1) The item id
// (2) The time
// (3) The position of the kart
// Using (1) means that not all boxes at a certain time for a kart
// will give the same box. Using (2) means that the item will
// change over time - even if the next item is displayed, it
// will mean a cheater has to wait, and because of the frequency
// of the time component it will also be difficult to get the
// item at the right time. Using (3) adds another cheat-prevention
// layer: even if a cheater is waiting for the right sequence
// of items, if he is overtaken the sequence will change.
//
// In order to increase the probability of correct client prediction
// in networking (where there might be 1 or 2 frames difference
// between client and server when collecting an item), the time
// is divided by 10, meaning even if there is one frame difference,
// the client will still have a 90% chance to correctly predict the
// item. We multiply the item with a 'large' (more or less random)
// number to spread the random values across the (typically 200)
// weights used in the PowerupManager - same for the position.
unsigned long random_number = item_state.getItemId()*31
+ world->getTicksSinceStart() / 10
+ position * 23;
// Use this random number as a seed of a PRNG (based on the one in
// bullet's btSequentialImpulseConstraintSolver) to avoid getting
// consecutive numbers. Without this the same item could be
// produced for a longer period of time, which would make this
// exploitable: someone could hack STK to display the item that
// can be collected for each box, and the pick the one with the
// 'best' item.
random_number = (1664525L * random_number + 1013904223L);
// Lower bits only have a short period, so mix in higher
// bits:
random_number ^= (random_number >> 16);
random_number ^= (random_number >> 8);
new_powerup = powerup_manager->getRandomPowerup(position, &n,
random_number);
// Always add a new powerup in ITEM_MODE_NEW (or if the kart
// doesn't have a powerup atm).