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Improved 'randomness' of item selection while keeping exploitability

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very low. Added documentation, moved item-distribution logging to
get more detailed information.
This commit is contained in:
hiker 2018-06-19 10:17:59 +10:00
parent 46b62de07e
commit 714665f444
2 changed files with 52 additions and 15 deletions
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45
src/items/powerup.cpp
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@ -468,13 +468,44 @@ void Powerup::hitBonusBox(const ItemState &item_state)
//seconds apart. If yes, then call getRandomPowerup again. If no, then break.
for (int i = 0; i < 20; i++)
{
// Determine the item based on index and time - random enough for
// the player, and reduces network synchronisation overhead.
// Dividing the time by 10 does not really allow exploiting the
// non-random selection (e.g. by displaying which item is collected
// next), since it's only around 83 ms - but it is bit more
// relaxed when client prediction should be a frame or so earlier.
int random_number = item_state.getItemId() + world->getTimeTicks() / 10;
// 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->getTimeTicks() / 10 + position*23;
new_powerup =
powerup_manager->getRandomPowerup(position, &n, random_number);
if (new_powerup != PowerupManager::POWERUP_RUBBERBALL ||

22
src/items/powerup_manager.cpp
View File

@ -383,7 +383,8 @@ void PowerupManager::WeightsData::precomputeWeights()
* The value returned matches the enum value of the random item if single.
* In case of triple-item, the value will be the enum value plus
* the number of existing powerups (= POWERUP_LAST-POWERUP_FIRST+1)
* \param rank The rank for which an item needs to be picked.
* \param rank The rank for which an item needs to be picked (between 0
* and number_of_karts-1).
* \param random_number A random number used to 'randomly' select the item
* that was picked.
*/
@ -392,10 +393,14 @@ int PowerupManager::WeightsData::getRandomItem(int rank, int random_number)
// E.g. for battle mode with only one entry
if(rank>(int)m_summed_weights_for_rank.size())
rank = m_summed_weights_for_rank.size()-1;
else if (rank<0) rank = 0; // E.g. battle mode
else if (rank<0) rank = 0; // E.g. battle mode, which has rank -1
const std::vector<int> &summed_weights = m_summed_weights_for_rank[rank];
// The last entry is the sum of all previous entries, i.e. the maximum
// value
#undef ITEM_DISTRIBUTION_DEBUG
#ifdef ITEM_DISTRIBUTION_DEBUG
int original_random_number = random_number;
#endif
random_number = random_number % summed_weights.back();
// Put the random number in range [1;max of summed weights],
// so for sum = N, there are N possible random numbers <= N.
@ -409,6 +414,12 @@ int PowerupManager::WeightsData::getRandomItem(int rank, int random_number)
// We align with the beginning of the enum and return
// We don't do more, because it would need to be decoded from enum later
#ifdef ITEM_DISTRIBUTION_DEBUG
Log::verbose("Powerup", "World %d rank %d random %d %d item %d",
World::getWorld()->getTimeTicks(), rank, random_number,
original_random_number, powerup);
#endif
return powerup + POWERUP_FIRST;
} // WeightsData::getRandomItem
@ -553,8 +564,7 @@ void PowerupManager::computeWeightsForRace(int num_karts)
* item for POSITION_BATTLE_MODE is returned. This function takes the weights
* specified for all items into account by using a list which contains all
* items depending on the weights defined. See updateWeightsForRace()
* \param pos Position of the kart (1<=pos<=number of karts) - ignored in
* case of a battle mode.
* \param pos Position of the kart (1<=pos<=number of karts).
* \param n Number of times this item is given to the kart.
* \param random_number A random number used to select the item. Important
* for networking to be able to reproduce item selection.
@ -564,10 +574,6 @@ PowerupManager::PowerupType PowerupManager::getRandomPowerup(unsigned int pos,
int random_number)
{
int powerup = m_current_item_weights.getRandomItem(pos-1, random_number);
#ifdef ITEM_DISTRIBUTION_DEBUG
Log::verbose("Powerup", "World %d pos %d random %d iten %d",
World::getWorld()->getTimeTicks(), pos, random_number, powerup);
#endif
if(powerup > POWERUP_LAST)
{
powerup -= (POWERUP_LAST-POWERUP_FIRST+1);