Slot Collisions
Collision Resolution Techniques
Anki OVERDRIVE Expansion Track Collision Kit. 4.6 out of 5 stars 551. 00 $29.99 $29.99. Best Seller in Slot Car Vehicle Race Sets. Any station can send only one frame at each slot. Also, the stations cannot transmit at any time whenever a frame is available. They should wait for the beginning of the next slot. However, there still can be collisions. If more than one frame transmits at the beginning of a slot, collisions occur. The collision duration is 1 slot. Collision resolution by chaining (closed addressing) Chaining is a possible way to resolve collisions. Each slot of the array contains a link to a singly-linked list containing key-value pairs with the same hash. New key-value pairs are added to the end of the list.
Collision in hashing
- In this, the hash function is used to compute the index of the array.
- The hash value is used to store the key in the hash table, as an index.
- The hash function can return the same hash value for two or more keys.
- When two or more keys are given the same hash value, it is called a collision. To handle this collision, we use collision resolution techniques.
Collision resolution techniques
There are two types of collision resolution techniques.
Preventing Double Events in a Time Slot. In many use cases you may need to limit the count of events per time slot. For example, you may need to deny creation of the 2nd event if some other event has already been defined at that time. Activating the monitoring for collisions. To control the number of events in a time slot, use the 'collision. In an SA protocol, each user transmits a packet at the time slot next to that in which the data packet arrived. When multiple users send packets at the same slot, i.e., the packets collide, these collided packets are discarded and retransmissions are scheduled.
- Separate chaining (open hashing)
- Open addressing (closed hashing)
Separate chaining
In this technique, a linked list is created from the slot in which collision has occurred, after which the new key is inserted into the linked list. This linked list of slots looks like a chain, so it is called separate chaining. It is used more when we do not know how many keys to insert or delete.
Time complexity
- Its worst-case complexity for searching is o(n).
- Its worst-case complexity for deletion is o(n).
Advantages of separate chaining
- It is easy to implement.
- The hash table never fills full, so we can add more elements to the chain.
- It is less sensitive to the function of the hashing.
Disadvantages of separate chaining
- In this, cache performance of chaining is not good.
- The memory wastage is too much in this method.
- It requires more space for element links.
Open addressing
Open addressing is collision-resolution method that is used to control the collision in the hashing table. There is no key stored outside of the hash table. Therefore, the size of the hash table is always greater than or equal to the number of keys. It is also called closed hashing.
The following techniques are used in open addressing:
- Linear probing
- Quadratic probing
- Double hashing
Linear probing
In this, when the collision occurs, we perform a linear probe for the next slot, and this probing is performed until an empty slot is found. In linear probing, the worst time to search for an element is O(table size). The linear probing gives the best performance of the cache but its problem is clustering. The main advantage of this technique is that it can be easily calculated.
Disadvantages of linear probing
- The main problem is clustering.
- It takes too much time to find an empty slot.
Quadratic probing
In this, when the collision occurs, we probe for i2thslot in ith iteration, and this probing is performed until an empty slot is found. The cache performance in quadratic probing is lower than the linear probing. Quadratic probing also reduces the problem of clustering.
Double hashing
In this, you use another hash function, and probe for (i * hash 2(x)) in the ith iteration. It takes longer to determine two hash functions. The double probing gives the very poor the cache performance, but there has no clustering problem in it.
Figure 12.1 Implementing a dynamic set by a direct-address table T. Each key in the universe U = {0,1, . . . , 9} corresponds to an index in the table. The set K = {2, 3, 5, 8} of actual keys determines the slots in the table that contain pointers to elements. The other slots, heavily shaded, contain NIL.
Figure 12.2 Using a hash function h to map keys to hash-table slots. Keys k2 and k5 map to the same slot, so they collide.
Figure 12.3 Collision resolution by chaining. Each hash-table slot T[j] contains a linked list of all the keys whose hash value is j. For example, h(k1) = h(k4) and h(k5) = h(k2) = h(k7).
Slot Collisions Games
Collision resolution by chaining
Slot Collisions Game
