We present a new class of resizable sequential and concurrent hash map algorithms directed at both uni-processor and multicore machines. The new hopscotch. I am currently experimenting with various hash table algorithms, and I stumbled upon an approach called hopscotch hashing. Hopscotch. We present a new resizable sequential and concurrent hash map algorithm directed at both uniprocessor and multicore machines. The algorithm is based on a.

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An advantage of using linked lists is that the size of the neighborhoods can be much larger without too much overhead on the total size of the bucket array.

The advantage of a bitmap or a linked list, as presented in the original paper, is that you only compare to keys of the haxhing that landed in the same original bucket as the query key.

Hopscotch hashing – Wikipedia

In Figure 3, all the buckets in the area of the swap candidates are in the neighborhoods of buckets that are before the beginning of the swap area. Se Kwon Lee permalink. At this point, I believe that long paragraphs of explanations would not be of any help. Software and thoughts by Emmanuel Goossaert. The first step to retrieve an entry is to determine its initial bucket.

Indeed, when looking up a key, this allows to quickly compare its hash with the ones in the bucket array, and only retrieve data from the secondary storage when the hash values are matching. Regarding storing the hashed keys in the bucket array, the main advantage is for when the data is stored in secondary storage HDD, SSD, etc.


In order to move the empty bucket E from the location where it was found into the neighborhood of B, it needs to be swapped with entries between the positions of B and E.

Hopscotch hashing | Code Capsule

The search for an empty bucket E then starts, using linear probing in the neighborhood of B. Nevertheless, with this code I was able to find out more about the practical behavior of hopscotch hashing, and some of the limitations that were not described in the original paper.

The desired property of the neighborhood is that the cost of finding an item in the buckets of the neighborhood is close to the cost of finding it in the bucket itself for example, by having buckets in the neighborhood fall within the same cache line. The neighborhood is thus a “virtual” bucket that has fixed size and overlaps with the next H-1 buckets.

This clustering behavior occurs around load factors of 0. Then a third search begins at bucket 7 in order to find a bucket whose initial bucket is less than or equal to 6.

Very Fast HashMap in C++: Hopscotch & Robin Hood Hashing (Part 1)

In the original paper, two representations of the neighborhoods were covered [2]. In order to insert a new entry, its key is hashed to find the initial bucket for the entry, denoted as B.


However in a table where many insertions and deletions occur, after some time I would expect the DELETED entry to be pushed to the border of the neighborhood, loosing its initial advantage. I found a typing error in the last part of the second line in section 2. Views Read Edit View history. The size of the bitmaps could be increased, but that would mean that the size of each bucket would be increased, which at some point would make the whole bucket array explode in memory.


In the paper, one of the proofs states: One advantage of hopscotch hashing is that it provides good performance at very high table load factors, even ones exceeding 0.

Implementation Variants Part 3: Unfortunately, linked lists are not very cache friendly. So now we can ditch the hop size, and hopscocth keep swapping elements exactly like robin hood hashing does. Note that the Hopscotch insertion algorithm discussed in Section 2.

Hopscotch Hashing

In this way, an item can be found quickly by looking at the word to hashinng which entries belong to the bucket, and then scanning through the constant number of entries most modern processors support special bit manipulation operations that make the lookup in the “hop-information” bitmap very fast.

Hopscotch After contemplating a while, I have come to the conclusion that Hopscotch is just a bad version of Robin Hood Hashing. Hopscotch hashing was introduced by Herlihy nashing al.

However, this does not prevent multiple buckets to cluster the overlapping area of their respective neighborhoods. This hasshing was last edited on 6 Octoberat For a perfectly full hashmap, where each bucket contains a corresponding entry, of the 32 hop bits there will be just a single bit that is set to 1.

Hopscotch hashing is a reordering scheme that can be used with the open addressing method for collision resolution in hash tables.