[quantadev]

A hash is a way of mapping a data array to a more compact representation that only has one output with the attribute of uniqueness and improbability of collision. This is the opposite of what embeddings are for, and what they do.

Embeddings are a way of mapping a data array to a different (and yes smaller) data array, but the goal is not to compress into one thing, but to spread out into an array of output, where each element of the output has meaning. Embeddings are the exact opposite of hashes.

Hashes destroy meaning. Embeddings create meaning. Hashes destroy structure in space. Embeddings create structures in space.

[nighthawk454]

A hash function in general is only a function that maps input to a fixed-length output. So embeddings are hash functions.

You’re probably thinking of cryptographic hashes, where avoiding collisions is important. But it’s not intrinsic. For example, Locality Sensitive Hashing where specific types of collisions are encouraged.

[quantadev]

Yes, some hash functions are intended to have collisions (like hash algorithms that are designed to put things into 'buckets' for searching for example). And you're correct to notice that by mentioning improbability of collision I'm talking about strong hashes in that sentence. But you can take my words literally nonetheless. When I say "hash" I mean all kinds of hashes. Strong and weak.

The existence of weaker hash algos actually moves you further away from your assertion (that semantic vectors are hashes) than closer to it. Weak hashes is about a small finite number of buckets in one dimension. Semantic vectors are an infinite continuum of higher dimensional space locations. These two concepts are therefore the exact opposite.

[nighthawk454]

I guess it depends on how loosely we take the definition. Wikipedia has it as just a function that maps variable length sequences to fixed length sequences. So by that definition most embedding networks fit.

Hashes are often assumed to be 1d, discrete valued, deterministic, uniformly distributed, and hard-to-reverse. And embeddings are often assumed to have semantic structure. Those two things certainly have some pretty different properties.

In the strict definitions, I’d say if hashing is just mapping to a fixed-size output space and an embedding is a projection/mapping of one space onto another (usually smaller) space, then they’re similar.

Some hash algorithms like SimHash or LSH use random projection onto sets of random hyperplanes to produce output vectors. Blurring the lines fairly well. You could even implement that as a NN with a single projection layer. Or indeed the torch.nn.Embedding class. Of course the outputs are usually then quantized or even binarized, but that’s more a use-case specific performance optimization not fundamental (and sometimes so are embeddings).

[quantadev]

Hashing is about destroying meaning, structure, and data, albeit in a special way for a special purpose. Semantic Vectors are about creating meaning, structure, and data.

The only similarity at all is that they're both an algorithm that maps from one domain to another. So your logic collapses into "All mapping functions are hashes, whenever the output domain is smaller than the input domain", which is obviously wrong. And it's additionally wrong because the output domain of a Semantic Vector is 1500 infinities (dimensions) larger than the input. So even as a "mapper" it's doing the inverse of what a hash does.

[nighthawk454]

No, mapping to a _fixed length code_, that’s it. Note that the output domain need not be smaller than the input domain either.

If your model takes a sequence of 1 or 10,000 or N tokens and returns a vector of fixed length, say 1500 dimensions, then it is a hash function of sorts.

> A hash function is any function that can be used to map data of arbitrary size to fixed-size values

https://en.m.wikipedia.org/wiki/Hash_function

[quantadev]

I mean you can even look at this from an entropy perspective. A good hash algo generates pure noise (high entropy), while a semantic vector generates structure (low entropy). These two concepts are as far apart as anything could be, no matter what metric of comparison you choose to use. You literally couldn't name two concepts that are further apart if you tried.