7 ms·

Those are examples of useful functions. They are not examples of useful hash functions.

• gabetax 3y ago
  Specifically, these functions to not provide "uniform distribution".
  • jagged-chisel 3y ago
    That isn’t a requirement of an algorithm called a hash function
    • gabetax 3y ago
      From the article: > For a function to be useful as a hash function, it must exhibit the property of uniform distribution It's also listed as the first property on the Wikipedia page: https://en.wikipedia.org/wiki/Hash_function#Uniformity https://en.wikipedia.org/wiki/Hash_function#Uniformity
      • jagged-chisel 3y ago
        I can’t find the text you quoted. The article starts off: > A hash function is any function that can be used to map data of arbitrary size to fixed-size values And the first sentence in the section you link says > A good hash function should map the expected inputs as evenly as possible over its output range. It doesn’t have to be “good” to be a hash function.
        • anamexis 3y ago
          That's not a meaningful definition of a hash function, then. def hash(val): return 0
          • jagged-chisel 3y ago
            Yes, this is a hash function. Whether it’s “useful” or “good” is the purview of the architect. Tangential edit: https://xkcd.com/221/ https://xkcd.com/221/
            • pcthrowaway 3y ago
              The best thing about that one is that it seeds the panel with a random number every time you refresh
          • throwmeaway1212 3y ago
            That has a pretty uniform distribution..
• derefr 3y ago
  I think you're conflating "functions used to build a hash table" with "functions used to build a dictionary ADT." A hash table doesn't have to serve the purpose of storing items in a dictionary ADT. A hash table where everything that's similar collides into the same buckets is just as useful for finding "a linked list of similar items" in O(1), as a hash table where everything gets its own unique bucket is useful for finding items in O(1).
• dimatura 3y ago
  It's true that these functions are in some sense very different to traditional hash functions, because they are designed to create "collisions" (or at least be similar) for elements that are similar. But I still think it is intuitive to use the term, maybe qualified as a "similarity hash" (I've also seen "perceptual hash" or "semantic hash"). In both cases we are describing a (potentially) large object with a short code. In normal hashes, the hash is supposed to be the same for objects that are exactly the same - in similarity hashes, that is softened to similar hashes for similar objects.
  • pcthrowaway 3y ago
    Functions that create collisions can still hashes can't they? Things like CRCs, Checksums are loosely considered hash functions: https://en.wikipedia.org/wiki/List_of_hash_functions https://en.wikipedia.org/wiki/List_of_hash_functions MD5SUM is a classic example of a hash function with a low (but still reasonable) chance of a collision