Testing n-gram hash functions

Daniel Lemire

License: Apache 2.0

What is this?

This is a set of C++ classes implementing various recursive n-gram hashing techniques, also called rolling hashing (http://en.wikipedia.org/wiki/Rolling_hash), including:

• Randomized Karp-Rabin (sometimes called Rabin-Karp)
• Hashing by Cyclic Polynomials (also known as Buzhash)
• Hashing by Irreducible Polynomials

Code sample

    const uint n(3);//hash all sequences of 3 characters
    const uint L(7); // you need 7 bits
    CyclicHash hf(n,L );
    for(uint32 k = 0; k<n;++k) {
              chartype c = ... ; // grab some character
              hf.eat(c); // feed it to the hasher
    }
    while(...) { // go over your string
       hf.hashvalue; // at all times, this contains the hash value
       chartype c = ... ;// point to the next character
       chartype out = ...; // character we want to forget
       hf.update(out,c); // update hash value
    }

Requirements

GNU GCC.

What should I do after I download it?

type:

    make

then

    ./unit

then

    ./speedtesting

Reference

Daniel Lemire, Owen Kaser: Recursive n-gram hashing is pairwise independent, at best, Computer Speech & Language, Volume 24, Issue 4, October 2010, Pages 698-710 http://arxiv.org/abs/0705.4676

Daniel Lemire, The universality of iterated hashing over variable-length strings, Discrete Applied Mathematics 160 (4-5), 2012. http://arxiv.org/abs/1008.1715