How to compute the hash code for a stream in the same way as List.hashCode()

Question

I just realized that implementing the following algorithm to compute the hash code for a stream is not possible using Stream.reduce(...). The problem is that the initial see

Answer 1

While, at the first glance, the hash code algorithm seems to be non-parallelizable due to its non-associativity, it is possible, if we transform the function:

((a * 31 + b) * 31 + c ) * 31 + d

to

a * 31 * 31 * 31 + b * 31 * 31 + c * 31 + d

which basically is

a * 31³ + b * 31² + c * 31¹ + d * 31⁰

or for an arbitrary List of size n:

1 * 31ⁿ + e₀ * 31ⁿ⁻¹ + e₁ * 31ⁿ⁻² + e₂ * 31ⁿ⁻³ +  …  + eₙ₋₃ * 31² + eₙ₋₂ * 31¹ + eₙ₋₁ * 31⁰

with the first 1 being the initial value of the original algorithm and eₓ being the hash code of the list element at index x. While the summands are evaluation order independent now, there’s obviously a dependency to the element’s position, which we can solve by streaming over the indices in the first place, which works for random access lists and arrays, or solve generally, with a collector which tracks the number of encountered objects. The collector can resort to the repeated multiplications for the accumulation and has to resort to the power function only for combining results:

static  Collector hashing() {
    return Collector.of(() -> new int[2],
        (a,o)    -> { a[0]=a[0]*31+Objects.hashCode(o); a[1]++; },
        (a1, a2) -> { a1[0]=a1[0]*iPow(31,a2[1])+a2[0]; a1[1]+=a2[1]; return a1; },
        a -> iPow(31,a[1])+a[0]);
}
// derived from http://stackoverflow.com/questions/101439
private static int iPow(int base, int exp) {
    int result = 1;
    for(; exp>0; exp >>= 1, base *= base)
        if((exp & 1)!=0) result *= base;
    return result;
}

 

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