Question

Consider:

String[] segments = {"asdf", "qwerty", "blahblah", "alongerstring", "w349fe3434"};
String fullString = "asdfqwertyblahblahalongerstringw349fe3434";

Is there an efficient way to combine the hashCode()s of each of element in segments in such a way that it equals the hashCode of fullString?

Obviously if I loop over every character in all the segments, I can come up with the same result as fullString.hashCode(), but that doesn’t take advantage of the cached hashCodes in each of the segment string objects. I would like to avoid looping over each character of each segment. Also I can’t cache the looped-over hashcode for segments, because sets of segments might be combined to create a full string.

So basically, I would like something that does this:

int segmentHash = 0;
for(int i = 0; i < segments.length; i++)
{
    segmentHash = combine(segmentHash, segments[i].hashCode());
}
assert(segmentHash == fullString.hashCode());

Possible?

Answer 1

Surprisingly, yes.

Looking how the String‘s hashCode is calculated (s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]), you need to do the following:

1. for appending one segment: multiply the hash code of the initial segment by 31^L, where L is the length of the second segment, and add the hash code of the second segment
2. do the same iteratively for the rest of segments.

Edit:
Of course, you need to know additionally the lengths of the segments. Without this information the calculation is obviously impossible.