Hash of a+b given hashes of a and b

[Google Code Exporter]

A comment in city.h says:
// By the way, for some hash functions, given strings a and b, the hash
// of a+b is easily derived from the hashes of a and b.  This property
// doesn't hold for any hash functions in this file.

But I didn't find how it can be done. It would be nice to have this feature. 
For example, it can be used to hash a data stream that becomes available one 
block at a time, but the block sizes are arbitrary, and we need both the hash 
of each block and the hash of the whole stream (invariant of block sizes).

Original issue reported on code.google.com by [e-mail] on 26 Mar 2014 at 9:54

[Google Code Exporter]

I didn't find how to change issue type from Defect to Enhancement.

Original comment by [e-mail] on 26 Mar 2014 at 9:56