Question

Let’s say I want to calculate a hash of a data structure, using a hash algorithm like MD5 which accepts a serial stream, for the purposes of equivalence checking. (I want to record the hash, then recalculate the hash on the same or an equivalent data structure later, and check the hashes to gauge equivalence with high probability.)

Are there standard methods of doing this?

Issues I can see that are problematic are

• if the data structure contains an array of binary strings, I can’t just concatenate them since [“abc”,”defg”] and [“ab”,”cdefg”] are not equivalent arrays
• if the data structure contains a collection that isn’t guaranteed to enumerate in the same order, e.g. a key-value dictionary {a: “bc”, d: “efg”, h: “ijkl”} which should be considered equivalent to a key-value pair {d: “efg”, h: “ijkl”, a: “bc”}.

Answer 1

For the first issue, also hash the lengths of the strings. This will differentiate their hashes.

For the second, sort the keys.