JS: Hashing function for combinable hashes from Objects

Question

I need to determine if objects are super/subsets of one another.

Given a function hash that generates fixed-length string hashes from arbitrary objects:

const fooHash = hash({foo: "foo"}) 
const barHash = hash({bar: "bar"}) 

const fooBarHash = hash({foo: "foo", bar: "bar"})

Is there a combine function, such that:

combine(fooHash, barHash) === fooBarHash

?

If yes, how would the combine function work?

Note that OP is asking for a function that reads "string hashes". — Louay Alakkad
– Louay Alakkad, Commented Dec 20, 2015 at 18:23
The combine function I am searching for should operate on the hashes, not on the objects. — Anatol
– Anatol, Commented Dec 20, 2015 at 18:23

Louay Alakkad · Accepted Answer · 2015-12-20 18:22:55Z

1

If the hashing function is reversible, then you can have the following combine function:

function combine(obj1hash, obj2hash) {
  return hash(merge(unhash(obj1hash), unhash(obj2hash)));
}

hash and unhash can be JSON.stringify and JSON.parse. merge is any function that merges two js objects like this one.

answered Dec 20, 2015 at 18:22

Louay Alakkad

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9 Comments

Louay Alakkad Over a year ago

if it's not possible to reverse the action, it's not doable. On the other hand, you can get away without converting back to POJO.

Anatol Over a year ago

That would work, but JSON.stringify will break on cyclical objects. Also, I didn't say so, but by hashes, I intended "fixed-size string hashes". For my use case, I need to avoid serializing the entire object, because I want to quickly compare a lot of objects. I'll update my question accordingly :)

Louay Alakkad Over a year ago

Yeah I thought about that. As mentioned it's probably not possible. This is a cryptography-related question and not a javascript one.

Louay Alakkad Over a year ago

I'm not an expert in cryptography, but I'd say 90% yes it's not possible.

Louay Alakkad Over a year ago

@anatol Let me suggest something else. What you need is a function contains(hash(obj1), hash(parent)) which returns true when parent is contained within obj1. This might have a better chance of finding a solution.

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