Hash method on multiple values?

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Occasionally, I find I need to sort some objects, grouping them by multiple values. I usually accomplish this by concatenating the values together, with an underscore or other delineator in between, and then use that as an array index.

// group all objects with a common parent_id, date, and type
foreach ($objects as $obj) {
    $hash = $obj->parent_id . '_' . $obj->date  . '_' . $obj->type;
    $sorted_objects[$hash][] = $obj;
}

...ick! There's got to be a better way than abusing PHP's loose typing and string concatenation. Is there any way to perform a hash on multiple values? It seems I should be able to just do something like this:

$hash = sha1_multiple($obj->parent-id, $obj->date, $obj->type);

Am I already using the best method, or is there a better way?

Since hash keys are strings you're probably doing it the best way already, via a hash of the string concatenation - that's how I've performed similar functions when I've needed to in the past.

How to hash multiple values, How to hash multiple values. To hash a string like "hello" , you choose a specific hash function like SHA-256, then pass the string to it, getting a  As tylo correctly notes, when we hash multiple values, the important feature that any method we choose should have is that it should be unambiguous: there should not be any way to construct two valid sets of values that produce the same hash value — or at least it should not be any easier than breaking the collision resistance of the underlying hash function by constructing two strings with the same hash value.

Using PHP's serialization makes it a bit neater, but less efficient:

function sha1_multiple() {
    $args = func_get_args();
    return sha1(serialize($args));
}

How to hash a list of multiple items?, As tylo correctly notes, when we hash multiple values, the important feature that any method we choose should have is that it should be  A hash function is any function that can be used to map data of arbitrary size to fixed-size values. The values returned by a hash function are called hash values, hash codes, digests, or simply hashes. The values are used to index a fixed-size table called a hash table.

In concat-then-hash, the hash of "Jim" and "Fisher" is the hash of "JimFisher". Reversing the inputs produces the hash of "FisherJim", a distinct string, so this is not vulnerable to an attack by reordering inputs. Is concat-then-hash then invulnerable to a second-preimage attack? Can you find an example of another input which collides with hash2_concat_then_hash("Jim", "Fisher")? Again, you don’t need to brute-force it! One example is hash2_concat_then_hash("Ji", "mFisher"), because "Ji" and "mFisher" also concatenate to "JimFisher".

For more info, take a look at how-to-hash-multiple-values

Best way to hash two values into one?, The most important thing you're missing is that you should combine the two strings with an injective function: a function such that every unique  To store this data in a lookup table / map , we need to create a HashMap with key as string and then associate multiple values with same key. We can do this by storing a List of Integers i.e. List<Integer> objects as values in HashMap. HashMap with List<T> Object as Value. Let’s Create a HashMap of String as Key and List<Integer> as value i.e.

Hash::MultiValue - Store multiple values per key, $hash is an object, but can be used as a hashref and DWIMs! This method croaks if there is no value or multiple values associated with the  Hash::MultiValue is an object (and a plain hash reference) that may contain multiple values per key, inspired by MultiDict of WebOb. RATIONALE In a typical web application, the request parameters (a.k.a CGI parameters) can be single value or multi values.

Hashes with Multiple Values Per Key - Perl Cookbook [Book], Hashes with Multiple Values Per Key Problem You want to store more than one value for each key. Solution Store an array reference in $hash{$key}, and put the values … - Selection The exists function can have two meanings: “Is there . Python | hash() method. In the new era of digital technology, Machine Learning, Artificial Intelligence and Cyber Security are a rising phenomenon. Python stands out as a language to implement much of the good sects of this. Python offers hash() method to encode the data into unrecognisable value.

Python hash() function, Python hash(), python hash function, python hash string, python hash Python Multiple Inheritance · 38. Python Apart from the above definition, hash value of an object should be cheap to calculate in terms of space and memory complexity. Typically, hash values are provided by hash function(s) that take one or more attributes of an object and return an integer that identifies the object. The hash value is used to compare two objects and assert whether the objects under comparison are the same. Containers like dict, set use hash values in their lookup operations.

Comments
  • It doesn't get any much faster than simple string concatenation
  • Even the SHA1 step shouldn't be necessary, unless you have a need for all your keys to be the same string length.