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digitalmars.D.learn - proper way to calculate toHash() for string

reply "aki" <aki google.com> writes:
I would like to know proper way to calculate
hash for given member fields.

class Foo {
	int value;
	string str;
	override nothrow  safe size_t toHash() {
		// calculate hash based on field value.
		// value and str in this example.
	}
}

boost::hash provides easy and systematic way
to calculate hash.
I found std.digest.crc is useful.

override nothrow  safe size_t toHash() {
	if (str is null) return value;
	ubyte[4] hash = crc32Of(str);
	return value^((hash[0]<<24)|(hash[1]<<16)|(hash[2]<<8)|hash[3]);
}

Please suggest me if anyone have an idea.

Regards, aki.
Jun 30 2015
next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Tuesday, 30 June 2015 at 14:19:39 UTC, aki wrote:

 I would like to know proper way to calculate
 hash for given member fields.

 class Foo {
 	int value;
 	string str;
 	override nothrow  safe size_t toHash() {
 		// calculate hash based on field value.
 		// value and str in this example.
 	}
 }

 boost::hash provides easy and systematic way
 to calculate hash.
 I found std.digest.crc is useful.

 override nothrow  safe size_t toHash() {
 	if (str is null) return value;
 	ubyte[4] hash = crc32Of(str);
 	return 
 value^((hash[0]<<24)|(hash[1]<<16)|(hash[2]<<8)|hash[3]);
 }

 Please suggest me if anyone have an idea.

 Regards, aki.


Well, technically, it really doesn't matter so long as it's 
consistent with opEquals (though having a hashing algorithm which 
has a low collision rate can definitely help performance; you 
still don't want it to be expensive though if you can help it). 
Effective Java had one that made sense which I'd probably use as 
a starting point if was going to write one, but I'd have to dig 
out the book to see what it was. It might make sense to add 
something to Phobos which took a list of member variables and 
generated an appropriate hash function for you, but we don't have 
anything like that right now. But if you really want to find a 
good hashing function, you'll probably need to go searching 
online. There's nothing special about D with regards to how 
hashing functions need to work, so it's probably pretty easy to 
find some good algorithms online. But someone here may already 
have a link that they can point you to.

- Jonathan M Davis
Jun 30 2015
prev sibling parent reply Jacob Carlborg <doob me.com> writes:
On 30/06/15 16:19, aki wrote:


 Please suggest me if anyone have an idea.


You can use TypeInfo.getHash [1] to get the hash of a given value. 
Something like:

string a = "foo";
typeid(a).getHash(&a)



-- 
/Jacob Carlborg
Jun 30 2015
parent "aki" <aki google.com> writes:
On Tuesday, 30 June 2015 at 19:36:24 UTC, Jacob Carlborg wrote:

 On 30/06/15 16:19, aki wrote:


 Please suggest me if anyone have an idea.


 You can use TypeInfo.getHash [1] to get the hash of a given 
 value. Something like:

 string a = "foo";
 typeid(a).getHash(&a)




Wow, this is what I was looking for.
It's generic way. Thank you for good suggestion.
Your 2 lines code is also a good explanation for the
manual of getHash. Because it's not obvious what is
the argument for the getHash.
Jun 30 2015