• Kevin Bailey <noone nowhere.com> Aug 28 2010
• torhu <no spam.invalid> Aug 28 2010
• bearophile <bearophileHUGS lycos.com> Aug 28 2010
• Pelle <pelle.mansson gmail.com> Aug 28 2010
• bearophile <bearophileHUGS lycos.com> Aug 28 2010
• BCS <none anon.com> Aug 28 2010

Kevin Bailey <noone nowhere.com> writes:

So I have a class containing two strings:

class Foo
{
	string s1, s2;

	...

and I'd like to add a toHash() member, but I can't find
the built-in string hash function. These don't work:

s1.toHash()
s1.toHash
s1.hash
s1.hash()
hash(s1)

yet strings can clearly be the key in a map.

I see that xml.d writes its own string hash function
but that just doesn't seem right.

Is there a way to get to the built-in ?

torhu <no spam.invalid> writes:

On 28.08.2010 16:37, Kevin Bailey wrote:
 So I have a class containing two strings:

 class Foo
 {
 	string s1, s2;

 	...

 and I'd like to add a toHash() member, but I can't find
 the built-in string hash function. These don't work:

 s1.toHash()
 s1.toHash
 s1.hash
 s1.hash()
 hash(s1)

 yet strings can clearly be the key in a map.

 I see that xml.d writes its own string hash function
 but that just doesn't seem right.

 Is there a way to get to the built-in ?


You can get to it through TypeInfo:
http://www.digitalmars.com/d/2.0/phobos/object.html#getHash

string a = "abc";

auto hash = typeid(a).getHash(&a);

bearophile <bearophileHUGS lycos.com> writes:

torhu:
 string a = "abc";
 auto hash = typeid(a).getHash(&a);


If higher performance is necessary, you may pre-compute part of that:

void main() {
    string a = "abc";
    auto hash1 = typeid(a).getHash(&a);
    auto stringHash = &(typeid(a).getHash);
    auto hash2 = stringHash(&a);
    assert(hash1 == hash2);
}

Bye,
bearophile

Pelle <pelle.mansson gmail.com> writes:

On 08/28/2010 10:25 PM, bearophile wrote:
 torhu:
 string a = "abc";
 auto hash = typeid(a).getHash(&a);


 If higher performance is necessary, you may pre-compute part of that:

 void main() {
      string a = "abc";
      auto hash1 = typeid(a).getHash(&a);
      auto stringHash =&(typeid(a).getHash);
      auto hash2 = stringHash(&a);
      assert(hash1 == hash2);
 }

 Bye,
 bearophile


I doubt that gives any performance gains. typeid(a).getHash should be a 
constant expression anyway, and I don't see any gains in my tiny 
benchmark test.

Perhaps it works better if a was an Object, since typeid for objects 
does more.

bearophile <bearophileHUGS lycos.com> writes:

Pelle:
 I doubt that gives any performance gains. typeid(a).getHash should be a 
 constant expression anyway, and I don't see any gains in my tiny 
 benchmark test.


My code shows a limited time difference:

import std.stdio: writeln;
import std.date: getUTCtime, ticksPerSecond;

void main() {
    enum int NLOOP = 30_000_000;
    string s = "and I'd like to add a toHash() member, but I can't find the
built-in string hash function.";

    {
        auto t0 = getUTCtime();
        auto stringHash =&(typeid(s).getHash);
        hash_t tot;
        for (int i; i < NLOOP; i++)
            tot += stringHash(&s);
        auto t1 = getUTCtime();
        writeln(tot, " ", (t1 - t0) / cast(double)ticksPerSecond);
    }

    {
        auto t0 = getUTCtime();
        hash_t tot;
        for (int i; i < NLOOP; i++)
            tot += typeid(s).getHash(&s);
        auto t1 = getUTCtime();
        writeln(tot, " ", (t1 - t0) / cast(double)ticksPerSecond);
    }
}


And the asm shows the first loop to contain one less instruction (dmd 2.048, -O
-release -inline), so the difference is small:

L42:    lea ECX,01Ch[ESP]
        mov EAX,02Ch[ESP]
        push    ECX
        call    EDI
        add ESI,EAX
        inc EBX
        cmp EBX,01C9C380h
        jb  L42


LA7:    lea EDX,01Ch[ESP]
        mov ECX,_D12TypeInfo_Aya6__initZ
        mov EAX,offset FLAT:_D12TypeInfo_Aya6__initZ
        push    EDX
        call    dword ptr 018h[ECX]
        add EDI,EAX
        inc EBX
        cmp EBX,01C9C380h
        jb  LA7

Bye,
bearophile

BCS <none anon.com> writes:

Hello bearophile,

 Pelle:
 
 I doubt that gives any performance gains. typeid(a).getHash should be
 a constant expression anyway, and I don't see any gains in my tiny
 benchmark test.
 


 
 import std.stdio: writeln;
 import std.date: getUTCtime, ticksPerSecond;
 void main() {
 enum int NLOOP = 30_000_000;
 string s = "and I'd like to add a toHash() member, but I can't
 find the built-in string hash function.";
 {
 auto t0 = getUTCtime();
 auto stringHash =&(typeid(s).getHash);
 hash_t tot;
 for (int i; i < NLOOP; i++)
 tot += stringHash(&s);
 auto t1 = getUTCtime();
 writeln(tot, " ", (t1 - t0) / cast(double)ticksPerSecond);
 }
 {
 auto t0 = getUTCtime();
 hash_t tot;
 for (int i; i < NLOOP; i++)
 tot += typeid(s).getHash(&s);
 auto t1 = getUTCtime();
 writeln(tot, " ", (t1 - t0) / cast(double)ticksPerSecond);
 }
 }
 And the asm shows the first loop to contain one less instruction (dmd
 2.048, -O -release -inline), so the difference is small:
 
 L42:    lea ECX,01Ch[ESP]
 mov EAX,02Ch[ESP]
 push    ECX
 call    EDI
 add ESI,EAX
 inc EBX
 cmp EBX,01C9C380h
 jb  L42
 LA7:    lea EDX,01Ch[ESP]
 mov ECX,_D12TypeInfo_Aya6__initZ


DMD seems to do a bad job of dead assignment removal at the ASM level. I've 
seen this befor where half the asm in large blocks was dead.

 mov EAX,offset FLAT:_D12TypeInfo_Aya6__initZ
 push    EDX
 call    dword ptr 018h[ECX]
 add EDI,EAX
 inc EBX
 cmp EBX,01C9C380h
 jb  LA7
 Bye,
 bearophile


... <IXOYE><