Dear Mono Devs,
I tested string hash code collisions in mono and .NET and saw that mono's
hash code algorithm for strings lead to almost the same collisions with java
whereas .NET seems to perform better with 0 collisions in both tests.
You can read about it here:
http://corsis.blogspot.com/2008/07/mono-string-hash-code-collisions.html
Just wanted to ask about the implications of these results and whether you
might plan to improve on the current state.
Best Regards,
Cetin Sert
P/s: below is the quick and probably poor quality test code used to get the
results for the blog post
http://corsis.de/mono/string-hashcode-collision-test.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace HashCollisions
{
class Program
{
static void Main(string[] args)
{
var arr = get3(3);
var l = arr.Length;
var x = 0; // total collisions
var y = 0; // colls
var q = false;
for (int i = 0; i < l; i++)
{
var c = arr[i];
var h = c.GetHashCode();
for (int j = 0; j < l; j++)
{
if (i != j)
{
var t = arr[j];
if (h == t.GetHashCode())
{
Console.Write(c);
Console.Write(", ");
Console.WriteLine(t);
y++;
break;
//
if (!q)
{
Console.Write(c);
Console.Write(", ");
y++;
}
x++;
Console.Write(t);
Console.Write(", ");
q = true;
}
}
}
if (q) Console.WriteLine();
q = false;
}
Console.WriteLine(l);
Console.WriteLine(l * l);
Console.WriteLine(y);
Console.WriteLine(x);
}
unsafe static string[] get(int size)
{
var s = set.Length;
var l = set.Length;
for (int i = 0; i < size-1; i++)
{
l *= l;
}
var all = new string[l];
char* itm = stackalloc char[size + 1];
int x = 0;
for (int i = 0; i < s; i++)
{
itm[0] = set[i];
for (int j = 0; j < s; j++)
{
itm[1] = set[j];
all[x++] = new string(itm);
}
}
return all;
}
unsafe static string[] get3(int size)
{
var s = set.Length;
var l = (int)Math.Pow(s, size);
var all = new string[l];
char* itm = stackalloc char[size + 1];
int x = 0;
for (int i = 0; i < s; i++)
{
itm[0] = set[i];
for (int j = 0; j < s; j++)
{
itm[1] = set[j];
for (int k = 0; k < s; k++)
{
itm[2] = set[k];
all[x++] = new string(itm);
}
}
}
return all;
}
static char[] set = init();
static char[] init()
{
var b1 = block('a', 'z');
var b2 = block('A', 'Z');
var b3 = block('0', '9');
var set = new char[b1.Length + b2.Length + b3.Length];
Array.Copy(b1, 0, set, 0, b1.Length);
Array.Copy(b2, 0, set, b1.Length, b2.Length);
Array.Copy(b3, 0, set, b1.Length + b2.Length, b3.Length);
return set;
}
static char[] block(char start, char end)
{
return block((int)start, (int)end + 1);
}
unsafe static char[] block(int start, int end)
{
var length = end - start;
var block = new char[length];
fixed (char *p = block)
{
char* x = p;
for (int i = start; i < end; i++, x++)
{
*x = (char)i;
Console.WriteLine(*x);
}
}
return block;
}
}
}_______________________________________________
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