Convert String to 64bit Integer

Introduction

.NET Framework provides the 'GetHashcode()' function returning a 32bit Integer. You can convert 'string' to 32bit Integer but it doesn't guarantee a unique number. Here is the function that converts string to a unique 64bit integer, avoids collision domain level string store, compare and trace.

Background

If you need to store a large amount of URLs in your database, you must define 'character' index for manipulation. If you can generate a matching 'one-string-to-one-numeric-value' that can use as a numeric 'Key' instead of a variant length of string.

Using the Code

1. Convert a variable length of string to fixed length of hashcode, and it must have fast hashing speed, so use .NET provided System.Security.Cryptography.SHA256CryptoServiceProvider.
2. Convert 32 byte hashcode to 8 byte integer, avoiding making a collision.

/// <summary>
/// Return unique Int64 value for input string
/// </summary>
/// <param name="strText"></param>
/// <returns></returns>
static Int64 GetInt64HashCode(string strText)
{
    Int64 hashCode = 0;
    if (!string.IsNullOrEmpty(strText))
    {
        //Unicode Encode Covering all characterset
          byte[] byteContents = Encoding.Unicode.GetBytes(strText);
        System.Security.Cryptography.SHA256 hash = 
		new System.Security.Cryptography.SHA256CryptoServiceProvider();
        byte[] hashText = hash.ComputeHash(byteContents);
        //32Byte hashText separate
        //hashCodeStart = 0~7  8Byte
        //hashCodeMedium = 8~23  8Byte
        //hashCodeEnd = 24~31  8Byte
        //and Fold
        Int64 hashCodeStart = BitConverter.ToInt64(hashText, 0);
        Int64 hashCodeMedium = BitConverter.ToInt64(hashText, 8);
        Int64 hashCodeEnd = BitConverter.ToInt64(hashText, 24);
        hashCode = hashCodeStart ^ hashCodeMedium ^ hashCodeEnd;
    }
    return (hashCode);
}        

Collision and Performance Test

Tested platform: Core2Duo, Windows 2003 Server SP2, .NET Framework 3.5 SP1

10,000,000 Times generate GetInt64HashCode

Collision: Not found

100,000 Times generate ElapsedTime: 830 milliseconds

10,000,000 Times generate .NET Framework provided object.GetHashCode

Collision: 4,150 found

100,000 Times generate ElapsedTime: 35 milliseconds

Points of Interest

I know that Cryptography.SHA256 does not provide perfect collision avoided hash value and compressed 32Byte to 8Byte can increase collision probability, but I think the above function shows enough performance and avoids collision.

Your reply will make the function more efficient and reliable.

This function now uses and collects large amount of URLs. There is no collision for 40,000,000 unique URLs.

History

• 20^th March, 2009: Initial post