Tag: 32-Bit

Maps a 64-Bit Hash Into a 32-Bit Space

using System.Security.Cryptography;
/// <summary>
///     Pros: ~no Collisions within int bit space ~(2,147,483,647)
///     Cons: Maintains a TinyDictionary(byte[],byte[]), non-deterministic across application or
///     method domains
///     Cannot Transform or reuse.
/// </summary>
public class Mapping64BitToHash32Bit : HashAlgorithm
{
    private readonly FNV1a64                        hasher = new FNV1a64();
    public readonly  TinyDictionary<byte[], byte[]> map    = new TinyDictionary<byte[], byte[]>(101, new ArrayComparer());
    private          byte[]                         h64;
    public override  int                            HashSize => 32;
    public override void Initialize()
    {
    }
    /// <inheritdoc />
    /// <summary>
    ///     Compute the 64 Bit hash value and add it and the original array to the map to create a unique position within the
    ///     TinyDictionary.
    /// </summary>
    protected override void HashCore(byte[] bytes, int ibStart, int cbSize)
    {
        h64 = hasher.ComputeHash(bytes, ibStart, cbSize);
        map.Add(h64, bytes);
    }
    /// <inheritdoc />
    /// <summary>
    ///     Return the unique position within the TinyDictionary as the hash value (index).
    /// </summary>
    protected override byte[] HashFinal()
    {
        HashValue = (byte[]) map.FindKeyIndex(h64).GetBytes().Clone();
        return HashValue;
    }
}