using System;
using Org.BouncyCastle.Crypto;

namespace Org.BouncyCastle.Crypto.Digests
{
	/**
	* Wrapper class that reduces the output length of a particular digest to
	* only the first n bytes of the digest function.
	*/
	public class ShortenedDigest
		: IDigest
	{
		private IDigest	baseDigest;
		private int		length;

		/**
		* Base constructor.
		*
		* @param baseDigest underlying digest to use.
		* @param length length in bytes of the output of doFinal.
		* @exception ArgumentException if baseDigest is null, or length is greater than baseDigest.GetDigestSize().
		*/
		public ShortenedDigest(
			IDigest	baseDigest,
			int		length)
		{
			if (baseDigest == null)
			{
				throw new ArgumentNullException("baseDigest");
			}

			if (length > baseDigest.GetDigestSize())
			{
				throw new ArgumentException("baseDigest output not large enough to support length");
			}

			this.baseDigest = baseDigest;
			this.length = length;
		}

		public string AlgorithmName
		{
			get { return baseDigest.AlgorithmName + "(" + length * 8 + ")"; }
		}

		public int GetDigestSize()
		{
			return length;
		}

		public void Update(byte input)
		{
			baseDigest.Update(input);
		}

		public void BlockUpdate(byte[] input, int inOff, int length)
		{
			baseDigest.BlockUpdate(input, inOff, length);
		}

		public int DoFinal(byte[] output, int outOff)
		{
			byte[] tmp = new byte[baseDigest.GetDigestSize()];

			baseDigest.DoFinal(tmp, 0);

	        Array.Copy(tmp, 0, output, outOff, length);

			return length;
		}

		public void Reset()
		{
			baseDigest.Reset();
		}

		public int GetByteLength()
		{
			return baseDigest.GetByteLength();
		}
	}
}
