namespace NBitcoin.BouncyCastle.Math.EC.Multiplier
{
	/**
    * Class implementing the WNAF (Window Non-Adjacent Form) multiplication
    * algorithm.
    */
	internal class WNafL2RMultiplier
		: AbstractECMultiplier
	{
		/**
         * Multiplies <code>this</code> by an integer <code>k</code> using the
         * Window NAF method.
         * @param k The integer by which <code>this</code> is multiplied.
         * @return A new <code>ECPoint</code> which equals <code>this</code>
         * multiplied by <code>k</code>.
         */
		protected override ECPoint MultiplyPositive(ECPoint p, BigInteger k)
		{
			// Clamp the window width in the range [2, 16]
			int width = System.Math.Max(2, System.Math.Min(16, GetWindowSize(k.BitLength)));

			WNafPreCompInfo wnafPreCompInfo = WNafUtilities.Precompute(p, width, true);
			ECPoint[] preComp = wnafPreCompInfo.PreComp;
			ECPoint[] preCompNeg = wnafPreCompInfo.PreCompNeg;

			int[] wnaf = WNafUtilities.GenerateCompactWindowNaf(width, k);

			ECPoint R = p.Curve.Infinity;

			int i = wnaf.Length;

			/*
             * NOTE: We try to optimize the first window using the precomputed points to substitute an
             * addition for 2 or more doublings.
             */
			if(i > 1)
			{
				int wi = wnaf[--i];
				int digit = wi >> 16, zeroes = wi & 0xFFFF;

				int n = System.Math.Abs(digit);
				ECPoint[] table = digit < 0 ? preCompNeg : preComp;

				// Optimization can only be used for values in the lower half of the table
				if((n << 2) < (1 << width))
				{
					int highest = LongArray.BitLengths[n];

					// TODO Get addition/doubling cost ratio from curve and compare to 'scale' to see if worth substituting?
					int scale = width - highest;
					int lowBits = n ^ (1 << (highest - 1));

					int i1 = ((1 << (width - 1)) - 1);
					int i2 = (lowBits << scale) + 1;
					R = table[i1 >> 1].Add(table[i2 >> 1]);

					zeroes -= scale;

					//Console.WriteLine("Optimized: 2^" + scale + " * " + n + " = " + i1 + " + " + i2);
				}
				else
				{
					R = table[n >> 1];
				}

				R = R.TimesPow2(zeroes);
			}

			while(i > 0)
			{
				int wi = wnaf[--i];
				int digit = wi >> 16, zeroes = wi & 0xFFFF;

				int n = System.Math.Abs(digit);
				ECPoint[] table = digit < 0 ? preCompNeg : preComp;
				ECPoint r = table[n >> 1];

				R = R.TwicePlus(r);
				R = R.TimesPow2(zeroes);
			}

			return R;
		}

		/**
         * Determine window width to use for a scalar multiplication of the given size.
         * 
         * @param bits the bit-length of the scalar to multiply by
         * @return the window size to use
         */
		protected virtual int GetWindowSize(int bits)
		{
			return WNafUtilities.GetWindowSize(bits);
		}
	}
}
