content: package local.org.bouncycastle.math.ec;
  
  import java.math.BigInteger;
  
  /**
   * Class implementing the WTNAF (Window
   * <code>&tau;</code>-adic Non-Adjacent Form) algorithm.
   */
  class WTauNafMultiplier implements ECMultiplier
  {
      /**
       * Multiplies a {@link local.org.bouncycastle.math.ec.ECPoint.F2m ECPoint.F2m}
       * by <code>k</code> using the reduced <code>&tau;</code>-adic NAF (RTNAF)
       * method.
       * @param p The ECPoint.F2m to multiply.
       * @param k The integer by which to multiply <code>k</code>.
       * @return <code>p</code> multiplied by <code>k</code>.
       */
      public ECPoint multiply(ECPoint point, BigInteger k, PreCompInfo preCompInfo)
      {
          if (!(point instanceof ECPoint.F2m))
          {
              throw new IllegalArgumentException("Only ECPoint.F2m can be " +
                      "used in WTauNafMultiplier");
          }
  
          ECPoint.F2m p = (ECPoint.F2m)point;
  
          ECCurve.F2m curve = (ECCurve.F2m) p.getCurve();
          int m = curve.getM();
          byte a = curve.getA().toBigInteger().byteValue();
          byte mu = curve.getMu();
          BigInteger[] s = curve.getSi();
  
          ZTauElement rho = Tnaf.partModReduction(k, m, a, s, mu, (byte)10);
  
          return multiplyWTnaf(p, rho, preCompInfo, a, mu);
      }
  
      /**
       * Multiplies a {@link local.org.bouncycastle.math.ec.ECPoint.F2m ECPoint.F2m}
       * by an element <code>&lambda;</code> of <code><b>Z</b>[&tau;]</code> using
       * the <code>&tau;</code>-adic NAF (TNAF) method.
       * @param p The ECPoint.F2m to multiply.
       * @param lambda The element <code>&lambda;</code> of
       * <code><b>Z</b>[&tau;]</code> of which to compute the
       * <code>[&tau;]</code>-adic NAF.
       * @return <code>p</code> multiplied by <code>&lambda;</code>.
       */
      private ECPoint.F2m multiplyWTnaf(ECPoint.F2m p, ZTauElement lambda,
              PreCompInfo preCompInfo, byte a, byte mu)
      {
          ZTauElement[] alpha;
          if (a == 0)
          {
              alpha = Tnaf.alpha0;
          }
          else
          {
              // a == 1
              alpha = Tnaf.alpha1;
          }
  
          BigInteger tw = Tnaf.getTw(mu, Tnaf.WIDTH);
  
          byte[]u = Tnaf.tauAdicWNaf(mu, lambda, Tnaf.WIDTH,
                  BigInteger.valueOf(Tnaf.POW_2_WIDTH), tw, alpha);
  
          return multiplyFromWTnaf(p, u, preCompInfo);
      }
  
      /**
       * Multiplies a {@link local.org.bouncycastle.math.ec.ECPoint.F2m ECPoint.F2m}
       * by an element <code>&lambda;</code> of <code><b>Z</b>[&tau;]</code>
       * using the window <code>&tau;</code>-adic NAF (TNAF) method, given the
       * WTNAF of <code>&lambda;</code>.
       * @param p The ECPoint.F2m to multiply.
       * @param u The the WTNAF of <code>&lambda;</code>..
       * @return <code>&lambda; * p</code>
       */
      private static ECPoint.F2m multiplyFromWTnaf(ECPoint.F2m p, byte[] u,
              PreCompInfo preCompInfo)
      {
          ECCurve.F2m curve = (ECCurve.F2m)p.getCurve();
          byte a = curve.getA().toBigInteger().byteValue();
  
          ECPoint.F2m[] pu;
          if ((preCompInfo == null) || !(preCompInfo instanceof WTauNafPreCompInfo))
          {
              pu = Tnaf.getPreComp(p, a);
              p.setPreCompInfo(new WTauNafPreCompInfo(pu));
          }
          else
          {
              pu = ((WTauNafPreCompInfo)preCompInfo).getPreComp();
          }
  
          // q = infinity
          ECPoint.F2m q = (ECPoint.F2m) p.getCurve().getInfinity();
          for (int i = u.length - 1; i >= 0; i--)
          {
              q = Tnaf.tau(q);
              if (u[i] != 0)
              {
                  if (u[i] > 0)
                  {
                      q = q.addSimple(pu[u[i]]);
                  }
                  else
                  {
                      // u[i] < 0
                      q = q.subtractSimple(pu[-u[i]]);
                  }
              }
          }
  
          return q;
      }
  }
  