text: package org.bouncycastle.math.ec;
  
  import java.math.BigInteger;
  
  import org.bouncycastle.math.raw.Nat;
  
  public class FixedPointCombMultiplier extends AbstractECMultiplier
  {
      protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
      {
          ECCurve c = p.getCurve();
          int size = FixedPointUtil.getCombSize(c);
  
          if (k.bitLength() > size)
          {
              /*
               * TODO The comb works best when the scalars are less than the (possibly unknown) order.
               * Still, if we want to handle larger scalars, we could allow customization of the comb
               * size, or alternatively we could deal with the 'extra' bits either by running the comb
               * multiple times as necessary, or by using an alternative multiplier as prelude.
               */
              throw new IllegalStateException("fixed-point comb doesn't support scalars larger than the curve order");
          }
  
          FixedPointPreCompInfo info = FixedPointUtil.precompute(p);
          ECLookupTable lookupTable = info.getLookupTable();
          int width = info.getWidth();
  
          int d = (size + width - 1) / width;
  
          ECPoint R = c.getInfinity();
  
          int fullComb = d * width;
          int[] K = Nat.fromBigInteger(fullComb, k);
  
          int top = fullComb - 1; 
          for (int i = 0; i < d; ++i)
          {
              int secretIndex = 0;
  
              for (int j = top - i; j >= 0; j -= d)
              {
                  int secretBit = K[j >>> 5] >>> (j & 0x1F);
                  secretIndex ^= secretBit >>> 1;
                  secretIndex <<= 1;
                  secretIndex ^= secretBit;
              }
  
              ECPoint add = lookupTable.lookup(secretIndex);
  
              R = R.twicePlus(add);
          }
  
          return R.add(info.getOffset());
      }
  }
  