function bnModPow()

in application/resources/libs/jsbn2.js [411:467]

• 52 lines of code
• 21 McCabe index (conditional complexity)

function bnModPow(e,m) {
  var i = e.bitLength(), k, r = nbv(1), z;
  if(i <= 0) return r;
  else if(i < 18) k = 1;
  else if(i < 48) k = 3;
  else if(i < 144) k = 4;
  else if(i < 768) k = 5;
  else k = 6;
  if(i < 8)
    z = new Classic(m);
  else if(m.isEven())
    z = new Barrett(m);
  else
    z = new Montgomery(m);

  // precomputation
  var g = new Array(), n = 3, k1 = k-1, km = (1<<k)-1;
  g[1] = z.convert(this);
  if(k > 1) {
    var g2 = nbi();
    z.sqrTo(g[1],g2);
    while(n <= km) {
      g[n] = nbi();
      z.mulTo(g2,g[n-2],g[n]);
      n += 2;
    }
  }

  var j = e.t-1, w, is1 = true, r2 = nbi(), t;
  i = nbits(e[j])-1;
  while(j >= 0) {
    if(i >= k1) w = (e[j]>>(i-k1))&km;
    else {
      w = (e[j]&((1<<(i+1))-1))<<(k1-i);
      if(j > 0) w |= e[j-1]>>(this.DB+i-k1);
    }

    n = k;
    while((w&1) == 0) { w >>= 1; --n; }
    if((i -= n) < 0) { i += this.DB; --j; }
    if(is1) {	// ret == 1, don't bother squaring or multiplying it
      g[w].copyTo(r);
      is1 = false;
    }
    else {
      while(n > 1) { z.sqrTo(r,r2); z.sqrTo(r2,r); n -= 2; }
      if(n > 0) z.sqrTo(r,r2); else { t = r; r = r2; r2 = t; }
      z.mulTo(r2,g[w],r);
    }

    while(j >= 0 && (e[j]&(1<<i)) == 0) {
      z.sqrTo(r,r2); t = r; r = r2; r2 = t;
      if(--i < 0) { i = this.DB-1; --j; }
    }
  }
  return z.revert(r);
}