[Click] random bit errors element

[Eddie Kohler]

Hi Alastair,

Thanks for the patch!  You missed one important fix, around line 168, which 
would need changes to produce a random number with 28 bits of randomness. 
There are a few other places too.  I've checked in a fixed (I hope) version 
that seems to work.  Thanks again!

Eddie


Alastair McKinley wrote:
> Hi everyone,
> 
> I would like to use the RandomBitErrors element to produce a lower bit
> error rate than currently possible.  At the moment its minimum bit error
> rate is 2x10-5.  So I've been trying to figure out how this element
> works and unfortunately in my lameness, I haven't quite got to the
> bottom of it.  I want to use 28 bits of fraction for the conversion of
> the bit error configuration string.
> 
> I've made a guess at how it might work and attached a diff.  Would
> someone be able to tell me if this will work correctly? 
> 
> Best regards and thanks,
> 
> Alastair
> 
> 
> 
> 
> --- randomerror.cc.bak	2006-02-23 12:08:51.000000000 +0000
> +++ randomerror.cc	2006-02-23 13:54:23.000000000 +0000
> @@ -83,9 +83,9 @@
>  void
>  RandomBitErrors::set_bit_error(unsigned bit_error)
>  {
> -  assert(bit_error <= 0x10000);
> +  assert(bit_error <= 0x10000000);
>    _p_bit_error = bit_error;
> -  unsigned non_bit_error = (0xFFFF - bit_error) + 1;
> +  unsigned non_bit_error = (0xFFFFFFF - bit_error) + 1;
>    // _p_error[i] is the probability that i bits are flipped.
>    // _p_error[i] = bit_error^i * (1-bit_error)^(8-i) * #combinations
>    
> @@ -93,13 +93,13 @@
>    for (int i = 0; i < 9; i++) {
>      uint64_t p = 0x100000000LL;
>      for (int j = 0; j < i; j++)
> -      p = (p * bit_error) >> 16;
> +      p = (p * bit_error) >> 28;
>      for (int j = i; j < 8; j++)
> -      p = (p * non_bit_error) >> 16;
> +      p = (p * non_bit_error) >> 28;
>      // account for # of combinations
>      p *= bit_flip_array_idx[i+1] - bit_flip_array_idx[i];
>      accum += p;
> -    _p_error[i] = (accum >> 16) & 0x1FFFF;
> +    _p_error[i] = (accum >> 28) & 0x1FFFFFFF;
>      if ((accum & 0xFFFFFFFF) >= 0x80000000)
>        _p_error[i]++;
>    }
> @@ -112,7 +112,7 @@
>    String kind_str = "flip";
>    bool on = true;
>    if (cp_va_parse(conf, this, errh,
> -		  cpUnsignedReal2, "bit error probability", 16, &bit_error,
> +		  cpUnsignedReal2, "bit error probability", 28, &bit_error,
>  		  cpOptional,
>  		  cpString, "action (set/clear/flip)", &kind_str,
>  		  cpBool, "active?", &on,
> @@ -130,7 +130,7 @@
>      return errh->error("bad action '%s' (must be 'set', 'clear', or 'flip')",
>  		       kind_str.c_str());
>    
> -  if (bit_error > 0x10000)
> +  if (bit_error > 0x10000000)
>      return errh->error("drop probability must be between 0 and 1");
>    if (bit_error == 0)
>      errh->warning("zero bit error probability (underflow?)");
> @@ -147,7 +147,7 @@
>  RandomBitErrors::simple_action(Packet *p_in)
>  {
>    // if no chance we'll flip a bit, return now
> -  if (!_on || _p_error[0] >= 0x10000)
> +  if (!_on || _p_error[0] >= 0x10000000)
>      return p_in;
>    
>    WritablePacket *p = p_in->uniqueify();
> @@ -184,7 +184,7 @@
>    int which = (intptr_t)vwhich;
>    RandomBitErrors *lossage = (RandomBitErrors *)f;
>    if (which == 0)
> -    return cp_unparse_real2(lossage->p_bit_error(), 16);
> +    return cp_unparse_real2(lossage->p_bit_error(), 28);
>    else if (which == 1) {
>      switch (lossage->kind()) {
>       case 0: return "clear";
> 
> 
> 
> 
> 
> 
> _______________________________________________
> click mailing list
> click at amsterdam.lcs.mit.edu
> https://amsterdam.lcs.mit.edu/mailman/listinfo/click