Thread overview  


April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Andrei Alexandrescu Wrote:
> On 04/10/2010 12:01 PM, Andrei Alexandrescu wrote:
> > Consider:
> >
> > byte c = a  b;
> >
> > Say you already know min_a, max_a, min_b, and max_b. How do you compute min_c and max_c? I thought of it for a bit and it seems quite tricky.
> >
> >
> > Thanks,
> >
> > Andrei
>
> I think this would work:
>
> uint maxOR(uint maxa, uint maxb) {
> if (maxa < maxb) return maxOR(maxb, maxa);
> uint candidate = 0;
> foreach (i, bit; byBitDescending(maxa)) {
> if (bit) continue;
> auto t = candidate  (1 << (31  i));
> if (t <= maxb) candidate = t;
> }
> return maxa  candidate;
> }
>
> The idea is to find the candidate that would fill as many zeros in maxa as possible.
>
> But this is inefficient. I'm wondering whether a simple bitwise manipulation expression could do the trick.
Don't you need to take into account the minimum of b also? That is, the candidate needs to be greater than minb. Because filling in more holes doesn't necessarily mean biggest number, I don't think it works.
Simple counter case:
a's range is 4 to 5
b's range is 4 to 4
The max value of a  b is 5.
However, I think your function will return 15.
Also, you may to use the max of the smaller number. For example:
a's range is 3 to 5
b's range is 4 to 4
The max number here is 7, but only when a is 3 and b is 4. With your method, you automatically assume the number with the highest max value should be at its max value.
I think this is going to need a recursive solution, but I haven't yet wrapped my brain around how it needs to work. I think you should start by assuming for a given range, mina to maxa, you know that the top 1bits from ~(mina ^ maxa) will always be in the result, same with b. If you eliminate those bits, the problem gets narrower. I feel like a good solution first eliminates those, then decides how to set the next bit, and recurses. I'm sure there's some sort of method to decide how to set the bit in order to avoid brute force, but I haven't thought about it enough.
Steve

April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Steven Schveighoffer  On 04/10/2010 09:51 PM, Steven Schveighoffer wrote:
> Andrei Alexandrescu Wrote:
>
>> On 04/10/2010 12:01 PM, Andrei Alexandrescu wrote:
>>> Consider:
>>>
>>> byte c = a  b;
>>>
>>> Say you already know min_a, max_a, min_b, and max_b. How do you
>>> compute min_c and max_c? I thought of it for a bit and it seems
>>> quite tricky.
>>>
>>>
>>> Thanks,
>>>
>>> Andrei
>>
>> I think this would work:
>>
>> uint maxOR(uint maxa, uint maxb) { if (maxa< maxb) return
>> maxOR(maxb, maxa); uint candidate = 0; foreach (i, bit;
>> byBitDescending(maxa)) { if (bit) continue; auto t = candidate 
>> (1<< (31  i)); if (t<= maxb) candidate = t; } return maxa 
>> candidate; }
>>
>> The idea is to find the candidate that would fill as many zeros in
>> maxa as possible.
>>
>> But this is inefficient. I'm wondering whether a simple bitwise
>> manipulation expression could do the trick.
>
> Don't you need to take into account the minimum of b also? That is,
> the candidate needs to be greater than minb. Because filling in more
> holes doesn't necessarily mean biggest number, I don't think it
> works.
Ha! Good point. I'd forgotten the initial requirement and considered both values to start at 0. They don't! Adam? :o)
Andrei

April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Andrei Alexandrescu  On Sat, 10 Apr 2010 22:53:42 0400, Andrei Alexandrescu <SeeWebsiteForEmail@erdani.org> wrote: > On 04/10/2010 09:51 PM, Steven Schveighoffer wrote: >> Andrei Alexandrescu Wrote: >> >>> On 04/10/2010 12:01 PM, Andrei Alexandrescu wrote: >>>> Consider: >>>> >>>> byte c = a  b; >>>> >>>> Say you already know min_a, max_a, min_b, and max_b. How do you >>>> compute min_c and max_c? I thought of it for a bit and it seems >>>> quite tricky. >>>> >>>> >>>> Thanks, >>>> >>>> Andrei >>> >>> I think this would work: >>> >>> uint maxOR(uint maxa, uint maxb) { if (maxa< maxb) return >>> maxOR(maxb, maxa); uint candidate = 0; foreach (i, bit; >>> byBitDescending(maxa)) { if (bit) continue; auto t = candidate  >>> (1<< (31  i)); if (t<= maxb) candidate = t; } return maxa  >>> candidate; } >>> >>> The idea is to find the candidate that would fill as many zeros in >>> maxa as possible. >>> >>> But this is inefficient. I'm wondering whether a simple bitwise >>> manipulation expression could do the trick. >> >> Don't you need to take into account the minimum of b also? That is, >> the candidate needs to be greater than minb. Because filling in more >> holes doesn't necessarily mean biggest number, I don't think it >> works. > > Ha! Good point. I'd forgotten the initial requirement and considered both values to start at 0. They don't! Adam? :o) Couldn't help it, this seems like such a fun problem, I had to solve it :) uint maxor(uint mina, uint maxa, uint minb, uint maxb) { uint bt = 1 << 31; uint result = 0; while(bt) { if((bt & maxa) && (bt & mina)) { result = bt; if((bt & minb) ^ (bt & maxb)) { return result  (bt1);// always can choose all 1s for rest of b } } else if((bt & maxb) && (bt & minb)) { result = bt; if((bt & mina) ^ (bt & maxa)) { return result  (bt1);// always can choose all 1s for rest of a } } else if(bt & (maxa  maxb)) { result = bt; if(bt & (maxa & maxb)) { // both maxa and maxb have bt set, and both mina and minb have // bt unset. This means we can choose this bit to be set on // either, and it is possible to have all 1s set for the other // for the rest of the bits. return result  (bt  1); } else if(bt & maxa) { // now that we are using this bit from maxa, we need to raise // mina so it becomes the smallest value with bt set. However, // we don't really care about bits above bt, so setting // it to 0 is fine. mina = 0; } else { minb = 0; } } // else, neither maxa or maxb have bt set, continue to next bit. bt >>= 1; } return result; } This only solves unsigned (tested all ranges against a brute force solution up to max values of 64, given the nature of binary, I don't think there can be any weird cases that wouldn't have problems at lower ranges. Note that in my solution, maxa and maxb are inclusive. Signed is probably trickier, suppose one is always negative and one is always positive! I'm satisfied solving the unsigned for now, maybe someone else can modify this solution to work with signed integers also :) Steve 
April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Andrei Alexandrescu  On Sat, Apr 10, 2010 at 09:53:42PM 0500, Andrei Alexandrescu wrote: > Ha! Good point. I'd forgotten the initial requirement and considered both values to start at 0. They don't! Adam? :o) Yeah, my big bruteforce tester program ran loops for min_a and min_b too. And it tripped an assert. core.exception.AssertError@or.d(60): [10, 10]. expected: 3, got 2 (min == 2) I hacked it by only asserting if the mins are zero too... but I did mention it a couple of times in my spam as the day has gone on. I'm going to lecture a bit in this post. It is mostly for my benefit  maybe if I write up the patterns and thoughts explicitly, something new will come to mind. My current solution uses a pattern I noticed: 0100 1000 There, the best you get is 1100. But, 0100 1100 Lets you get to 1111. The reason is that duplicated one in there means you can trade a 100 for a 011, and still OR in the 100 thanks to the duplicate. So, I take the two numbers and OR them together, but then, if there is a bit duplicated, I trade one of them for all the following ones, and or that in too. That leads to the one liner solution: return a  ((1 << numbits(a&b))  1)  b; We OR the two values, then use AND to find duplicated bits. We take the biggest one and trade it out (a power of two minus one gives us all those ones). If there are no duplicated bits, a&b == 0, and 1 << 1 == 1, and 1  1 = 0, so it means no change, the correct solution! Now, here's the problem: what if all those ones are unavailable due to the minimum value? The example I've been using is (4, 4). It would be nice to trade one of those 4's for a 3, but if the min value for each is also 4, this is impossible. The formula above would give 7, but this limitation means you can't get bigger than 4. A minimum of one is fine still  it is the same as zero due to how OR works. The problem is a minimum of two or more. This cuts off ones from the right. Oh, but not necessarily! You get a one on the end for ALL odd numbers, so unless the min == max && !(max&1), you get the one on the end. So, the bigger the gap, the smaller our mask. Since the interval is inclusive, the most significant set bits on each max can never be masked out. We'll use that as the starting point and write up a loop solution. We start at the right  the ones place  and or it in if the bit is available. It looks like the another AND filter applied to the AND that's there, but instead of using numbits(a&b), we should use numbits(something_min). Which one are we carrying those ones from? Either, really. So as long as either one works, we should be ok. Let's try it. Nope :( The max is now too small. Gah, I need to get to bed. Here's what I have: ========= uint max_c(uint max_a, uint max_b, uint min_a, uint min_b) { uint tmp = max_a  max_b; uint true_min = min(min_a, min_b); if(min_a <= 1  min_b <= 1) tmp = ((1 << numbits(max_a&max_b))  1); else { // THIS PORTION IS WRONG tmp = ((1 << numbits(max_a&max_b))  1); uint mask = min(msb(max_a), msb(max_b))  1; for(uint v = 1; v < mask; v <<= 1) { if(true_min  v && !(true_min & v)) mask &= ~v; } tmp &= ~mask; } return tmp; } =========  Adam D. Ruppe http://arsdnet.net 
April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Adam D. Ruppe  I normally lurk here, because I don't have any projects that use D and thus I haven't really learned it. But I just wanted to say that this problem, and the three threads of solutions, constitute one of the most fascinating discussions I've seen on the newsgroups in quite a while  probably because I can almost solve the basic problem myself (though doubtless very suboptimally).
Anyway, the strangeness of this particular problem is really very entrancing. I could sit and think about it all day  except that it makes my brain hurt ;).
On 20100410 21:45, Adam D. Ruppe wrote:
> On Sat, Apr 10, 2010 at 09:53:42PM 0500, Andrei Alexandrescu wrote:
>> Ha! Good point. I'd forgotten the initial requirement and considered
>> both values to start at 0. They don't! Adam? :o)
>
> Yeah, my big bruteforce tester program ran loops for min_a and min_b too.
> And it tripped an assert.
>
> core.exception.AssertError@or.d(60): [10, 10]. expected: 3, got 2 (min == 2)
>
> I hacked it by only asserting if the mins are zero too... but I did mention
> it a couple of times in my spam as the day has gone on.
>
> I'm going to lecture a bit in this post. It is mostly for my benefit  maybe
> if I write up the patterns and thoughts explicitly, something new will come
> to mind.
>
>
> My current solution uses a pattern I noticed:
>
> 0100
> 1000
>
> There, the best you get is 1100.
>
> But,
>
> 0100
> 1100
>
> Lets you get to 1111. The reason is that duplicated one in there means you
> can trade a 100 for a 011, and still OR in the 100 thanks to the duplicate.
>
> So, I take the two numbers and OR them together, but then, if there is
> a bit duplicated, I trade one of them for all the following ones, and
> or that in too.
>
> That leads to the one liner solution: return a  ((1<< numbits(a&b))  1)  b;
>
> We OR the two values, then use AND to find duplicated bits. We take the
> biggest one and trade it out (a power of two minus one gives us all those
> ones).
>
> If there are no duplicated bits, a&b == 0, and 1<< 1 == 1, and 1  1 = 0,
> so it means no change, the correct solution!
>
>
>
> Now, here's the problem: what if all those ones are unavailable due to the
> minimum value? The example I've been using is (4, 4). It would be nice to
> trade one of those 4's for a 3, but if the min value for each is also 4,
> this is impossible. The formula above would give 7, but this limitation
> means you can't get bigger than 4.
>
> A minimum of one is fine still  it is the same as zero due to how OR works.
> The problem is a minimum of two or more. This cuts off ones from the right.
>
> Oh, but not necessarily! You get a one on the end for ALL odd numbers, so
> unless the min == max&& !(max&1), you get the one on the end.
>
> So, the bigger the gap, the smaller our mask. Since the interval is inclusive,
> the most significant set bits on each max can never be masked out.
>
>
> We'll use that as the starting point and write up a loop solution. We start
> at the right  the ones place  and or it in if the bit is available.
>
> It looks like the another AND filter applied to the AND that's there, but
> instead of using numbits(a&b), we should use numbits(something_min).
>
> Which one are we carrying those ones from? Either, really. So as long as
> either one works, we should be ok.
>
> Let's try it.
>
> Nope :( The max is now too small.
>
> Gah, I need to get to bed. Here's what I have:
>
> =========
> uint max_c(uint max_a, uint max_b, uint min_a, uint min_b) {
> uint tmp = max_a  max_b;
>
> uint true_min = min(min_a, min_b);
> if(min_a<= 1  min_b<= 1)
> tmp = ((1<< numbits(max_a&max_b))  1);
> else {
> // THIS PORTION IS WRONG
> tmp = ((1<< numbits(max_a&max_b))  1);
> uint mask = min(msb(max_a), msb(max_b))  1;
>
> for(uint v = 1; v< mask; v<<= 1) {
> if(true_min  v&& !(true_min& v))
> mask&= ~v;
> }
>
> tmp&= ~mask;
> }
> return tmp;
> }
> =========

April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Andrei Alexandrescu  How about this?
uint fill_bits(uint min_v, uint max_v) {
uint mask = 0;
for (int i = 0; i < 32; ++i) {
if ((min_v  (1 << i)) <= max_v) mask = (1 << i);
}
return mask;
}
max_c = min(
max_a  fill_bits(min_b, max_b),
max_b  fill_bits(min_a, max_a));

Rainer Deyke  rainerd@eldwood.com

April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Rainer Deyke  Rainer Deyke wrote: > How about this? > > uint fill_bits(uint min_v, uint max_v) { > uint mask = 0; > for (int i = 0; i < 32; ++i) { > if ((min_v  (1 << i)) <= max_v) mask = (1 << i); > } > return mask; > } > > max_c = min( > max_a  fill_bits(min_b, max_b), > max_b  fill_bits(min_a, max_a)); > > That proposal looks at the two pairs of a and b separately. For example, works with min_a and max_a, and decides a bit pattern. What if the allowable bit pattern for the b pair has 0 bits that would be filled with an value of a that was missed by fill_bits for a? Imagine a value of a, that has little number of 1 bits, but one of those bits happen to be the one that fills the hole in b... This problem has nothing directly to do with values, it has something to do with parts of values. The parts of values cannot be decided by looking at only a pair. Here are some values that the algorithm fails with ("mask" is printed before returning from fill_bits) : binary decimal  mask 0000000011 3 mask 0111111111 511 min_a 0000100100 36 max_a 0101100011 355 min_b 0000000001 1 max_b 0000000100 4 calculated 0101100011 355 WRONG! empirical 0101100111 359 emp_max_a 0101100011 355 emp_max_b 0000000100 4 The last two lines are telling: An unsuspected value of b happens to be the one that produces the maximum value of ab. Ali 
April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Ali Çehreli  On 4/11/2010 11:16, Ali Çehreli wrote: > Rainer Deyke wrote: >> How about this? >> >> uint fill_bits(uint min_v, uint max_v) { >> uint mask = 0; >> for (int i = 0; i < 32; ++i) { >> if ((min_v  (1 << i)) <= max_v) mask = (1 << i); >> } >> return mask; >> } >> >> max_c = min( >> max_a  fill_bits(min_b, max_b), >> max_b  fill_bits(min_a, max_a)); >> >> > > That proposal looks at the two pairs of a and b separately. For example, works with min_a and max_a, and decides a bit pattern. > > What if the allowable bit pattern for the b pair has 0 bits that would be filled with an value of a that was missed by fill_bits for a? Imagine a value of a, that has little number of 1 bits, but one of those bits happen to be the one that fills the hole in b... The intention of fill_bits is to create a number that contains all of the bits of all of the numbers from min_v to max_v. In other words: min_v  (min_v + 1)  (min_v + 2)  ...  (max_v  1)  max_v It does this by considering each bit separately. For each bit 'i', is there a number 'n' with that bit set such that 'min_v <= n <= max_v'? 'min_v  (1 << i)' is my attempt at calculating the smallest number with bit 'i' set that satisfies 'min_v  (1 << i)'. This is incorrect. Correct would be '(min_v & (1 << i)) ? min_v : ((min_v >> i) << i)  (1 << i)'. My other mistake is this bit: max_c = min( max_a  fill_bits(min_b, max_b), max_b  fill_bits(min_a, max_a)); This is my attempt to get a tighter fit than 'fill_bits(min_a, max_a)  fill_bits(min_b, max_b)'. It doesn't work correctly, as you have pointed out. Here is my revised attempt, with those errors corrected: uint fill_bits(uint min_v, uint max_v) { uint mask = min_v; for (int i = 0; i < 32; ++i) { if ((min_v & (1 << i)) == 0) { if ((((min_v >> i) << i)  (1 << i)) <= max_v) { mask = (1 << i); } } } return mask; } max_c = fill_bits(min_a, max_a)  fill_bits(min_b, max_b);  Rainer Deyke  rainerd@eldwood.com 
April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Rainer Deyke  Rainer Deyke wrote: > The intention of fill_bits is to create a number that contains all of > the bits of all of the numbers from min_v to max_v. But no value in the range may have all those bits set. As a result, ab may have less bits set than fill_bits returns. > In other words: > > min_v  (min_v + 1)  (min_v + 2)  ...  (max_v  1)  max_v That's the problem: Only one of those values is used in ab at a time. Here is the test code that I've been using when I tried to solve this problem: import std.random; import std.stdio; void report(string label, uint value) { writefln("%25s %032b %08x %10s", label, value, value, value); } // ... unittest { foreach (i; 0 .. 1000) { uint max_a = uniform(0, 1024); uint min_a = uniform(0, max_a + 1); uint max_b = uniform(0, 1024); uint min_b = uniform(0, max_b + 1); uint calculated_max_c = max_c_Deyke_2(min_a, max_a, min_b, max_b); report("min_a", min_a); report("max_a", max_a); report("min_b", min_b); report("max_b", max_b); report("calculated", calculated_max_c); uint emp_max_a; uint emp_max_b; uint empirical_max_c; foreach (a; min_a .. max_a + 1) { foreach(b; min_b .. max_b + 1) { if ((a  b) > empirical_max_c) { emp_max_a = a; emp_max_b = b; empirical_max_c = a  b; } } } if (empirical_max_c != calculated_max_c) { report("WRONG! empirical", empirical_max_c); report("emp_max_a", emp_max_a); report("emp_max_b", emp_max_b); break; } } } Yes, it's incomplete, but does catch problems. :) So far, Steven Schveighoffer's maxor() is the only one that passes the tests. (There may be other solutions that I've missed.) Ali 
April 11, 2010 Re: value range propagation for _bitwise_ OR  

 
Posted in reply to Ali Çehreli Attachments:
 Ali Çehreli wrote: > Yes, it's incomplete, but does catch problems. :) So far, Steven > Schveighoffer's maxor() is the only one that passes the tests. (There > may be other solutions that I've missed.) > I suggest you triplecheck your implementations of everyone's algorithms since the counterexample you found for my algorithm is wrong (i.e my code gives the correct result). Jerome  mailto:jeberger@free.fr http://jeberger.free.fr Jabber: jeberger@jabber.fr 
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