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Bit Shifting Operators that Wrap

Jun 15, 2011 at 1:34am
LB (13399)
01101001 << 3 = 01001011
01101001 >> 4 = 10010110
Sadly this is not true, the bits that go off the edge of the byte just get cut off. This is useful, though. However, wrapping around is also useful - your challenge is to come up with a use for shift left and shift right operators that wrap bits around, and then actually write functions for them that do it like above. Choose any variable type you prefer; char, short, long, long long, etc. and use any method you prefer, even if it involves strings - though using only bitwise operators would be quite interesting to do :)

I myself accept this challenge, so I will respond with my own code once I have it working.
Jun 15, 2011 at 2:34am
helios (17607)
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//Uncomment if the sizes of all unsigned types are powers of two.
//#define POWER_SIZES

template <typename T>
T rotate_left(T x,T amount){
	const unsigned bits=sizeof(T)*8;
#if POWER_SIZES
	amount&=bits-1;
#else
	amount%=bits;
#endif
	if (amount)
		return (x<<amount)|(x>>(bits-amount));
	return x;
}

template <typename T>
T rotate_right(T x,T amount){
	const unsigned bits=sizeof(T)*8;
#if POWER_SIZES
	amount&=bits-1;
#else
	amount%=bits;
#endif
	if (amount)
		return (x>>amount)|(x<<(bits-amount));
	return x;
}
As usual, bitwise operations exhibit wonky behavior when performed on signed operands.
Last edited on Jun 15, 2011 at 2:40am
Jun 15, 2011 at 2:45am
jsmith (5804)
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int rol( int x, unsigned char b ) {
	asm ( "roll %1, %0" : "=g"(x) : "cI"(b) : );
	return x;
}

int ror( int x, unsigned char b ) {
	asm ( "rorl %1, %0" : "=g"(x) : "cI"(b) : );
	return x;
}


Jun 15, 2011 at 3:11am
closed account (3hM2Nwbp)
Excellent example of when inline assembly is appropriate, jsmith!
Jun 15, 2011 at 4:01am
m4ster r0shi (2201)
I use bitwise operators and tail recursion.

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#include <iostream>
#include <bitset>
#include <string>

typedef unsigned char Byte;
typedef unsigned int Number;

Byte rotate_left(Byte b, Number n)
{
    if (n == 0) return b;

    return rotate_left((b << 1) | (b >> 7), (n & 7) - 1);
}

Byte rotate_right(Byte b, Number n)
{
    if (n == 0) return b;

    return rotate_right((b >> 1) | (b << 7), (n & 7) - 1);
}

int main()
{
    Byte b = std::bitset<8>(std::string("10000001")).to_ulong();

    b = rotate_right(b, 1000000000 + 2);

    std::cout << std::bitset<8>(b).to_string() << std::endl;

    b = rotate_left(b, 1000000000 + 4);

    std::cout << std::bitset<8>(b).to_string() << std::endl;

    return 0;
}
Edit & run on cpp.sh

EDIT: It's slower, but cooler :D
Last edited on Jun 15, 2011 at 5:26am
Jun 15, 2011 at 5:15am
LB (13399)
Of all these cool methods, mine is quite clunky and is a very direct approach. It also doesn't work correctly when shifting left...
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#include <iostream>

template<typename T>
class BitWrapper
{
	T v;
public:
	BitWrapper() : v(0) {}
	BitWrapper(const BitWrapper<T>& bw) : v(bw.v) {}
	BitWrapper(const T& V) : v(V) {}
	BitWrapper<T>& operator=(const BitWrapper<T>& bw){ v = bw.v; return(*this); }
	BitWrapper<T>& operator=(const T& V){ v = V; return(*this); }
	operator T()const{ return(v); }

	BitWrapper<T>& operator<<=(const int& by) //Doesn't work correctly...
	{
		if(by < 0){ return((*this) >>= (0-by)); }
		else if(by != 0)
		{
			for(unsigned long i = 0; i < unsigned long(by); ++i)
			{
				T temp = v & (T(1) << (sizeof(T)*8 - 1));
				(v <<= 1) += (temp != 0 ? 1 : 0);
			}
		}
		return(*this);
	}
	BitWrapper<T> operator<<(const int& by)const
	{
		BitWrapper<T> temp (*this);
		return(temp <<= by);
	}
	BitWrapper<T>& operator>>=(const int& by)
	{
		if(by < 0){ return((*this) <<= (0-by)); }
		else if(by != 0)
		{
			for(unsigned long i = 0; i < unsigned long(by); ++i)
			{
				T temp = v & 1;
				(v >>= 1) += temp << (sizeof(T)*8 - 1);
			}
		}
		return(*this);
	}
	BitWrapper<T> operator>>(const int& by)const
	{
		BitWrapper<T> temp (*this);
		return(temp >>= by);
	}
};

template<typename T>
std::ostream& operator<<(std::ostream& out, const BitWrapper<T>& bw)
{
	for(unsigned long i = 0; i < sizeof(T)*8; ++i)
	{
		T on = T(bw) & (1 << i);
		out << (on != 0 ? '1' : '0');
	}
	return(out);
}

int main()
{
	const BitWrapper<unsigned char> bits = 150;
	std::cout << bits << " << 3 = " << (bits << 3) << std::endl;
	std::cout << bits << " >> 4 = " << (bits >> 4) << std::endl;

	std::cin.sync();
	std::cin.ignore();
}
 
01101001 << 3 = 00101101<--WRONG
01101001 >> 4 = 10010110
Edit & run on cpp.sh
Jun 15, 2011 at 6:47am
helios (17607)
I wanted to automatically generate the strings, but I don't really have the time.
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#include <iostream>
#include <sstream>
#include <vector>
#include <string>
#include <algorithm>
#include <bitset>

const char *transformations[]={
	"||||||||\n"
	"||||||||\n",

	"| | | | | | | |      \n"
	" \\ \\ \\ \\ \\ \\ x       \n"
	"  \\ \\ \\ \\ \\ x \\      \n"
	"   \\ \\ \\ \\ x \\ \\     \n"
	"    \\ \\ \\ x \\ \\ \\    \n"
	"     \\ \\ x \\ \\ \\ \\   \n"
	"      \\ x \\ \\ \\ \\ \\  \n"
	"       x \\ \\ \\ \\ \\ \\ \n"
	"      / \\ \\ \\ \\ \\ \\ \\\n"
	"      | | | | | | | |\n",

	"| | | | | | | |    \n"
	" \\ \\ \\ \\ \\ x /     \n"
	"  \\ \\ \\ \\ x x      \n"
	"   \\ \\ \\ x x \\     \n"
	"    \\ \\ x x \\ \\    \n"
	"     \\ x x \\ \\ \\   \n"
	"      x x \\ \\ \\ \\  \n"
	"     / x \\ \\ \\ \\ \\ \n"
	"    / / \\ \\ \\ \\ \\ \\\n"
	"    | | | | | | | |\n",

	"| | | | | | | |  \n"
	" \\ \\ \\ \\ x / /   \n"
	"  \\ \\ \\ x x /    \n"
	"   \\ \\ x x x     \n"
	"    \\ x x x \\    \n"
	"     x x x \\ \\   \n"
	"    / x x \\ \\ \\  \n"
	"   / / x \\ \\ \\ \\ \n"
	"  / / / \\ \\ \\ \\ \\\n"
	"  | | | | | | | |\n",

	"| | | | | | | |\n"
	" \\ \\ \\ x / / / \n"
	"  \\ \\ x x / /  \n"
	"   \\ x x x /   \n"
	"    x x x x    \n"
	"   / x x x \\   \n"
	"  / / x x \\ \\  \n"
	" / / / x \\ \\ \\ \n"
	"/ / / / \\ \\ \\ \\\n"
	"| | | | | | | |\n",

	"  | | | | | | | |\n"
	"   \\ \\ x / / / / \n"
	"    \\ x x / / /  \n"
	"     x x x / /   \n"
	"    / x x x /    \n"
	"   / / x x x     \n"
	"  / / / x x \\    \n"
	" / / / / x \\ \\   \n"
	"/ / / / / \\ \\ \\  \n"
	"| | | | | | | |  \n",

	"    | | | | | | | |\n"
	"     \\ x / / / / / \n"
	"      x x / / / /  \n"
	"     / x x / / /   \n"
	"    / / x x / /    \n"
	"   / / / x x /     \n"
	"  / / / / x x      \n"
	" / / / / / x \\     \n"
	"/ / / / / / \\ \\    \n"
	"| | | | | | | |    \n",

	"      | | | | | | | |\n"
	"       x / / / / / / \n"
	"      / x / / / / /  \n"
	"     / / x / / / /   \n"
	"    / / / x / / /    \n"
	"   / / / / x / /     \n"
	"  / / / / / x /      \n"
	" / / / / / / x       \n"
	"/ / / / / / / \\      \n"
	"| | | | | | | |      \n"
};

inline int read_left(int x){
	if (x<10)
		return x;
	x/=10;
	return x/10;
}

inline int read_right(int x){
	if (x<10)
		return x;
	x/=10;
	return x%10;
}

inline int cross_and_combine(int left,int right){
	//std::cout <<"cross_and_combine("<<right<<","<<left<<")="<<right*100+left*10<<"\n";
	return right*100+left*10;
}

void print_step(const std::vector<int> &v){
	for (size_t a=0;a<v.size();a++){
		int i=v[a];
		if (i<0)
			std::cout <<' ';
		else if (i<10)
			std::cout <<i;
		else if (i==10)
			std::cout <<'A';
		else if (i==100)
			std::cout <<'B';
		else if (i==110)
			std::cout <<'C';
		else
			std::cout <<' ';
		std::cout <<':';
	}
	std::cout <<'\n';
}

unsigned char rotate(unsigned char x,int amount){
	bool left;
	if (amount<0){
		left=0;
		amount=-amount;
	}else
		left=1;
	amount&=7;
	std::vector<std::string> strings;
	{
		std::stringstream stream(transformations[amount]);
		std::string line;
		while (std::getline(stream,line))
			strings.push_back(line);
	}
	if (!left){
		for (size_t a=0;a<strings.size();a++){
			std::string &i=strings[a];
			std::reverse(i.begin(),i.end());
			for (size_t b=0;b<i.size();b++){
				if (i[b]=='\\')
					i[b]='/';
				else if (i[b]=='/')
					i[b]='\\';
			}
		}
	}
	std::vector<int> step,
		next_step;
	for (size_t a=0;a<strings[0].size();a++){
		if (strings[0][a]=='|'){
			step.push_back(!!(x&0x80));
			x<<=1;
		}else
			step.push_back(-1);
	}
	print_step(step);
	for (size_t a=1;a<strings.size()-1;a++){
		std::string &s=strings[a];
		for (size_t z=0;z<s.size();z++)
			std::cout <<s[z]<<' ';
		std::cout <<std::endl;
		next_step.clear();
		for (size_t b=0;b<s.size();b++){
			switch (s[b]){
				case '\\':
					next_step.push_back(read_right(step[b-1]));
					break;
				case '/':
					next_step.push_back(read_left(step[b+1]));
					break;
				case 'x':
					next_step.push_back(cross_and_combine(read_right(step[b-1]),read_left(step[b+1])));
					break;
				default:
					next_step.push_back(-1);
			}
		}
		step=next_step;
		print_step(step);
	}
	x=0;
	{
		std::string &s=strings.back();
		for (size_t a=0;a<s.size();a++){
			if (s[a]=='|'){
				x<<=1;
				x|=step[a];
			}
		}
	}
	return x;
}

int main(){
	unsigned char n=172;
	std::cout <<std::bitset<8>(n).to_string()<<std::endl;
	n=rotate(n,-3);
	std::cout <<std::bitset<8>(n).to_string()<<std::endl;
	return 0;
}
Edit & run on cpp.sh
Last edited on Jun 15, 2011 at 6:49am
Jun 15, 2011 at 6:56am
m4ster r0shi (2201)
I hate you.
Jun 15, 2011 at 7:06am
helios (17607)
:-)
I also thought about simulating a small electronic circuit with two sets of flip-flops arranged into two shift registers, but it seemed messy and I wasn't sure how to terminate it. The above would be the wiring connecting the two shift registers.
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