Code Snippets: July 2010

Friday, July 16, 2010

Finding the greatest power of 2 less than given integer

assuming that n is a 32-bit integer


if((n&(n-1))!=0) {
 n |= n >> 1; 
 n |= n >> 2; 
 n |= n >> 4; 
 n |= n >> 8; 
 n |= n >> 16;
 n=n+1;
}

Efficiently calculating integer powers of float


 static float pow(float base, int exp) {
  float result = 1;
  if(exp<0) return 1.0f/pow(base,-exp);
  while (exp!=0) {
   if ((exp & 1)!=0) result *= base;
   exp >>= 1;
   base *= base;
  }
  return result;
 }

Displaying fraction in HTML

Showing something like this : ²²⁄₇


function fraction(a, b) {
     return "<span style=\"font-size:80%;letter-spacing:-1px;\"><sup>"+a+"</sup>⁄<sub>"+b+"</sub> </span>";
}

Thursday, July 15, 2010

C++ code printing itself


#include <iostream>
#include <string>
using namespace std;
int f;void s(char *m);void o(char *m){char *p=m,c=*p;while(*p>0&&(c=*p++))
if(!f||c!=(char)10)cout<<c;else cout<<(char)92<<(char)110;f=1;}
char m[310];int main(){s(m);o(m);cout<<(char)34;o(m);
cout<<(char)34<<(char)41<<(char)59<<(char)125<<endl;return 0;}
void s(char *m){strcpy(m,"#include <iostream>\n#include <string>\nusing namespace std;\nint f;void s(char *m);void o(char *m){char *p=m,c=*p;while(*p>0&&(c=*p++))\nif(!f||c!=(char)10)cout<<c;else cout<<(char)92<<(char)110;f=1;}\nchar m[310];int main(){s(m);o(m);cout<<(char)34;o(m);\ncout<<(char)34<<(char)41<<(char)59<<(char)125<<endl;return 0;}\nvoid s(char *m){strcpy(m,");}

Simple Metronom for Windows

Only for WIN32 console. not sure conio.h is deprecated


#include <windows.h>
#include <conio.h>
#include <stdio.h>

void main() {
 while(!kbhit()) {
  puts("\x07");
  Sleep(750);
 }
}

Finding Wi-Fi IP address of iphone

It will work for any unix-like system- might need to change "en0" to something else


#include <ifaddrs.h>
const char* http_ip_address(void)
{
 struct ifaddrs *interfaces = NULL;
 struct ifaddrs *temp_addr = NULL;
 int success = 0;
 
 // retrieve the current interfaces - returns 0 on success
 success = getifaddrs(&interfaces);
 if (success != 0) return NULL;
 
 // Loop through linked list of interfaces
 temp_addr = interfaces;
 while(temp_addr != NULL)
 {
  if(temp_addr->ifa_addr->sa_family == AF_INET)
  {
   // Check if interface is en0 which is the wifi connection on the iPhone
   if(!strcmp(temp_addr->ifa_name,"en0"))
   {
    freeifaddrs(interfaces);
    return inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr);
   }
  }
  
  temp_addr = temp_addr->ifa_next;
 }
 
 // Free memory
 freeifaddrs(interfaces); 
 return NULL;
}

Wednesday, July 14, 2010

Converting unix timestamp to "~ seconds ago", etc.

Written in PHP


function datestring($date) {
 $diff=time()-$date;
 if($diff<60) {
  $date=$diff."seconds ago";
 } else if($diff<3600) {
  $date=round($diff/60)."minutes ago";
 } else if($diff<86400) {
  $date=round($diff/3600)."hours ago";
 } else {
  $date=round($diff/86400)."days ago";
 }
 return $date;
}

Fast Fourier Transform


// fft function- the length n has to be a power of 2
// v[] is composed of imaginary and real parts of the input:
// [ imag[0], real[0], imag[1], real[1], .... ]
// the function is in-place thus the result is in the original array.

void FFT(float v[], int n) {
 int        ip, k, length;
 float      theta, pi = 3.1415926535897932384f;
 float      wr, wi, ur, ui, tr, ti, tmp;
 
 for (int i=0, j=0; i < n-1; i++,j+=k) {
  if (i<j) {
   tmp = v[i*2]; v[i*2] = v[j*2]; v[j*2] = tmp;
   tmp = v[i*2+1]; v[i*2+1] = v[j*2+1]; v[j*2+1] = tmp;
  }
  for(k=n/2;k<:=j;k>>=1) j -= k;
 }
 for (int li=1; li<:n; li*=2 ){
  length = 2*li;
  theta  = pi/li;
  ur   = 1.0f;
  ui   = 0.0f;
  if ( li == 1 ) {
   wr = -1.0f;
   wi =  0.0f;
  } else if ( li == 2 ) {
   wr =  0.0f;
   wi =  1.0f;
  } else {
   wr = cos(theta);
   wi = sin(theta);
  }
  for (int j = 0; j <: li; j++ ) {
   for (int i = j; i <: n; i += length ) {
    ip=i+li;
    tr=v[ip*2]*ur-v[ip*2+1]*ui; ti=v[ip*2]*ui+v[ip*2+1]*ur;
    v[ip*2]=v[i*2]-tr;   v[ip*2+1]=v[i*2+1]-ti;
    v[i*2]+=tr;     v[i*2+1]+=ti;
   }
   ur = ur*wr - ui*wi;
   ui = ui*wr + ur*wi;
  }
 }
}

Intersection of two line segments

The JAVA code below finds intersection of two line segments (x00,y00)-(x01,y01) and (x10,y10)-(x11,y11)

It returns float array of the coordinates of the intersection, or null if there is no intersection,


public static float[] intersection(float x00, float y00, float x01, float y01,
           float x10, float y10, float x11, float y11) {

 float ABx=x01-x00, ABy=y01-y00;
 float CDx=x11-x10, CDy=y11-y10;
 float BCx=x10-x01, BCy=y10-y01;
 float BDx=x11-x01, BDy=y11-y01;
 float DAx=x00-x11, DAy=y00-y11;
 float DBx=x01-x11, DBy=y01-y11;
 
 // check if the line segments are not crossing
 if((ABx*BCy-ABy*BCx)*(ABx*BDy-ABy*BDx)>=0) return null;
 if((CDx*DAy-CDy*DAx)*(CDx*DBy-CDy*DBx)>=0) return null;
 float d=(x00-x01)*(y10-y11)-(y00-y01)*(x10-x11);
 if(d==0) return null;
 
 float x=((x00*y01-y00*x01)*(x10-x11)-(x00-x01)*(x10*y11-y10*x11))/d;
 float y=((x00*y01-y00*x01)*(y10-y11)-(y00-y01)*(x10*y11-y10*x11))/d;
 float result[]={x,y};
 return result;
}

Getting current timestamp in UNIX


static double now_ms(void) { 
    struct timeval tv; 
    gettimeofday(&tv, NULL); 
    return tv.tv_sec*1000. + tv.tv_usec/1000.; 
}

Syntax Highlighting Test


#include <stdio.h>

int main(int argc, char ** argv) {
    printf("Hello World\n");
    return 0;
}


<link href='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Styles/SyntaxHighlighter.css' rel='stylesheet' type='text/css' />
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shCore.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushCpp.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushPython.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushPhp.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushJava.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushJScript.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushXml.js' type='text/javascript'/>
<script src='http://www-personal.umich.edu/~jongwook/SyntaxHighlighter/Scripts/shBrushCss.js' type='text/javascript'/>
<script type='text/javascript'>
dp.SyntaxHighlighter.BloggerMode();
dp.SyntaxHighlighter.HighlightAll();
</script>

Code Snippets