Principia Programatica: Greg Miller's Blog

I just saw a question regarding closures in JavaScript regarding code similar to the following:

for (i=0;i<n;i++){
     foo.bind( function() { bar(i); } );
}

The question was why does every closure execute bar(i) with i as the value of n for every instance, rather than whatever the value of i was when the closure was defined. He had reckoned that i was being passed by reference than by copy and found that making a copy of i in a variable local to the closure solved the problem.

But what he’s really done is confused a closure with an eval() statement. If he had written the code similar to eval(“function(){bar(“+i+”);”); he would have gotten what he wanted. The difference being that when you use eval() the variable i is determined when the eval() is executed. But with a closure, i is determined when the closure is executed, and every time the closure is executed.

It’s also important to note that the i referenced inside the enclosure is not a reference to the i in the outer loop, rather it actually IS the same i in the outer loop. No parameter passing is going on.

One other possible solution is to define the closure to take a parameter, and pass i in as a parameter to the enclosure:

for (i=0;i<n;i++){
     foo.bind( function(x) { bar(x); } )(i);
}

I had an old Red Had Linux system I wanted to convert to a Virtual Machine. The typical method recommended by most people is to back up the system, then restore it inside the VM. The problem I had with that is that you end up wasting a lot of time creating an image you don’t need, plus you need a storage device large enough to hold the image which can be accessed from both systems.

My solution was to create the VM with a virtual drive the size I needed. Then use a Knoppix image to initially boot the VM, once you get to a command line, use the commands below.

On the receiving server:

sudo nc -p 2222 -l | dd of=/dev/hda


On the system to be copied:

sudo dd if=/dev/hda | nc 192.168.0.56 2222


If your CPU on each device is faster than the network, you could also gain some speed by piping the data stream through bzip2. E.g. “sudo nc -p 2222 -l | bzip2 -d | dd of=/dev/hda” and “sudo dd if=/dev/hda | bzip2 -c | nc 192.168.0.56 2222”. Whether or not it’s worth it depends on the speed of your CPUs and the speed of your network.

It’s not a requirement that you boot the system being imaged from a CD, nor that you shut down any services. But, if you don’t, it’s possible you might end up with some files that aren’t in a valid state from the application’s point of view. There’s no real method I know of to monitor the progress of the copy, but you can run “fdisk /dev/hda” to verify the copy at least got started. If the copy started OK, then you’ll see partition information the same as on the system being imaged. You can also run “ifconfig” and watch the “RX bytes” field to get an idea of how much data has been transferred, but the counter will eventually overflow and start back at 0.

I had just ran into an issue attempting to activate my copy of Windows 2008 Server downloaded from MSDN. The error expands to “DNS Entry not found”, although all networking features work fine. The issue was that it was attempting to validate against an Activation Server rather than Microsoft’s website. The solution was to open “System” in the Control Panel, down at the bottom of that window is a link titled “Change Product Key”. I entered my key from the MSDN website and Windows activated automatically after the key was entered.

From what I’ve seen, the problem most people have with the difference between these two JavaScript functions isn’t knowing what they do, but it’s knowing that they both exist. I’ve seen lots of people constantly call setTimeout() calling the same function over and over where the last line of the function calls setTimeout(), but setInterval() is probably what they really wanted, they just didn’t know it existed. But I’ve also seen lots of code that calls setInterval() to execute a function after a specific delay, and the first thing that function does is call clearInterval(), so they obviously really wanted the functionality of setTimeout(). I guess it’s mostly a result of people finding one thing that gets the job done, and never looking for anything else.

The difference between the two functions is fairly trivial. setTimeout() executes the function only once, unless you call it again; and setInterval() continuously calls the function until clearInterval() is called. The only disadvantage to using setTimeout() when you really wanted the functionality of setInterval is that the time between each call will vary depending on how long it takes your code to execute (and sometimes you actually want that feature). There is some danger in using setInterval() and hoping your code is fast enough to call clearInterval() before the interval time passes. In such a case, the next call is queued and will execute your function any number of times depending on how many times the interval time passed while your code was running.

An experiment was conducted in order to study the differences between programmers, engineers, and mathematicians. One programmer, one engineer, and one mathematician were all locked in separate classrooms for one week. Each classroom contained all of the normal things you’d find in a classroom: chalkboard, chalk, desks, etc. In addition to that each classroom contained water, one can of food (enough to feed a person for a week), but no can opener. At the end of the week, they opened the programmer’s door first. The programmer walked out. When they looked in the classroom they saw the can beat to hell, but opened. When they opened the engineer’s door, the engineer walked out. In the classroom were a bunch of equations and diagrams written on the board, one small dent in the can which was all that was required to open it. Lastly they opened the mathematician’s door. No one came out. They looked in and saw the mathematician laying dead on the floor, the can of food untouched, and a long series of equations on the chalkboard. But at the end of the equations was written the phrase: “A solution exists”.

When you develop a Firefox extension you’re required to specify the minimum and maximum versions of Firefox which your extension is known to work with. This is done by specifying a minVersion and maxVersion in the install.rdf file inside your plugin’s archive. Since my ClearSkyPlugin isn’t likely to break in any future version of Firefox, I attempted to bypass the version checking by setting the minVersion to 1.0, and the maxVersion to 9.0.
Of course, today Firefox 3.0 was released and my extension was detected as incompatible. After some twiddling I managed to figure out I had to set the minVersion to 1.0.* to get things to work.

If you’re having trouble getting your downloaded extensions to work, you might just disable extension version checking altogether.

This uses reflection to avoid the clerical task of adding a project reference:

System.Type t = System.Type.GetTypeFromProgID("SAPI.SpVoice");
object o = System.Activator.CreateInstance(t);
t.InvokeMember("Speak", System.Reflection.BindingFlags.InvokeMethod, null, o, new object[] { "test", 0 });

If you don’t mind adding a reference, then the whole thing can be done in one line:

new SpeechLib.SpVoice().Speak("This is a test", SpeechLib.SpeechVoiceSpeakFlags.SVSFDefault);

The documentation for System.nanoTime() (and the function’s name itself) leads developers to believe it’s a much more accurate timer than anything else Java provides. Depending on which set of documentation you read, it claims to use “the most precise available system timer”. While I can’t speak for any architecture other than x86, I can state that System.nanoTime() is broken for multi-CPU or multi-core x86 systems, and generally unreliable on all x86 systems. The problem lies in the RDTSC instruction which retrieves the number of CPU ticks since the CPU started. On multi-core systems, each core will have its own tick count, and they will not match, so every time your process switches CPUs, you get a different measurement. The issue is compounded by the fact that some power management systems actually alter the CPU’s frequency to save power, which breaks the functionality even on single core, single CPU systems.

In general, do not use System.nanoTime() in any program which you don’t plan to strongly control how it is run, and the environment it is run in.

Below is an implementation I did of DES back in the early 90’s while I was still in college. It’s one of the first complex C programs I wrote. It served its purpose back in the day, but now more advanced algorithms and APIs have eliminated the need for such things. It’s been heavily optimized, more so than is practical in today’s world, so you probably couldn’t get it to compile even if you wanted to use it. I post it here to reminisce on my programming ability at the time, and to give an example of the complexity/simplicity of implementing an encryption algorithm.

    1 #include <stdio.h>
    2 
    3 static int keyout[17][48];
    4 
    5 void des_init(),lshift(),cypher(),des_encrypt(),des_descrypt();
    6 
    7 void des_init(unsigned char *key){
    8  unsigned char c[28],d[28];
    9  static int pc1[56] = {57,49,41,33,25,17,9,
   10               01,58,50,42,34,26,18,
   11               10,02,59,51,43,35,27,
   12               19,11,03,60,52,44,36,
   13               63,55,47,39,31,23,15,
   14               07,62,54,46,38,30,22,
   15               14,06,61,53,45,37,29,
   16               21,13,05,28,20,12,04};
   17  static int pc2[48] = {14,17,11,24,1,5,
   18               3,28,15,6,21,10,
   19               23,19,12,4,26,8,
   20               16,7,27,20,13,2,
   21               41,52,31,37,47,55,
   22               30,40,51,45,33,48,
   23               44,49,39,56,34,53,
   24               46,42,50,36,29,32};
   25  static int nls[17] = {
   26   0,1,1,2,2,2,2,2,2,1,2,2,2,2,2,2,1};
   27 
   28  static int cd[56],keyb[64];
   29  static int cnt,n=0;
   30  register int i,j;
   31 
   32  for(i=0;i<8;i++) /*Read in key*/
   33   for(j=0;j<8;j++) keyb[n++]=(key[i]>>j&0x01);
   34 
   35  for(i=0;i<56;i++) /*Permuted choice 1*/
   36   cd[i]=keyb[pc1[1]-1];
   37  for(i=0;i<28;i++){
   38   c[i]=cd[i];
   39   d[i]=cd[i+28];
   40  }
   41  for(cnt=1;cnt<=16;cnt++){
   42   for(i=0;i<nls[cnt];i++){
   43    lshift(c); lshift(d);
   44   }
   45   for(i=0;i<28;i++){
   46    cd[i]=c[i];
   47    cd[i+28]=d[i];
   48   }
   49   for(i=0;i<48;i++) /*Permuted Choice 2*/
   50    keyout[cnt][i]=cd[pc2[i]-1];
   51  }
   52 }
   53 
   54 static void lshift(unsigned char shft[]){
   55  register int temp,i;
   56 
   57  temp=shft[0];
   58  for(i=0;i<27;i++) shft[i]=shft[i+1];
   59  shft[27]=temp;
   60 }
   61 
   62 static void cypher(int *r, int cnt, int *fout){
   63  static int expand[48],b[8][6],sout[8],pin[48];
   64  register int i,j;
   65  static int n,row,col,scnt;
   66  static int p[32]={
   67     16,7,20,21,29,12,28,17,1,15,23,26,
   68      5,18,31,10,2,8,24,14,32,27,3,9,
   69     19,13,30,6,22,11,4,25};
   70 
   71  static int   e[48] = {32,1,2,3,4,5,
   72               4,5,6,7,8,9,
   73               8,9,10,11,12,13,
   74               12,13,14,15,16,17,
   75               16,17,18,19,20,21,
   76               20,21,22,23,24,25,
   77               24,25,26,27,28,29,
   78               28,29,30,31,32,1};
   79 
   80  static char s[8][64] = {
   81     14,4,13,1,2,15,11,8,3,10,6,12,5,9,0,7, /*s1*/
   82      0,15,7,4,14,2,13,1,10,6,12,11,9,5,3,8,
   83      4,1,14,8,13,6,2,11,15,12,9,7,3,10,5,0,
   84     15,12,8,2,4,9,1,7,5,11,3,14,10,0,6,13,
   85     15,1,8,14,6,11,3,4,9,7,2,13,12,0,5,10, /*s2*/
   86      3,13,4,7,15,2,8,14,12,0,1,10,6,9,11,5,
   87      0,14,7,11,10,4,13,1,5,8,12,6,9,3,2,15,
   88     13,8,10,1,3,15,4,2,11,6,7,12,0,5,14,9,
   89     10,0,9,14,6,3,15,5,1,13,12,7,11,4,2,8, /*s3*/
   90     13,7,0,9,3,4,6,10,2,8,5,14,12,11,15,1,
   91     13,6,4,9,8,15,3,0,11,1,2,12,5,10,14,7,
   92      1,10,13,0,6,9,8,7,4,15,14,3,11,5,2,12,
   93      7,13,14,3,0,6,9,10,1,2,8,5,11,12,4,15,/*s4*/
   94     13,8,11,5,6,15,0,3,4,7,2,12,1,10,14,9,
   95     10,6,9,0,12,11,7,13,15,1,3,14,5,2,8,4,
   96      3,15,0,6,10,1,13,8,9,4,5,11,12,7,2,14,
   97      2,12,4,1,7,10,11,6,8,5,3,15,13,0,14,9,/*s5*/
   98     14,11,2,12,4,7,13,1,5,0,15,10,3,9,8,6,
   99      4,2,1,11,10,13,7,8,15,9,12,5,6,3,0,14,
  100     11,8,12,7,1,14,2,13,6,15,0,9,10,4,5,3,
  101     12,1,10,15,9,2,6,8,0,13,3,4,14,7,5,11, /*s6*/
  102     10,15,4,2,7,12,9,5,6,1,13,14,0,11,3,8,
  103      9,14,15,5,2,8,12,3,7,0,4,10,1,13,11,6,
  104      4,3,2,12,9,5,15,10,11,14,1,7,6,0,8,13,
  105      4,11,2,14,15,0,8,13,3,12,9,7,5,10,6,1,/*s7*/
  106     13,0,11,7,4,9,1,10,14,3,5,12,2,15,8,6,
  107      1,4,11,13,12,3,7,14,10,15,6,8,0,5,9,2,
  108      6,11,13,8,1,4,10,7,9,5,0,15,14,2,3,12,
  109     13,2,8,4,6,15,11,1,10,9,3,14,5,0,12,7, /*s8*/
  110      1,15,13,8,10,3,7,4,12,5,6,11,0,14,9,2,
  111      7,11,4,1,9,12,14,2,0,6,10,13,15,3,5,8,
  112      2,1,14,7,4,10,8,13,15,12,9,0,3,5,6,11
  113  };
  114 
  115  for(i=0;i<48;i++) expand[i]=r[e[i]-1]; /*Expansion Function*/
  116  for(i=n=0;i<8;i++) {
  117   for(j=0;j<6;j++,n++) b[i][j]=expand[n]^keyout[cnt][n];
  118  }
  119 
  120  /*Selection functions*/
  121 
  122   for(scnt=n=0;scnt<8;scnt++){
  123    row=(b[scnt][0]<<1)+b[scnt][5];
  124    col=(b[scnt][1]<<3)+(b[scnt][2]<<2)+(b[scnt][3]<<1)+b[scnt][4];
  125    sout[scnt]=s[scnt][(row<<4)+col];
  126    for(i=3;i>=0;i--){
  127     pin[n]=sout[scnt]>>i;
  128     sout[scnt]=sout[scnt]-(pin[n++]<<i);
  129    }
  130   }
  131   for(i=0;i<32;i++) fout[i]=pin[p[i]-1]; /*Permutation Function*/
  132 }
  133 
  134 static int p[64] = {58,50,42,34,26,18,10,2,
  135               60,52,44,36,28,20,12,4,
  136               62,54,46,38,30,22,14,6,
  137               64,56,48,40,32,24,16,8,
  138               5 = {58,50,42,34,26,18,10,2,
  139               60,52,44,36,28,20,12,4,
  140               62,54,46,38,30,22,14,6,
  141               64,56,48,40,32,24,16,8,
  142               57,49,41,33,25,17,9,1,
  143               59,51,43,35,27,19,11,3,
  144               61,53,45,37,29,21,13,5,
  145               63,55,47,39,31,23,15,7};
  146 
  147 static int invp[64]={
  148  40, 8,48,16,56,24,64,32,39, 7,47,15,55,23,63,31,
  149  38, 6,46,14,54,22,62,30,37, 5,45,13,53,21,61,29,
  150  36, 4,44,12,52,20,60,28,35, 3,43,11,51,19,59,27,
  151  34, 2,42,10,50,18,58,26,33, 1,41, 9,49,17,57,25};
  152 
  153 void des_encrypt(unsigned char *input){
  154  static unsigned char out[64];
  155  static int inputb[64],lr[64],l[32],r[32];
  156  static int fn[32];
  157  static int cnt,n;
  158  register int i,j;
  159 
  160  for(i=n=0;i<8;i++)
  161   for(j=0;j<8;j++) inputb[n++]=(input[i]>>j&0x01);
  162 
  163  for(i=0;i<64;i++){ /*Initial Permutation*/
  164   lr[i]=inputb[p[i]-1];
  165   if(i<32) l[i]=lr[i];
  166   else r[i-32]=lr[i];
  167  }
  168  for(cnt=1;cnt<=16;cnt++){ /*Main encryption loop*/
  169   cypher(r,cnt,fn);
  170   for(i=0;i<32;i++){
  171    j=r[i];
  172    r[i]=l[i]^fn[i];
  173    l[i]=j;
  174   }
  175  }
  176  for(i=0;i<32;i++){
  177   lr[i]=r[i];
  178   lr[i+32]=l[i];
  179  }
  180  for(i=0;i<64;i++) out[i]=lr[invp[i]-1]; /*Inverse IP*/
  181 
  182  for(i=1;i<=8;i++)
  183   for(j=1;j<=8;j++) input[i-1]=(input[i-1]<<1)|out[i*8-j];
  184 }
  185 
  186 void des_decrypt(unsigned char *input){
  187  static unsigned char out[64];
  188  static int inputb[64],lr[64],l[32],r[32];
  189  static int fn[32];
  190  static int cnt,rtemp,n;
  191  register int i,j;
  192 
  193  for(i=n=0;i<8;i++)
  194   for(j=0;j<8;j++) inputb[n++]=(input[i]>>j&0x01);
  195 
  196  for(i=0;i<64;i++){ /*Initial Permutation*/
  197   lr[i]=inputb[p[i]-1];
  198   if(i<32) l[i]=lr[i];
  199   else r[i-32]=lr[i];
  200  }
  201  for(cnt=16;cnt>0;cnt--){ /*Main decryption loop*/
  202   cypher(r,cnt,fn);
  203   for(i=0;i<32;i++){
  204    rtemp=r[i];
  205    if(l[i]==1 && fn[i]==1) r[i]=0;
  206    else r[i]=(l[i] || fn[i]);
  207    l[i]=rtemp;
  208   }
  209  }
  210  for(i=0;i<32;i++){
  211   lr[i]=r[i];
  212   lr[i+32]=l[i];
  213  }
  214  for(i=0;i<64;i++) out[i]=lr[invp[i]-1]; /*Inverse IP*/
  215 
  216  for(i=1;i<=8;i++)
  217   for(j=1;j<=8;j++) input[i-1]=(input[i-1]<<1) | out[i*8-j];
  218 }
  219 
  220 int main(int argc, char *argv[]){
  221  unsigned char *key;
  222  unsigned char data[8];
  223  int n;
  224  FILE *in;
  225  FILE *out;
  226 
  227  if (argc!=4) {
  228   printf("\r\nUsage:  des [e][d] <source file> <destination file>\r\n");
  229   return 1;
  230  }
  231 
  232  key=(unsigned char*)getpass("Enter Key:");
  233  des_init(key);
  234 
  235  if((in=fopen(argv[2],"rb"))==NULL){
  236   fprintf(stderr,"\r\nCould not open input file: %s",argv[2]);
  237   return 2;
  238  }
  239 
  240  if((out=fopen(argv[3],"wb"))==NULL){
  241   fprintf(stderr,"\r\nCould not open output file: %s",argv[3]);
  242   return 3;
  243  }
  244 
  245  if(argv[1][0]=='e'){
  246   while ((n=fread(data,1,8,in)) >0){
  247    des_encrypt(data);
  248    printf("data enctyted");
  249    if(fwrite(data,1,8,out) < 8){
  250     fprintf(stderr,"\r\nError writing to output file\r\n");
  251     return(3);
  252    }
  253   }
  254  }
  255 
  256  if(argv[1][0]=='d'){
  257   while ((n=fread(data,1,8,in)) >0){
  258    des_decrypt(data);
  259    if(fwrite(data,1,8,out) < 8){
  260     fprintf(stderr,"\r\nError writing to output file\r\n");
  261     return(3);
  262    }
  263   }
  264  }
  265 
  266  fclose(in); fclose(out);
  267  return 0;
  268 }ntf(stderr,"\r\nError writing to output file\r\n");
  269     return(3);
  270    }
  271   }
  272  }
  273 
  274  fclose(in); fclose(out);
  275  return 0;
  276 }

Final in Java is used to declare a class that can’t be subclassed, a method that can’t be overridden, or …

A final variable means the value won’t change, but the value can still be determined at run time:

final int i;
if ( j > 0 ){
   i =1;
} else {
   i = 2;
}

You get a compile time error if the final variable isn’t definitely unassigned anytime you assign to it:

final int i;
if(j>0){
   i=1;
}
if(a<10){
   i=1;
}

In C#, a "const" must be determined at compile time.