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Feb 28th, 2007, 2:52 PM
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#1 |
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King of Portal
Join Date: Sep 2005
Posts: 431
Rep Power: 4
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Implementation of CRC32
ok now I realize this isn't the best or most efficient or even a way that I should implement CRC32, BUT I figure if I can do it this way first then the other better ways will be easier to understand (or at the very least please afford me that discretion). I'm basically trying to compute the crc32 hash of the letter 'a' using ActionScript. Now irrelevant of the actual language, this actionscript code doesn't use any fancy Flash stuff other than the function trace() which outputs results in a console-like fashion for debugging. So here's the code:
trace(crc32('a'));
function crc32(input:String):String {
var hexInput:String = '';
for(var i:Number = 0; i < input.length; i++) {
hexInput += dechex(input.charCodeAt(i));
}
var binaryInput:String = '';
for(var i:Number = 0; i < hexInput.length; i++) {
switch(hexInput.charCodeAt(i)) {
case 48:
binaryInput += '0000';
break;
case 49:
binaryInput += '0001';
break;
case 50:
binaryInput += '0010';
break;
case 51:
binaryInput += '0011';
break;
case 52:
binaryInput += '0100';
break;
case 53:
binaryInput += '0101';
break;
case 54:
binaryInput += '0110';
break;
case 55:
binaryInput += '0111';
break;
case 56:
binaryInput += '1000';
break;
case 57:
binaryInput += '1001';
break;
case 97:
binaryInput += '1010';
break;
case 98:
binaryInput += '1011';
break;
case 99:
binaryInput += '1100';
break;
case 100:
binaryInput += '1101';
break;
case 101:
binaryInput += '1110';
break;
case 102:
binaryInput += '1111';
break;
}
}
binaryInput += '00000000000000000000000000000000';
var binaryGenerator:String = '100000100110000010001110110110111';
for(var i:Number = 0; i < binaryInput.length - binaryGenerator.length; i++) {
if(binaryInput.charAt(i) == '1') {
for(var j:Number = 0; j < binaryGenerator.length; j++) {
if(binaryInput.charAt(i + j) != binaryGenerator.charAt(j)){
binaryInput = replace(binaryInput, i + j, '1');
}else{
binaryInput = replace(binaryInput, i + j, '0');
}
}
}
}
return binaryInput.substr(binaryInput.length - (binaryGenerator.length - 1));
}
function replace(original:String, index:Number, replacement:String):String {
if(index == 0){
return replacement.concat(original.substr(1));
}
if(index == original.length - 1){
return original.substr(0, original.length - 1) + replacement;
}
return original.substr(0, index) + replacement + original.substr(index + 1);
}
function dechex(val:Number):String {
if(val < 0){
val += Math.pow(2, 32);
}
var output:String = '';
for(var i:Number = 0; i < 8; i++){
var remainder:Number = val % 16;
val = (val - remainder) / 16;
switch(remainder){
case 10:
output = 'a' + output;
break;
case 11:
output = 'b' + output;
break;
case 12:
output = 'c' + output;
break;
case 13:
output = 'd' + output;
break;
case 14:
output = 'e' + output;
break;
case 15:
output = 'f' + output;
break;
default:
output = remainder + output;
break;
}
}
do{
if(output.charCodeAt(0) == 48){
output = output.substr(1, output.length - 1);
}
}while(output.charCodeAt(0) == 48);
return output;
}Here's an explanation on crc32 The actual value I'm trying to achieve is 0xe8b7be43
__________________
Lo, there do I see my father. 'Lo, there do I see My mother, and my sisters, and my brothers. 'Lo, there do I see The line of my people... Back to the beginning. 'Lo, they do call to me. They bid me take my place among them. In the halls of Valhalla... Where the brave... May live... ...forever.. GrimBB | Mimesis |
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Mar 1st, 2007, 11:41 PM
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#2 |
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King of Portal
Join Date: Sep 2005
Posts: 431
Rep Power: 4
|
As per this page I just discovered that you can append the crc32 checksum to the original message and that if you compute the crc32 value of that then the result should be 0. I did this with my own program and sure enough the result was 0. Soooooooo now I'm even more confused 'cause it seemingly works correctly. What am I doing wrong?
 This is so discouraging 
__________________
Lo, there do I see my father. 'Lo, there do I see My mother, and my sisters, and my brothers. 'Lo, there do I see The line of my people... Back to the beginning. 'Lo, they do call to me. They bid me take my place among them. In the halls of Valhalla... Where the brave... May live... ...forever.. GrimBB | Mimesis |
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Mar 3rd, 2007, 2:13 PM
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#3 |
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King of Portal
Join Date: Sep 2005
Posts: 431
Rep Power: 4
|
I changed the script a bit and now it will actually verify the checksum by appending it to the original message and computing the remainder.
trace('//////////////////////');
trace('// Compute CheckSum //');
trace('//////////////////////');
var mess:String = 'a';
var myArray:Array = new Array(mess);
myArray.push(crc32(myArray));
trace('/////////////////////');
trace('// Verify CheckSum //');
trace('/////////////////////');
crc32(myArray);
function crc32(input:Array):String {
var hexInput:String = '';
for(var i:Number = 0; i < input[0].length; i++) {
hexInput += dechex(input[0].charCodeAt(i));
}
var binaryInput:String = '';
for(var i:Number = 0; i < hexInput.length; i++) {
switch(hexInput.charCodeAt(i)) {
case 48:
binaryInput += '0000';
break;
case 49:
binaryInput += '0001';
break;
case 50:
binaryInput += '0010';
break;
case 51:
binaryInput += '0011';
break;
case 52:
binaryInput += '0100';
break;
case 53:
binaryInput += '0101';
break;
case 54:
binaryInput += '0110';
break;
case 55:
binaryInput += '0111';
break;
case 56:
binaryInput += '1000';
break;
case 57:
binaryInput += '1001';
break;
case 97:
binaryInput += '1010';
break;
case 98:
binaryInput += '1011';
break;
case 99:
binaryInput += '1100';
break;
case 100:
binaryInput += '1101';
break;
case 101:
binaryInput += '1110';
break;
case 102:
binaryInput += '1111';
break;
}
}
var iterations:Number = binaryInput.length;
var binaryGenerator:String = '100000100110000010001110110110111'; //Big-endian
switch(input.length){
case 1:
for(var i:Number = 0; i < binaryGenerator.length - 1; i++){
binaryInput += '0';
}
break;
case 2:
binaryInput += input[1];
break;
default:
return 'Error';
break;
}
for(var i:Number = 0; i < iterations; i++) {
trace(' ' + binaryInput);
hexInput = '';
for(var j:Number = 0; j < i; j++){
hexInput += ' ';
}
if(binaryInput.charAt(i) == '1') {
trace('- ' + hexInput + binaryGenerator);
}else{
for(var j:Number = 0; j < binaryGenerator.length; j++){
hexInput += '0';
}
trace('- ' + hexInput);
}
if(binaryInput.charAt(i) == '1') {
for(var j:Number = 0; j < binaryGenerator.length; j++) {
if(binaryInput.charAt(i + j) != binaryGenerator.charAt(j)){
binaryInput = replace(binaryInput, i + j, '1');
}else{
binaryInput = replace(binaryInput, i + j, '0');
}
}
}
hexInput = '';
for(var j:Number = 0; j < binaryInput.length; j++){
hexInput += '-';
}
trace(' ' + hexInput);
}
trace(' ' + binaryInput);
return binaryInput.substr(binaryInput.length - (binaryGenerator.length - 1));
}
function replace(original:String, index:Number, replacement:String):String {
if(index == 0){
return replacement + original.substr(1);
}
if(index == original.length - 1){
return original.substr(0, original.length - 1) + replacement;
}
return original.substr(0, index) + replacement + original.substr(index + 1);
}
function dechex(val:Number):String {
if(val < 0){
val += Math.pow(2, 32);
}
var output:String = '';
for(var i:Number = 0; i < 8; i++){
var remainder:Number = val % 16;
val = (val - remainder) / 16;
switch(remainder){
case 10:
output = 'a' + output;
break;
case 11:
output = 'b' + output;
break;
case 12:
output = 'c' + output;
break;
case 13:
output = 'd' + output;
break;
case 14:
output = 'e' + output;
break;
case 15:
output = 'f' + output;
break;
default:
output = remainder + output;
break;
}
}
do{
if(output.charCodeAt(0) == 48){
output = output.substr(1, output.length - 1);
}
}while(output.charCodeAt(0) == 48);
return output;
}////////////////////// // Compute CheckSum // ////////////////////// 0110000100000000000000000000000000000000 - 000000000000000000000000000000000 ---------------------------------------- 0110000100000000000000000000000000000000 - 100000100110000010001110110110111 ---------------------------------------- 0010000000110000010001110110110111000000 - 100000100110000010001110110110111 ---------------------------------------- 0000000010101000011001001101101100100000 - 000000000000000000000000000000000 ---------------------------------------- 0000000010101000011001001101101100100000 - 000000000000000000000000000000000 ---------------------------------------- 0000000010101000011001001101101100100000 - 000000000000000000000000000000000 ---------------------------------------- 0000000010101000011001001101101100100000 - 000000000000000000000000000000000 ---------------------------------------- 0000000010101000011001001101101100100000 - 000000000000000000000000000000000 ---------------------------------------- 0000000010101000011001001101101100100000 ///////////////////// // Verify CheckSum // ///////////////////// 0110000110101000011001001101101100100000 - 000000000000000000000000000000000 ---------------------------------------- 0110000110101000011001001101101100100000 - 100000100110000010001110110110111 ---------------------------------------- 0010000010011000001000111011011011100000 - 100000100110000010001110110110111 ---------------------------------------- 0000000000000000000000000000000000000000 - 000000000000000000000000000000000 ---------------------------------------- 0000000000000000000000000000000000000000 - 000000000000000000000000000000000 ---------------------------------------- 0000000000000000000000000000000000000000 - 000000000000000000000000000000000 ---------------------------------------- 0000000000000000000000000000000000000000 - 000000000000000000000000000000000 ---------------------------------------- 0000000000000000000000000000000000000000 - 000000000000000000000000000000000 ---------------------------------------- 0000000000000000000000000000000000000000
__________________
Lo, there do I see my father. 'Lo, there do I see My mother, and my sisters, and my brothers. 'Lo, there do I see The line of my people... Back to the beginning. 'Lo, they do call to me. They bid me take my place among them. In the halls of Valhalla... Where the brave... May live... ...forever.. GrimBB | Mimesis |
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