js decodes the image base64 encoded string and outputs the image example_javascript skills

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/////////////////////////////
//base64 encoded GIF image decoding
//By Mozart0
//2005/10/29
////////////////////

//Create a GIF class object
/ /Class GIF is defined inside this function
// str64: Base64 encoded string of gif file
// Returns the created GIF object on success
// Returns null on failure
function getGif(str64){
var bytes=decodeBase64(str64);
if(!bytes){
alert("Error: Invalid Base64 encoding");
return null;
}
var gif =new GIF();
for(var i=0;igif.version =String.fromCharCode(bytes[i]);
if(gif.version.slice( 0,3)!="GIF"){
alert("Error: non-Gif image format");
return null;
}
gif.width=bytes[i]|(bytes [i 1]gif.height=bytes[i 2]|(bytes[i 3]var f=bytes[i 4];
gif.colorResolution=(f>>4&0x7) 1;
gif.sorted=(f&0x8)?true:false;
gif.backgroundIndex=bytes[i 5];
gif.pixelAspectRadio= bytes[i 6];
if(f&0x80){
gif.globalPalette=[];
i =getPalette(i 7,bytes,gif.globalPalette,2}
i =7;
for(var j=i;jif(bytes[j]==0x21&&bytes[j 1]==0xf9)
break;
if(j==bytes.length){
for(;iif(bytes[i]==0x2c)
break;
if(i==bytes.length){
alert("Error: Image data not found");
return null;
}
var f=new GIF_Frame();
if(!getSingleFrame(i,f))
return null;
else
gif.frames.push(f);
}
else{
i=j;
do{
var f=new GIF_Frame();
var t=getSingleFrame(i,f);
if(!t)
return null;
gif.frames.push (f);
for(i =t;iif(bytes[i]==0x21&&bytes[i 1]==0xf9)
break;
}
while(i}
return gif;

//Internal process, generate color table
function getPalette(pos,s,d,len) {
len*=3;
for(var i=pos;id.push('#' (s[i] (s[i 1] (s [i 2]return len;
}

//Internal process, integration Data segment
function getBlock(pos,s,d){
var p=pos;
while(len=s[p]){
for(var i=0;id.push(s[p i]);
p =len;
}
return p-pos;
}

//Internal process, get One frame of data
function getSingleFrame(pos,frame){
var i=pos;
if(bytes[i]==0x21){
i =3;
if(bytes[ i]&1)
frame.transparentIndex=bytes[i 3];
frame.delay=bytes[i 1]|(bytes[i 2]for(i = 5;iif(i==bytes.length){
alert("Error: Image identifier not found");
return 0;
}
}
frame.offsetX=bytes[i 1]|(bytes[i 2]frame.offsetY=bytes[i 3]| (bytes[i 4]frame.width=bytes[i 5]|(bytes[i 6]frame.height=bytes[i 7 ]|(bytes[i 8]var f=bytes[i 9];
i =10;
if(f&0x40)
frame.interlace=true;
if(f&0x20)
frame.sorted=true;
if(f&0x80){
frame.colorResolution=(f&0x7) 1;
frame.localPalette=[];
i =getPalette(i,bytes,frame.localPalette,1}
else{
frame.colorResolution=gif.colorResolution;
frame.localPalette=gif.globalPalette ;
}
var lzwLen=bytes[i ] 1;
i =getBlock(i,bytes,frame.data);
frame.data=decodeLzw(frame.data,lzwLen);
return frame.data?i-pos:0;
}

//Define the data structure for storing GIF files
//Provide method showInfo and return image information
function GIF( ){
this.version=""; //Version number
this.width=0; //Logical screen width
this.height=0; //Logical screen height
this.colorResolution =0; //Color depth
this.sorted=false; //Global color table classification flag
this.globalPalette=null; //Global color table
this.backgroundIndex=-1; //Background Color index
this.pixelAspectRadio=0; //Pixel aspect ratio
this.frames=[]; //For each frame of the image, see GIF_Frame
this.showInfo=function(sep){ //Display Image information, sep is the line separator
if(!sep)
sep="n";
var s="Gif infomation:" sep "------------ -------";
s =subInfo(this) sep;
for(var i=0;is =sep "frames " i "----------" subInfo(this.frames[i]);
return s;
function subInfo(o){
var s="";
for (var i in o){
if(i=="showInfo"||i=="draw")
continue;
s =sep i ":";
if(typeof( o[i])=="object")
s =(o[i]?o[i].length:"null");
else
s =o[i];
}
return s;
}
}
}

//Define the data structure to store a frame of image
//Provide method draw, drawing
function GIF_Frame (){
this.offsetX=0; //X-direction offset
this.offsetY=0; //Y-direction offset
this.width=0; //Image width
this.height=0; //Image height
this.localPalette=null; //Local color table
this.colorResolution=0; //Color depth
this.interlace=false; / /Interlaced flag
this.sorted=false; //Local color table classification flag
this.data=[]; //Image data, storing the integer index of each pixel color
this.transparentIndex=-1 ; //Transparent color index
this.delay=0; //Frame delay
this.draw=function(parent,zoom){
if(!this.data.length)
return ;
if(!parent)
parent=document.body;
if(!zoom)
zoom=1;
if(parent.clientWidthparent.style.width=this.width*zoom;
if(parent.clientHeightparent.style.height=this.height*zoom;
var id=" ImgDefaultDraw";
var img=document.getElementById(id);
if(img)
delete parent.removeChild(img);
img=document.createElement("DIV");
img.id=id;
parent.appendChild(img);
img.style.position="absolute";
var t=document.createElement("DIV");
t. style.overflow="hidden";
t.style.position="absolute";
defLayout(this.data,this.localPalette,this.width,this.height,img,t,zoom);
delete t;
}
}
}

//Base64 decoding
//strIn, input string
//returns an array successfully, each element Contains one byte of information
// Returns null on failure
function decodeBase64(strIn){
if(!strIn.length||strIn.length%4)
return null;
var str64 =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 /=";
var index64=[];
for(var i=0;iindex64[str64.charAt(i)]= i;
var c0,c1,c2,c3,b0,b1,b2;
var len=strIn.length;
var len1=len;
if(strIn.charAt(len-1 )=='=')
len1-=4;
var result=[];
for(var i=0,j=0;i c0=index64[strIn.charAt(i)];
c1=index64[strIn.charAt(i 1)];
c2=index64[strIn.charAt(i 2)];
c3=index64 [strIn.charAt(i 3)];
b0=(c0>4);
b1=(c1>2 );
b2=(c2result.push(b0&0xff);
result.push(b1&0xff);
result.push(b2&0xff);
}
if(len1!=len){
c0=index64[strIn.charAt(i)];
c1=index64[strIn.charAt(i 1)];
c2=strIn. charAt(i 2);
b0=(c0>4);
result.push(b0&0xff);
if(c2!='='){
c2=index64[c2];
b1=(c1>2);
result.push(b1&0xff);
}
}
return result;
}

//LZW decoding function for GIF
//arrBytes is the source data, nBits is the initial encoding number of bits
//Successfully returns the array, each Elements include a color index
// Returns null on failure
function decodeLzw(arrBytes,nBits){
var cc=1var eoi=cc 1;
var table=[],mask=[],result=[];
for(var i=0;itable[i]=(i>>8&0xf) .toString(16)
(i>>4&0xf).toString(16) (i&0xf).toString(16);
for(i=2,mask[1]=1;imask[i]=mask[i-1]var bc=nBits;
var pos=0,temp=0,tleft=0,code=0 ,old=0;
while(true){
while(tlefttemp=temp|(arrBytes[pos ]tleft =8;
}
code=temp&mask[bc];
tleft-=bc;
temp>>=bc;
if(code==eoi)
break;
if (code==cc){
table.length=cc 2;
bc=nBits;
old=code;
continue;
}
var t="";
if(codet=table[code];
if(old!=cc)
table.push(table[old] t.slice(0,3) );
}
else if(oldt=table[old] table[old].slice(0,3);
table.push(t);
}
else{
alert("Error: Invalid image data");
return null;
}
old=code;
for(var i=0; iresult.push(parseInt(t.substr(i,3),16))
if(table.length==1bc ;
}
return result;
}

//According to the byte array data layout, complete the drawing with the least number of divs
function defLayout(data,palette,width ,height,image,block,zoom){
var map=new Array(height);
for(var i=0;imap[i]=new Array( width);
for(var j=0;jmap[i][j]=data[i*width j];
}
var i,j, i1,i2,j1,j2,c;
for(i=0;ifor(j=0;jif(map[i][ j]==0x100){
j ;
continue;
}
c=map[i][j];
for(i1=i 1;i1for(j1=j 1;j1for(i2=i;i2if(i2break;
}
for(i2=i;i2for(j2=j;j2map[i2][j2]=0x100;
var x=block.cloneNode(true);
x.style.left=j*zoom;
x.style.top=i*zoom;
x.style.width=(j1-j)*zoom;
x.style.height=(i1-i)*zoom;
x.style.backgroundColor=palette[c];
image.appendChild(x);
j=j1;
}
}


<script> <BR>function main(){ <BR>var t=new Date().getTime(); <BR>var xmldom=document.getElementById("imgData"); <BR>var gif=getGif("R0lGODlhDwAPAKECAAAAzMzM/////wAAACwAAAAADwAPAAACIISPeQHsrZ5ModrLlN48CXF8m2iQ3YmmKqVlRtW4MLwWACH H09wdGltaXplZCBieSBVbGVhZCBTbWFydFNhdmVyIQAAOw=="); <BR>var info=document.getElementById("info"); <BR>info.innerHTML=gif.showInfo("<br>"); <BR>t=new Date().getTime(); <BR>gif.frames[0].draw(document.getElementById("canvas"),1); <BR>info.innerHTML ="<br>缁</script>