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<html>
<head>
<title>A brainfuck interpreter in Javascript</title>
<script type='text/javascript'>
//first we create the brainfuck environment

//empty array of 30000 elements
var ar=new Array(30000);
for(var n=0;n<30000;n++)
{
    ar[n]=0;
}
//current location in array
var pointer=0;
//the text of the program we're running (for parsing)
var prog="";
//the current character in program we're running
var run_index=0;

//these are internal functions for some dynamic content using divs and such

//input returns the entered text
function input()
{
    text=document.getElementById("input_area").value.substr(0,1);
    document.getElementById("input_area").value=text.substr(1);
    //return the ascii value of the text
    return text.charCodeAt();
}
function output(str)
{
    document.getElementById("output_area").value+=str;
}

//okay, so these are functions for each of brainfuck's commands
function inc_point() //>
{
    if(pointer<29999)
    {
        pointer=pointer+1;
    }
    else
    {
        //I suppose I should give an error message
        output("Err: Pointer past end of array");
        run_index=prog.length; //stop program execution, this was a fatal error
        return;
    }
}
function dec_point() //<
{
    if(pointer>0)
    {
        pointer=pointer-1;
    }
    else
    {
        //same deal as before
        output("Err: Pointer before start of array");
        run_index=prog.length; //stop program execution, this was a fatal error
        return;
    }
}
function inc_byte() //+
{
    ar[pointer]=ar[pointer]+1;
}
function dec_byte() //-
{
    ar[pointer]=ar[pointer]-1;
}
function out_byte() //.
{
    output(String.fromCharCode(ar[pointer]));
}
function in_byte() //,
{
    //input is made to accept one byte only
    ar[pointer]=input();
}
function jmp_f() //[
{
    if(ar[pointer]==0)
    {
        while(prog.charAt(run_index)!="]")
        {
            if(run_index<prog.length)
            {
                run_index++;
            }
            else
            {
                output("Err: Program ends within a conditional");
                run_index=prog.length; //stop program execution, this was a fatal error
                return;
            }
        }
        //we didn't return, so we found the end of the conditional, handle that
        run_index++;
    }
    else
    {
        //else continue normal execution
        run_index++;
    }
}
function jmp_b() //]
{
    if(ar[pointer]!=0)
    {
        while(prog.charAt(run_index)!="[")
        {
            if(run_index>0)
            {
                run_index--;
            }
            else
            {
                output("Err: No matching [ for a given ]");
                run_index=prog.length; //stop program execution, this was a fatal error
                return;
            }
        }
        //we didn't return, so we found the start of the conditional, handle that
        run_index++;
    }
    else
    {
        //else continue normal execution
        run_index++;
    }
}

//okay, now the parser
function bf_eval(text)
{
    //resets all defaults to ignore all previous programs' environments
    run_index=0;
    pointer=0;
    for(var n=0;n<30000;n++)
    {
        ar[n]=0;
    }

    prog=text;
    document.getElementById("output_area").value="";
    document.getElementById("input_area").value="";
    document.getElementById("code_area").value="";

    while(run_index<prog.length)
    {
//      alert(prog.charAt(run_index)); //debug
        switch(prog.charAt(run_index))
        {
            case ">":
                inc_point();
                run_index++;
                break;
            case "<":
                dec_point();
                run_index++;
                break;
            case "+":
                inc_byte();
                run_index++;
                break;
            case "-":
                dec_byte();
                run_index++;
                break;
            case ".":
                out_byte();
                run_index++;
                break;
            case ",":
                in_byte();
                run_index++;
                break;
            case "[":
                jmp_f();
                break;
            case "]":
                jmp_b();
                break;
        }
    }
}

</script>
</head>
<body>
<form>
    <textarea rows="10" cols="80" id="code_area" ondblclick="bf_eval(this.value);"></textarea><br>
    <input id="input_area"></input><br>
    <textarea rows="10" cols="80" id="output_area"></textarea>
</form>
</body>