freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/rosetta-code/execute-brain.md

title	id	challengeType	forumTopicId
Execute Brain****	59e0a8df964e4540d5abe599	5	302261

Description

Write a function to implement a Brain**** interpreter. The function will take a string as a parameter and should return a string as the output. More details are given below: RCBF is a set of Brainf*** compilers and interpreters written for Rosetta Code in a variety of languages. Below are links to each of the versions of RCBF. An implementation need only properly implement the following instructions:

Command	Description
>	Move the pointer to the right
<	Move the pointer to the left
+	Increment the memory cell under the pointer
-	Decrement the memory cell under the pointer
.	Output the character signified by the cell at the pointer
,	Input a character and store it in the cell at the pointer
[	Jump past the matching ] if the cell under the pointer is 0
]	Jump back to the matching [ if the cell under the pointer is nonzero
Any cell size is allowed, EOF (End-O-File) support is optional, as is whether you have bounded or unbounded memory.

Instructions

Tests

tests:
  - text: <code>brain(bye)</code> should return a string
    testString: assert(typeof brain(bye) === 'string');
  - text: <code>brain("++++++[>++++++++++<-]>+++++.")</code> should return "A"
    testString: assert.equal(brain("++++++[>++++++++++<-]>+++++."),"A");
  - text: <code>brain(bye)</code> should return <code>Goodbye, World!\\r\\n</code>
    testString: assert.equal(brain(bye), 'Goodbye, World!\r\n');
  - text: <code>brain(hello)</code> should return <code>Hello World!\\n</code>
    testString: assert.equal(brain(hello), "Hello World!\n");
  - text: <code>brain(fib)</code> should return <code>1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89</code>
    testString: assert.equal(brain(fib), "1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89");

Challenge Seed

function brain(prog) {

}

Before Test

let fib=`+

++

+++

++++

+>+>>

>>++++

+++++++

++++++++

+++++++++

++++++++++

++++++>++++

++++++++++++

+++++++++++++

+++<<<<<<[>[>>

>>>>+>+<<<<<<<-

]>>>>>>>[<<<<<<<

+>>>>>>>-]<[>++++

++++++[-<-[>>+>+<<

<-]>>>[<<<+>>>-]+<[

>[-]<[-]]>[<<[>>>+<<

<-]>>[-]]<<]>>>[>>+>+

<<<-]>>>[<<<+>>>-]+<[>

[-]<[-]]>[<<+>>[-]]<<<<

<<<]>>>>>[++++++++++++++

+++++++++++++++++++++++++

+++++++++.[-]]++++++++++<[

->-<]>+++++++++++++++++++++

+++++++++++++++++++++++++++.

[-]<<<<<<<<<<<<[>>>+>+<<<<-]>

>>>[<<<<+>>>>-]<-[>>.>.<<<[-]]

<<[>>+>+<<<-]>>>[<<<+>>>-]<<[<+

>-]>[<+>-]<<<-]`;
let hello='++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.'
let bye='++++++++++[>+>+++>++++>+++++++>++++++++>+++++++++>++++++++++>+++++++++++>++++++++++++<<<<<<<<<-]>>>>+.>>>>+..<.<++++++++.>>>+.<<+.<<<<++++.<++.>>>+++++++.>>>.+++.<+++++++.--------.<<<<<+.<+++.---.';

Solution

function brain(prog){
  var output="";
	var code; // formatted code
  var ip = 0; // current instruction within code
  var nest = 0; // current bracket nesting (for Out button)
  var ahead = []; // locations of matching brackets

  var data = [0]; // data array (mod by +, -)
  var dp = 0; // index into data (mod by <, >)

  var inp = 0; // current input character (fetch with ,)
  var quit = 0;
	var commands = {
	'>':function() { if (++dp >= data.length) data[dp]=0 },
	'<':function() { if (--dp < 0) quit++ },
	'+':function() { ++data[dp] },
	'-':function() { --data[dp] },
	'[':function() { if (!data[dp]) ip = ahead[ip]; else ++nest },
	']':function() { if ( data[dp]) ip = ahead[ip]; else --nest },
	',':function() {
		var c = document.getElementById("input").value.charCodeAt(inp++);
		data[dp] = isNaN(c) ? 0 : c; // EOF: other options are -1 or no change
	},
	'.':function() {
    		output+=String.fromCharCode(data[dp]);
    		/*var s = document.getElementById("output").innerHTML)
    		 + String.fromCharCode(data[dp]);
    		s = s.replace(/\n/g,"<br>").replace(/ /g,"&amp;nbsp;");
    		document.getElementById("output").innerHTML = s;*/
    	},
    };

	let ar=prog.split('');
	var st = [], back, error = -1;
	for (ip=0; ip<ar.length; ip++) {
		switch(ar[ip]) {
		case '[':
			st.push(ip);
			break;
		case ']':
			if (st.length == 0) error = ip;
			back = st.pop();
			ahead[ip] = back;
			ahead[back] = ip;
			break;
		}
	}

	for(ip=0;ip<ar.length;ip++){
    if(commands.hasOwnProperty(ar[ip]))
		  commands[ar[ip]]();
	}

	return output;
}