jsBefunge

Befunge.js

function Befunge(code) {
  var output = '';
  var stack = [];
  var codeArray = code.split('\n');
  var x = 0;
  var y = 0;
  var d = code.charAt(0);
  var next = d;
  var cmd = 'NORMAL';
  var loop = true;
  var v;

  while (loop) {
    switch(d) {
      case '>':
        x += 1;
        break;
      case '<':
        x -= 1;
        break;
      case '^':
        y -= 1;
        break;
      case 'v':
        y += 1;
        break;
      default:
        d = '>';
        stack.push(next);
        continue;
    }

    try {
      next = codeArray[y][x];
    } catch(e) {
      next = null;
      break;
    }
    if (next === '"') {
      cmd = cmd === 'STRING' ? 'NORMAL' : 'STRING';
      continue;
    }
    if (cmd === 'STRING') {
      stack.push( next.charCodeAt(0) );
      continue;
    } else {
      switch ( next ) {
        case '?':
         next = '><^v'[Math.floor(Math.random() * 4)];
         break;
        case '_':
          d = +stack.pop() === 0 ? '>': '<';
          continue;
          // break;
        case '|':
          d = +stack.pop() === 0 ? 'v': '^';
          continue;
          // break;
      }
      if ( '<>^v'.indexOf(next) >= 0 ) {
        d = next;
        continue;
      } else {
        switch(next) {
          case ':':
            next = stack.pop();
            if ( typeof next === 'undefined' ) {
              stack.push( 0 );
            } else {
              stack.push(next);
              stack.push(next);
            }
            continue;
          case '.':
            output += parseInt( stack.pop() );
            continue;
          case ',':
            output += String.fromCharCode( + stack.pop() );
            continue;
          case '`':
            next = stack.pop();
            stack.push( +(+next < +stack.pop()) );
            continue;
          case '!':
            next = !stack.pop();
            stack.push( +next );
            continue;
          case '\\':
            // Swap two values on top of the stack.
            next = stack.pop();
            stack.unshift( stack.pop() );
            stack.push(next);
            stack.push( stack.shift() );
            continue;
          case '$':
            stack.pop();
            continue;
          case 'p':
            stack.unshift( stack.pop() ); // y...vx
            stack.unshift( stack.pop() ); // xy...v
            v = String.fromCharCode(+stack.pop()); // xy...
            stack.push( stack.shift() ); // y...x
            next = codeArray[ +stack[0] ].split(''); // current line to array
            next[+stack.pop()] = v;
            codeArray[ +stack.shift() ] = next.join('');
            continue;
          case 'g':
            next = codeArray[ +stack.pop() ][ +stack.pop() ];
            stack.push( next.charCodeAt(0) );
            continue;
          case '#':
            x += {'<': -1, '>': 1}[d] || 0;
            y += {'^': -1, 'v': 1}[d] || 0;
            continue;
          case ' ':
            continue;
          case '@':
            loop = false;
            break;
        }
      }

      if ( next && '+-*/%'.indexOf(next) >= 0 ) {
        stack.unshift( stack.pop() );
        next = parseInt( +eval(stack.pop() +next+ +stack.shift() ) || 0 );
      }
    }
    stack.push(next);
  }

  return output;
}