befunge.js

function formatCode(codeStr) {
  var arrLines = codeStr.split('\n');
  var code = [];
  for (var y = 0; y < arrLines.length; y++) {
    var line = arrLines[y].split('');
    code[y] = [];
    for (var x = 0; x < line.length; x++) {
      var ins = line[x];
      code[y][x] = ins;
    }
  }
  return code;
}

function interpret(inputCode) {
  var output = [];
  var code = formatCode(inputCode);
  var stack = [];
  var done = false;
  var x = 0;
  var y = 0;
  var direction = 'r'; // as per Befunge-93 spec
  var stringMode = false;
  var skip = false;
  
  function doMovement() {
    if (direction === 'u') {
      y--;
      if (y < 0) {
        y = code.length - 1;
      }
    } else if (direction === 'd') {
      y++
      if (y >= code.length) {
        y = 0;
      }
    } else if (direction === 'r') {
      x++;
      if (x >= code[y].length) {
        x = 0;
      }
    } else if (direction === 'l') {
      x--;
      if (x < 0) {
        x = code[y].length - 1;
      }
    }
  }
  
  function getFromStorage(x, y) {
    if (y >= code.length) {
      return ' ';
    }
    if (x >= code[y].length) {
      return ' ';
    }
    return code[y][x].charCodeAt(0);
  }
  
  function putIntoStorage(x, y, val) {
    if (y >= code.length) {
      for (var i = code.length; i <= y; i++) {
        code[i] = [];
      }
    }
    if (x >= code[y].length) {
      for (var i = code[y].length; i <= x; i++) {
        code[y][i] = ' ';
      }
    }
    code[y][x] = val;
  }
  
  // continue until program over
  while (! done) {
    var instruction = code[y][x];
    
    // string mode enabled, ignore everything except "
    if (stringMode) {
      if (instruction === '"') {
        stringMode = false;
      } else {
        var val = instruction.charCodeAt(0);
        stack.push(val);
      }
      doMovement();
      continue;
    }
    
    // skip this instruction
    if (skip) {
      skip = false;
      doMovement();
      continue;
    }
    
    // process the instructions
    switch (instruction) {
      case '0':
      case '1':
      case '2':
      case '3':
      case '4':
      case '5':
      case '6':
      case '7':
      case '8':
      case '9':
        // because lazy
        var val = parseInt(instruction);
        stack.push(val);
        break;
      case '+':
        var a = stack.pop();
        var b = stack.pop();
        stack.push(a + b);
        break;
      case '-':
        var a = stack.pop();
        var b = stack.pop();
        stack.push(b - a);
        break;
      case '*':
        var a = stack.pop();
        var b = stack.pop();
        stack.push(a * b);
        break;
      case '/':
        var a = stack.pop();
        var b = stack.pop();
        if (a === 0) {
          stack.push(0);
        } else {
          stack.push(~~(b / a));
        }
        break;
      case '%':
        var a = stack.pop();
        var b = stack.pop();
        if (a === 0) {
          stack.push(0);
        } else {
          stack.push(b % a);
        }
        break;
      case '!':
        var a = stack.pop();
        if (a === 0) {
          stack.push(1);
        } else {
          stack.push(0);
        }
        break;
      case '`':
        var a = stack.pop();
        var b = stack.pop();
        if (b > a) {
          stack.push(1);
        } else {
          stack.push(0);
        }
        break;
      case '>':
        direction = 'r';
        break;
      case '<':
        direction = 'l';
        break;
      case '^':
        direction = 'u';
        break;
      case 'v':
        direction = 'd';
        break;
      case '?':
        var options = 'rldu';
        var choice = ~~(Math.random() * 4);
        direction = options[choice];
        break;
      case '_':
        var a = stack.pop();
        direction = (a === 0) ? 'r' : 'l';
        break;
      case '|':
        var a = stack.pop();
        direction = (a === 0) ? 'd' : 'u';
        break;
      case '"':
        stringMode = true;
        break;
      case ':':
        if (stack.length === 0) {
          stack.push(0);
        } else {
          stack.push(stack[stack.length - 1]);
        }
        break;
      case '\\':
        var a = stack.pop();
        var b = 0;
        if (stack.length > 0) {
          b = stack.pop();
        }
        stack.push(a);
        stack.push(b);
        break;
      case '$':
        stack.pop();
        break;
      case '.':
        var a = stack.pop();
        output.push('' + a);
        break;
      case ',':
        var a = stack.pop();
        output.push(String.fromCharCode(a));
        break;
      case '#':
        skip = true;
        break;
      case 'p':
        var valY = stack.pop();
        var valX = stack.pop();
        var v = stack.pop();
        putIntoStorage(valX, valY, String.fromCharCode(v));
        break;
      case 'g':
        var valY = stack.pop();
        var valX = stack.pop();
        var val = getFromStorage(valX, valY);
        stack.push(val);
        break;
      case '@':
        done = true;
        break;
      case ' ':
        break;
      default:
        console.log('unknown instruction: ', instruction);
        break;
    }
    
    doMovement();
  }
  
  return output.join('');
}