var untilde = function(str) {

return str.replace(/~./g, function(m) {
  switch (m) {
    case "~0":
      return "~";
    case "~1":
      return "/";
  }
  throw new Error("Invalid tilde escape: " + m);
});

}

var traverse = function(obj, pointer, value) {

// assert(isArray(pointer))
var part = untilde(pointer.shift());
if(!obj.hasOwnProperty(part)) {
  return null;
}
if(pointer.length !== 0) { // keep traversin!
  return traverse(obj[part], pointer, value);
}
// we're done
if(typeof value === "undefined") {
  // just reading
  return obj[part];
}
// set new value, return old value
var old_value = obj[part];
if(value === null) {
  delete obj[part];
} else {
  obj[part] = value;
}
return old_value;

}

var validate_input = function(obj, pointer) {

if(typeof obj !== "object") {
  throw new Error("Invalid input object.");
}

if(pointer === "") {
  return [];
}

if(!pointer) {
  throw new Error("Invalid JSON pointer.");
}

pointer = pointer.split("/");
var first = pointer.shift();
if (first !== "") {
  throw new Error("Invalid JSON pointer.");
}

return pointer;

}

var get = function(obj, pointer) {

pointer = validate_input(obj, pointer);
if (pointer.length === 0) {
  return obj;
}
return traverse(obj, pointer);

}

var set = function(obj, pointer, value) {

pointer = validate_input(obj, pointer);
if (pointer.length === 0) {
  throw new Error("Invalid JSON pointer for set.")
}
return traverse(obj, pointer, value);

}

exports.get = get exports.set = set