var FN_ARGS = /^function\s*[^\(]*\(\s*([^\)]*)\)/m;
var FN_ARG_SPLIT = /,/;
var FN_ARG = /^\s*(_?)(.+?)\1\s*$/;
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;

// TODO: should I keep these assertions?
function assertArg(arg, name, reason) {
  if (!arg) {
    throw new Error("Argument '" + (name || '?') + "' is " + (reason || "required"));
  }
  return arg;
}

function assertArgFn(arg, name, acceptArrayAnnotation) {
  if (acceptArrayAnnotation && angular.isArray(arg)) {
    arg = arg[arg.length - 1];
  }

  assertArg(angular.isFunction(arg), name, 'not a function, got ' +
      (arg && typeof arg == 'object' ? arg.constructor.name || 'Object' : typeof arg));
  return arg;
}

module.exports = function (fn) {
  var $inject,
      fnText,
      argDecl,
      last;

  if (typeof fn == 'function') {
    if (!($inject = fn.$inject)) {
      $inject = [];
      fnText = fn.toString().replace(STRIP_COMMENTS, '');
      argDecl = fnText.match(FN_ARGS);
      argDecl[1].split(FN_ARG_SPLIT).forEach(function(arg) {
        arg.replace(FN_ARG, function(all, underscore, name) {
          $inject.push(name);
        });
      });
      fn.$inject = $inject;
    }
  } else if (angular.isArray(fn)) {
    last = fn.length - 1;
    assertArgFn(fn[last], 'fn');
    $inject = fn.slice(0, last);
  } else {
    assertArgFn(fn, 'fn', true);
  }
  return $inject;
};