Convenience `firestore.rules`'s function.

firestore.rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    // Create `Path` from array of string.
    function documentPath(paths) {
      return path([
        ['databases', database, 'documents'].join('/'),
        paths.join('/')
      ].join('/'));
    }

    // Requested User is authenticated.
    function isAuthenticated() {
      return request.auth != null;
    }

    // Requested User is authenticated and `auth.uid` equal to `userID`
    function isUserAuthenticated(userID) {
      return request.auth.uid == userID;
    }

    // Returns request.resource.data
    function incomingData() {
      return request.resource.data;
    }

    // Returns resource.data.
    function existingData() {
      return resource.data;
    }

    // Retrieve data from `path`.
    function getData(path) {
      return get(path).data;
    }

    // Retrieve data from `path` (for atomic operation such as batch and transaction).
    function getAfterData(path) {
      return getAfter(path).data;
    }

    // Whether request.time matches the specified time. (Use `FieldValue.serverTimestamp()`
    // (FieldValue.serverTimestamp()を使うことで一致する)
    function isRequestedTime(time) {
      return time == request.time;
    }

    // string validation
    function validateString(text, min, max) {
      return text is string
        && min <= text.size()
        && text.size() <= max;
    }

    // number validation
    function validateNumber(num, min, max) {
      return num is number
        && min <= num
        && num <= max;
    }

    // Whether the value is the same before and after the change
    function isNotChanged(key) {
      return incomingData()[key] == existingData()[key];
    }
  }
}