Firestore document size calculation

main.dart

import "dart:typed_data";

/// An example collection from firestore.
/// Covers most datatypes of firestore list in this page.
/// https://firebase.google.com/docs/firestore/storage-size#document-size
/// Reference field is omitted for now. But shold be easy to implement with the help of cloud_firestore package.
final collection = [
  {
    "name": "John",
    "lastname": "Doe",
    "alien": false,
    "dob": DateTime.now(),
    "height": 172,
    "weight": 65.5,
    "misc": ["area-56", 25, 90.5],
    "links": {"twitter": "@jd", "google": "jd@google.com"},
    "avatar": Uint8List.fromList([1,4,6,7,7])
  },
  {
    "name": "John",
    "lastname": "Doe2",
    "alien": false,
    "dob": DateTime.now(),
    "height": 172,
    "weight": 65.5,
    "misc": ["area-56", 25, 90.5],
    "links": {"twitter": "@jd", "google": "jd@google.com"},
    "avatar": Uint8List.fromList([1,4,4,5])
  }
];

/// create an extension on string to return in firestore size
extension on String {
  int fstSize() {
    return this.length + 1;
  }
}

/// extension to create firestore size on num. Covers both int and double
extension on num {
  int fstSize() {
    return 8;
  }
}

/// extension to create firestore size on DateTime
extension on DateTime {
  int fstSize() {
    return 8;
  }
}

///
/// FireStoreSize is aimed at calculating the document size for firestore documents.
/// This implementation is based on the reference size from the following page.
/// https://firebase.google.com/docs/firestore/storage-size#document-size
///
class FireStoreSize {
  /// Return size in bytes of the provided firestore document [fsDocument].
  int getFireStoreDocumentSize(Map<String, dynamic> fsDocument) {
    int netSizeInBytes = 0;

    fsDocument.entries.forEach((entry) {
      // First add the key size to the [netSizeInBytes]
      netSizeInBytes += entry.key.fstSize();
      // assign value to local variable
      final val = entry.value;
      // Next depending on the type of the field add the respective size to the [netSizeInBytes] variable.
      // Recursively call [getFireStoreDocumentSize] method to size of map fields as it represents the firestore map fields.
      if (val is Map<String, dynamic>) {
        netSizeInBytes += this.getFireStoreDocumentSize(val);
      } else if (val is List) {
        val.forEach((item) {
          if (item is Map<String, dynamic>) {
            // For map fields again use a recursive call
            netSizeInBytes += getFireStoreDocumentSize(item);
          } else {
            // For generic fields use the [_fieldSize] method to retrieve field specific size in bytes            
            netSizeInBytes += fieldSize(item);
          }
        });
      }else{
        netSizeInBytes += fieldSize(val);
      }
    });
    return netSizeInBytes;
  }

  /// Returns field type specific size in bytes.
  /// This method doesnt return reference field sizes as it requires cloud firestore package of dart or flutter to verify the type.
  /// The [Uint8List] type is used instead of [Blob] from the cloud firetor package for berevity. 
  int fieldSize(dynamic val) {
    // num,String and DateTime has the respective extension function [fstSize] to return firestoelsere specific sizes.
    if (val is String) {            
      return val.fstSize();
    }else if (val is num) {            
      return val.fstSize();
    }else if (val is DateTime) {            
      return val.fstSize();
    }else if (val is Uint8List) {
      // Bytes field should return the size in the bytes.
      return val.lengthInBytes;
    } else if (val == null) {
      // null fields have size of 1 byte by default in firestore.
      return 1;
    }
    return 0;
  }
}

void main() {
  
  /// Prepare a [FireStoreSize] instance to access the utility functions calculating size of a firestore document.
  final fss = FireStoreSize();
  
  
  /// Iterate through the collection of documents are print the size in bytes.
  /// If the logic is correct it should print 
  collection.asMap().forEach((id, document){
    print("Size of document $id is ===> ${fss.getFireStoreDocumentSize(document)}-Bytes");
    print("----------------");
    
  });
}