calculate-bson-size.js

function calculateColumnSize(column) {
  let size = 0;
  size += 4; // 4 bytes for int32 for a document size
  // `_id` key-value where _id value is a string
  // 10 bytes + ? = 1(value type) + 3(`_id` key length) + 1(key terminator) + 4(int32 string length) + ?(value length) + 1(string value terminator)
  size += 10 + column._id.length;
  // `type` key value with 'string' as value
  // 17 bytes = 1(value type) + 4(`type` key length) + 1(key terminator) + 4(int32 string length) + 6(`string` length) + 1(string value terminator)
  size += 17;
  // `values` array with string or null values
  // it will be encoded as key-value first
  // 8 bytes + ? = 1(array type) + 6(`values` key size) + 1(key terminator) + ?(array BSON)
  // array BSON size 5bytes + ? = 4(int32 length) + ?(key-value pairs encoded) +1(BSON terminator)
  // totally 13 bytes for what we know + key-value pairs encoded
  size += 13;
  for (let i = 0, len = column.values.length; i < len; i++) {
    const value = column.values[i];
    // 1byte for value type encoded
    size += 1;
    // size = key length + 1 byte for key terminator
    size += String(i).length + 1;
    // encoding values
    if (value === null) {
      // for null we do not need to count anything else
    } else if (typeof value === 'string') {
      // size = 4(int32 string length) + ?(value length) + 1(value string terminator)
      size += 4 + Buffer.from(value).size + 1;
    } else {
      throw new Error('Not implemented');
    }
  }
  // 1 byte for document terminator
  size += 1;
  return size;
}