Determining ArduinoJSON document size based on file size

Question

I'm trying to effectively allocate a doc's size, based on the size of the file saved in flash of ESP8266. Is there a way?

For example: file.size() X 1.5

• Majenko: I would think it very much depends on what the content is. If you have a lot of string data then it can correlate quite well. If you have a lot of numeric data then I would imagine the internal representation could be a lot smaller than the textual file representation of the same data.
• guyd: @Majenko Since some parameter files are stored on flash (which also can be change in future), one size's can be 256, and other is 1250. My goal is not to allocate max of all

Answer 1

There is unfortunately no way to predict the size of the JsonDocument from the file size alone.

As a workaround, I suggest that you allocate a very large memory pool and shrink it after deserialization; like so:

DynamicJsonDocument doc(ESP.getMaxAllocHeap());
deserializeJson(doc, file);
doc.shrinkToFit();

Indeed, this program consumes more memory than strictly required, but only for a fraction of a second.

See also:

• How to determine the capacity of the JsonDocument?
• BasicJsonDocument::shrinkToFit()

• guyd: Is only for DynamicJsonDocument?
• orithena: @guyd shrinkToFit is available in BasicJsonDocument, too. But not in JsonDocument or StaticJsonDocument. See the docs to determine which one to use in your use case.
• timemage: Depending on what is actually being done, the filtering feature may also be helpful.
• Benoit Blanchon: @guyd, you asked how to determine the document size at run time, so StaticJsonDocument is not an option.