Invalid results when using big numbers about calculator

[LorkScorguar]

I also have this issue on latest calculator app. It seems that most calculator apps have this issue, even Google.
15 numbers is the max precision we can get

[michallepicki]

Recently Microsoft open-sourced their calcuator app, and inside we can find a library that allows for

infinite precision for basic operations (addition, subtraction, multiplication, division) and 32 digits of precision for advanced operations (square root, transcendental operators)

The calculator is written in C++ but glancing over the library I think it's using only C so it should be possible to use it relatively easily.

Another example:
12345679 * 12345679 * 81 = 12345678987654320, but it was expected 12345678987654321.

The problem is that the built-in data type, double, is used for calculations. The built-in types are good for software calculations like iterations over the transition matrix, when a range of values ​​is known. But to use them for working with user data, when the range is obviously unknown and accuracy is important at the output, is bad practice.

The solution is quite simple: use third-party data types for calculations using long arithmetic.
Like MPRF (Examples of good calculators you can see there in "Software Using MPFR")

https://en.wikipedia.org/wiki/List_of_arbitrary-precision_arithmetic_software

For Vala there is a building to libgcrypt: https://gitlab.gnome.org/GNOME/vala-extra-vapis/blob/master/gcrypt.vapi
See MPI

Or see https://github.com/GNOME/gnome-calculator/tree/master/vapi