According to https://github.com/rawbytess/hissab, it's not even close to being an alternative to Wolfram. Hissab is described as "A strict, unit-aware natural-language calculator" and its syntax looks nothing like Wolfram. It reminds me of Wolfram Alpha, though.
Rubi is just rule based integration. So its like looking up a book of integral formulas and encoding them as rules. It does mean you need a minimum number of other features like partial fraction decomposition, polynomial factoring so it demonstrates some capability. Many of the other Mathematica like CAS end up using the Rubi rules themselves.
But indefinite integration is just a small aspect of CAS capabilities. What about integration over a line or surface, definite integration and dealing with singularities, differential equations, solving equations under assumptions, simplifying equations.
According to https://github.com/rawbytess/hissab, it's not even close to being an alternative to Wolfram. Hissab is described as "A strict, unit-aware natural-language calculator" and its syntax looks nothing like Wolfram. It reminds me of Wolfram Alpha, though.
Its not trying to achieve a feature parity with wolfram or mimic it. Its just an alternative to do calculations and may meet somewhere in the middle.
Here are some other alternatives (some internally use free Wolfram engine):
Reimplementation in Rust: https://github.com/ad-si/Woxi
WLJS Notebook: https://wljs.io
VS Code extension: https://github.com/vanbaalon/wolfbook
How do they stack up doing actual computer algebra things like symbolic integration?
https://www.12000.org/my_notes/CAS_integration_tests/reports...
Note that alternative open source solvers like Fricas fail 10x the integrals in that corpus.
Rubi is just rule based integration. So its like looking up a book of integral formulas and encoding them as rules. It does mean you need a minimum number of other features like partial fraction decomposition, polynomial factoring so it demonstrates some capability. Many of the other Mathematica like CAS end up using the Rubi rules themselves.
But indefinite integration is just a small aspect of CAS capabilities. What about integration over a line or surface, definite integration and dealing with singularities, differential equations, solving equations under assumptions, simplifying equations.
The bottom two are based on Wolfram engine itself, so would fair the same as Mathematica. I do not know about the first one.