[feature] LP round-trip tests

[GermanHeim]

Is your feature request related to a problem? Please describe.
Currently, the LP writer/reader doesn't have any round-trip tests.

Describe the solution you'd like
Build models covering senses/bounds/domains/objective names, write to LP string, read back, assert equivalence.
Improves confidence in the reader without needing solvers.

Additional context
This issue is tagged good first issue specifically to help new contributors get hands-on with the codebase and wants to contribute in the long term. Please do not use AI for this issue.

[NikitaPomies]

Hello again,

I have been working on this issue and find some other pbs along the way with oximo-io write_lp

You can fix an Integer variable to a non integer value. It does not raise any error + write the infeasible lp

fn integer_var_fixing_to_non_int_bug() {
    let m = Model::new("lp_model");
    variable!(m, z, Integer);
    m.fix(z, 0.3); // We should raise an error
    objective!(m, Min, 2 * z);
    let dir = PathBuf::from(env!("CARGO_TARGET_TMPDIR"));
    write_lp(&m, &mut File::create(dir.join("integer_var_fixing.lp")).expect("create LP file"))
        .expect("dump LP");
}

You cannot fix a a Binary variable at all. See line 1040 of oximo-io/src/lp.rs

#[test]
fn binary_var_fixing_to_non_int_bug() {
    let m = Model::new("lp_model");
    variable!(m, z, Binary);
    m.fix(z, 1); // Currently not written due to a loo
    objective!(m, Min, 2 * z);
    let dir = PathBuf::from(env!("CARGO_TARGET_TMPDIR"));
    write_lp(&m, &mut File::create(dir.join("binary_var_fixing.lp")).expect("create LP file"))
        .expect("dump LP");
}

will write :

\* OXIMO LP export - model: lp_model *\
Minimize
 obj: 2 z
Subject To
Binaries
 z
End

You cannot properly fix a semi-integer variable

#[test]
fn semi_continous_var_fixing_to_non_int_bug() {
    let m = Model::new("lp_model");
    variable!(m, z, SemiContinuous(1));
    m.fix(z, 2.0); // We should raise an error
    objective!(m, Min, 2 * z);
    let dir = PathBuf::from(env!("CARGO_TARGET_TMPDIR"));
    write_lp(&m, &mut File::create(dir.join("semi_cont_fixing.lp")).expect("create LP file"))
        .expect("dump LP");
}

\* OXIMO LP export - model: lp_model *\
Minimize
 obj: 2 z
Subject To
Bounds
 1 <= z <= 2
Semi-Continuous
 z
End

Want me to fix that in the same PR ?

[GermanHeim]

Hi @NikitaPomies, yes, that would be great, thank you!
Also, it would be nice if you could fix your Git commit config so that you are credited as a contributor.

[NikitaPomies]

Hello, my git config should be fixed now.

I made a first PR in #80 to prevent the user fixing variable to non coherent values.

More incoming