oximo-io

Model I/O for oximo: MPS and LP writers.

Converts an oximo [oximo_core::Model] to standard text formats for exchanging models with external solvers and tools. Both formats support LP and MILP only. Nonlinear expressions raise [IoError::Nonlinear].

Support for NL files for nonlinear programming (NLP) and mixed-integer nonlinear programming (MINLP) is planned.

Usage

Enabled by default via the io feature on the umbrella oximo crate:

[dependencies]
oximo = "0.1"  # io is on by default

To opt out:

[dependencies]
oximo = { version = "0.1", default-features = false, features = ["highs"] }

To use this crate directly:

[dependencies]
oximo-io   = "0.1"
oximo-core = "0.1"

Quick example

use oximo::prelude::*;
use oximo::io::{to_mps_string, to_lp_string};

let m = Model::new("knapsack");
let x = m.var("x").lb(0.0).build();
let y = m.var("y").lb(0.0).ub(4.0).build();

m.constraint("c1", (x + 2.0 * y).le(14.0));
m.constraint("c2", (3.0 * x - y).ge(0.0));
m.maximize(3.0 * x + 4.0 * y);

let mps = to_mps_string(&m)?;
let lp  = to_lp_string(&m)?;
println!("{mps}");

Formats

MPS

Fixed-format MPS (fixed-column, 10-char field width). Widely supported by commercial and open-source solvers.

Feature	Behavior
Objective row	Named `OBJ`, maximization models are negated with a `* sense: maximize` comment so re-importers can recover the original sense
Integer variables	Wrapped in `INTORG`/`INTEND` markers
Bounds	`FR` (free), `MI`+`UP` (lower=-inf), `LO`/`UP` as needed. Default lb=0 omitted
Constant terms	Objective constant written to `RHS OBJ`, constraint constants folded into `RHS`

use oximo_io::{write_mps, to_mps_string};
use std::fs::File;
use std::io::BufWriter;

// To string
let s = to_mps_string(&model)?;

// To file
let mut f = BufWriter::new(File::create("model.mps")?);
write_mps(&model, &mut f)?;

LP (CPLEX LP format)

Human-readable CPLEX LP format. Sections emitted: header comment, Minimize/Maximize, Subject To, Bounds (non-default only), General, Binaries, End.

Feature	Behavior
Objective sense	`Minimize` / `Maximize` keyword, no negation needed
Integer variables	`General` section (integer/semi-integer), `Binaries` section
Bounds	Free variables declared with `free`; default lb=0, ub=+inf omitted
Objective constant	Written as a comment if non-zero

use oximo_io::{write_lp, to_lp_string};
use std::fs::File;
use std::io::BufWriter;

// To string
let s = to_lp_string(&model)?;

// To file
let mut f = BufWriter::new(File::create("model.lp")?);
write_lp(&model, &mut f)?;

Errors

All functions return Result<_, IoError>:

Variant	Cause
`IoError::NoObjective`	Model has no objective set
`IoError::Nonlinear`	Objective or constraint contains nonlinear nodes (`Mul` with two non-constant children, `Pow`, `Sin`, etc.)
`IoError::Io(e)`	Underlying `std::io::Error` from the writer

License

MIT OR Apache-2.0

oximo-io 0.1.0