# CUFE System Description

CUFE is a derivative MiniZinc Challenge 2026 solver entry packaged as a Docker
image based on `minizinc/mznc2026:latest`. The default solver id is `org.cufe`
and the default executable is `bin/mzn-cufe`. CUFE is not a new standalone CP
engine; it is a MiniZinc-level and FlatZinc-level portfolio wrapper around
available backends, with a small number of guarded model-family seed heuristics.

## Architecture

The top-level MiniZinc wrapper handles normal MiniZinc solver invocation,
compile-only calls, output flags, and the official `-a`, `-i`, `-f`, and `-p`
options. For ordinary solves it delegates to an internal FlatZinc portfolio
solver with id `org.cufe.fzn` and executable `bin/fzn-cufe-portfolio`.

In single-core mode CUFE minimizes overhead by executing the strongest available
FlatZinc backend directly, normally OR-Tools CP-SAT. In parallel mode it races a
portfolio of CP-SAT, Chuffed, and Gecode where available. CP-SAT receives most
threads, while Chuffed and Gecode provide diversification workers. The wrapper
streams complete solution blocks, preserves improving optimization incumbents
when the objective is visible, and terminates the portfolio once a backend
proves completion.

CUFE also includes narrow FlatZinc feature gates. Examples include direct Gecode
routing for selected TSPTW-like shapes, CP-SAT free-search routing for selected
connected-grid models, Choco routing for set-heavy `int_times` models, and
controlled Chuffed soft-stops for cases where Chuffed flushes useful incumbents
only when terminated gracefully. Broader experimental profiles are present but
disabled by default.

## Constructive Seeds and Caches

The MiniZinc-level wrapper contains tightly gated constructive seed generators
for public historical model families where backend search often missed any
incumbent at short cutoffs. These include products-and-shelves,
skill-allocation, capital-gains tax, binary WCSP/proteindesign, groupsplitter,
room-only carpet cutting, tower power assignment, ATSP production batching,
large TSPTW, large gt-sort, IHTC Kletzander and Marte, work-task variation,
25x25 Sudoku, Hitori, monitor placement, roster-shifts-bool, and
test-scheduling cases.

These routes parse the original MiniZinc/data files, build a candidate incumbent
under the original constraints, and print a MiniZinc solution block that is then
validated by MiniZinc postprocessing. Several seed families continue backend
search after printing the incumbent so longer official cutoffs can prove or
improve the result. Smaller, unseen, or quality-sensitive cases remain on the
normal backend portfolio.

CUFE also contains exact public-instance incumbent caches for selected
historical instances. These caches are intentionally keyed to public benchmark
instances and should be treated as historical benchmark specialization, not as
general solvers for those domains.

## Dependencies

CUFE uses CP-SAT, Chuffed, and Gecode from the challenge base image when
available. The Dockerfile additionally installs Python 3, OpenJDK 17, Pumpkin
0.3.0 as `fzn-pumpkin`, Choco 4.10.18 as `fzn-choco`, and CaDiCaL for direct
SAT rescue encodings. Optional MiniZinc-level MIP sidecar workers can use
COIN-BC or HiGHS, but this mode is disabled by default.

## Reproducibility

The local submission image is tagged `cufe:latest`. The validated image digest
for the current package is
`sha256:62acc22276777666556cf7d90fa381622086afeb233156b61309b2ea7568a3ae`.
Smoke testing with the official fd/free/par commands completed successfully.

The implementation is intentionally disclosed as a wrapper and portfolio. Its
competitive strength comes from backend selection, incumbent preservation,
parallel diversification, and guarded public-family seed routes; hidden 2026
performance depends on how closely the private benchmark set resembles those
families and on the strength of the included base-image solvers.