Yuck is a constraint-based local-search solver with FlatZinc interface
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What is Yuck?

  • Yuck is a constraint solver powered by local search.
  • Yuck's approach to problem solving is comparable to Comet [HM05] and OscaR/CBLS [BMFP15].
  • Yuck can be used as a library or as a FlatZinc interpreter that integrates with the MiniZinc IDE.
  • Yuck is provided under the terms of the Mozilla Public License 2.0.
  • Yuck's design goal is not to compete with highly tuned LP, CP, or SAT solvers, but to provide an alternative for cases where the use of such solvers is inappropriate or seems risky, namely when the input data might be inconsistent with the constraints (so the solver is expected to build around what is given), or when the requirements are hard to model and it seems easier to provide domain-specific constraints for local search than for tree search.
  • Yuck is written in Scala and exploits the Scala library's immutable collection classes for implementing global constraints.

Current state

  • Yuck implements simulated annealing along with some basic annealing schedules and some schedule combinators.
  • Yuck supports lexicographic cost functions with both minimization and maximization goals.
  • Yuck allows to timebox and parallelize solvers by means of solver combinators.
  • Yuck supports the interruption and the resumption of solvers to facilitate the presentation of intermediate results.
  • Yuck implements boolean, integer, and integer set variables.
  • Yuck supports implicit solving by means of constraint-specific neighbourhoods.
  • Yuck's constraint library is far from complete but already provides the most frequently used constraints.
  • Yuck's FlatZinc frontend supports most of FlatZinc and many global MiniZinc constraints, see FlatZinc support.
  • Yuck is developer-friendly and easy to extend, see Contributing.
  • Yuck was submitted to the 2016 MiniZinc challenge where it coped quite well with 12 out of 20 problems.
  • In the 2017 MiniZinc challenge, Yuck ranked second among local-search solvers.

FlatZinc support

Yuck's FlatZinc frontend supports all of FlatZinc except for float variables and float constraints.

Yuck provides dedicated solvers for the following global MiniZinc constraints:

  • all_different, all_different_except_0
  • at_least, at_most
  • bin_packing, bin_packing_capa, bin_packing_load
  • count_eq, count_geq, count_gt, count_leq, count_lt, count_neq
  • cumulative
  • disjunctive
  • exactly
  • global_cardinality and friends
  • inverse
  • lex_less, lex_lesseq
  • maximum
  • member
  • minimum
  • nvalue
  • regular
  • table

Yuck provides dedicated neighbourhoods for the following global MiniZinc constraints:

  • all_different
  • inverse

When used as a FlatZinc interpreter, Yuck proceeds as follows:

  • It performs a limited amount of constraint propagation to reduce variable domains before starting local search.
  • It eliminates variables by exploiting equality constraints.
  • It identifies and exploits functional dependencies to reduce the number of decision variables.
  • It uses an annealing schedule that interleaves adaptive cooling with geometric reheating.
  • In move generation, it concentrates on variables that are involved in constraint violations.
  • It uses restarting to increase robustness: When a solver terminates without having reached its objective, it gets replaced by a new one starting out from another random assignment.
  • When Yuck is configured to use multiple threads, restarting turns into parallel solving: Given a thread pool and a stream of solvers with a common objective, Yuck submits the solvers to the thread pool and, when one of the solvers provides a solution that satisfies the objective, Yuck discards all running and pending solvers.

Download and installation

Yuck requires a Java 8 (or higher) runtime environment.

Download the latest release, unzip the package in a suitable location, and change to the resulting folder. The start scripts reside in the bin subfolder; you can run Yuck with either ./bin/yuck (on UNIX-based systems) or bin\yuck.bat (on Windows). Yuck's MiniZinc library resides in the mzn subfolder.

Command-line interface

To solve a problem with Yuck, invoke Yuck with the FlatZinc file on the command line, for example:

yuck zebra.fzn

animal = array1d(0..4, [4, 1, 2, 5, 3]);
colour = array1d(0..4, [3, 5, 4, 1, 2]);
drink = array1d(0..4, [5, 2, 3, 4, 1]);
nation = array1d(0..4, [3, 4, 2, 1, 5]);
smoke = array1d(0..4, [3, 1, 2, 4, 5]);

Yuck's output complies to the requirements of the FlatZinc 1.6 specification (see section 6).

Use the --help option to obtain a list of all options.

MiniZinc IDE integration

To configure Yuck as a backend for the MiniZinc IDE, proceed as follows:

  • In the MiniZinc IDE, go to the Configuration tab.
  • From the Solver dropdown list, select Add new solver and a window will open.
  • In that window, provide values for the following parameters:
    • Name: Yuck
    • Executable: Give the full path to the Yuck start script, see above.
    • MiniZinc library path: Give the full path to Yuck's MiniZinc library, see above, and prefix it with -I. (There must NOT be a space between -I and the path.)
  • Then press the Add button.
  • Back on the Configuration tab, select Yuck from the dropdown list.
  • Switch from Default behavior to User-defined behavior.
  • If your hardware allows, increase the number of threads.

(Tested on Ubuntu and Windows 10.)

Future work

  • Allow for the composition of lexicographic cost functions in MiniZinc through constraint and goal annotations
  • Implement among, diffn, circuit, subcircuit, ...
  • Provide soft constraints
  • Refactor and improve up-front propagation
  • Reduce dependence on integration testing by adding more unit tests


Yuck is developer-friendly:

  • It's source code is clean and well-documented.
  • It provides extensive and configurable logging.
  • Test coverage is good.

Yuck is easy to extend:

  • It's core machinery can be asked to look for misbehaving constraints while running a solver.
  • New variable types can be added without touching the core machinery and without impairing the type safety of the system.

To build and run Yuck, you need sbt.

Use sbt eclipse to create an Eclipse project.


To build and rebuild Yuck and its documentation, use the sbt standard targets:

  • sbt compile compiles all sources that need compilation.
  • sbt doc creates the ScalaDoc documentation from the sources.
  • sbt clean removes all artifacts of compiling and building.


Yuck tests are based on JUnit 4 and MiniZinc 2.1.x.

  • make unit-tests builds and runs all unit tests.
  • make minizinc-tests runs all FlatZinc frontend tests.
  • make minizinc-examples exercises Yuck on a subset of the MiniZinc 1.6 examples.
  • make ci-tests runs all these tests.

FlatZinc generation is fully automated and happens on the fly.

All test cases other than unit tests leave a log in the local tmp folder.

Optimization results undergo automated verification on the basis of the G12 solver (which comes with MiniZinc).


There are two ways to run Yuck:

  • Stage and run it:

     sbt stage
     ./bin/yuck --help
  • Run it through sbt:

    sbt run --help

Coding style

Yuck code should follow the Scala Style Guide with two exceptions:

  • Indentation should be four instead of two spaces.
  • Method calls should always be chained with the dot operator, so don't use infix notation.

In addition, the following rules apply:

  • Lines should not be much longer than 120 characters.
  • Don't give a result type when overriding a definition.


[BMFP15] G. Björdal, J.-N. Monette, P. Flener, and J. Pearson. A Constraint-Based Local Search Backend for MiniZinc. Constraints, 20(3):325-345, 2015.

[HM05] P. V. Hentenryck and L. Michel. Constraint-Based Local Search. MIT Press, 2005.