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Language-independent, search-based program repair -- just your cup of tea! ☕
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Darjeeling is a language-agnostic search-based program repair tool. Unlike other repair tools, such as GenProg, SPR, and Nopol, Darjeeling delegates the responsibility of generating patches, obtaining coverage, analyzing code, and executing tests to other services. Once those auxillary concerns are removed, what is left is a lightweight framework for composing and executing repair algorithms: Darjeeling.


  • Language Agnostic: delegates syntax transformation and static analysis to other services.
  • Containerization: uses BugZoo to quickly and safely evaluate patches without executing arbitrary code on your machine.
  • Custom Repair Templates: uses Rooibos to support rich, custom repair templates for arbitrary languages.
  • Asynchronous Evaluation: accelerates patch evaluation by spreading the load across multiple threads.
  • Test Redundancy Checking: uses coverage information to skip test executions that can't be affected by a given patch.
  • Test Ordering: numerous test ordering schemes reduce the cost of patch evaluation by prioritizing likely failing tests.
  • Equivalent Patch Detection: uses static analysis to remove duplicate transformations from the search space.



To use Darjeeling, Docker must be installed on your machine, and your user account must be a member of the docker group in order to avoid problems related to insufficient privileges . Python 3.5 or greater and pip3 must also be installed; Darjeeling will not work with older versions of Python 3 nor will it work with any versions of Python 2.

To unlock all of Darjeeling's features, including template-based repair, Rooibos and Rooibosd must be installed.

Optional Extras

We strongly recommend that you install either virtualenv or pipenv to contain your installation of Darjeeling and to avoid conflicting with system packages. Both of packages can be installed via pip as follows:

$ pip install virtualenv
$ pip install pipenv

Using virtualenv, you should create a virtual environment for Darjeeling either in a new directory or at the root of your clone of this repository:

$ virtualenv name_of_directory

To enter the virtual environment:

$ cd name_of_directory
$ source activate
(venv) $ ...

To exit the virtual environment:

(venv) $ deactivate


To install the latest stable release of Darjeeling from PyPI from inside the virtual environment:

(venv) $ pip install darjeeling

Alternatively, to install from source, execute the following inside the virtual environment:

(venv) $ pip install .


Darjeeling exposes a command-line interface (CLI) for performing program repair, as demonstrated below. The CLI provides a single command, repair, which accepts the path to a Darjeeling configuration file format, described below.

$ darjeeling repair my-config.yml

Configuration File Format (v1.0)

The Darjeeling configuration file format is written in YAML. Below is an example of a configuration file.

version: '1.0'
snapshot: 'manybugs:python:69223-69224'
language: c
seed: 0
threads: 16
  type: spectrum
  metric: tarantula
    - foo.c
  type: exhaustive
    - type: delete-statement
    - type: replace-statement
    - type: prepend-statement
  ignore-equivalent-prepends: yes
  ignore-dead-code: yes
  ignore-string-equivalent-snippets: yes
  candidates: 5000
  time-minutes: 3600

Below, we describe the top-level options exposed by the configuration file:

  • version: the version of the Darjeeling configuration file format that was used to write the file.
  • snapshot: the name of the BugZoo snapshot that should be used to provide the bug as a Docker container.
  • seed: a seed for the random number generator.
  • threads: number of threads over which the repair workload should be distributed.
  • limits: limits on the resources that may be consumed during the search.


The language property specifies the language used by the program under repair. Although Darjeeling supports multiple languages, it is not yet possible to fix bugs that involve more than one language.

Below is a list of the languages that are fully supported by Darjeeling. Darjeeling can automatically perform static analysis and compute coverage information for each of these languages.

  • C: c
  • C++: cpp

The text option (i.e., language: text) may be used to ignore the language of the program under repair and to treat each file as a text file. When this option is used, users will need to manually provide coverage information, and static analysis will not be performed.


The localization section provides instructions for localizing the fault inside the program under repair. Currently, the configuration file format supports a single type of fault localization: spectrum-based fault localization, which assigns a suspiciousness value to each line in the program under repair based on the number of passing and failing tests that touch that line. A suspiciousness metric is used to compute individual suspiciousness values. The configuration file exposes a number of metrics via its metric property:

  • tarantula
  • genprog
  • jaccard
  • ochiai

The localization section also exposes an exclude-files property, which may be used to exclude certain files from the fault localization. Each file should be given by its location relative to the source directory for the program under repair. In the example below, the files foo.c and bar.c are excluded from the fault localization.

  - foo.c
  - bar.c

Individual source code lines can also be excluded using the exclude-lines property, as shown below. The exclude-lines property states which lines should be excluded from specified files. In the example below, lines 1, 2, 3 and 4 from foo.c, and lines 4, 6, 7 from bar.c are excluded from the fault localization.

  foo.c: [1, 2, 3, 4]
  bar.c: [4, 6, 7]

The fault localization can also be restricted to only consider certain files by using the restrict-to-files property, as shown below.

  - foo.c

Similarly, the fault localization can also be restricted to individual source code lines using the restrict-to-lines property:

  foo.c: [11, 14, 16]


The algorithm section outlines the search algorithm that should be used to search the space of candidate repairs. A description of the types of search algorithm exposed by the configuration file format is given below.

  • exhaustive: iterates over all single-transformation patches within the search space until the termination criteria are met.
  • genetic: implements a customisable genetic algorithm, inspired by GenProg.


The transformations section describes the space of program transformations from which candidate patches should be composed. The schemas property of this section specifies a list of the program transformation schemas, along with any parameter values for those schemas, that should may be used to construct concrete program transformations. Each entry in the schemas list must specify a type.

The configuration format supports three "classical" statement-based transformation schemas based on those introduced by GenProg: delete-statement, replace-statement, and prepend-statement; swap-statement has not been implemented at the time of writing. To learn more about why Darjeeling uses prepend-statement rather than the traditional append-statement schema, see the Darjeeling design document. Below is an example of schemas property that uses all of the classical statement-based schemas.

  - type: delete-statement
  - type: replace-statement
  - type: prepend-statement

The configuration format also supports custom repair templates via match-rewrite patterns for Rooibos. Below is an example of a simple repair template that replaces all calls to foo with calls to bar.

- type: template
  match: "foo(:[1])"
  rewrite: "bar(:[1])"

The type property is set to template to indicate that this schema represents a Rooibos-based repair template. The match and rewrite sections are used to specify match and rewrite patterns, respectively.

Darjeeling also provides support for naive line-based transformations, given below, which can be used for programs that use languages that are not fully supported (i.e., programs that use the text language).

- type: delete-line
- type: insert-line
- type: replace-line


The optimizations section is used to toggle various optimizations available to the repair process. By default, all optimizations are enabled. Below is a list of optimizations that can be toggled by the configuration file.

  • use-scope-checking: ensures that all variable and function references that occur in a given transformation are visible from the scope into which they are being inserted.
  • use-syntax-scope-checking: ensures that any keywords introduced by a transformation (e.g., break and continue) are permitted by their surrounding context.
  • ignore-dead-code: prevents the insertion of code that exclusively writes to dead variables.
  • ignore-equivalent-prepends: uses an approach inspired by instruction scheduling to prevent equivalent insertions of code.
  • ignore-untyped-returns: prevents insertion of a return statement into a context where the type of the retval is incompatible with the return type of the enclosing method or function.
  • ignore-string-equivalent-snippets: transforms donor code snippets into their canonical form, thus preventing the insertion of string-equivalent snippets.
  • ignore-decls: prevents transformations that are either applied to declaration statements, or else solely introduce a declaration statement.
  • only-insert-executed-code: prevents the insertion of code that has not been executed by at least one test case.


The resource-limits section of the configuration file is used to impose limits on the resources that may be consumed during the search. The search will be terminated upon hitting any of these limits. The limits specified in this section of the configuration file may be overridden by command-line options. If a limit for a particular resource is not given in either the configuration file or as a command-line argument, then the use of that resource will be unbounded (i.e., no limit will be imposed).

Below is a list of the resource limits that may be specified in the configuration file:

  • candidates: the maximum number of candidate patches that may be evaluated. May be overriden at the command line by the --max-candidates option.
  • time-minutes: the maximum length of wall-clock time that may be spent searching for a patch, given in minutes. May be overriden at the command line by the --max-time-mins option.

Search Algorithms

This section describes the different search algorithms that are supported by Darjeeling.


The exhaustive search algorithm exhaustively searches over all legal single-transformation patches within the search space until the termination criteria are fulfilled.


The genetic search algorithm implements a genetic algorithm that is inspired by the one used by GenProg, a formative search-based program repair tool for C. Below is an excerpt from a configuration file that uses a genetic search algorithm.

  type: genetic
  population: 80
  generations: 20
  tournament-size: 3
  mutation-rate: 0.6
  crossover-rate: 0.1
  test-sample-size: 0.4

Below is a list of the parameters that are exposed by genetic:

  • population: the size of the (initial) population. Used to control the number of individuals that are selected as parents.
  • generations: the maximum number of generations.
  • tournament-size: the size of the tournament when performing tournament selection to choose parents. Larger tournament sizes lead to an increased selective pressure.
  • mutation-rate: the probability of an individual mutation event.
  • crossover-rate: the probability of an individual crossover event between two parents.
  • test-sample-size: controls test sampling. When test sampling is enabled, the fitness of an individual is computed using a randomly selected subset of the test suite, rather than the entire test suite. (More specifically, test sampling selects a subset of the passing tests whilst keeping all of the failing tests.) The value of test-sample-size is used to specify the size of the subset (or sample). If test-sample-size is given as a float, then it will be treated as a fraction. If test-sample-size is given as an integer, then its value will be used as the absolute number of (passing) tests that should be included in the sample. If test-sample-size is omitted or set to null, test sampling will be disabled.
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