In order of appearance: Ioannis Petros Samiotis, Thomas Millross, Sander Bosma and Jente Hidskes.
This chapter describes the software architecture of Neovim: an open source code editor based on Vim. This analytical essay will provide interested readers with objective and relevant insights into the challenges and architectural decisions of the Neovim development effort. Neovim’s software architecture is assessed within the Rozanski and Woods  framework. The system stakeholders are detailed and categorised. Then the context and development viewpoints are described, followed by an analysis from the perspectives of variability and evolution. Finally the technical debt of the system is assessed and quantified.
Table of Contents
- Stakeholder Analysis
- Context Viewpoint
- Development Viewpoint
- Variability Perspective
- Evolutionary Perspective
- Technical Debt
Neovim is an extensible code editor, written mainly in C. The project is a fork of the famous code editor Vim. It is a modal editor, which means the keypresses are interpreted differently depending on the current mode. The main modes are normal, insert and visual mode. The keyboard shortcuts and commands allow users to edit and navigate through files faster than is possible with regular editors.
Early on the project established its goals of making Vim development more open to the community, refactoring the Vim codebase and removing misfeatures. The vision page mentions that Neovim is intended "for users who want the good parts of Vim, and more". The community is active and welcoming to new users and contributors alike.
This chapter presents an analysis of Neovim's software architecture based on the book Software Systems Architecture by Rozanski and Woods . We begin by considering the groups and individuals influencing the project through a stakeholder analysis. We then present a context viewpoint, which defines the relationships between Neovim and its environment. An in-depth study of Neovim's codebase is presented in the development viewpoint section. Next, two perspectives are explored: the variability and evolutionary perspectives. Finally, we discuss the technical debt in Neovim's codebase, before concluding.
We present here a stakeholder analysis of the Neovim project. To compile information, we began by documenting our existing knowledge of the stakeholders involved and supplemented this knowledge with data sources including: GitHub issues and pull requests, Gitter conversations and the online documentation. For categorisation, we took the Rozanski and Woods stakeholder list  as a basis, then removed the categories irrelevant to Neovim. Finally we estimated the power and interest of each group through structured discussion within our team.
Neovim users are ultimately the reason the project exists. Professional data scientists, sysadmins and devops experts are known to favour Vim. Users could be divided based on their skill-level into beginners and advanced users. Beginners may struggle initially with Neovim's minimalist interface, and frequently consult the help and documentation, striving to improve. The advanced group have traversed the steep learning curve, developed muscle-memory in their fingers, and memorised countless keyboard shortcuts. Design decisions for Neovim primarily emphasise the maximisation of editing efficiency for the advanced group. Beginners are encouraged to improve their skills, for instance using vim-tutor-mode.
Many people have contributed to the Neovim codebase as developers. The ~14 core developers include @justinmk the Benevolent Dictator For a Limited time, the lead developer @ZyX-I and @jamessan, with @tarruda recently becoming much less active. These core developers also fulfil additional roles, such as maintainers and assessors, keeping the project running whilst ensuring contributions adhere to coding standards and follow the style guide. There is not a dedicated team of testers; new tests are added by developers whenever significant changes are submitted.
Developers can further be divided into subcategories, in recognition of their differing concerns and motivations. Related project & plugin developers are essential for the growth of a healthy software ecosystem in the Neovim environment. Communication and overlap between these development teams helps to ensure that requirements are met and clean interfaces are maintained without regression. Vim developers may have submitted code still in use by Neovim, or recent bug-fixes that have been ported. Neovim developers also actively assist Vim developers by porting relevant patches to their codebase.
There are no official support staff or dedicated communicators.
Volunteers provide support through a
wiki which contains the documentation,
and a user manual which can be accessed through the
:help command. Individual support is also offered through
Gitter, IRC, Google
Reddit. Everyone who replies on these
channels could be considered as support staff.
Suppliers are the stakeholders that develop distributions and packages. They take decisions on whether to distribute Neovim for their platforms and hence can potentially influence the future popularity of the software. Neovim accepts donations through Bountysource, so the donators could be regarded as acquirers. However, they have no formal decision making power. Finally, the competitors are an important stakeholder, described in the context viewpoint section.
To visualise the relative power and interest of the stakeholders, a power-interest grid is provided in Figure 1. The power axis indicates influence in the decision making process and project direction. The interest axis represents the ongoing attention paid by a stakeholder to the project development. Stakeholders in the upper right corner should be closely managed, while those in the lower left require minimal engagement. The stakeholders in the upper left corner should be kept satisfied and those in the lower right should be kept informed about developments, to maintain their interest.
Figure 1: the power-interest grid
The context viewpoint describes "the relationships, dependencies, and interactions between the system and its environment" . We highlight Neovim's project scope, then use a context model to elaborate on the relationships with external entities.
System Scope and Responsibilities
Neovim is a code editor for those users who prefer to have a simple but powerful environment. Its responsibilities include opening text files and allowing the user to edit these files. Neovim assists the user in navigating these files and understanding the code contained within them by offering syntax highlighting and other features. An embedded scripting language called Vimscript is included, to enable extension and configuration by the user (as described in the variability perspective).
Note that Neovim is not striving to be an integrated development environment (IDE). While it offers the user a vast array of extensibility that could provide IDE-like features, the intention is to remain a simple but powerful code editor.
Neovim's context model is illustrated in Figure 2. This model depicts Neovim in the centre, surrounded by the external entities it interacts with. These entities are described below, beginning at the competitors and rotating clockwise.
Figure 2: the context model of Neovim
Neovim has competition from mainstream code editors such as Atom, Sublime Text and Notepad++. Vim and GNU Emacs are the main competitors from the same niche and require a small elaboration:
- Vim and Neovim are of course quite similar. Neovim introduced new features, some of which are now also found in Vim. Many differences are also found under the hood and in the way the communities operate.
- GNU Emacs is an extensible, customisable text editor -- and much more. At its core, Emacs is a Lisp interpreter that just so happens to support text editing. Plugins can change almost everything; there is an entire ecosystem providing functionality such as calendars, project planners, PDF readers and even internet browsers.
Neovim has an extensive community, known for being open and friendly to newcomers. See the stakeholder analysis for an overview.
Neovim runs on a variety of operating systems: macOS, Linux, FreeBSD, OpenBSD and even Android. Neovim chose not to inherit Vim's support for obsolete operating systems. Windows support is the focus of the current release target.
Plugins can extend the functionality of the editor or overwrite its default behaviour; detail is available in the variability perspective.
Neovim is built on top of existing C libraries that handle low level operations. These dependencies are enumerated in Table 1.
|jemalloc||an efficient replacement for the system provided
|gettext||used to localise the interface to the user's locale|
|unibilium||Neovim's built-in UI is a terminal user interface. Unibilium provides information about the supported features of the terminal emulator within which it runs|
|libtermkey||to decode keypresses, enabling the functionality described above|
|libvterm||provides the built-in terminal emulator|
|libuv||abstracts the operating system layer|
|MessagePack||exposes Neovim's API, allowing clients to be written in any programming language|
|LuaJIT||provides support for Lua scripting|
Table 1: dependencies of Neovim
Static analysis tools help developers to identify bugs, style deviations and technical debt. Three different static analysis tools are run on the Neovim codebase. Coverity is the most advanced static analysis tool used by Neovim and integrates with both GitHub and Travis. Clang Static Analyzer is the static analysis tool offered by the Clang compiler. Finally, clint is used to check for coding style violations and suspicious patterns.
Neovim's architecture (as described in the development viewpoint) enables the creation of graphical UIs and other clients that cannot be implemented in Vim. For instance, VimR replaces Neovim's built-in terminal UI with a graphical UI native to macOS. NyaoVim also provides a modern graphical UI for Neovim using Electron, but registers itself as a client over MessagePack instead of replacing the built-in terminal UI. SolidOak is an aspiring IDE for Rust that embeds Neovim as its editor component. Finally, Neovim-Qt is a client graphical UI like Nyaovim, using Qt5 instead of Electron. Neovim-Qt will be the default graphical UI on Windows.
Source Code Control and Issue Management
Neovim's entire development infrastructure is hosted on GitHub. Coveralls is used to keep track of code coverage. SourceGraph is offered as an alternative way of browsing the code online and Waffle is offered as an alternative project management tool.
Build and Continuous Integration
Neovim replaces the autotools build system with one implemented in CMake, which is arguably easier to work with, particularly on Windows. Neovim uses AppVeyor for Continuous Integration builds on Windows and TravisCI for macOS and Linux builds.
Neovim comes from a long line of editors. ed was the line editor provided by the Unix operating system. It inspired the creation of ex, a line editor for Unix systems that took advantage of video terminals. vi was born as the visual mode for ex. Over the years, vi became the standard Unix text editor. Vim (Vi IMproved) is a clone of vi providing more features, such as syntax highlighting and an extended range of ex commands. Finally, Neovim is a fork of Vim.
A development viewpoint describes "the architecture that supports the software development process" . We describe a number of codeline models, then present the module structure model.
This section is mainly structured after the four activities from the "Codeline Models" chapter in Rozanski & Woods . Additionally, it contains a section relating to the standardisation of testing.
A description of the top-level folders of the Neovim repository is shown in Table 2 below.
|build||generated folder containing all object files and binaries|
|cmake||CMake recipes to generate the Makefiles required during the building process|
|config||CMake and versioning configuration|
|contrib||useful files for contributors: autocompletion configurations, Doxygen settings, etc.|
|man||source for displaying manual pages (through the
|runtime||all run-time data: syntax highlighting, indentation scripts, icons, in-program documentation, plugins, etc.|
|scripts||scripts for building, releasing, etc.|
|src/nvim||C source code including some related files organised into subdirectories and some unorganised loose files. See the module structure model for more information|
|test||testing related files; functional tests are placed in test/functional, unit tests are placed in test/unit. The structure of test/unit is identical to src/nvim|
|third-party||CMake recipes to build third party dependencies|
|unicode||files containing Unicode definitions|
Table 2: description of the top-level folders
Neovim uses a combination of GNU Make and CMake to automate the building process. Make
ensures that source code is compiled in the correct order; it uses a Makefile
which lists dependencies between targets and specifies the steps required to
build a target. Since it is difficult to write Makefiles that are independent of
the system configuration (e.g. installed compiler and libraries), CMake is used
to dynamically create Makefiles specific to the system on which it runs.
Once a user has cloned the Neovim repository from GitHub, they simply
make command. This will automatically call both CMake and Make as
Each time a commit is made to the master branch, Travis-CI automatically builds the Doxygen and user documentation. It also generates the Clang Static Analyzer, translation, clint, vim-patch and Coverity reports. Finally, a nightly release is made which contains the Linux and MacOS binaries. Windows binaries are generated by AppVeyor. The nightly releases are mostly for testing. At major milestones, an official release is tagged which contains only the source code. Users can either compile Neovim themselves or wait until distribution maintainers update their packages to provide the new version.
Neovim uses the standard GitHub pull-based development model for collaboration. The project is split into multiple repositories. All files mentioned in the subsections above are located inside the Neovim repository. Related projects, such as plugin clients, continuous integration support and forks of dependencies are in GitHub repositories under the same organisation.
Standardisation of Testing
Neovim categorises test into unit and functional tests. Unit tests are compiled into a shared library and executed through the LuaJIT FFI library, which enables Lua code to use the C functions and data structures of the Neovim code. Functional tests, on the other hand, do not interface with the C code directly, but instead use the API via remote procedure calls (RPCs). While the unit tests usually only check the return values of functions, the functional tests can also check for example whether the resulting screen displays the correct state. These tests can be run locally. They are also automatically executed when changes are pushed to GitHub, together with clint to detect coding style violations and Coveralls for test coverage (as in coverage and testing debt).
Module Structure Model
A module structure model shows the organisation of the source files into modules that contain related code . Such a structure provides an overview of the source code which guides developers to understand and navigate the codebase.
The module structure model for Neovim in Figure 3 displays a simplified conceptual overview for two reasons:
- the codebase is complex and exhibits a structure which is difficult to discern due to inheriting 25 years worth of updates and changes without a significant refactoring (see technical debt). Neovim was created to address this issue, and this work is ongoing.
- C is a low-level language, but low-level analysis does not provide the ideal overview for documenting the module organisation.
Within the model, each box represents a module containing the names of some of the relevant source files of that module. The arrows show inter-module relationships, and the relative height differences graphically represent the layering.
Figure 3: the module structure model
We begin with the Modes module, which has a thick border. Neovim models a pushdown automaton that changes state on receipt of input from the operating system. A state in this case refers to a mode (such as insert mode) or normal mode) that implements an interface. The modes guide the pushdown automaton's state changes and Neovim's behaviour, and hence the modes drive all other modules.
A mode operates on a buffer (Vim jargon for a loaded file) to edit its contents. The Files and Buffers module contains the code required to load files into memory, for which it needs to handle I/O, character encoding and multi-byte characters. Because I/O is inherently operating system specific, this module interacts with the Operating System Interface module. This module provides abstractions over filesystem access, signals, input handling and other low-level operations that vary across operating systems.
When a buffer is changed from any mode, the mode notifies the Screen module. This module is responsible for maintaining an internal representation of what should be visible on the UI. For each running Neovim instance, there is one such screen that displays one or more windows (a view into a buffer) in tab pages (a layout of windows). An architectural decision taken by Neovim is that all code related to GUIs is removed from the core. Instead, there is a User Interface module that reflects the Screen module. Synchronisation between the User Interface and the Screen module is handled through the Events module: the Screen module publishes events that in turn drive the User Interface module.
The API module exposes the User Interface and other modules over the MessagePack RPC module. GUIs or other clients can subscribe to this RPC channel to for example interact with the running Neovim instance, or control it over a (local) network connection.
Finally, the Local Clients module contains clients that bypass the RPC interface and directly communicate with the User Interface module. The built-in Terminal UI (TUI) is an example of such a local client.
This section highlights three types of software variability: compile-time, load-time and run-time variability . The choice of which type of variability to utilise is a design decision which balances disk-space requirements, run-time performance and usability. Neovim uses all three types to some degree.
Compile-time options influence the binary created by the build process; the chosen options are compiled into the binary, the others removed. This results in a reduction in binary size, but also an increase in performance as these options now do not have to be evaluated at run-time. A classic example of compile-time variability is enabling or disabling certain feature sets, an option heavily used by Vim. Neovim has removed these feature sets and instead compiles most features unconditionally. The remaining compile-time variability options include inherently compile-time dependent OS-specific code that cannot be abstracted and the built-in TUI that can be disabled.
Neovim is highly customisable, with over 300 configurable settings such as syntax highlighting, colour schemes, indentation width, and default case sensitivity of searches. Key bindings can also be personalised for the user. These settings can all be configured at run-time or added to the initialisation file for load-time variability. As this file is interpreted as Vimscript, it can also be used for implementation of plugins.
Neovim plugins can be broadly divided into two categories: remote plugins and Vimscript plugins. VimAwesome lists over 15000 Vimscript plugins, with the majority designed originally for Vim. Most are compatible with Neovim. After Neovim implemented asynchronicity, Vim developed an incompatible alternative implementation. Asynchronous plugins are thus an exception: they are not compatible unless support is manually added. However, most plugins are purely synchronous and can thus be used by both systems.
Remote plugins can communicate with the Neovim API via MessagePack. To simplify remote plugin development, Neovim-specific API clients are currently available in 17 languages, although in theory any language implementing MessagePack (of which there are over 50) can be used to develop new remote plugins. Since remote plugins are introduced by Neovim, these plugins do not work on Vim.
Inter-plugin dependence is uncommon, although some such as vim-airline are designed to integrate with others. Plugin conflicts are also rare, with the possible exception of overlapping key bindings.
External User Interface
Neovim can also be embedded into other programs through the RPC API. The external GUI starts Neovim in headless mode using a command line parameter (another form of load-time variability). Instead of drawing the screen, Neovim sends the screen state over the RPC API to the external UI. Conversely, key strokes are received by the external GUI and sent across the API to Neovim. The RPC layer can also by bypassed directly by replacing the built-in TUI with another UI (see VimR in related projects).
The evolutionary perspective takes a view on the ability of the system's architecture to be flexible . We take a slightly different approach and consider the changes required of Vim's architecture from the perspective of @tarruda when he decided to fork Vim. We use the following process, adapted from Rozanski & Woods : characterise the evolution needs, assess the current ease of evolution and rework the architecture. We conclude by evaluating the extent to which @tarruda's architectural goals have been met.
After two decades of evolution, Vim had accumulated a complex codebase that few people understood. Bram Molenaar was possibly the only person capable of maintaining Vim's codebase, leaving him reluctant to merge new features due to the risk of regression. Vim was thus unable to keep up with the improvement and evolution demands of its users.
@tarruda realised this and decided it was time to rework Vim's architecture to increase its flexibility and reduce the time taken to implement new features. He characterised Vim's evolutionary requirements using his prior experience contributing to Vim, as in Table 3.
|Simplify maintenance to support new features and make it easier to provide bug fixes||Functional||Large-scale, high risk||Not immediately required, rather a long-term goal|
|Enable the implementation of modern user interfaces separate from the core of the editor||Functional, environment||Large-scale, high risk||Required sooner rather than later|
|Increase extensibility (see the variability perspective)||Functional, environment||Large-scale, high risk||Required almost immediately|
Table 3: Neovim's change requirements when forked from Vim
For each change, @tarruda specified a plan to help in estimating the ease of evolution:
- To simplify maintenance, Neovim would migrate to a CMake-based build system, remove legacy support and compile-time features and remove platform-specific code in favour of libuv. This is a difficult and high-risk requirement that would change the codebase as a whole. Luckily, much of it could be automated using existing tools.
- To enable modern user interfaces and increase extensibility, Neovim would implement a job control mechanism over MessagePack. This is again a difficult requirement that would require substantial refactoring of the codebase. However, there was already prior work and experience from attempting to bring asynchronous plugins to Vim.
Since then, Neovim's developers have refactored Vim's architecture in a step-wise fashion, limiting changes to distinct sub-systems. They strived to encapsulate functionality within well-defined modules (see the module structure model) with separate concerns. Where possible, these modules were abstracted behind interfaces. A focus on unit and functional tests within a continuous integration framework helped towards ensuring the changes were reliable.
Evaluation of the Initial Project Goals
Neovim is now three years old; how have these initial goals been met? In November 2016, Neovim developers stated that at least 20,000 new lines of C code had been written, with 2200 new tests in addition to Vim's own test suite. By that time, 273 contributors had committed more changes than Vim had received in twelve years. For a complete and up-to-date overview of differences between Vim and Neovim, see the vim-differences manual page.
The asynchronous plugins and built-in terminal emulator are prime examples of new features that have been implemented. All the work on the foundation is just now starting to bear its fruit with the appearance of many external GUI clients and several clients making use of the asynchronicity to implement features previously impossible in Vim. As new features have been implemented without the addition of a significant support burden, it is clear that the changes have led to simplified maintenance requirements. All of this is not without technical debt, however, as the following section will show.
Additionally, Vim 8.0 was announced in December 2016 as the first release in ten years, sporting many of the features initially found only in Neovim. This suggests that the new competition was the inspiration and motivation for this vital renewal in development effort.
This section explores technical debt in the Neovim codebase. Before entering into the details, let us first consider the perspective that the Neovim project exists partly as an answer to the technical debt accumulated by the Vim project. This blog describes examples of technical debt found in Vim's codebase, before highlighting Neovim as the solution.
It is clear that Neovim developers understand Neovim within this context. The lead maintainer @justinmk demonstrates his familiarity with the technical debt metaphor on numerous occasions (1, 2, 3, 4). Furthermore, on the progress page in the Wiki a lot of items are listed that are directly or indirectly related to paying technical debt. Finally, during a period of limited new features, in response to the question of "is Neovim in maintenance mode?", he responds "Yes. We're paying down technical debt".
Coverity Scan Static Analysis
Coverity Scan is an advanced static analysis tool that helps developers find and fix defects. It is executed periodically to scan Neovim's source code. At the time of writing, there are 89 outstanding issues out of a total of 458 found since the project's inception. 40 of these are dismissed as false positives and 329 have been fixed. The last scan (March 5, 2017) concluded a defect density (defined as the number of defects per 1,000 lines of code) of 0.33.
Some of the 89 outstanding issues were first detected in 2014. All open Coverity issues can be categorised as technical debt, since all of these defects are known but nobody has managed to fix them (or determined if they are false positives).
In Figure 4 we present the defect density of Neovim over a period of time as generated by Coverity. As can be seen, Neovim's complexity fluctuates around the average defect density of open source software projects of similar size (between 100,000 and 499,999 lines of code). The spike visible around January 2017 is likely a misconfiguration or something going wrong on Coverity; a defect density of 14 would mean there are total of 3500 defects, which does not correlate to the amount of changes made in this period.
Figure 4: Neovim's defect density over time, courtesy of Coverity Scan
A popular and well-studied  metric for measuring code complexity is the Cyclomatic Complexity metric (CC). This quantifies the number of independent paths through the source code of a particular function. The explanatory power of CC is frequently challenged [4, 5]. However this discussion is beyond the scope of this essay, and we will assume the metric has scientific validity and informational value with regards to technical debt for the remainder of this section: a high CC is an indicator for technical debt.
Coverity marks all functions with a CC of greater than 15 as being too complex. The Coverity scan of March 5 2017 marked 728 functions as being too complex, out of 5128. The average CC of all the functions is 10, while the average of the functions marked too complex is 47. The highest CC found in Neovim is 728. While the threshold of 15 is debatable , it is clear that some of Neovim's functions are way too complex.
"ToDo" Style Placeholders
A complementary approach to measuring technical debt is counting the occurrences of "TODO", "FIXME", "XXX" et cetera within the codebase. This approach relies upon the commenting habits and conventions of the developers, and does not provide a comprehensive understanding of the technical debt of a project. However, because Neovim has had mostly the same group of maintainers over its lifetime, we assume that this metric provides some insight into the evolution of technical debt in Neovim.
The occurrences were counted by searching for the strings mentioned above, which
were chosen from looking through comments and searching for common "ToDo" style
variants. The results were inspected and the following files were excluded to
remove many false positives:
*.txt and all
binary files. It is uncertain whether all categories contribute equally to
technical debt, but quantifying their difference is not simple. Therefore,
Figure 5 only displays the sum of all these occurrences for
various versions of the project.
Figure 5: Number of occurrences of "TODO", "FIXME" and "XXX" over several versions of the project
It can be seen that the technical debt gradually seems to increase as the project evolves. A sharp increase can be observed in the current (unreleased) version, although this number may decrease before an official release is made.
Coverage and Testing Debt
Neovim uses Coveralls to keep track of the total test coverage. From Coveralls we can observe that coverage increased gradually from 57% in 2014 at Neovim's inception to 76% in March 2017. This can be explained by Neovim's policy of adding tests whenever significant changes are made.
Instead of identifying testing debt in the actual tests' code, we can identify a rather large case of possible testing debt on a higher level. "New style" tests imported from Vim are written in Vimscript, whereas tests made for Neovim are written in Lua. The gain from rewriting these tests is small in comparison to the effort required and hence they are imported as-is. The result is having tests written in two different programming languages, which can be considered testing debt.
There are also "old style" legacy tests that are left over from the fork of Vim. These are written using yet another framework and are actively encouraged to be rewritten using Lua, as can be seen from the many pull requests named "Migrate legacy test", e.g. #2988. In this sense, Neovim is working to pay off some of its testing debt.
The goal of this chapter was to present Neovim and examine its software architecture. We did so by documenting the stakeholders, giving the context and development viewpoints, considering the variability and evolutionary perspectives and discussing Neovim's technical debt. We have shown that despite the clean simplicity of the UI, there is significant complexity hiding under the surface. Challenging architectural decisions have been taken in order to satisfy a variety of stakeholders.
Through our research, we have discovered a conscientious community of hardworking volunteers, dedicated to improving the text editor they love. The Neovim codebase, like any other, is not perfect. There are still issues and ongoing improvements. However, we may conclude that the Neovim team has improved the Vim experience and brought Vim into the twenty-first century.
We would like to thank:
- @justinmk and the community for actively welcoming new contributors, assisting with our contributions and clarifying some technical details for the module structure model
- the dedicated teaching assistants @sandervdo and @valmai for their invaluable feedback and encouragement
- our peers that reviewed the chapter prior to completion, helpfully highlighting areas of improvement
- professors @avandeursen and @azaidman for designing and managing such an original and valuable course and assignment.
- Rozanski, N., & Woods, E. (2011). Software systems architecture: working with stakeholders using viewpoints and perspectives. Addison-Wesley.
- Apel, S., Batory, D., Kästner, C., & Saake, G. (2013). Feature-Oriented Software Product Lines: Concepts and Implementations. Springer.
- McCabe, T. J. (1976). A complexity measure. IEEE Transactions on software Engineering, (4), 308-320.
- Shepperd, M. (1988). A critique of cyclomatic complexity as a software metric. Software Engineering Journal, 3(2), 30-36.
- Graylin, J., Hale, J. E., Smith, R. K., David, H., Kraft, N. A., & Charles, W. A. R. D. (2009). Cyclomatic complexity and lines of code: empirical evidence of a stable linear relationship. Journal of Software Engineering and Applications, 2(03), 137.
- Watson, A. H., Wallace, D. R., & McCabe, T. J. (1996). Structured testing: A testing methodology using the cyclomatic complexity metric (Vol. 500, No. 235). US Department of Commerce, Technology Administration, National Institute of Standards and Technology.