Search-Based Software Testing (SBST) tools can automatically generate tests to achieve high code coverage; however, a systematic understanding of why they fail in specific situations is necessary. This thesis addresses this gap by developing a comprehensive taxonomy of coverage failures through an empirical analysis of the three most prominent SBST tools: Pynguin (Python), SynTest (JavaScript), and EvoSuite (Java). By classifying and analysing failure patterns across these tools and language paradigms, this research provides a foundational framework to diagnose shortcomings, prioritise future development, and enhance the practical effectiveness of automated test generation.

Testing of software is crucial to the quality of the final product manual test assertion creation has become a significant bottleneck in the development process, which delays release. Having shown promise in generating assertions automatically, Large language models (LLMs) have showed promise in generating assertions automatically. This is due to their fluency in both natural languages and code, as well as the fact that they produce tests a lot faster than a developer would. However, LLMs must reckon with deployment issues that come with the high computation time and latency of large models, or the limited functionality of their smaller, locally-executable counterparts. Knowledge distillation, a technique that aims to "transfer knowledge" from a teacher model to a student one, can thus enable the potential of smaller and faster models. This drives the research to explore the effectiveness of knowledge distillation in developing a smaller and efficient model for assertion generation. With CodeT5 as the teacher model, the student model learns from the teacher. The student is iteratively trained in epochs, validated on unseen data. The metrics used to evaluate include assertion accuracy, similarity to teacher model output and ground truth, model size, inference time, with the goal to quantify the trade-offs and determine the feasibility of distilled models for practical assertion generation. We presented and analyzed the results we achieved. The capability the student showed was around 1/3 of that of the teacher, which suggest a potential for creating efficient, yet reliable assertion generation tools.

Effective test assertions are important for software quality, but their creation is time-consuming. While Large Language Models (LLMs) show promise in automated assertion generation, their size, cost, resource demands, and need for online connection often render them impractical for widespread developer use. Knowledge Distillation (KD) offers a solution to bridge that gap by transferring capabilities from a large "teacher" LLM to smaller "student" models (SLMs). However, the majority of the ground work on KD has been focused on classification tasks and not on generative problems. This paper investigates the feasibility of a test assertion generation task using response-based Knowledge Distillation (KD) from a CodeT5-base teacher. We specifically explore the impact of three parameters on assertion quality and model efficiency - those being student model size (number of layers), pretraining initialization, and loss weighting. Our results demonstrate that distilled small student models (231 MB), particularly those initialized from pretrained checkpoints and fine-tuned with specific loss weight (α = 0.5) for the ground truth and distillation losses, can retain a significant portion of the teacher's assertion generation performance when considering the defined metrics - achieving around 83.9% of the CodeBERTScore of the teacher with just 25.9% of the size. This work provides empirical insights into creating specialized SLMs for test assertion generation, highlighting practical configurations for deployment in development environments.

The XRP Ledger (XRPL) relies on a Byzantine fault-tolerant consensus algorithm to ensure global agreement on transactions across distributed nodes. Despite its critical financial role, the implementation remains under-tested. While prior work has shown the potential of evolutionary testing to uncover potential consensus violations in XRPL, the role of genetic operator selection in this process remains unexplored. We address this research gap by presenting a comparative evaluation of four evolutionary configurations that differ in their balance of exploration and exploitation. The system is tested by injecting network delays to simulate adverse conditions and trigger violations. Our results show that the balance of exploration and exploitation affects the performance of bug detection: configurations that favor exploitation, complemented by subtle exploration, yield the most favorable results. In addition, we contribute an extensible testing method tailored to XRPL but applicable to other distributed systems.

The XRP Ledger Consensus Protocol is a Byzantine fault-tolerant algorithm that enables the XRP Ledger to reach agreement on which transactions to apply, supporting millions of transactions daily. While the protocol is correct by design, its practical implementation is vulnerable to concurrency-related bugs triggered by nondeterministic message delivery between distributed validator nodes. These bugs are subtle and difficult to expose through conventional testing. In this paper, we investigate the use of evolutionary concurrency testing combined with a priority-based message scheduling strategy to explore different message interleavings. Specifically, we evaluate multiple fitness functions and assess their ability to guide the search toward buggy executions. Our results show that EvoPriority, which applies evolutionary testing to priority-based schedules, is difficult to guide toward buggy executions regardless of the fitness function used. Although it is capable of uncovering violations on a bug-seeded versions of the XRP Ledger Consensus Protocol, its performance is similar to randomized concurrency testing.

The decentralized nature of blockchain systems makes them prone to concurrency bugs, which are difficult to detect. There exist testing techniques to find these bugs, such as systematic exploration of the solution space, but these techniques are difficult to scale. Evolutionary algorithms have been proposed as an effective solution to find these bugs. In this research, we aim to discover the influence of evolutionary operators in the bug detection performance of evolutionary algorithms. We test this on the XRP Ledger Consensus Protocol (XRP LCP) using priority-based event representation. We present Groot, an evolutionary algorithm that is implemented using a modified version of the Rocket framework. We experimented with two combinations of operators: the Simulated Binary Crossover (SBX) operator with the Gaussian mutation operator and the Laplace Crossover (LX) operator with the Makinen, Periaux and Toivanen Mutation (MPTM) operator. We evaluated these setups using effectiveness and efficiency and compared them to a random baseline. We used a bug-seeded version of the XRP LCP to run the experiments of these setups. We discovered that all setups are capable of detecting bugs in the XRP LCP. The results indicate that the effectiveness and efficiency is not influenced by the choice of these operators in a significant way. We discuss that possible reasons for these discoveries include noise in the fitness function, event representation limitations and configuration choices that may have contributed to these results.

Distributed systems, such as blockchains, can have bugs around edge-cases that are hard to detect or trigger. Previous publications have introduced guided-search testing approaches that are able to find edge cases more efficiently than through conducting a systematic and exhaustive search. In this paper, we compare the effectiveness of fitness functions in evolutionary testing frameworks. For this we evaluate time and proposal fitness. While evolutionary testing frameworks are not new to the domain of concurrency testing consensus algorithms, the impact of the fitness functions that underpin them remains poorly understood. We use the XRPL consensus algorithm as a case study to evaluate the fitness functions using the Rocket testing framework. For this, we make use of seeded versions of XRPL. All evaluated fitness functions have been able to detect the bugs we seeded in the source code of the XRPL consensus algorithm. We show the validity of various fitness functions in trying to find the bug and analyze effects in the interplay between the time and proposal fitness functions we examine.

Effective software testing relies on the quality and correctness of test assertions. Recent Large Language Models (LLMs), such as CodeT5+, have shown significant promise in automating assertion generation tasks; however, their substantial computational resource demands limit their practical deployment in common development environments like laptops or local IDEs. To address this challenge, this work explores knowledge distillation to derive smaller, more efficient student models from a larger, pre-trained CodeT5+ teacher. While knowledge distillation has been successfully applied to general code models, its specific application to creating lightweight, locally-deployable models for test assertion generation remains a recognized research gap. Using a dataset that includes assertion input-output pairs and teacher logits, we systematically investigate the impact of different distillation loss components—soft logits loss and hard target losses—on student performance. Our findings demonstrate the practical viability of this approach: a distilled 220M parameter student model can be nearly 3x faster and consume over 40% less memory than its 770M teacher, while retaining approximately 78% of the original’s assertion generation quality as measured by CodeBLEU. These results offer practical insights and a clear pathway for deploying efficient yet effective assertion-generation models suitable for local developer workflows.

Writing clear, semantically rich test assertions remains a major bottleneck in software development. While large pre-trained models such as CodeT5 excel at synthesizing assertions, their size and latency make them impractical for on-premise or resourceconstrained workflows. In this work, we introduce a knowledgedistillation pipeline that transfers knowledge from CodeT5-base, a pre-trained encoder–decoder Transformer model based on the T5 architecture, into sub-1 GB student models tailored specifically for test-assertion generation. Our pipeline combines response-based distillation using soft labels and hard-label fine-tuning, and incorporates custom student architectures comparing pre-trained models vs random initialization, along with targeted regularization techniques. We instantiate students at various size points and conduct an empirical evaluation on standard assertion benchmarks, measuring exact-match accuracy, similarity, RAM footprint, and CPU and GPU inference latency. Our best 230 MB student retains over 80% of the teacher’s assertion-generation accuracy on exact matches and over 90% of the similarity, while having an inference time of under 3 seconds on a single consumer-grade CPU, with a 75% reduction in RAM usage. These results demonstrate that distilled code-LLMs can deliver near-teacher assertion quality under tight memory and latency constraints, paving the way for fully on-device IDE integration and low-overhead continuous-integration workflows.

Writing test cases is an important yet complex task. Search-Based Software Testing (SBST) is an automated test case generation technique that aims to help developers by creating high-coverage test cases. Despite its strengths, a major limitation of this technique is that it often struggles with generating test cases that contain complex method sequences, as they have no semantic understanding of which methods are related. The recent advancement of Large Language Models (LLMs) offers a potential solution due to their natural language capabilities and applicability to software engineering tasks. However, LLMs often end up with lower coverage than SBST when directly compared due to lacking the output diversity that is required in software testing. This opens an opportunity to combine the exploratory power of SBST, with the semantic understanding of LLMs to make the test case generation process more effective in terms of test coverage and test structure. This thesis investigates the combination of these methodologies with our hybrid approach, LLM-Seeded Evolutionary Testing (LSET), which uses LLMs to generate tests that contain complex method sequences, and introduces this structure into the SBST process by serving as the starting point (seeds) of the algorithm to be evolved further. We conducted an empirical evaluation on a benchmark consisting of 35 JavaScript classes and found a significant branch coverage increase on 19 and 14 classes when compared to SBST and LLM-only baselines. However, when taking the combination of final SBST and LLM test suites, this gap reduces to 1 out of 35 classes. Beyond coverage, we also found a positive effect on structure when compared to tests generated by SBST, as the structure provided by the LLM tests can be further evolved to reach deeper branches while maintaining readability.

Blockchain systems rely on consensus protocols to ensure agreement among nodes even in the presence of malicious or faulty nodes. A consensus protocol that provides safety and liveness guarantees under such conditions is known as a Byzantine fault‑tolerant (BFT) protocol. Various whitepapers describe the design and implementation of BFT protocols, providing formal proofs of their safety and liveness properties. However, in practice such protocols are difficult to implement correctly and often contain subtle logic errors that cause consensus violations. Ensuring correctness is especially important for the XRP Ledger—a public, decentralized blockchain that processes transactions for the widely used cryptocurrency XRP. Thorough testing of consensus protocols is crucial, but systematic testing is challenging and time-consuming due to the large number of possible network and timing configurations.
In this paper, we present a replication package for evaluating randomized Byzantine fault tolerance testing on the XRP Ledger Consensus Protocol (LCP). We implement the ByzzFuzz search algorithm and compare it to naive random testing. Additionally, we investigate the impact of different hyperparameter configurations on the performance of the ByzzFuzz algorithm. Our experimental results demonstrate that both naive random testing and the ByzzFuzz algorithm detect seeded bugs in the XRP LCP, with ByzzFuzz algorithm uncovering more agreement violations. Finally, we identify the most effective hyperparameter configurations for the ByzzFuzz algorithm.

Software testing is a vital yet time consuming process during the development lifecycle, often causing engineers to limit its use in practice. In order to encourage active software testing, researchers have shown significant advances in automatic unit test case gener- ation with approaches such as search-based testing (i.e., EvoSuite) and large language models (i.e., ChatGPT). However, while the first suffers with exploring edge cases of the input space, the latter still suffers from hallucinations during code synthesis, limiting the use of both solutions. This research aims to overcome these limitations by utilizing the strengths of both techniques, which are effective test structure generation and program inference, respectively. In particular, the assertions of initial unit tests generated by EvoSuite are augmented using ChatGPT-4o, with the aim of improving the mutation score, and hence the overall test suite effectiveness. We evaluate our solution, called evoLLve’M, on a benchmark of 20 Java classes from the SourceForge110 Corpus and compare it to only using EvoSuite, which is considered the state-of-the-art ap- proach. Results show that evoLLve’M outperforms EvoSuite in 25% of the classes for mutation score, without negatively impacting other classes. It boosts the total number of killed mutations by 3%, achieving the most improvement for mutations types of increments and null returns, being 26.9% and 8.9%, respectively.

Mutation testing is a way to test the effectiveness of a test suite for catching bugs in a given piece of code. Writing these tests manually can be cumbersome and time-consuming. Automated tools can be used to generate tests that achieve a high mutation score. The output of these tools is often very hard to understand for humans, and therefore rarely used as actual test suites for software programs. Because LLMs have been shown to be able to generate programs that can be more easily understood by humans, we ask if these LLMs can be used for improving or generating tests for the purpose of mutation testing. Some LLMs run in the cloud, while others run locally. Cloud-based LLMs such as ChatGPT or Copilot are not always an option because of privacy concerns, speed, or regulations, but do not require possession of hardware. Local LLMs do not have the privacy concerns, but sometimes require large amounts of hardware to be available. This paper will focus on local LLMs that can be run in a computationally restricted environment. We present an automated approach to use a local LLM to improve the mutation score of existing test suites. We compare three different models (DeepSeek Coder, Code Llama and Codestral), evaluated on publicly available datasets. Using this approach, we were able to successfully generate unit tests that, combined with the existing manually written tests, are able to increase the mutation score around one third to half of the time depending on the model.

The automated generation of test suites is crucial for enhancing software quality and efficiency. Manually writing tests is time-consuming and accounts for about 15% of project time while tests generated by automated tools like EvoSuite and Pynguin often lack readability and comprehensibility. Recent research suggests that Large Language Models (LLMs) might offer a promising alternative. This paper investigates the effectiveness of Llama3 70B in generating unit test cases for Java and Python projects. We compared Llama3 against EvoSuite and Pynguin by measuring the mutation score of test suites generated for a corpus of 20 Java and 20 Python classes. Our findings reveal that EvoSuite significantly outperforms Llama3 in terms of mutation score, achieving an average mutation score of 81.05% versus Llama3's 66.26%. Conversely, Llama3 surpasses Pynguin, with scores of 51.95% and 42.73% respectively. These results highlight that while Llama3 is not superior to EvoSuite, it shows potential as a viable tool for test generation, especially for dynamically typed languages like Python. Further, empirical observations indicate that Llama3 requires significantly more time to generate tests compared to EvoSuite and Pynguin. This study underscores the need for continued research to optimize LLMs for software testing and improve their efficiency and accuracy.

Manually crafting test suites is time-consuming and susceptible to bugs. The automation of this process has the potential to make this task more appealing. While current tools like EvoSuite manage to obtain high coverages, their generated tests are not always readable. Recent literature indicates that Large Language Models (LLMs) could address readability and comprehension issues. Our objective in this study is to explore the capabilities of ChatGPT-3.5-Turbo in enhancing existing Java unit tests. We have designed an algorithm that sends multiple prompts to the LLM and overwrites the test cases with the ones received from GPT-3.5. Thus, we have assessed its performance by measuring the initial mutation score of the test suite with the new coverage. The benchmark consists of 16 non-trivial Java classes, on which we performed 80 runs of our algorithm. The results indicate that after one run, GPT-3.5 increases mutation coverage by 23% on average for isolated classes. However, for classes with dependencies, it is less reliable, often producing code with run-time or compile-time errors. Through this paper, we hope to emphasize the importance of ongoing research in this domain to optimize LLMs for providing better test cases.

Over the last few years, Large Language Models have become remarkably popular in research and in daily use with GPT-4o being the most advanced model from OpenAI as of the publishing of this paper. We assessed its performance in unit test generation using mutation testing. 20 Java classes were selected from the SF110 Corpus of classes, and for each 10 different test classes were generated. After we resolved build errors and removed failing assertions, the evaluation using Pitest produced around 71% of mutation coverage on average on the sample dataset. Manually fixing the failing assertions increased the overall mutation score to 75%. Nonetheless, one of the main drawbacks was the need to manually resolve problems that the GPT-4o responses produced, such as code hallucination and incorrect assumptions about the classes under test.

Software testing, a critical phase in the software development lifecycle, is often hindered by the time-intensive and costly manual creation of test cases. While automating test case generation could mitigate these challenges, its adoption in the industry has been limited due to difficulties in comprehending the generated test cases. To address this, our study presents an approach for clustering test cases and evaluates its impact on the comprehensibility of test suites through empirical research. Our approach clusters test cases based on their covered objectives, grouping together those with similar attributes to enhance developer understanding. The core of our empirical research evaluates developer agreement with our clustering method and contrasts the comprehensibility of clustered versus non-clustered test suites. Findings suggest a broad agreement among developers in favor of our clustering approach, with clustered test suites facilitating faster software maintenance tasks. Notably, the effectiveness of task completion remained comparable between both suite types. In summary, our research introduces and validates an innovative test case clustering strategy, striving to enhance the comprehensibility of automatically generated test suites.

Kotlin is a programming language best known for its interoperability with Java, as well as the measurable improvements it offers over it. Since it became Android’s go-to language in 2019, the popularity and impact of Kotlin have risen greatly. Amidst this surge in popularity, the Kotlin developer team is working on a new version of the compiler that introduces sweeping changes to the ecosystem. Traditional compiler testing is a manual and laborious task that requires extensive developer effort and expertise. In an attempt to mitigate this, researchers have invested great resources in developing and perfecting automated compiler testing tools over the last decades. These approaches generate new pieces of code to test the behavior of compilers, which is assessed through differential testing. However, the usage of heuristics as guidance for the generative process is not well understood, and no approach that generates Kotlin code from scratch currently exists. In this thesis, we propose a novel method of enriching standard grammar specifications with language-targeted semantic context that is integrated in the sampling process. We structure generated code hierarchically and use it as the base of an evolutionary computation framework. Within this framework, we introduce two classes of algorithms that are novel to the field of compiler fuzzing, based on syntactic diversity and semantic proximity, respectively. We carry out an empirical analysis spanning 200K generated Kotlin files, which we analyzed through different Kotlin compiler versions. Our results uncovered five previously unreported categories of bugs, which we reported to the Kotlin compiler developer team. The developers verified and replicated our instances on the current re- lease of the Kotlin compiler, and have assigned target release dates for fixes within the current major version of the compiler. The study also provides new insight into the effects of heuristic-specific hyperparameters such as expression simplicity, dissimilarity measurements, and target selection.

Software testing is an important yet time consuming task in the software development life cycle. Artificial Intelligence (AI) algorithms have been used to automate this task and have proven to be proficient at it. This research focuses on the automated testing of JavaScript programs, and builds upon the existing SynTest framework that is the current state of the art, with the Dynamic Many Objective Sorting Algorithm (DynaMOSA) being the best performing AI algorithm for test case generation. DynaMOSA uses the Non-Dominated Sorting Algorithm - II (NSGA-II) as its base algorithm, and adds modifications to it. This paper investigates whether the use of the Pareto Envelope Based Search Algorithm - II (PESA-II) as the base algorithm results in improved performance. The contributions of this research includes a modified PESA-II integrated into the SynTest framework, using inspiration from DynaMOSA. Moreover, we answer the question "How does the modified PESA- II perform compared to DynaMOSA in generating test cases for JavaScript programs?" The performance of the algorithms is measured based on the (branch and method) coverage of the test cases generated for a suite of JavaScript classes. The results show that the modified version of PESA-II outperforms the base version. However, neither manage to outperform DynaMOSA.

On the intuitive level, software testing is important because it assures the quality of the software used by humans. However, ensuring this quality is not an easy task because as the complexity of the software increases, so do the efforts to test it. Search-based software testing is an active research field that develops and explores tools for automatic test case generation. Their work involves using meta-heuristic search optimisation approaches such as evolutionary algorithms
from the evolutionary computation community and applying them to test case generation. The crossover between the evolutionary computation domain and the test case generation one produced DynaMOSA, a state-of-the-art evolutionary algorithm for generating test cases. In an attempt to
produce another well-performing algorithm, this paper performs another crossover between the two communities and creates DynaMOSARVEA - a product of a Reference Vector Guided Evolutionary Algorithm (RVEA) and DynaMOSA.
The conducted empirical study showed that although DynaMOSARVEA does not outperform DynaMOSA, it did outperform RVEA, thus demonstrating the value brought by domain-specific knowledge.