Content-Based Textual File Type Detection at Scale

01/21/2021 ∙ by Francesca Del Bonifro, et al. ∙ University of Bologna 7

Programming language detection is a common need in the analysis of large source code bases. It is supported by a number of existing tools that rely on several features, and most notably file extensions, to determine file types. We consider the problem of accurately detecting the type of files commonly found in software code bases, based solely on textual file content. Doing so is helpful to classify source code that lack file extensions (e.g., code snippets posted on the Web or executable scripts), to avoid misclassifying source code that has been recorded with wrong or uncommon file extensions, and also shed some light on the intrinsic recognizability of source code files. We propose a simple model that (a) use a language-agnostic word tokenizer for textual files, (b) group tokens in 1-/2-grams, (c) build feature vectors based on N-gram frequencies, and (d) use a simple fully connected neural network as classifier. As training set we use textual files extracted from GitHub repositories with at least 1000 stars, using existing file extensions as ground truth. Despite its simplicity the proposed model reaches 85 high number of recognized classes (more than 130 file types).



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1. Introduction

“Which programming language is it written in?” is among the first questions a developer would ask about any piece of software. In software engineering research, “programming language” is a common variable to correlate against—researchers have measured language productivity (Maxwell et al., 1996; Maxwell and Forselius, 2000; Rodríguez et al., 2012), trends (Rabai et al., 2011; Urma et al., 2014), usability (McIver, 2000), and code quality (Kochhar et al., 2016; Ray et al., 2017; Berger et al., 2019), to name just a few. Developers would know by heart the language their code is written in, and will also easily identify the main programming language used in any given code base. But when it comes to large code bases, which often mix and match multiple programming languages; or when it comes to large-scale analyses (Dyer et al., 2013; Ma et al., 2019), encompassing entire collaborative development forges if not the full body of publicly available source code (Abramatic et al., 2018), manual detection is not an option. In those cases researchers tend to rely on either off-the-shelf language detection tools (e.g., GitHub’s Linguist (GitHub, Inc., 2011)), or on metadata exposed by code hosting platforms (Katz, 2018), which were in turn extracted using the same tools.

File extensions are highly predictive of programming languages and, more generally, file types. As such detection tools tend to heavily rely on them to determine file types (Ganesan and Foti, 2019). While very effective in the general case, doing so is problematic when file extensions are either not available (e.g., code snippets posted on the Web or embedded in document, executable scripts, etc.) or wrong (either on purpose or by mistake).

In this paper we study the problem of detecting the file type of textual files commonly found in software source code. Specifically, we will answer the following research question:

RQ: is it possible to accurately detect the extension of textual files commonly found in software version control systems repositories, based solely on file contents?

We will interpret “solely” in a strict sense, depriving ourselves of the use of any a prioriheuristic on the content of the files that will be encountered—i.e., our detection approach will not know a priori the keywords in the grammar of any given programming language, nor how shebang (#!/usr/bin/perl) or editor mode lines (%% mode: latex) are usually written in files. This is important for several reasons. First, programming languages (which are the most significant part of textual file types encountered in code bases) evolve and diverge syntactically significantly over time (Wexelblat, 1981; Bergin Jr and Gibson Jr, 1996; Ryder and Hailpern, 2007; Sammet, 1972; Gabbrielli and Martini, 2010). As it interesting (Shustek, 2006) and is now becoming increasingly more possible (Di Cosmo and Zacchiroli, 2017) to analyze historically relevant software source code dating back several decades, heuristics built today will fail on old code and heuristics spanning several generations of file formats will be fragile and hard to maintain in the long run. Second, extensionless file type detection shed some light on the intrinsic recognizability of textual file types, and programming languages in particular, which is an interesting topic.

As a consequence of the above we refuse to use as ground truth about file types the notion of programming language—which, as we have seen, is actually not well-defined in datasets spanning several decades, or that might contain syntactically invalid files, files that mixes multiple programming languages, etc; also, we did not want to rely on any pre-existing language detection tool to not negatively compound accuracy. Instead, we use file extensions encountered in real code bases as file type classes and attempt to detect them based on file content only. To stress this important point: file extensions will only be used as labels in training sets and to measure accuracy, but not as file features in the detection phase (as that would replicate the weakness of current tools that we want to address).

We propose a simple model for the task at hand: (a) use a language-agnostic tokenizer for textual files, (b) group tokens in n-grams of length 1 to 2, (c) build feature vectors based on n-gram frequencies, and (d) use a simple fully connected neural network as the actual classifier. As initial training set we use  16 M textual files extracted from popular GitHub repositories (as determined by “stars” (Borges and Valente, 2018)), using associated file extensions as labels; we further cleanup the dataset by filtering out binary files and downsampling classes so that the number of samples per class are analogous. In spite of its simplicity, the proposed classification approach reach an aggregate precision of 91% on a total of 133 file type classes, outperforming state-of-the-art tools and approaches in precision, amount of considered classes, or both. Thanks to its simplicity, the model is also fast to (re)train and deploy, making the proposed approach more maintainable in the long run. The remaining of the paper is organized as follows. In Section 2 we present the experimental dataset. The proposed classification model is introduced in Section 3 and experimentally validated in Section 4. Threats to validity are discussed in Section 5. Comparisons with related work, both qualitative and quantitative, are given in Section 6. We discuss future work and wrap up the paper in Section 7. A complete replication package for this paper is available from Zenodo at (DOI: 10.5281/zenodo.3813163).

2. Dataset

As experimental dataset we want files coming from public version control system. We retrieved a snapshot of GitHub dated 2017-01-27 and made available by the Software Heritage (Abramatic et al., 2018; Di Cosmo and Zacchiroli, 2017) project,111 which contains all source code files extracted from all commits of GitHub projects ranked with 1000 or more “stars” (Borges and Valente, 2018). It is a significant dataset, weighting 141 GB after compression and file-level deduplication, containing 15 M unique files. Each included file is identified by a SHA1 cryptographic checksum; a mapping between (unique) files and the corresponding file names as encountered on GitHub is provided. Due to deduplication it might happen that the same SHA1 is associated to different filenames, associating it to different extensions. The number of different extensions in the mapping is 546 K. However looking at extension frequencies in the dataset it is immediately clear that many of them are either typos or uncommon extension assignments. We set a threshold of for extension frequencies in order to filter out non statistically significant extensions, thus obtaining 220 extensions, still remaining in the 15 M ballpark for number of files, confirming the validity of the selection criteria.

A second filtering step consisted in excluding binary files, as our main purpose for this paper is classifying textual files. As criterion for this we have considered files as a sequence of raw bytes (as no explicit encoding information is stored by Git), put a threshold of 20% printable characters, and excluded all files (and associated extensions) with more non printable characters than that. After this filtering step we obtained 133 different extensions and 13 M files. A very small percentage of this set (less than 0.4%) consists of files associated to more than one extension. Rather than dealing with multi-label classification, and given its low occurrence, we have excluded multi-extension files from the dataset. We have obtained this way a multi-class, single-label classification problem with a dataset of files and 133 classes.

Figure 1. Extension distribution in our corpus.
Figure 2. Word cloud for token frequency distribution.

Most of the extensions in the polished dataset are commonly used as extensions for programming or markup languages (the extensions with the highest frequencies were y , \verb rb , \verb html , \verbo , h , , \verb java , \verb js , \verb c ), but te dataset also contains extensions typically associated to textual files of other nature. Moreover, even though we ignored rare and weird extensions, the frequency range remains wide, with some classes containing many examples while others just a few, as shown in Figure 2

). This represents a serious training issue for most the supervised learning approaches, since models obtained with unbalanced datasets tend to overfit and do not generalize well 

(Bishop, 2007). In the following we describe how we solved address this problem and built a more balanced and fair train set.

2.1. Balancing the dataset

The dataset as obtained thus far is not balanced, since the most frequent extension ( ) has a frequeny of while the least frequent one (cl ) has a frequency of $10^{-4}$. Severalechniques can be used for dataset balancing, and they are usually divided in two major classes: oversampling and undersampling (Maimon and Rokach, 2010). In the first case, new instances for the minority classes are generated until they reach a population similar to those of majority classes. The new instances are either copies of the existing examples or synthesized by using statistical properties computed on the minority classes. In our case, since the unbalancing is very significant, we could be forced to use the same examples (or the same information extracted from few examples) too many times, increasing the risk of overfitting. We did not consider the option of synthesizing artificial samples to avoid introducing biases, given understanding how intrinsically recognizable are real textual files found in VCS is part of our goal. Rather, we have applied an undersampling technique consisting in (sub)sampling the various classes randomly to obtain the same number of elements for each class. Of course, with this approach we are limited by the number of instances of the less populated class. As a result we ended up with a train set containing total examples and in which each class has the same number of instances (950). In addition to the training set we also kept, as it customary, a test set and a validation set, these sets preserve the original dataset classes distribution in order to represent a real situation.

In order to check model resistance to VCS evolution after a few years we have also used a second, more recent, test set, consisting of new files which do not appear in the first dataset, corresponding to the top 1000 GitHub repositories as archived by Software Heritage on 2019-10-08222 The second dataset set was prepared with the same methodology used for the first one, and contained 121 of the previously treated 133 extensions.

3. The model

Figure 3. File type classification model.

The proposed classification model is developed in several steps. first the content of the input file is divided into tokens which are then used to define a reference vocabulary V. Then, as common practice in many NLP applications, we also construct of a vocabulary containing 2-grams: These are sub-sequences of 2 tokens extracted from the files, once these are “tokenized”, that is represented as sequences of tokens according to the vocabulary V. At this stage we can extract the actual feature vectors from the files by considering the frequency of words and of 2-grams in the file text333Not all possible words and 2-grams are considered, of course, but only those appearing in the vocabularies V and , respectively.. Finally the classifier which makes predictions on the basis of the feature vectors is defined, by using a simple neural network whose structure is depicted in Figure 3); these steps are detailed in the remainder of this section.

3.1. Tokenization

Since our files are available as sequences of bytes, to treat them as texts we first need to decode them using a suitable character encoding. We used the ASCII encoding and considered only the characters which are the most common in source code. All non-ASCII characters are mapped to an unique, special value. After this conversion we can consider our files as simple text files, i.e., sequences of characters, on top of which we can define a notion of token. Differently from what is usually done in Natural Language Processing (NLP), case sensitivity is relevant in our setting, hence preserve character case-ness. Also, while in NLP punctuation symbols are discarded, they are crucial in source code, so we consider them as tokens.

444With the exception if _ which is considered an alphanumeric character, as it is often part of identifiers in source code. Hence the following:

Definition 3.1 ().

Given a sequence of characters , a token (or equivalently a word) in is defined as follows:

  • Any character representing a punctuation symbol is a token

  • Any sub-sequence of which is delimited by (characters representing) punctuation symbols and/or white spaces, and which does not contain punctuation symbols and/or white spaces is a token.

Thus, for example, the string ‘a=b’ is interpreted as a sequence of the three tokens ‘a’, ‘=’ and ‘b’. For model manageability we cannot consider all the possible words that occur in any file. A common technique for addressing this is looking at the frequencies of the tokens that occur in the trainset, and assemble a vocabulary consisting of all the tokens whose frequency is higher than a given threshold. However, due to heterogeneity of our dataset, this approach could cause the exclusion of tokens that are quite common only for specific classes, thus causing poor performance.555This was experimentally verified on our corpus Hence we computed the frequencies for each class and included in the vocabulary the tokens with frequency higher than for each class. To mitigate overfitting risks we have defined the vocabulary V only by using a part of the trainset (still a balanced set), which was then excluded from the set used to train the network (Singhi and Liu, 2006). Many files in code bases include at their beginning and/or end explicit information about the file content, in the form of shebang lines (e.g., #!/usr/bin/perl) or editor mode lines ((%% mode: latex)). This information can be really helpful for the classification task, but it can also compromise the performance the model, since this information could gain too much relevance with respect to other features, inducing poor results on files which lack it. For this reason, when collecting tokens to build the vocabulary and during training we excluded a portion of tokens from both the beginning and end of files.

The resulting vocabulary V contains 465 tokens, which are represented with their own identity. The special token “UNK” represents any unknown, out-of-vocabulary (OOV) token. Figure 2 contains a word cloud (where word size is proportional to the frequency of the token in the dataset) representation of the tokens in the final vocabulary except for punctuation symbols.

3.2. n-grams

Information about the relative position of tokens in a text can be richer than information about isolated tokens. There exist various algorithms and techniques that can capture different kind (e.g., short or long) of relation among tokens. Some of them like, CNN and LSTM (Wenpeng et al., 2017; Yang et al., 2016), can capture meaningful relations, automatically but they are also computational quite expensive. Simpler approaches are based on n-grams, i.e., sub-sequences of tokens extracted from a sequence of tokens defined according to a given vocabulary V. Taking into account n-grams, instead of individual tokens, it is possible to identify co-occurrences of tokens, extracting more information about the actual text structure. By increasing the n-grams length (the value ) it becomes possible to capture longer and more complex relations among tokens, at the cost of increased computational costs and increased overfitting risk—since longer n-grams tend to become tightly bound to the text they are derived from. For this work we have used bigrams, i.e., , which turned out to be a good choice in the performance/overfitting spectrum. We have also experimented models with trigrams which have worse performance.

To define the bigram vocabulary we relied on the same approach used for building the token vocabulary V. We define as the set of all bigrams whose frequency is higher than ) in each class, by considering the same dataset subset used to define V. Bigrams that do not belong to are mapped to the unknown bigram ‘’. For each input file one can now build the feature vector , which will be the representation of in our model. To build this vector we first decode from bytes to characters with the ASCII encoding and then tokenize the result using vocabulary V. We then compute, for each token in V, the frequency of that token among ’s tokens. We do the same for bigrams: for each bigram in , its frequency among ’s tokens is determined. Finally, we enumerate the elements of the set determined a fixed order for them. The feature vector is built by assigning the computed frequency for of -th element of the ordered version of to the -th component of the vector itself. will represent the file in the following. Its cardinality is . This process is applied to each file in the dataset; the resulting vectors will be used as inputs for the classification algorithm. The same has been performed taking into account trigrams too but the best results have been achieved without them.

3.3. Classifier

As classifier we use a Deep Fully Connected Layers Neural Network which has 5063 input units, 133 output units (which correspond to the possible extensions that we consider) and 3 hidden layers with 1000, 800, and 700 units, respectively, with a dropout rate of 0.5 for each layer. The model structure is shown in Figure 3.

The classification problem is multi-class, single label. Hence we use in the output layer the softmaxactivation function

which normalizes the values obtained from the previous layers with respect to the available classes. Output values represent the probabilities that a given file belongs to each class. As we have multiple classes, we use the

categorical cross entropyloss function. For each instance passed to the model, i.e. the -th one, the loss function takes the form where represents the actual ground truth label (in the form of a one-hot vector) and represents the predicted probability output vector. In the training phase we use the Adam optimizer (Kingma and Ba, 2014) with learning rate , which converge to good results (see Section 4

) after 8 epochs of training. During training the model parameters were progressively modified in order to improve the similarity of the predicted

vectors to the correspondent ground truth vectors.

Given the relatively high number of classes in our problem, erroneous classification is likely to happen. To investigate it we keep track of classification errors (on the validation set) in two different ways. We will use the following assumptions and notations: is the set of classes, denotes our validation set, while for , iff is the ground truth label of and iff is the label assigned by the classifier to . Moreover, when we perform a prediction for an input we obtain a probability value for each possible class . The predicted class is the one which has the highest probability value, that is, iff for each , . First, we measure how many of the examples of a given class are classified in a wrong way. This is done by introducing, for each pair of classes and , a quantity indicating how many times a file whose label (i.e., ground truth) is is classified in the class . More precisely we define .

A second way to register classification errors consists in considering also the second, third, fourth, and fifth best choices for classifying an example and see whether some of them significantly co-occur with the predicted class. More precisely, given , assume that and that , for and for each other . In this case we say that are the top five classifications for , written , for short. Then we normalise these five values by defining and analogously for the other four values.

Then, for each pair of classes and , we define and we normalize this value by considering by the total number of times in which the i-th class has been predicted, as follows: .

Given these quantities and we set two thresholds for their values, and respectively. Given a pair of classes and , if or we consider the classes and somehow related in the predictions and we say that they belong to the same “confusion group”. By setting the thresholds and we obtain the confusion groups shown in Table 1. The labels which do not appear in the table are those that do not pass the thresholds, meaning that the classifier do not incur into relevant ambiguity for them.

Group ID Extensions Group ID Extensions
bash , \verbsh , ps1 , \verbafter , jet , \verbkt ,
template &
$1$ & \verb
markdown ,
md \\
$2$ & \verb
cmake , cmd , \verbyaml , yml , \verbrst , txt , \verbbaseline ,
bat &
$3$ & \verb
dts ,
dtsi \\
$4$ & \verb
ctp ,
php &
$5$ & \verb
ml ,
mli \\
$6$ & \verb
csproj ,
ilproj &
$7$ & \verb
jl ,
j \\
$8$ & \verb
rb , cr , \verbexs
h , \verbino ,
hpp \\
$10$ & \verb
m4 ,
ac &
$11$ & \verb
cpp ,
cc \\
$12$ & \verb
clj ,
cljs &
$13$ & \verb
swift , sil , \verbgyb
tsx , \verbjsx , js , \verbts , htm , \verbhtml cjsx , \verbcoffee
css , \verbscss after , \verbkt
Table 1. Relevant extension confusion groups. Note how some groups contain labels that commonly refer to the same or similar file types, justifying the origin of the ambiguity.

4. Results

The architecture underlying the proposed model is quite simple in comparison to other machine learning approaches used to treat text. Contrarily to complete automatic feature learning algorithms, such as those used in Convolutional, Recursive, or Attention Neural Networks, computations in our model are faster, both for training and prediction. In particular, the learning task can be completed in a relatively short time: it took around 10 hours of training for 8 epochs to train the parameters of the model and to reach of accuracy on the validation and test sets. Predictions are made transforming the input feature vectors by means of simple operations such as vector multiplications, sums, and activation functions applications based on the parameters learned during the training phase. The test set consists of elements that we extracted from the original dataset after the pre-processing described in Section 2

except for the balancing step as this could bring to unfair evaluations and the resulting set would not represent the real world statistical distribution of classes, excluding of course the elements that were used in the train and validation sets. Various performance measures are used here and they are all based on the confusion matrix which is computed on the test set and whose generic element

contains the number of files of class (extension) which are classified as . The average accuracy value obtained on all the classes by the model is . We report in Table 2 the values for precision , recall and -score for each of the considered classes. The extensions are shown in descending order of frequency within the original test set.

Ext P R F     Ext P R F     Ext P R F
.js 0.93 0.69 0.79     .vb 0.95 0.99 0.97     .gyp 0.32 1.0 0.48
.c 0.96 0.94 0.95     .asciidoc 0.64 0.95 0.76     .bat 0.42 0.72 0.53
.html 0.98 0.87 0.92     .gradle 0.75 0.95 0.84     .erl 0.6 1.0 0.75
.java 0.99 0.97 0.98     .cr 0.34 0.92 0.5     .gemspec 0.78 1.0 0.88
.h 0.93 0.71 0.81     .lua 0.7 0.95 0.81     .fish 0.58 0.95 0.72
.py 0.99 0.93 0.96     .ex 0.83 0.96 0.89     .i 0.09 0.89 0.16
.go 0.99 0.96 0.97     .ilproj 0.79 0.95 0.86     .texi 0.93 1.0 0.96
.md 0.97 0.72 0.83     .dtsi 0.64 0.88 0.74     .template 0.09 0.48 0.15
.rb 0.98 0.85 0.91     .props 0.79 0.97 0.87     .pl 0.62 0.95 0.75
.json 0.95 0.95 0.95     .vcxproj 0.97 0.99 0.98     .ac 0.68 0.99 0.81
.cpp 0.74 0.59 0.66     .clj 0.96 0.99 0.97     .groovy 0.23 0.91 0.37
.ts 0.65 0.82 0.73     .markdown 0.13 0.73 0.22     .mak 0.86 0.96 0.91
.php 0.96 0.89 0.92     .symbols 0.88 0.99 0.93     .vbproj 0.54 0.98 0.7
.cs 0.98 0.96 0.97     .hs 0.78 0.99 0.87     .pkgproj 0.85 0.98 0.91
.rs 0.97 0.97 0.97     .dts 0.75 0.69 0.72     .sql 0.16 0.93 0.27
.cc 0.58 0.77 0.66     .el 0.95 0.99 0.97     .j 0.22 0.98 0.36
.xml 0.96 0.93 0.94     .proto 0.53 0.99 0.69     .tpl 0.12 0.77 0.21
.glif 1.0 1.0 1.0     .toml 0.6 0.98 0.74     .rake 0.1 0.95 0.18
.txt 0.82 0.52 0.64     .pbxproj 0.98 1.0 0.99     .textile 0.2 0.99 0.33
.kt 0.96 0.72 0.82     .exs 0.56 0.95 0.7     .webidl 0.59 0.99 0.74
.scala 0.96 0.95 0.95     .mk 0.79 0.91 0.85     .bash 0.22 0.77 0.34
.swift 0.92 0.85 0.88     .sil 0.71 0.97 0.82     .cjsx 0.35 0.97 0.51
.yml 0.77 0.81 0.79     .after 0.19 0.74 0.3     .pb 0.39 1.0 0.56
.css 0.88 0.9 0.89     .erb 0.19 0.77 0.3     .builds 0.93 1.0 0.96
.rst 0.59 0.89 0.71     .jade 0.27 0.9 0.42     .vcproj 0.92 1.0 0.96
.sh 0.84 0.83 0.83     .gyb 0.33 0.97 0.49     .xcscheme 1.0 1.0 1.0
.csproj 0.96 0.9 0.93     .log 0.57 0.91 0.7     .ngdoc 0.1 0.96 0.18
.coffee 0.82 0.92 0.87     .ipynb 0.43 0.98 0.6     .perl 0.66 0.98 0.79
.scss 0.85 0.93 0.89     .cmake 0.29 0.95 0.44     .eslintrc 0.13 0.98 0.23
.phpt 0.68 0.94 0.79     .ps1 0.78 0.96 0.86     .sbt 0.61 0.97 0.75
.jsx 0.26 0.79 0.39     .pyx 0.43 0.95 0.59     .handlebars 0.1 0.97 0.18
.hpp 0.2 0.83 0.32     .tmpl 0.24 0.67 0.35     .iml 0.62 1.0 0.77
.xht 0.79 0.97 0.87     .m4 0.74 0.95 0.83     .rml 0.79 1.0 0.88
.tsx 0.34 0.81 0.48     .check 0.24 0.81 0.37     .cmd 0.28 0.81 0.42
.jl 0.9 0.79 0.84     .il 0.77 1.0 0.87     .zsh 0.2 0.94 0.33
.htm 0.41 0.93 0.57     .am 0.54 0.96 0.69     .tcl 0.65 0.98 0.78
.dart 0.74 0.98 0.84     .adoc 0.58 0.95 0.72     .xib 0.55 1.0 0.71
.ml 0.98 0.95 0.96     .mli 0.68 1.0 0.81     .jet 0.05 0.91 0.09
.m 0.72 0.95 0.82     .sln 0.99 1.0 0.99     .dsp 0.94 1.0 0.97
.haml 0.92 0.98 0.95     .sass 0.69 0.96 0.8     micro avg. 0.85 0.85 0.85
.yaml 0.46 0.65 0.54     .w32 0.31 0.99 0.47     macro avg. 0.64 0.91 0.71
Table 2. Performance measures for the encoder architecture on the testset (2019)

We obtained good overall results and we report at the end of the table also micro- and macro-average. The former is useful in order to take into account the number of instances per class: classes with the higher number of examples will have a heavier influence in the average value than the less popular ones the. In fact, micro-average is defined as where are the true positives and are the false positives . Macro-average is defined without taking into account the number of instances per class, i.e., every class will equally influence the average. It appears that the main problem faced by the model is the presence of very similar classes that could actually be treated as the same class, for example both .yaml and .yml are used for files written in YAML, which can introduce errors in the predictions. Also, there are classes whose text contents can be very general and therefore could not be easily recognized, such as files with the .txt extension.

5. Threats to Validity

External validity. Even if the proposed classifier has been designed and tested on a fairly large set of files, the used dataset falls short of the entire corpus of source code distributed via publicly accessible version control systems. Datasets that approximate that corpus (Abramatic et al., 2018; Di Cosmo and Zacchiroli, 2017; Ma et al., 2019) do exist and could potentially be more challenging to tackle because: (1) they will include more extension/classes that did not occur within this paper datasets, (2) they can include noisier data as repository popularity is likely a proxy of project quality in terms of coding practices, and (3) they can include files from more varied chronological epochs—also programming languages always evolve and new ones emerge increasing the variety of classes to be considered. Nevertheless our design choices were oriented to address this kind of problems, as we discussed before, and we aim to use the proposed approach on these larger datasets as future work.

Internal validity. Various (non domain-specific) heuristics have been applied. First, filtering of non-textual files has been performed based on the percentage of non-printable characters within a random sample of the datasets; different samples or thresholds might affect stated results. On the same front, the choice of using the ASCII encoding to detect printable characters due to the lack of encoding information. An alternative approach would have been guessing the used encoding (using libraries such as chardet); it is not clear which biases either approach would introduce, if any.

The vocabularies V and are based on tokens/bigrams frequency distributions defined separately on different classes. This can introduce model biases and could be mitigated by using separate datasets to define the vocabularies and to train the neural network. Hyper parameters tuning has been performed as an iterative process, certainly not exhaustive. In spite of the achieved good results it is possible that different choices for hyper parameters and/or neural network topology would score even better. At the end of previous section we mentioned some possible threats due to extensions classification, in particular some of the extension classes that we treated as different might actually refer to the same abstract file type, which can cause confusion for the model and it makes the training task harder as a class share its instances with other classes. We maintain it is difficult to mitigate this last threat without relying on domain-specific knowledge that we wanted to avoid using, at least as a first approximation in the present study.

6. Related Work

Big code

Information retrieval (Frakes and Baeza-Yates, 1992) have been applied extensively to all sorts of software development artifacts, including source code. The most active and recent trend in the field goes under the label of “big code” and consists in the application of machine learning techniques to software development artifacts. In the remainder of this section we focus on related work in the area of file type and programming language detection. For big code work outsite that field of application we refer to the work of Allamanis et al. (Allamanis et al., 2018) that surveys an impressive number of applications and approaches in the field.

Language detection. Several approaches have explored for for programming language detection. van Dam and Zaytsev (van Dam and Zaytsev, 2016) tested various programming language classifiers on source files extracted from GitHub for 19 language classes and libelling them using Linguist as a source of truth. They obtained a value of 0.97 for

-score, precision, and recall. Klein et al. 

(Klein et al., 2011) performed language recognition among 25 language classes using source code from GitHub and labeling files using both file extensions and the GitHub language detection tool (Linguist discussed below). Only files for which file extension and Linguist-detected language match have been included in their trainset. Then, a feature vector is extracted from source code using features such has parentheses used, comments, keywords, etc., and used for training. The obtained accuracy is 50%. Ugurel et al. (Ugurel et al., 2002)

proposed various mechanisms for the classification of source code, programming language, and topics, based on support vector machines (SVM). On the language front they were able to discriminate among 10 different programming languages with 89% accuracy. Their data were retrieved from various source code archives available at the time (it was 2002, pre-GitHub). With respect to the aforementioned studies the approach presented in this paper is simpler, performs better in terms of accuracy, and handles significantly more (5–10x) file type classes. Reyes et al. 

(Reyes et al., 2016)

used a long short-term memory (LSTM) algorithm to train and test a programming language classifier on 3 different datasets, one from Rosetta Code 

(Nanz and Furia, 2015) (391 classes), GitHub archive (338), and a custom dataset (10). The obtained accuracies were, respectively, 80%, 29%, and 100%. They also compared their results to Linguist, finding Linguist scored worse except for the second dataset in which it reached 66% accuracy. The comparison between Rayes et al. and the approach presented in the present paper is interesting. The custom dataset confirms what was already apparent from previous comparisons: one can do much better in terms of accuracy reducing the number of language classes (and as little as 10 classes is not enough for our stated purpose). The other two datasets exhibit larger diversity than ours ( 300 classes v. 100), but perform very differently. We score better than the (more controlled) Rosetta Code dataset and much better than the GitHub dataset. Our approach is simpler in terms of architecture than theirs and we expect it to perform better in terms of training and recognition time (as LSTM tends to be slow to train)—but we have not benchmarked the two approaches in comparable settings so this remains a qualitative assessment at this stage. Linguist (GitHub, Inc., 2011)

is an open source language detection tool developed by GitHub. Its own accuracy is reported by GitHub 

(Ganesan and Foti, 2019) as being around 85%. The accuracy of studies that have used Linguist as ground truth should then be diminished accordingly. Additionally, Linguist relies on file extensions as a feature and its accuracy drops significantly when they are missing (Ganesan and Foti, 2019), as it is the case in our problem statement. The work by Gilda (Gilda, 2017)

is very similar to ours. File extensions are used as ground truth for source code extracted from GitHub and a word-level convolutional neural network (CNN) is used as classifier. The model reached 97% accuracy and is able to classify 60 different languages. We perform significantly better in terms of diversity (2x classes), but slightly worse in terms of accuracy (-8%).

File-type detection. Previous work has also been done on the more general problem of classifying file types that might also be binary formats. In the field of digital forensic Fitzgerald et al. (Fitzgerald et al., 2012) performed classification for 24 file classes using byte-level 1-grams and bigrams to build feature vectors fed to a SVM algorithm, reaching 47% accuracy on average. Gopal et al. (Gopal et al., 2011) used and compared similar approaches for the same task, but for 316 classes including 213 binary formats and 103 textual ones. They reached 90% micro-average and 60% macro-average -score. This relatively big gap between the two average measures hints at significant differences in the classes (e.g., frequencies in the test set). Binary file type detection is a significantly different problem than ours. There one can rely on file signatures (also known as “magic numbers”), as popular Unix libraries like libmagic and the accompanying file utility do. Such approaches are viable and could be very effective for binary files, but they are less maintainable in the long run (as the database of heuristics should be maintained lest it becomes stale) and less effective on textual file formats, where magic numbers are either missing or easily altered.

7. Conclusion

In this work we considered the problem of predicting textual file types for files commonly found in version control systems (VCS), relying solely on file content without any a priori domain knowledge or predetermined heuristic. The problem is relatively novel (as most existing language/file type detectors rely heavily on extensions) and relevant in contexts where extensions are missing or cannot be trusted, and shed light into the intrinsic recognizability of textual files used in software development repositories. We propose a simple model to solve the problem based on an universal word tokenizer, word-level n-grams of length 1 to 2, feature vectors based on n-gram frequency, and a fully connected neural network as classifier. We applied the model to a large dataset extracted from GitHub spanning 133 well represented file type classes. In spite of its simplicity the model performed very well, nearing 85% average accuracy, and outperforming previous work in either accuracy, number of supported classes, or both. We expect that model simplicity will make it more maintainable in the future and less computationally expensive to train and run than alternatives based on learning algorithms such as CNN, LSTM, or Attention.

Concerning future work, a straightforward next step is scaling up experimentation of the proposed model, moving from the high-starred GitHub dataset we used for this work to larger and more diverse datasets such as Software Heritage (Abramatic et al., 2018). In that context we will have significantly more starting classes and hopefully enough samples in each of them for enlarging the set of labels actually used in training. As we observed, extensions alone are ambiguous in many cases and this poses challenges in training and evaluation. To mitigate this issue it is worth exploring the possibility of inserting narrow domain knowledge about file extensions that often go together. It is not clear whether doing so, partly backtracking the “no domain knowledge” assumption of this work, would be worth the effort in terms of increased accuracy; hence, it is worth exploring. An alternative approach for improving over the current handling of model confusion is adding a second tier of classifiers, one for each class of ambiguous extensions, a popular technique in NLP. There are various methods to combine the predictions from the first and second level classifiers which should be explored. We have briefly explored the topic of accuracy degradation in the lack of retraining at a 2-year distance. A more general characterization would be interesting to have and is feasible to obtain it by exploiting historical software archives and/or VCS timestamp information. Such a characterization will allow to devise data-driven approaches for when and how to retrain file type and language detection tools in a world in which programming languages constantly evolve.

The authors would like to thank Éric de La Clergerie and Benoit Sagot for insightful discussions on automated techniques for natural language detection, without which this work would not have been possible.


  • (1)
  • Abramatic et al. (2018) Jean-François Abramatic, Roberto Di Cosmo, and Stefano Zacchiroli. 2018. Building the Universal Archive of Source Code. Commun. ACM 61, 10 (October 2018), 29–31.
  • Allamanis et al. (2018) Miltiadis Allamanis, Earl T Barr, Premkumar Devanbu, and Charles Sutton. 2018. A survey of machine learning for big code and naturalness. ACM Computing Surveys (CSUR) 51, 4 (2018), 81.
  • Berger et al. (2019) Emery D Berger, Celeste Hollenbeck, Petr Maj, Olga Vitek, and Jan Vitek. 2019. On the impact of programming languages on code quality: a reproduction study. ACM Transactions on Programming Languages and Systems (TOPLAS) 41, 4 (2019), 1–24.
  • Bergin Jr and Gibson Jr (1996) Thomas J Bergin Jr and Richard G Gibson Jr. 1996. History of programming languages—II. ACM.
  • Bishop (2007) Christopher M. Bishop. 2007. Pattern recognition and machine learning, 5th Edition. Springer.
  • Borges and Valente (2018) Hudson Borges and Marco Tulio Valente. 2018. What’s in a GitHub Star? Understanding Repository Starring Practices in a Social Coding Platform. Journal of Systems and Software 146 (2018), 112–129.
  • Di Cosmo and Zacchiroli (2017) Roberto Di Cosmo and Stefano Zacchiroli. 2017. Software Heritage: Why and How to Preserve Software Source Code. In iPRES 2017: 14th International Conference on Digital Preservation.
  • Dyer et al. (2013) Robert Dyer, Hoan Anh Nguyen, Hridesh Rajan, and Tien N Nguyen. 2013. Boa: A language and infrastructure for analyzing ultra-large-scale software repositories. In Proceedings of the 2013 International Conference on Software Engineering. IEEE Press, 422–431.
  • Fitzgerald et al. (2012) Simran Fitzgerald, George Mathews, Colin Morris, and Oles Zhulyn. 2012. Using NLP techniques for file fragment classification. Digital Investigation 9 (2012), S44–S49.
  • Frakes and Baeza-Yates (1992) William B. Frakes and Ricardo Baeza-Yates. 1992. Information retrieval: Data structures & algorithms. Prentice Hall.
  • Gabbrielli and Martini (2010) Maurizio Gabbrielli and Simone Martini. 2010. Programming Languages: Principles and Paradigms. Springer.
  • Ganesan and Foti (2019) Kavita Ganesan and Romano Foti. 2019. C# or Java? TypeScript or JavaScript? Machine learning based classification of programming languages. GitHub, Inc. blog post: retrieved 2020-01-06.
  • Gilda (2017) Shlok Gilda. 2017. Source code classification using Neural Networks. In 2017 14th International Joint Conference on Computer Science and Software Engineering (JCSSE). IEEE, 1–6.
  • GitHub, Inc. (2011) GitHub, Inc. 2011. Linguist: Language Savant. retrieved 2020-01-06.
  • Gopal et al. (2011) Siddharth Gopal, Yiming Yang, Konstantin Salomatin, and Jaime Carbonell. 2011. Statistical learning for file-type identification. In 2011 10th international conference on machine learning and applications and workshops, Vol. 1. IEEE, 68–73.
  • Katz (2018) Jeremy Katz. 2018. Open Source Repository and Dependency Metadata.
  • Kingma and Ba (2014) Diederik P Kingma and Jimmy Ba. 2014. Adam: A method for stochastic optimization. Technical Report 1412.6980. arXiv.
  • Klein et al. (2011) David Klein, Kyle Murray, and Simon Weber. 2011. Algorithmic programming language identification. Technical Report 1106.4064. arXiv.
  • Kochhar et al. (2016) Pavneet Singh Kochhar, Dinusha Wijedasa, and David Lo. 2016. A large scale study of multiple programming languages and code quality. In 2016 IEEE 23rd International Conference on Software Analysis, Evolution, and Reengineering (SANER), Vol. 1. IEEE, 563–573.
  • Ma et al. (2019) Yuxing Ma, Chris Bogart, Sadika Amreen, Russell Zaretzki, and Audris Mockus. 2019. World of code: an infrastructure for mining the universe of open source VCS data. In Proceedings of the 16th International Conference on Mining Software Repositories. IEEE Press, 143–154.
  • Maimon and Rokach (2010) Oded Maimon and Lior Rokach (Eds.). 2010. Data Mining and Knowledge Discovery Handbook, 2nd ed. Springer.
  • Maxwell and Forselius (2000) Katrina D Maxwell and Pekka Forselius. 2000. Benchmarking software development productivity. IEEE Software 17, 1 (2000), 80–88.
  • Maxwell et al. (1996) Katrina D Maxwell, Luk Van Wassenhove, and Soumitra Dutta. 1996. Software development productivity of European space, military, and industrial applications. IEEE Transactions on Software Engineering 22, 10 (1996), 706–718.
  • McIver (2000) Linda McIver. 2000. The effect of programming language on error rates of novice programmers.. In Proceedings of the 12th Annual Workshop of the Psychology of Programming Interest Group, PPIG 2000, Cosenza, Italy, April 10-13, 2000. 15.
  • Nanz and Furia (2015) Sebastian Nanz and Carlo A Furia. 2015. A comparative study of programming languages in rosetta code. In 2015 IEEE/ACM 37th IEEE International Conference on Software Engineering, Vol. 1. IEEE, 778–788.
  • Rabai et al. (2011) Latifa Ben Arfa Rabai, Yan Zhi Bai, and Ali Mili. 2011. A quantitative model for software engineering trends. Information Sciences 181, 22 (2011), 4993–5009.
  • Ray et al. (2017) Baishakhi Ray, Daryl Posnett, Premkumar Devanbu, and Vladimir Filkov. 2017. A large-scale study of programming languages and code quality in GitHub. Commun. ACM 60, 10 (2017), 91–100.
  • Reyes et al. (2016) Julio Reyes, Diego Ramírez, and Julio Paciello. 2016.

    Automatic Classification of Source Code Archives by Programming Language: A Deep Learning Approach. In

    2016 International Conference on Computational Science and Computational Intelligence (CSCI). IEEE, 514–519.
  • Rodríguez et al. (2012) Daniel Rodríguez, MA Sicilia, E García, and Rachel Harrison. 2012. Empirical findings on team size and productivity in software development. Journal of Systems and Software 85, 3 (2012), 562–570.
  • Ryder and Hailpern (2007) Barbara G. Ryder and Brent Hailpern (Eds.). 2007. Proceedings of the Third ACM SIGPLAN History of Programming Languages Conference (HOPL-III), San Diego, California, USA, 9-10 June 2007. ACM.
  • Sammet (1972) Jean E Sammet. 1972. Programming languages: history and future. Commun. ACM 15, 7 (1972), 601–610.
  • Shustek (2006) Leonard J. Shustek. 2006. What Should We Collect to Preserve the History of Software? IEEE Annals of the History of Computing 28, 4 (2006), 110–112.
  • Singhi and Liu (2006) Surendra K Singhi and Huan Liu. 2006. Feature subset selection bias for classification learning. In Proceedings of the 23rd international conference on Machine learning. ACM, 849–856.
  • Ugurel et al. (2002) Secil Ugurel, Robert Krovetz, and C Lee Giles. 2002. What’s the code?: automatic classification of source code archives. In Proceedings of the eighth ACM SIGKDD international conference on Knowledge discovery and data mining. ACM, 632–638.
  • Urma et al. (2014) Raoul-Gabriel Urma, Dominic A. Orchard, and Alan Mycroft (Eds.). 2014. Proceedings of the 1st Workshop on Programming Language Evolution, PLE@ECOOP 2014, Uppsala, Sweden, July 28, 2014. ACM.
  • van Dam and Zaytsev (2016) Juriaan Kennedy van Dam and Vadim Zaytsev. 2016. Software language identification with natural language classifiers. In 2016 IEEE 23rd International Conference on Software Analysis, Evolution, and Reengineering (SANER), Vol. 1. IEEE, 624–628.
  • Wenpeng et al. (2017) Yin Wenpeng, Kann Katharina, Yu Mo, and Schutze Hinrich. 2017. Comparative Study of CNN and RNN for Natural Language Processing.
  • Wexelblat (1981) Richard L Wexelblat. 1981. History of programming languages. Academic Press.
  • Yang et al. (2016) Zichao Yang, Diyi Yang, Chris Dyer, Xiaodong He, Alex Smola, and Eduard Hovy. 2016. Hierarchical attention networks for document classification. In Proceedings of the 2016 conference of the North American chapter of the association for computational linguistics: human language technologies. 1480–1489.