Is there a fast Java library to search for a string and its position in file?

Question

I need to search a big number of files (i.e. 600 files, 0.5 MB each) for a specific string.

I'm using Java, so I'd prefer the answer to be a Java library or in the worst case a library in a different language which I could call from Java.

I need the search to return the exact position of the found string in a file (so it seems Lucene for example is out of the question).

I need the search to be as fast as possible.

EDIT START:

The files might have different format (i.e. EDI, XML, CSV) and contain sometimes pretty random data (i.e. numerical IDs etc.). This is why I preliminarily ruled out an index-based searching engine.

The files will be searched multiple times for similar but different strings (i.e. for IDs which might have similar length and format, but they will usually be different).

EDIT END

Any ideas?

Answer 1

600 files of 0.5 MB each is about 300MB - that can hardly be considered big nowadays, let alone large. A simple string search on any modern computer should actually be more I/O-bound than CPU-bound - a single thread on my system can search 300MB for a relatively simple regular expression in under 1.5 seconds - which goes down to 0.2 if the files are already present in the OS cache.

With that in mind, if your purpose is to perform such a search infrequently, then using some sort of index may result in an overengineered solution. Start by iterating over all files, reading each block-by-block or line-by-line and searching - this is simple enough that it barely merits its own library.

Set down your performance requirements, profile your code, verify that the actual string search is the bottleneck and then decide whether a more complex solution is warranted. If you do need something faster, you should first consider the following solutions, in order of complexity:

• Use an existing indexing engine, such as Lucene, to filter out the bulk of the files for each query and then explicitly search in the (hopefully few) remaining files for your string.

• If your files are not really text, so that word-based indexing would work, preprocess the files to extract a term list for each file and use a DB to create your own indexing system - I doubt you will find an FTS engine that uses anything else than words for its indexing.

• If you really want to reduce the search time to the minimum, extract term/position pairs from your files, and enter those in your DB. You may still have to verify by looking at the actual file, but it would be significantly faster.

PS: You do not mention at all what king of strings we are discussing about. Does it contain delimited terms, e.g. words, or do your files contain random characters? Can the search string be broken into substrings in a meaningful manner, or is it a bunch of letters? Is your search string fixed, or could it also be a regular expression? The answer to each of these questions could significantly limit what is and what is not actually feasible - for example indexing random strings may not be possible at all.

EDIT:

From the question update, it seems that the concept of a term/token is generally applicable, as opposed to e.g. searching for totally random sequences in a binary file. That means that you can index those terms. By searching the index for any tokens that exist in your search string, you can significantly reduce the cases where a look at the actual file is needed.

1. You could keep a term->file index. If most terms are unique to each file, this approach might offer a good complexity/performance trade-off. Essentially you would narrow down your search to one or two files and then perform a full search on those files only.

2. You could keep a term->file:position index. For example, if your search string is "Alan Turing". you would first search the index for the tokens "Alan" and "Turing". You would get two lists of files and positions that you could cross-reference. By e.g. requiring that the positions of the token "Alan" precede those of the token "Turing" by at most, say, 30 characters, you would get a list of candidate positions in your files that you could verify explicitly.

I am not sure to what degree existing indexing libraries would help. Most are targeted towards text indexing and may mishandle other types of tokens, such as numbers or dates. On the other hand, your case is not fundamentally different either, so you might be able to use them - if necessary, by preprocessing the files you feed them to make them more palatable. Building an indexing system of your own, tailored to your needs, does not seem too difficult either.

You still haven't mentioned if there is any kind of flexibility in your search string. Do you expect being able to search for regular expressions? Is the search string expected to be found verbatim, or do you need to find just the terms in it? Does whitespace matter? Does the order of the terms matter?

And more importantly, you haven't mentioned if there is any kind of structure in your files that should be considered while searching. For example, do you want to be able to limit the search to specific elements of an XML file?

Answer 2

Unless you have an SSD, your main bottleneck will be all the file accesses. Its going to take about 10 seconds to read the files, regardless of what you in Java.

If you have an SSD, reading the files won't be a problem, and the CPU speed in Java will matter more.

If you can create an index for the files this will help enormously.