Created: 2008-11-15 18:22
Updated: 2016-05-08 17:05


Binary Reading and Writing Combinators for Scala

bin4s is a combinator library for reading and writing arbitrary file and stream formats.

Why does it exist?

In my work I needed to parse and generate some proprietary binary formats, which were completely undocumented. I had some C code that generated them and some Java that parsed them. I found both the C and the Java tricky to understand, but persevered and hacked together valid parsers and generators.

I thought that I should try not to add a 3rd implementation that's difficult to understand, and I should try not to repeat the format in both the parsing and generating code. So I came up with bin4s. Here's the main principle:

If you have a Format[T] (which can read and write Ts), you can combine it with a Format[U] to form a Format[(T, U)]. Obviously combining a few gets unwieldy and lacks information about what each type parameter means, so you can describe how to get from a T and a U to a V, and back again, to create a Format[V]. There are some less abstract examples of this in the next section.

How do I use it?

As an example, we'll consider a Person class. A person has a String name, String address and Date dateOfBirth. A Date has an int year, int month and int day. Format.string and Format.integer are provided in Binary4Scala. We can combine these to produce a Format[Person]. First let's tackle Format[Date].

Format.integer.andThen(Format.integer) returns a Format[(Int, Int)]. But Date has 3 Ints.. Format.integer.andThen(Format.integer).andThen(Format.integer) returns a Format[((Int, Int), Int)];. Handy, but not quite a Format[Date]. To make a Format[Date] from it we need to tell bin4s about a way of converting between 3 Ints and Dates, in both directions (consider that a Format can read and write). Luckily a Format has a map method that takes an XFunction, which describes these conversions.

val dateFormat: Format[Date] = Format.integer andThen Format.integer andThen Format.integer map Date.xFunction

So similarly we can combine this with Format.string to create a Format[Person]:

val personFormat: Format[Person] = Format.string andThen Format.string andThen dateFormat map Person.xFunction

Then to get a ByteBuffer containing that data, we can say: val personData = personFormat(somePerson); To read a ByteBuffer into a Person we can say: val person = personFormat.unapply(buffer);

A more realistic example, perhaps, is a block of bytes preceded by its length as an int.

val lengthEncodedBytes = Format[(Int, Array[Byte])] = Format.integer bind Format.byteArray

Unfortunately to use lengthEncodedBytes, we have to pass in the length separately to the array, so typical uses would look like:

val buffer = lengthEncodedBytes(array.length, array)

Clearly it would be better if we could make this a Format[Array[Byte]]. Luckily we can, by providing an XFunction describing the conversion from a Array[Byte] to a (Int, Array[Byte]), and vice-versa.

val lengthEncodedBytes: Format[Array[Byte]] = Format.integer bind Format.byteArray map someXFunction2

If you are using bin4s, please let me know, and I'll do what I can to help you. Other than that, it will evolve as and when I use it, which might be daily or never.

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