scodec / scodec-stream

Binding between scodec and FS2



Scodec-stream is a library for streaming binary encoding and decoding. It is built atop scodec and fs2. Here's a brief example of its use:

import scodec.{codecs => C}
import{decode => D, StreamDecoder}
import scodec.bits.ByteVector

val frames: StreamDecoder[ByteVector] = D.once(C.int32)
 .flatMap { numBytes => D.once(C.bytes(numBytes)).isolate(numBytes) }

val s: Stream[fs2.util.Task, ByteVector] =

When consumed, s will memory map in the contents of "largefile.bin", then decode a stream of frames, where each frame is expected to begin with a number of bytes specified as a 32-bit signed int (the int32 codec), followed by a frame payload of that many bytes. Nothing happens until the s stream is consumed, and s will ensure the FileChannel is closed in the event of an error or normal termination of the consumer. See the guide for further information and discussion of streaming encoding.

Decoding speeds have been observed at 100 MB/s for some realistic examples (decoding MPEG packets from a .pcap file), though decoding speed will generally depend on how fine-grained the decoding is.



This project is licensed under a 3-clause BSD license.

The scodec mailing list contains release announcements and is generally a good place to go for help. Also consider using the scodec tag on StackOverflow.

People are expected to follow the Typelevel Code of Conduct when discussing scodec on the Github page, Gitter channel, mailing list, or other venues.

Concerns or issues can be sent to Michael Pilquist ( or to Typelevel.

Getting Binaries

See the releases page on the website.

Sign up for the mailing list if you want to be notified of future releases.


The library provides two main types,[A], with helper functions in the package object, and[A], with helper functions in the package object. The[A] type just pairs an encoder with a decoder.


The model of a StreamDecoder[A] is a Stream[Cursor,A], where Cursor[X] is a BitVector => Attempt[DecodeResult[X]] state action. Thus, a StreamDecoder[A] produces a stream of A values, using a BitVector input that it may inspect and update as it emits values. The 'current' BitVector is sometimes called the 'cursor position' or just 'cursor' throughout this documentation.

The combinators for building StreamDecoder are fairly typical of what one might see in a monadic parser combinator library. Assuming we've imported{decode,StreamDecoder} and scodec.codecs:

  • decode.once: A scodec.Decoder[A] => StreamDecoder[A] which promotes a regular (strict) decoder to a StreamDecoder which decodes a single value, emits it, advances the cursor by the number of decoded bits, then halts. Any decoding failures are raised in the output stream, wrapped in DecodingError. Example: decode.once(codecs.int32) has type StreamDecoder[Int], and parses a single signed 32-bit integer.
  • decode.tryOnce: A scodec.Decoder[A] => StreamDecoder[A] which promotes a regular (strict) decoder to a StreamDecoder. Unlike decode.once, decoding failures are not raised. Instead, the cursor position is left at its current location and the stream halts with no emitted elements.
  • d1 ++ d2: Run d1 and emit all its decoded values, then run d2 on the remaining input and emit its values. Example: decode.once(codecs.int32) ++ decode.advance(12) decodes a single signed Int, then advances the cursor by 12 bits.
  • d1 or d2 (alternately d1 | d2): Runs d1, and if it emits no elements and raises no errors, runs d2. Example: decode.tryOnce(codecs.variableSizeBytes(codecs.int32L, codecs.utf8)) | decode.emit("Joe Sixpack") will try parsing UTF-8 encoded string, which begins with a length in bytes encoded as a little-endian encoded Int, followed by that many bytes for the string itself, and if this fails, will leave the cursor at its current location and emit the string "Joe Sixpack".
  • d.many: Run d for as long as there is nonempty input, emitting the stream of decoded values. Example, decode.once(codecs.int64).many will parse a stream of 64-bit signed integers. This can also be written as decode.many(codecs.int64).
  • d.isolate(numBits) and d.isolateBytes(numBytes): Useful when constructing nested decoders, runs d on the first numBits number of bits or numBytes number of bytes of the input, then advances the cursor by that many bits or bytes.
  • d flatMap f: Given a d: StreamDecoder[A] and an f: A => StreamDecoder[B], produce a StreamDecoder[B] by running d, then using each resulting A to choose a StreamDecoder[B] to run to produce the output. For example, decode.many(int32) flatMap { n => decode.once(codecs.fixedSizeBytes(n, codecs.utf8)) } will be a StreamDecoder[String] which produces a stream of String outputs by repeatedly reading a signed 32-bit integer, n, then decoding a UTF-8 encoded string consisting of n bytes.
  • d.peek: Run d: StreamDecoder[A], but after d completes, reset the cursor to its original, pre-d position.

There are also various combinators for statefully transforming the output of a StreamDecoder[A], or interleaving multiple decoders, namely d pipe proc (alternately d |> proc) and (d1 tee d2)(f). See the API docs for more details.

Running decoders

A decoder, d: StreamDecoder[A] may be run on an input bits: BitVector via d.decode(bits), and there are various convenience functions, including decodeInputStream, decodeMmap, and decodeChannel, for safely decoding from a, a memory-mapped file, or java.nio.ReadableByteChannel. These convenience functions will allocate a lazy BitVector backed by the given source and ensure the input is closed when the consumer finishes consuming the decoded stream.

Decoders are given the full input starting from the current cursor location - any chunking of the input BitVector is not visible to decoders, which simplifies their implementation as decoders need not worry about being given partial input. Thus, when promoting a d: Decoder[A] to a StreamDecoder[A] via decode.once(d), d will be run on the full remainder of the current input. (Calling decode.once(d).isolate(42) will give d only the first 42 bits of the input, starting from the current cursor location, and will advance the cursor by 42 bits on completion.)

When a StreamDecoder[A] advances the cursor, the head of the underlying BitVector may be garbage collected, making it easy to write decoders that operate in constant memory. There are a few caveats:

  • d.peek will retain the current cursor location in memory for as long as d takes to run, so it can reset this cursor location after d completes. Thus, it should be used only in restricted scopes, like decode.once(codecs.int32).peek, which reads a single Int and leaves it unconsumed. (Whereas decode.many(int32).peek will read an entire stream of Int values, and leave them unconsumed, requiring as much memory as is needed to represent all the integers in the stream)
  • decode.ask: StreamDecoder[BitVector] obtains the full input starting from the current cursor location. Obviously, stashing this BitVector somewhere may prevent the head of the BitVector from being garbage collected.


The model of a StreamEncoder[A] is a Stream.Handle[Pure, A] => Pull[Pure, BitVector, (Stream.Handle[Pure, A], StreamEncoder[A])]. Thus, a StreamEncoder[A] receives a stream of A values and transforms this stream (possibly statefully) into a stream of BitVector chunks. Encoding failures are raised within the pull as a EncodingError.

We are currently still discovering nice combinators for building up StreamEncoder values. Assuming you have imported{encode,StreamEncoder} and scodec.codecs, the key combinators so far are:

  • encode.once: A scodec.Encoder[A] => StreamEncoder[A] which encode a single A to the output, then halts. For example, encode.once(codecs)
  • encode.many or e.many: A scodec.Encoder[A] => StreamEncoder[A] which encodes multiple A values to the output, halting only when the input A stream is exhausted. Example: encode.many(codecs.int16) encodes zero or more integers, each of which is expected to fit within 16 bits.
  • e1 ++ e2: Encode values with e1 until it halts, then encode values with e2 until it halts. Example: encode.once(codecs.int32) ++ encode.many(codecs.int16) encodes one Int as a 32-bit signed Int, followed by zero or more Int values encoded as 16-bit signed ints. (If an input does not fit in 16 bits, an encoding error is raised within the pull)

Since encoders only retain references to previously received values if they do so explicitly, it is easy to write streaming encoders to operate in constant memory.

Running encoders

An e: StreamEncoder[A] may be applied to any s: Stream[F,A] via e.encode(s), which returns a Stream[F,BitVector] that can be dumped to a file using normal fs2 I/O combinators. We welcome contributions of helper functions for common encoding cases.

The representation of StreamEncoder means it can be used to transform the result of a StreamDecoder, for instance:

import scodec.codecs
import fs2.util.Task

// skip first 64 bits, then decode
val d: StreamDecoder[Int] = decode.advance(64) ++ decode.many(codecs.int32)
val e: StreamEncoder[Int] = encode.many(codecs.int16)

val t: Task[Unit] =
 e.encode { d.decodeMmap(new FileInputStream("largefile.bin").getChannel) }
  .to("smallerfile.bin")) // TODO Update this for fs2 when IO module is ready

// at the end of the universe

Calling t.unsafeRun will do a streaming decode of the "largefile.bin" file, skipping the first 64 bits, then a stream of signed 32 bit ints, which it downsamples to 16 bits and streams to the output file "smallerfile.bin", raising an EncodingError in the event of a format error or if an integer from the input fails to fit within 16 bits.