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Implement a decoder

Steps to add new decoder

  • Create a directory format/<name>
  • Copy some similar decoder, format/format/bson.go is quite small, to format/<name>/<name>.go
  • Cleanup and fill in the register struct, rename format.BSON and add it to format/fromat.go and don't forget to change the string constant.
  • Add an import to format/all/all.go

Some general tips

  • Main goal is to produce a tree structure that is user-friendly and easy to work with. Prefer a nice and easy query tree structure over nice decoder implementation.
  • Use same names, symbols, constant number bases etc as in specification. But maybe in lowercase to be jq/JSON-ish.
  • Decode only ranges you know what they are. If possible let "parent" decide what to do with unknown bits by using *Decode*Len/Range/Limit functions. fq will also automatically add "unknown" fields if it finds gaps.
  • Try to not decode too much as one value. A length encoded int could be two fields, but maybe a length prefixed string should be one. Flags can be struct with bit-fields.
  • Map as many value as possible to more symbolic values.
  • Endian is inherited inside one format decoder, defaults to big endian for new format decoder
  • Make sure zero length or no frames found etc fails decoding
  • If format is in the probe group make sure to validate input to make it non-ambiguous with other decoders
  • Try keep decoder code as declarative as possible
  • Split into multiple sub formats if possible. Makes it possible to use them separately.
  • Validate/Assert
  • Error/Fatal/panic
  • Is format probeable or not
  • Can new formats be added to other formats
  • Does the new format include existing formats

Development tips

I ususally use -d <format> and v while developing, that way you will get a decode tree even if it fails. v gives verbose output and also includes stacktrace.

go run fq.go -d <format> v file

If the format is inside some other format it can be handy to first extract the bits and run the decode directly. For example if working a aac_frame decoder issue:

fq '.tracks[0].samples[1234] | tobytes' file.mp4 > aac_frame_1234
fq -d aac_frame v aac_frame_1234

Sometimes nested decoding fails then maybe a good way is to change the parent decoder to use d.RawLen() etc instead of d.FormatLen() etc temporary to extract the bits. Hopefully there will be some option to do this in the future.

When researching or investinging something I can recommend to use watchexec, modd etc to make things more comfortable. Also using vscode/delve for debugging should work fine once launch args are setup etc.

watchexec "go run fq.go -d aac_frame v aac_frame"

Some different ways to run tests:

# run all tests
make test
# run all go tests
go test ./...
# run all tests for one format
go test -run TestFQTests/mp4 ./format/
# write all actual outputs
make actual
# write actual output for specific tests
WRITE_ACTUAL=1 go run -run ...
# color diff
DIFF_COLOR=1 go test ...

To lint source use:

make lint

Generate documentation. Requires FFmpeg and Graphviz:

make doc

TODO: make fuzz

Debug

Split debug and normal output even when using repl:

Write log package output and stderr to a file that can be tail -f:ed in another terminal:

LOGFILE=/tmp/log go run fq.go ... 2>>/tmp/log

gojq execution debug:

GOJQ_DEBUG=1 go run -tags debug fq.go ...

Memory and CPU profile (will open a browser):

make memprof ARGS=". file"
make cpuprof ARGS=". test.mp3"

From start to decoded value

main:main()
    cli.Main(default registry)
        interp.New(registry, std os interp implementation)
        interp.(*Interp).Main()
            interp.jq _main/0:
                args.jq _args_parse/2
                populate filenames for input/0
                interp.jq inputs/0
                    foreach valid input/0 output
                        interp.jq open
                            funcs.go _open
                        interp.jq decode
                            funcs.go _decode
                                decode.go Decode(...)
                                    ...
                        interp.jq eval expr
                            funcs.go _eval
                        interp.jq display
                            funcs.go _display
                                for interp.(decodeValueBase).Display()
                                    dump.go
                                        print tree
                                empty output

bitio and other io packages

*os.File, *bytes.Buffer
^
ctxreadseeker.Reader defers blocking io operations to a goroutine to make them cancellable
^
progressreadseeker.Reader approximates how much of a file has been read
^
aheadreadseeker.Reader does readahead caching
^
| (io.ReadSeeker interface)
|
bitio.Reader (implements bitio.Bit* interfaces)
^
| (bitio.Bit* interfaces)
|
bitio.Buffer convenience wrapper to read bytes from bit reader, create section readers etc
SectionBitReader
MultiBitReader

jq oddities

jq -n '[1,2,3,4] | .[null:], .[null:2], .[2:null], .[:null]'

Setup docker desktop with golang windows container

git clone https://github.com/StefanScherer/windows-docker-machine.git
cd windows-docker-machine
vagrant up 2016-box
cd ../fq
docker --context 2016-box run --rm -ti -v "C:${PWD//\//\\}:C:${PWD//\//\\}" -w "$PWD" golang:1.17.5-windowsservercore-ltsc2016

Implementation details

Dependencies and source origins

  • gojq fork that can be found at https://github.com/wader/gojq/tree/fq
    Issues and PR:s related to fq:
    #43 Support for functions written in go when used as a library
    #46 Support custom internal functions
    #56 String format query with no operator using %#v or %#+v panics #65 Try-catch with custom function
    #67 Add custom iterator function support which enables implementing a REPL in jq
    #81 path/1 behaviour and path expression question
    #86 ER: basic TCO #109 jq halt_error behaviour difference
    #113 error/0 and error/1 behavior difference
    #117 Negative number modulus *big.Int behaves differently to int
    #118 Regression introduced by "remove fork analysis from tail call optimization (ref #86)"
    #122 Slow performance for large error values that ends up using typeErrorPreview()
    #125 improve performance of join by make it internal
    #141 Empty array flatten regression since "improve flatten performance by reducing copy"

  • readline fork that can be found at https://github.com/wader/readline/tree/fq

  • gopacket for TCP and IPv4 reassembly

  • mapstructure for convenient JSON/map conversion

  • go-difflib for diff tests

  • golang.org/x/text for text encoding conversions

  • float16.go to convert bits into 16-bit floats

Release process

Run and follow instructions:

make release=1.2.3