Implementation details

fq uses a gojq fork that can be found at https://github.com/wader/gojq/tree/fq (the "fq" branch)
cli readline uses raw mode so blocks ctrl-c to become a SIGINT

Decoder implementation help

Main goal in the end is to produce a tree that is user-friendly and easy to work with. Prefer a nice and easy to use tree over nice decoder implementation.
Use same names, symbols, constant number base etc as in specification.
Decode only ranges you know what it is. If possible let "parent" decide what to do with unknown bits by using *Decode*Len/Range/Limit funcitions.
Try to no 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 usage symbolic values.
Endian inherited inside one format decoder, defaults to big endian for new format decoder
Make sure zero length or no frames etc found fails decoding
Try 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

Tests

Write new actual output:

make actual # write all actual outputs
WRITE_ACTUAL=1 go run -run ... # write only specific tests

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]'

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