## Implement a decoder ### Steps to add new decoder - Create a directory `format/` - Copy some similar decoder, `format/format/bson.go` is quite small, to `format//.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 ### Decoder API `*decode.D` reader methods use this name convention: `?(?>|Fn>)(...[, scalar.Mapper...]) ` - If starts with `Field` a field will be added and first argument will be name of field. If not it will just read. - `?>|Fn>` a reader or a reader function - `?>` Read bits using some decoder. - `U16` unsigned 16 bit integer. - `UTF8` UTF8 with byte length as argument. - `Fn>` read using a `func(d *decode.D) ` function. - This can be used to implement own custom readers. All `Field` functions takes a var args of `scalar.Mapper`:s that will be applied after reading. `` are these types: | `` | Go type | jq type | | -------- | ------- | ------- | | U | uint64 | number | | S | int64 | number | | F | float64 | number | | Str | string | string | | Bool | bool | boolean | | Nil | nil | null | TODO: there are some more (BitBuf etc, should be renamed) To add a struct or array use `d.FieldStruct(...)` and `d.FieldArray(...)`. TODO: nested formats, buffers, own decoders, scalar mappers TODO: seeking, framed/limited/range decode For example this decoder: ```go // read 4 byte UTF8 string and add it as "magic", return a string d.FieldUTF8("magic", 4) // create a new struct and add it as "headers", returns a *decode.D d.FieldStruct("headers", func(d *decode.D) { // read 8 bit unsigned integer, map it and add it as "type", returns a uint64 d.FieldU8("type", scalar.UToSymStr{ 1: "start", // ... }) }) ``` will produce something like this: ```go *decode.Value{ Parent: nil, V: *decode.Compound{ IsArray: false, // is struct Children: []*decode.Value{ *decode.Value{ Name: "magic", V: scalar.S{ Actual: "abcd", // read and set by UTF8 reader }, Range: ranges.Range{Start: 0, Len: 32}, }, *decode.Value{ Parent: &... // ref parent *decode.Value>, Name: "headers", V: *decode.Compound{ IsArray: false, // is struct Children: []*decode.Value{ *decode.Value{ Name: "type", V: scalar.S{ Actual: uint64(1), // read and set by U8 reader Sym: "start", // set by UToSymStr scalar.Mapper }, Range: ranges.Range{Start: 32, Len: 8}, }, }, }, Range: ranges.Range{Start: 32, Len: 8}, }, }, }, Range: ranges.Range{Start: 0, Len: 40}, } ``` and will look like this in jq/JSON: ```json { "magic": "abcd", "headers": { "type": "start" } } ``` #### `*decode.D` type This is the main type used during decoding. It keeps track of: - A current array or struct [`*decode.Value`](#decodevalue-type) where fields will be added. - Current bit reader - Current default endian - Decode options New [`*decode.D`](#decoded-type) are created during decoding when `d.FieldStruct` etc is used. It is also a kitchen sink of all kind functions for reading various standard number and string encodings etc. Decoder authors do not have to create them. #### `*decode.Value` type Is what [`*decode.D`](#decoded-type) produces and it used to represent the decoded structure. Can be array, struct, number, string etc. It is the underlaying type used by `interp.DecodeValue` that implements `gojq.JQValue` to expose it as various jq types, which in turn is used to produce JSON. It stores: - Parent [`*decode.Value`](#decodevalue-type) unless it's a root. - A decoded value, a [`scalar.S`](#scalars-type) or [`*decode.Compound`](#decodecompound-type) (struct or array) - Name in parent struct or array. If parent is a struct the name is unique. - Index in parent array. Not used if parent is a struct. - A bit range. Also struct and array have a range that is the min/max range of its children. - A bit reader where the bit range can be read from. Decoder authors will probably not have to create them. #### `scalar.S` type Keeps track of - Actual value. Decoded value represented using a go type like `uint64`, `string` etc. For example a value reader by a utf8 or utf16 reader both will ends up as a `string`. - Symbolic value. Optional symbolic representation of the actual value. For example a `scalar.UToSymStr` would map an actual `uint64` to a symbolic `string`. - String description of the value. - Number representation The `scalar` package has `scalar.Mapper` implementations for all types to map actual to whole [`scalar.S`](#scalars-type) value `scalar.ToScalar` or to just to set symbolic value `scalar.ToSym`. There is also mappers to just set values or to change number representations `scalar.Hex`/`scalar.SymHex` etc. Decoder authors will probably not have to create them. But you might implement your own `scalar.Mapper` to modify them. #### `*decode.Compound` type Used to store struct or array of [`*decode.Value`](#decodevalue-type). Decoder authors do not have to create them. ## Development tips I ususally use `-d ` and `dv` while developing, that way you will get a decode tree even if it fails. `dv` gives verbose output and also includes stacktrace. ```sh go run fq.go -d dv 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: ```sh fq '.tracks[0].samples[1234] | tobytes' file.mp4 > aac_frame_1234 fq -d aac_frame dv 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 dv aac_frame" ``` Some different ways to run tests: ```sh # 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 WRITE_ACTUAL=1 go test ./... # 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](https://github.com/FFmpeg/FFmpeg) and [Graphviz](https://gitlab.com/graphviz/graphviz): ```sh 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: ```sh LOGFILE=/tmp/log go run fq.go ... 2>>/tmp/log ``` gojq execution debug: ```sh GOJQ_DEBUG=1 go run -tags debug fq.go ... ``` Memory and CPU profile (will open a browser): ```sh 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.IOBitReader (implements bitio.Bit* interfaces) SectionBitReader MultiBitReader ``` ## jq oddities ``` jq -n '[1,2,3,4] | .[null:], .[null:2], .[2:null], .[:null]' ``` ## Setup docker desktop with golang windows container ```sh 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 - fq uses a gojq fork that can be found at https://github.com/wader/gojq/tree/fq (the "fq" branch) - fq uses a readline fork that can be found at https://github.com/wader/readline/tree/fq (the "fq" branch) - cli readline uses raw mode to blocks ctrl-c to become a SIGINT ## Dependencies and source origins - [gojq](https://github.com/itchyny/gojq) fork that can be found at https://github.com/wader/gojq/tree/fq
Issues and PR:s related to fq:
[#43](https://github.com/itchyny/gojq/issues/43) Support for functions written in go when used as a library
[#46](https://github.com/itchyny/gojq/pull/46) Support custom internal functions
[#56](https://github.com/itchyny/gojq/issues/56) String format query with no operator using %#v or %#+v panics [#65](https://github.com/itchyny/gojq/issues/65) Try-catch with custom function
[#67](https://github.com/itchyny/gojq/pull/67) Add custom iterator function support which enables implementing a REPL in jq
[#81](https://github.com/itchyny/gojq/issues/81) path/1 behaviour and path expression question
[#86](https://github.com/itchyny/gojq/issues/86) ER: basic TCO [#109](https://github.com/itchyny/gojq/issues/109) jq halt_error behaviour difference
[#113](https://github.com/itchyny/gojq/issues/113) error/0 and error/1 behavior difference
[#117](https://github.com/itchyny/gojq/issues/117) Negative number modulus *big.Int behaves differently to int
[#118](https://github.com/itchyny/gojq/issues/118) Regression introduced by "remove fork analysis from tail call optimization (ref #86)"
[#122](https://github.com/itchyny/gojq/issues/122) Slow performance for large error values that ends up using typeErrorPreview()
[#125](https://github.com/itchyny/gojq/pull/125) improve performance of join by make it internal
[#141](https://github.com/itchyny/gojq/issues/141) Empty array flatten regression since "improve flatten performance by reducing copy" - [readline](https://github.com/chzyer/readline) fork that can be found at https://github.com/wader/readline/tree/fq - [gopacket](https://github.com/google/gopacket) for TCP and IPv4 reassembly - [mapstructure](https://github.com/mitchellh/mapstructure) for convenient JSON/map conversion - [go-difflib](https://github.com/pmezard/go-difflib) for diff tests - [golang.org/x/text](https://pkg.go.dev/golang.org/x/text) for text encoding conversions - [float16.go](https://android.googlesource.com/platform/tools/gpu/+/gradle_2.0.0/binary/float16.go) to convert bits into 16-bit floats ## Release process Run and follow instructions: ``` make release=1.2.3 ```