fq/doc/dev.md

## 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

### Decoder API

`*decode.D` reader methods use this name convention:

`<Field>?(<reader<length>?>|<type>Fn>)(...[, scalar.Mapper...]) <type>`

- If starts with `Field` a field will be added and first argument will be name of field. If not it will just read.
- `<reader<length>?>|<type>Fn>` a reader or a reader function
  - `<reader<length>?>` Read bits using some decoder.
    - `U16` unsigned 16 bit integer.
    - `UTF8` UTF8 with byte length as argument.
  - `<type>Fn>` read using a `func(d *decode.D) <type>`  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.

`<type>` are these types:

| `<type>` | 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.<type>ToScalar` or to just to set symbolic value `scalar.<type>ToSym<type>`. 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 <format>` 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 <format> 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<br>
Issues and PR:s related to fq:<br>
[#43](https://github.com/itchyny/gojq/issues/43) Support for functions written in go when used as a library<br>
[#46](https://github.com/itchyny/gojq/pull/46) Support custom internal functions<br>
[#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<br>
[#67](https://github.com/itchyny/gojq/pull/67) Add custom iterator function support which enables implementing a REPL in jq<br>
[#81](https://github.com/itchyny/gojq/issues/81) path/1 behaviour and path expression question<br>
[#86](https://github.com/itchyny/gojq/issues/86) ER: basic TCO
[#109](https://github.com/itchyny/gojq/issues/109) jq halt_error behaviour difference<br>
[#113](https://github.com/itchyny/gojq/issues/113) error/0 and error/1 behavior difference<br>
[#117](https://github.com/itchyny/gojq/issues/117) Negative number modulus *big.Int behaves differently to int<br>
[#118](https://github.com/itchyny/gojq/issues/118) Regression introduced by "remove fork analysis from tail call optimization (ref #86)"<br>
[#122](https://github.com/itchyny/gojq/issues/122) Slow performance for large error values that ends up using typeErrorPreview()<br>
[#125](https://github.com/itchyny/gojq/pull/125) improve performance of join by make it internal<br>
[#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
```