Kostya's Boring Codec World

The weather here remains hellish so there’s little I can do besides suffering from it.

And yet I’ve spent two hours this morning to implement the main part of NihAV — set implementation. The other crucial part is options handling but I’ll postpone it for later since I can write proof of concept code without it.

Here’s a list of NihAV design guidelines I plan to follow:

• Naming: component functions should be called na_component_create(), na_component_destroy(), na_component_do_something().
• More generally, prefixing: public functions are prefixed with na_, public headers start with na as well e.g. libnadata/naset.h. The rest is private to the library. Even other NihAV libraries should use only public interfaces, otherwise you get ff_something and avpriv_that called from outside and in result you have MPlayer.
• NihAV-specific error codes to be used everywhere. Having AVERROR(EWHATEVER) and AVERROR_WHATEVER together is ridiculous. Especially when you have to deal with some error codes being missing from some platform and other being nonportable (what if on nihOS they decided to map ENOMEM to 42? Can you trust error code returned by service run on some remote platform then?).

And here’s how actual set interface looks like:
[sourcecode language=”c”]
#ifndef NA_DATA_SET_H
#define NA_DATA_SET_H

#include “nacommon.h”

struct NASet *na_set_create(NALibrary *lib);
void na_set_destroy(struct NASet *set);
int na_set_add(struct NASet *set, const char *key, void *data);
void* na_set_get(struct NASet *set, const char *key);
void na_set_del(struct NASet *set, const char *key);

struct NASetIterator;

typedef struct NASetEntry {
const char *key;
void *data;
} NASetEntry;

struct NASetIterator* na_set_iterator_get(struct NASet *set);
void na_set_iterator_destroy(struct NASetIterator* it);
int na_set_iterator_next(struct NASetIterator* it, NASetEntry *entry);
void na_set_iterator_reset(struct NASetIterator* it);

#endif
[/sourcecode]

As you can see, it’s nothing special, just basic set (well, it’s really dictionary but NIH terminology applies to this project) manipulation functions plus an iterator to scan through it — quite useful for e.g. showing all options or invoking all registered parsers. Implementation wise it’s simple hash table with djb2 hash.

Here’s how the main NihAV header looks and it should remain the same (maybe I’ll add error codes there as well but that’s it):

[sourcecode language=”c”]
#ifndef NA_COMMON_H
#define NA_COMMON_H

#include
#include

struct NASet;

enum NAOptionType {
NA_OPT_NULL = 0,
NA_OPT_FLAGS,
NA_OPT_INT,
NA_OPT_DOUBLE,
NA_OPT_STRING,
NA_OPT_BINARY,
NA_OPT_POINTER,
};

typedef union NAOptionValue {
int64_t i64;
uint64_t u64;
double dbl;
const char *str;
struct bin {
const char *ptr;
size_t size;
} bin;
const void *ptr;
} NAOptionValue;

typedef struct NAOption {
const char *name;
enum NAOptionType type;
NAOptionValue value;
} NAOption;

enum NAOptionInterfaceType {
NA_OPT_IF_ANY,
NA_OPT_IF_MINMAX,
NA_OPT_IF_ENUMERATED,
};

typedef struct NAOptionInterface {
const char *name;
const char *explanation;
enum NAOptionType type;
enum NAOptionInterfaceType if_type;
NAOptionValue min_val, max_val;
NAOptionValue *enums;
} NAOptionInterface;

typedef struct NALibrary {
void* (*malloc)(size_t size);
void* (*realloc)(void *ptr, size_t new_size);
void (*free)(void *ptr);

struct NASet *components;
} NALibrary;

#define NA_CLASS_MAGIC 0x11AC1A55

typedef struct NAClass {
uint32_t magic;
const char *name;
const NAOptionInterface *opt_if;
struct NASet *options;
NALibrary *library;

void (*cleanup)(NAClass *c);
} NAClass;

void na_init_library(NALibrary *lib);
void na_init_library_custom_alloc(NALibrary *lib,
void* (*new_malloc)(size_t size),
void* (*new_realloc)(void *ptr, size_t new_size),
void (*new_free)(void *ptr));
int na_lib_add_component(NALibrary *lib, const char *cname, void *component);
void *na_lib_query_component(NALibrary *lib, const char *cname);
void na_clean_library(NALibrary *lib);

int na_class_set_option(NAClass *c, NAOption *opt);
const NAOption* na_class_query_option(NAClass *c, const char *name);
void na_class_unset_option(NAClass *c, const char *name);
void na_class_destroy(NAClass *c);

#endif
[/sourcecode]

So what we have here is essentially three main entities NihAV will use for everything: NALibrary, NAClass and NAOption.

NALibrary is the core that manages the rest. As you can see it has a collection of components that, as discussed in the previous post, will contain the set of instances implementing tasks (e.g. codecs, de/compressors, hashes, de/muxers etc.) and this library object also contains allocator for memory management. This way it can be all pinned to the needed instance, e.g. once I’ve seen a code that had used libavcodec in two separate modules — for video and audio of course — and those two modules didn’t know a thing about each other (and were dynamically loaded too). Note to self: implement filtered loading for components, e.g. when initialising libnacodec only audio decoders will be registered or when initialising libnacompr only decoders are registered etc. etc.

The second component is NAClass. Every public component of NihAV beside NALibrary will be an instance of NAClass. Users are not supposed to construct one themselves, there will be utility functions for doing that behind the scenes (after all, you don’t need this object directly, you need a component in NALibrary doing what you want).

And the third component is what makes it more extensible without adding many public fields — NAOption for storing parameters in a class and NAOptionInterface for defining what options that class accepts.

Expected flow is like this:

1. NALibrary instance is created;
2. needed compontents are registered there (by creating copies inside the library tied to it — see the next to last field in NAClass);
3. when an instance is queried, a copy is created for that operation (the definition is quite small and you should not do it often so it should not be a complete murder);
4. user sets the options on the obtained instance;
5. user uses aforementioned instance to do work (coding/decoding, muxing, whatever);
6. user invokes destructor for the instance;
7. NALibrary instance is destroyed.

There will be some exceptions, i.e. probing should be done stateless by simply walking over the set of probe objects and invoking probe() there without creating a new instances. And something similar for decoder detection too — current libavcodec way with registering and initialising all decoders is an overkill.

This is how it should be. Volunteers to implement? None? Not even myself?! Yes, I thought so.

As you might have noticed, NihAV development is not going very fast (or at all — thanks to certain people and companies (where I’d never worked and have no desire to work at) that made me lost a desire to program anything) but at least I think somewhat on NihAV design.

So, here’s how the base should look:

NALibrary
   -> <named collection of NihAV components>
     -> NAClass instance that does some task

So, first you create NALibrary that is used to hold everything else. The main content of this library is a set of named collections corresponding to the tasks (e.g. “io” for I/O handlers, “demux” for demuxers, “compr” for compressors etc. etc.). Each collection holds objects based on NAClass that do some specific task — implement file or network I/O, demux AVI or Bink, compress into deflate format etc. All of this is supposed to be hidden from the final user though — it’s NihAV libraries that do all the interaction with NALibrary, they know their collection name and what type of components is stored there. So when you ask for ASF demuxer, the function na_demuxer_find() will access "demux" collection in the provided NALibrary and then it will try to find a demuxer with name "asf" there. NAClass provides common interface for object manipulation — name querying, options setting, etc.

And a word about demuxers — the more I think about it the more I’m convinced that they should output both packets and streams. This is not just for user inconvenience, it also helps chaining demuxers (nothing prevents people from putting raw DV into ASF and then muxing that into MOV with ASF packets containing DV packets — nothing except common sense but it’s too rare to rely upon).

Morale: if you want to implement multimedia framework start with hash table implementation.

P.S. As for implementation language I’ll still stick to C. Newer programming languages like Rust or Swift or that one with retarded gopher have the problem of being not well-widespread, i.e. what if I’m using somewhat outdated Ubuntu or Debian — especially on ARM — where I don’t want to mess with compiler (cross)compilation? Plus it’s likely I’ll make mistakes that will be hard for me to debug and constructions to work around (I doubt modern languages like passing void* on public interface that’s cast to something else inside the function implementation). Of course it’s a matter of experience but I’d rather train on something smaller scale first for a new language.

Before people reading this blog (all 0 of them) start asking about it — yes, I’ve started implementing NihAV, it will take a lot of time so don’t expect it to be finished or even usable this decade at least (too little free time, even less interest and too much work needed to be done to have it at least somewhat usable for anything).

Here’s the intended structure:

• libnaarch — arch-specific stuff here, like little/big endian handling, specific speedup tricks etc. Do not confuse with libnaosstuff — I’m not interested in non-POSIX systems.
• libnacodec — codecs will belong here.
• libnacompr — decompression and compression routines belong here.
• libnacrypto — cryptographic stuff (hashes, cyphers, ROT-13) belongs here.
• libnadata — data structures implementations.
• libnaformat — muxers and demuxers.
• libnamath — mathematics-related stuff (fixedpoint calculations, fractional math etc).
• libnaregistry — codecs registry. Codec information is stored here — both overall codec infomation (name to description mapping) and codec name search by tag. That means that e.g. FOURCC to codec name mapping from AVI or MOV are a part of this library instead of remaining demuxer-specific.
• libnautil — utility stuff not belonging to any other library.

Remark to myself: maybe it’s worth splitting out libnadsp so it won’t clutter libnacodec.

Probably I’ll start with a simple stuff — implement dictionary support (for options), AVI demuxer and some simple decoder.

Originally it should’ve been Bender Bending Rodriguez on the Moon (implying that he’ll build his own NihAV with …) but since I lack drawing skills (at school I’ve managed to draw something more or less realistic only once), here’s the alternative logo drawn by professional coder in Inkscape in couple of minutes.

Somehow I believe that building a castle on a swamp is a good metaphor for this enterprise as well (and much easier to draw too).