Kostya's Boring Codec World

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.

This entry was posted on Sunday, June 14th, 2015 at 12:03 pm and is filed under NihAV. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.