OBS解读-动态循环缓冲区的实现

  在音视频处理时，缓冲的设计十分重要，关系到通话的质量和实时性。本文主要介绍OBS中关于缓冲的设计，具体实现如下

struct circlebuf {
void   *data;
size_t size;


size_t start_pos;
size_t end_pos;
size_t capacity;
};


static inline void circlebuf_init(struct circlebuf *cb)
{
memset(cb, 0, sizeof(struct circlebuf));
}


static inline void circlebuf_free(struct circlebuf *cb)
{
bfree(cb->data);
memset(cb, 0, sizeof(struct circlebuf));
}


static inline void circlebuf_reorder_data(struct circlebuf *cb,
size_t new_capacity)
{
size_t difference;
uint8_t *data;


if (!cb->size || !cb->start_pos || cb->end_pos > cb->start_pos)
return;


difference = new_capacity - cb->capacity;
data = (uint8_t*)cb->data + cb->start_pos;
memmove(data+difference, data, cb->capacity - cb->start_pos);
cb->start_pos += difference;
}


static inline void circlebuf_ensure_capacity(struct circlebuf *cb)
{
size_t new_capacity;
if (cb->size <= cb->capacity)
return;


new_capacity = cb->capacity*2;
if (cb->size > new_capacity)
new_capacity = cb->size;


cb->data = brealloc(cb->data, new_capacity);
circlebuf_reorder_data(cb, new_capacity);
cb->capacity = new_capacity;
}


static inline void circlebuf_reserve(struct circlebuf *cb, size_t capacity)
{
if (capacity <= cb->capacity)
return;


cb->data = brealloc(cb->data, capacity);
circlebuf_reorder_data(cb, capacity);
cb->capacity = capacity;
}


static inline void circlebuf_upsize(struct circlebuf *cb, size_t size)
{
size_t add_size = size - cb->size;
size_t new_end_pos = cb->end_pos + add_size;


if (size <= cb->size)
return;


cb->size = size;
circlebuf_ensure_capacity(cb);


if (new_end_pos > cb->capacity) {
size_t back_size = cb->capacity - cb->end_pos;
size_t loop_size = add_size - back_size;


if (back_size)
memset((uint8_t*)cb->data + cb->end_pos, 0, back_size);


memset(cb->data, 0, loop_size);
new_end_pos -= cb->capacity;
} else {
memset((uint8_t*)cb->data + cb->end_pos, 0, add_size);
}


cb->end_pos = new_end_pos;
}


/** Overwrites data at a specific point in the buffer (relative).  */
static inline void circlebuf_place(struct circlebuf *cb, size_t position,
const void *data, size_t size)
{
size_t end_point = position + size;
size_t data_end_pos;


if (end_point > cb->size)
circlebuf_upsize(cb, end_point);


position += cb->start_pos;
if (position >= cb->capacity)
position -= cb->capacity;


data_end_pos = position + size;
if (data_end_pos > cb->capacity) {
size_t back_size = data_end_pos - cb->capacity;
size_t loop_size = size - back_size;


if (back_size)
memcpy((uint8_t*)cb->data + position, data, loop_size);
memcpy(cb->data, (uint8_t*)data + loop_size, back_size);
} else {
memcpy((uint8_t*)cb->data + position, data, size);
}
}


static inline void circlebuf_push_back(struct circlebuf *cb, const void *data,
size_t size)
{
size_t new_end_pos = cb->end_pos + size;


cb->size += size;
circlebuf_ensure_capacity(cb);


if (new_end_pos > cb->capacity) {
size_t back_size = cb->capacity - cb->end_pos;
size_t loop_size = size - back_size;


if (back_size)
memcpy((uint8_t*)cb->data + cb->end_pos, data,
back_size);
memcpy(cb->data, (uint8_t*)data + back_size, loop_size);


new_end_pos -= cb->capacity;
} else {
memcpy((uint8_t*)cb->data + cb->end_pos, data, size);
}


cb->end_pos = new_end_pos;
}


static inline void circlebuf_push_front(struct circlebuf *cb, const void *data,
size_t size)
{
cb->size += size;
circlebuf_ensure_capacity(cb);


if (cb->start_pos < size) {
size_t back_size = size - cb->start_pos;


if (cb->start_pos)
memcpy(cb->data, (uint8_t*)data + back_size,
cb->start_pos);


cb->start_pos = cb->capacity - back_size;
memcpy((uint8_t*)cb->data + cb->start_pos, data, back_size);
} else {
cb->start_pos -= size;
memcpy((uint8_t*)cb->data + cb->start_pos, data, size);
}
}


static inline void circlebuf_peek_front(struct circlebuf *cb, void *data,
size_t size)
{
assert(size <= cb->size);


if (data) {
size_t start_size = cb->capacity - cb->start_pos;


if (start_size < size) {
memcpy(data, (uint8_t*)cb->data + cb->start_pos,
start_size);
memcpy((uint8_t*)data + start_size, cb->data,
size - start_size);
} else {
memcpy(data, (uint8_t*)cb->data + cb->start_pos, size);
}
}
}


static inline void circlebuf_peek_back(struct circlebuf *cb, void *data,
size_t size)
{
assert(size <= cb->size);


if (data) {
size_t back_size = (cb->end_pos ? cb->end_pos : cb->capacity);


if (back_size < size) {
size_t front_size = size - back_size;
size_t new_end_pos = cb->capacity - front_size;


memcpy((uint8_t*)data + (size - back_size), cb->data,
back_size);
memcpy(data, (uint8_t*)cb->data + new_end_pos,
front_size);
} else {
memcpy(data, (uint8_t*)cb->data + cb->end_pos - size,
size);
}
}
}


static inline void circlebuf_pop_front(struct circlebuf *cb, void *data,
size_t size)
{
circlebuf_peek_front(cb, data, size);


cb->size -= size;
cb->start_pos += size;
if (cb->start_pos >= cb->capacity)
cb->start_pos -= cb->capacity;
}


static inline void circlebuf_pop_back(struct circlebuf *cb, void *data,
size_t size)
{
circlebuf_peek_front(cb, data, size);


cb->size -= size;
if (cb->end_pos <= size)
cb->end_pos = cb->capacity - (size - cb->end_pos);
else
cb->end_pos -= size;
}


static inline void *circlebuf_data(struct circlebuf *cb, size_t idx)
{
uint8_t *ptr = (uint8_t*)cb->data;
size_t offset = cb->start_pos + idx;


if (idx > cb->size)
return NULL;


if (offset >= cb->capacity)
offset -= cb->capacity;


return ptr + offset;
}