int aws_ring_buffer_acquire_up_to()

in source/ring_buffer.c [128:229]

• 80 lines of code
• 18 McCabe index (conditional complexity)

int aws_ring_buffer_acquire_up_to(
    struct aws_ring_buffer *ring_buf,
    size_t minimum_size,
    size_t requested_size,
    struct aws_byte_buf *dest) {
    AWS_PRECONDITION(requested_size >= minimum_size);
    AWS_PRECONDITION(aws_ring_buffer_is_valid(ring_buf));
    AWS_PRECONDITION(aws_byte_buf_is_valid(dest));

    if (requested_size == 0 || minimum_size == 0 || !ring_buf || !dest) {
        AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
        AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
        return aws_raise_error(AWS_ERROR_INVALID_ARGUMENT);
    }

    uint8_t *tail_cpy;
    uint8_t *head_cpy;
    AWS_ATOMIC_LOAD_TAIL_PTR(ring_buf, tail_cpy);
    AWS_ATOMIC_LOAD_HEAD_PTR(ring_buf, head_cpy);

    /* this branch is, we don't have any vended buffers. */
    if (head_cpy == tail_cpy) {
        size_t ring_space = ring_buf->allocation_end == NULL ? 0 : ring_buf->allocation_end - ring_buf->allocation;

        size_t allocation_size = ring_space > requested_size ? requested_size : ring_space;

        if (allocation_size < minimum_size) {
            AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
            AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
            return aws_raise_error(AWS_ERROR_OOM);
        }

        /* go as big as we can. */
        /* we don't have any vended, so this should be safe. */
        AWS_ATOMIC_STORE_HEAD_PTR(ring_buf, ring_buf->allocation + allocation_size);
        AWS_ATOMIC_STORE_TAIL_PTR(ring_buf, ring_buf->allocation);
        *dest = aws_byte_buf_from_empty_array(ring_buf->allocation, allocation_size);
        AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
        AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
        return AWS_OP_SUCCESS;
    }
    /* you'll constantly bounce between the next two branches as the ring buffer is traversed. */
    /* after N + 1 wraps */
    if (tail_cpy > head_cpy) {
        size_t space = tail_cpy - head_cpy;
        /* this shouldn't be possible. */
        AWS_ASSERT(space);
        space -= 1;

        size_t returnable_size = space > requested_size ? requested_size : space;

        if (returnable_size >= minimum_size) {
            AWS_ATOMIC_STORE_HEAD_PTR(ring_buf, head_cpy + returnable_size);
            *dest = aws_byte_buf_from_empty_array(head_cpy, returnable_size);
            AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
            AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
            return AWS_OP_SUCCESS;
        }
        /* after N wraps */
    } else if (tail_cpy < head_cpy) {
        size_t head_space = ring_buf->allocation_end - head_cpy;
        size_t tail_space = tail_cpy - ring_buf->allocation;

        /* if you can vend the whole thing do it. Also prefer head space to tail space. */
        if (head_space >= requested_size) {
            AWS_ATOMIC_STORE_HEAD_PTR(ring_buf, head_cpy + requested_size);
            *dest = aws_byte_buf_from_empty_array(head_cpy, requested_size);
            AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
            AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
            return AWS_OP_SUCCESS;
        }

        if (tail_space > requested_size) {
            AWS_ATOMIC_STORE_HEAD_PTR(ring_buf, ring_buf->allocation + requested_size);
            *dest = aws_byte_buf_from_empty_array(ring_buf->allocation, requested_size);
            AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
            AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
            return AWS_OP_SUCCESS;
        }

        /* now vend as much as possible, once again preferring head space. */
        if (head_space >= minimum_size && head_space >= tail_space) {
            AWS_ATOMIC_STORE_HEAD_PTR(ring_buf, head_cpy + head_space);
            *dest = aws_byte_buf_from_empty_array(head_cpy, head_space);
            AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
            AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
            return AWS_OP_SUCCESS;
        }

        if (tail_space > minimum_size) {
            AWS_ATOMIC_STORE_HEAD_PTR(ring_buf, ring_buf->allocation + tail_space - 1);
            *dest = aws_byte_buf_from_empty_array(ring_buf->allocation, tail_space - 1);
            AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
            AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
            return AWS_OP_SUCCESS;
        }
    }

    AWS_POSTCONDITION(aws_ring_buffer_is_valid(ring_buf));
    AWS_POSTCONDITION(aws_byte_buf_is_valid(dest));
    return aws_raise_error(AWS_ERROR_OOM);
}