@TEMPLATE decoder_tmpl.c
Decode With Drops Example
=========================
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INTRODUCTION
This is an example utility which drops a series of frames, as specified
on the command line. This is useful for observing the error recovery
features of the codec.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INTRODUCTION

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ EXTRA_INCLUDES
#include <time.h>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ EXTRA_INCLUDES

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ HELPERS
struct parsed_header
{
    char key_frame;
    int version;
    char show_frame;
    int first_part_size;
};

int next_packet(struct parsed_header* hdr, int pos, int length, int mtu)
{
    int size = 0;
    int remaining = length - pos;
    /* Uncompressed part is 3 bytes for P frames and 10 bytes for I frames */
    int uncomp_part_size = (hdr->key_frame ? 10 : 3);
    /* number of bytes yet to send from header and the first partition */
    int remainFirst = uncomp_part_size + hdr->first_part_size - pos;
    if (remainFirst > 0)
    {
        if (remainFirst <= mtu)
        {
            size = remainFirst;
        }
        else
        {
            size = mtu;
        }

        return size;
    }

    /* second partition; just slot it up according to MTU */
    if (remaining <= mtu)
    {
        size = remaining;
        return size;
    }
    return mtu;
}

void throw_packets(unsigned char* frame, int* size, int loss_rate, int* thrown, int* kept)
{
    unsigned char loss_frame[256*1024];
    int pkg_size = 1;
    int count = 0;
    int pos = 0;
    int loss_pos = 0;
    struct parsed_header hdr;
    unsigned int tmp;
    int mtu = 100;

    if (*size < 3)
    {
        return;
    }
    putc('|', stdout);
    /* parse uncompressed 3 bytes */
    tmp = (frame[2] << 16) | (frame[1] << 8) | frame[0];
    hdr.key_frame = !(tmp & 0x1); /* inverse logic */
    hdr.version = (tmp >> 1) & 0x7;
    hdr.show_frame = (tmp >> 4) & 0x1;
    hdr.first_part_size = (tmp >> 5) & 0x7FFFF;

    /* don't drop key frames */
    if (hdr.key_frame)
    {
        int i;
        *kept = *size/mtu + ((*size % mtu > 0) ? 1 : 0); /* approximate */
        for (i=0; i < *kept; i++)
            putc('.', stdout);
        return;
    }

    while ((pkg_size = next_packet(&hdr, pos, *size, mtu)) > 0)
    {
        int loss_event = ((rand() + 1.0)/(RAND_MAX + 1.0) < loss_rate/100.0);
        if (*thrown == 0 && !loss_event)
        {
            memcpy(loss_frame + loss_pos, frame + pos, pkg_size);
            loss_pos += pkg_size;
            (*kept)++;
            putc('.', stdout);
        }
        else
        {
            (*thrown)++;
            putc('X', stdout);
        }
        pos += pkg_size;
    }
    memcpy(frame, loss_frame, loss_pos);
    memset(frame + loss_pos, 0, *size - loss_pos);
    *size = loss_pos;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ HELPERS

Usage
-----
This example adds a single argument to the `simple_decoder` example,
which specifies the range or pattern of frames to drop. The parameter is
parsed as follows:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ USAGE
if(argc!=4 && argc != 5)
    die("Usage: %s <infile> <outfile> <N-M|N/M|L,S>\n", argv[0]);
{
    char *nptr;
    n = strtol(argv[3], &nptr, 0);
    mode = (*nptr == '\0' || *nptr == ',') ? 2 : (*nptr == '-') ? 1 : 0;

    m = strtol(nptr+1, NULL, 0);
    if((!n && !m) || (*nptr != '-' && *nptr != '/' &&
        *nptr != '\0' && *nptr != ','))
        die("Couldn't parse pattern %s\n", argv[3]);
}
seed = (m > 0) ? m : (unsigned int)time(NULL);
srand(seed);thrown_frame = 0;
printf("Seed: %u\n", seed);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ USAGE


Dropping A Range Of Frames
--------------------------
To drop a range of frames, specify the starting frame and the ending
frame to drop, separated by a dash. The following command will drop
frames 5 through 10 (base 1).

  $ ./decode_with_drops in.ivf out.i420 5-10


Dropping A Pattern Of Frames
----------------------------
To drop a pattern of frames, specify the number of frames to drop and
the number of frames after which to repeat the pattern, separated by
a forward-slash. The following command will drop 3 of 7 frames.
Specifically, it will decode 4 frames, then drop 3 frames, and then
repeat.

  $ ./decode_with_drops in.ivf out.i420 3/7


Extra Variables
---------------
This example maintains the pattern passed on the command line in the
`n`, `m`, and `is_range` variables:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ EXTRA_VARS
int              n, m, mode;                                        //
unsigned int     seed;
int              thrown=0, kept=0;
int              thrown_frame=0, kept_frame=0;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ EXTRA_VARS


Making The Drop Decision
------------------------
The example decides whether to drop the frame based on the current
frame number, immediately before decoding the frame.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PRE_DECODE
/* Decide whether to throw parts of the frame or the whole frame
   depending on the drop mode */
thrown_frame = 0;
kept_frame = 0;
switch (mode)
{
case 0:
    if (m - (frame_cnt-1)%m <= n)
    {
        frame_sz = 0;
    }
    break;
case 1:
    if (frame_cnt >= n && frame_cnt <= m)
    {
        frame_sz = 0;
    }
    break;
case 2:
    throw_packets(frame, &frame_sz, n, &thrown_frame, &kept_frame);
    break;
default: break;
}
if (mode < 2)
{
    if (frame_sz == 0)
    {
        putc('X', stdout);
        thrown_frame++;
    }
    else
    {
        putc('.', stdout);
        kept_frame++;
    }
}
thrown += thrown_frame;
kept += kept_frame;
fflush(stdout);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PRE_DECODE