frame.c

	int format_i = 0;
	char *next_format = NULL, *last_format = NULL;

	last_format = ast_strdupa(list);
	while(last_format) {
		if((next_format = strchr(last_format, ','))) {
			*next_format = '\0';
			next_format++;
		}
		if ((format_i = ast_getformatbyname(last_format)) > 0) {
			if (mask) {
				if (allowing)
					(*mask) |= format_i;
				else
					(*mask) &= ~format_i;
			}
			/* can't consider 'all' a prefered codec*/
			if(pref && strcasecmp(last_format, "all")) {
				if(allowing)
					ast_codec_pref_append(pref, format_i);
				else
					ast_codec_pref_remove(pref, format_i);
			} else if(!allowing) /* disallow all must clear your prefs or it makes no sense */
				memset(pref, 0, sizeof(struct ast_codec_pref));
		} else
			ast_log(LOG_WARNING, "Cannot %s unknown format '%s'\n", allowing ? "allow" : "disallow", last_format);

		last_format = next_format;
	}
}

static int g723_len(unsigned char buf)
{
	switch(buf & TYPE_MASK) {
	case TYPE_DONTSEND:
		return 0;
		break;
	case TYPE_SILENCE:
		return 4;
		break;
	case TYPE_HIGH:
		return 24;
		break;
	case TYPE_LOW:
		return 20;
		break;
	default:
		ast_log(LOG_WARNING, "Badly encoded frame (%d)\n", buf & TYPE_MASK);
	}
	return -1;
}

static int g723_samples(unsigned char *buf, int maxlen)
{
	int pos = 0;
	int samples = 0;
	int res;
	while(pos < maxlen) {
		res = g723_len(buf[pos]);
		if (res <= 0)
			break;
		samples += 240;
		pos += res;
	}
	return samples;
}

static unsigned char get_n_bits_at(unsigned char *data, int n, int bit)
{
	int byte = bit / 8;       /* byte containing first bit */
	int rem = 8 - (bit % 8);  /* remaining bits in first byte */
	unsigned char ret = 0;
	
	if (n <= 0 || n > 8)
		return 0;

	if (rem < n) {
		ret = (data[byte] << (n - rem));
		ret |= (data[byte + 1] >> (8 - n + rem));
	} else {
		ret = (data[byte] >> (rem - n));
	}

	return (ret & (0xff >> (8 - n)));
}

static int speex_get_wb_sz_at(unsigned char *data, int len, int bit)
{
	static int SpeexWBSubModeSz[] = {
		0, 36, 112, 192,
		352, 0, 0, 0 };
	int off = bit;
	unsigned char c;

	/* skip up to two wideband frames */
	if (((len * 8 - off) >= 5) && 
		get_n_bits_at(data, 1, off)) {
		c = get_n_bits_at(data, 3, off + 1);
		off += SpeexWBSubModeSz[c];

		if (((len * 8 - off) >= 5) && 
			get_n_bits_at(data, 1, off)) {
			c = get_n_bits_at(data, 3, off + 1);
			off += SpeexWBSubModeSz[c];

			if (((len * 8 - off) >= 5) && 
				get_n_bits_at(data, 1, off)) {
				ast_log(LOG_WARNING, "Encountered corrupt speex frame; too many wideband frames in a row.\n");
				return -1;
			}
		}

	}
	return off - bit;
}

static int speex_samples(unsigned char *data, int len)
{
	static int SpeexSubModeSz[] = {
		0, 43, 119, 160, 
		220, 300, 364, 492, 
		79, 0, 0, 0,
		0, 0, 0, 0 };
	static int SpeexInBandSz[] = { 
		1, 1, 4, 4,
		4, 4, 4, 4,
		8, 8, 16, 16,
		32, 32, 64, 64 };
	int bit = 0;
	int cnt = 0;
	int off = 0;
	unsigned char c;

	while ((len * 8 - bit) >= 5) {
		/* skip wideband frames */
		off = speex_get_wb_sz_at(data, len, bit);
		if (off < 0)  {
			ast_log(LOG_WARNING, "Had error while reading wideband frames for speex samples\n");
			break;
		}
		bit += off;

		if ((len * 8 - bit) < 5) {
			ast_log(LOG_WARNING, "Not enough bits remaining after wide band for speex samples.\n");
			break;
		}

		/* get control bits */
		c = get_n_bits_at(data, 5, bit);
		bit += 5;

		if (c == 15) { 
			/* terminator */
			break; 
		} else if (c == 14) {
			/* in-band signal; next 4 bits contain signal id */
			c = get_n_bits_at(data, 4, bit);
			bit += 4;
			bit += SpeexInBandSz[c];
		} else if (c == 13) {
			/* user in-band; next 5 bits contain msg len */
			c = get_n_bits_at(data, 5, bit);
			bit += 5;
			bit += c * 8;
		} else if (c > 8) {
			/* unknown */
			break;
		} else {
			/* skip number bits for submode (less the 5 control bits) */
			bit += SpeexSubModeSz[c] - 5;
			cnt += 160; /* new frame */
		}
	}
	return cnt;
}


int ast_codec_get_samples(struct ast_frame *f)
{
	int samples=0;
	switch(f->subclass) {
	case AST_FORMAT_SPEEX:
		samples = speex_samples(f->data, f->datalen);
		break;
	case AST_FORMAT_G723_1:
                samples = g723_samples(f->data, f->datalen);
		break;
	case AST_FORMAT_ILBC:
		samples = 240 * (f->datalen / 50);
		break;
	case AST_FORMAT_GSM:
		samples = 160 * (f->datalen / 33);
		break;
	case AST_FORMAT_G729A:
		samples = f->datalen * 8;
		break;
	case AST_FORMAT_SLINEAR:
		samples = f->datalen / 2;
		break;
	case AST_FORMAT_LPC10:
                /* assumes that the RTP packet contains one LPC10 frame */
		samples = 22 * 8;
		samples += (((char *)(f->data))[7] & 0x1) * 8;
		break;
	case AST_FORMAT_ULAW:
	case AST_FORMAT_ALAW:
		samples = f->datalen;
		break;
	case AST_FORMAT_ADPCM:
	case AST_FORMAT_G726:
		samples = f->datalen * 2;
		break;
	default:
		ast_log(LOG_WARNING, "Unable to calculate samples for format %s\n", ast_getformatname(f->subclass));
	}
	return samples;
}