update_model(int symbol,unsigned char oldch)
{
	int i;
	int index;
	unsigned char *freq_ptr;
	unsigned char freq;
	int old_weight_code;
	unsigned weight_gain;
	unsigned char temp_freq;
	long temp_total_freq;
	FrequencyInfo *temp_freq_info;
	unsigned char low_symbol;

	low_symbol = (unsigned char)(symbol_translation[symbol] & 255);

	char_total = (unsigned char)(char_total + low_symbol);

	old_weight_code = -1;
	temp_freq_info = frequency_info[oldch];
	freq_ptr = temp_freq_info->freq;
	index = symbol/2;
	if (symbol % 2 == 0) /* if a high symbol */
		{
		temp_freq = 
			(unsigned char)((freq_ptr[index] & HIGH_MASK) >> HIGH_SHIFT);
		if (temp_freq == 0)
			{
			freq_ptr[index] += INIT_INDEX << HIGH_SHIFT;
			temp_freq_info->total_freq += 
			translate[INIT_INDEX] - translate[0];
			}
		else if (char_total > upgrade_threshold[temp_freq])
			{
			old_weight_code = temp_freq;
			freq_ptr[index] += HIGH_INCREMENT; /* bump it */
			}
		}
	else /* if a low one */
		{
		temp_freq = (unsigned char)((freq_ptr[index] & LOW_MASK) >> LOW_SHIFT);
		if (temp_freq == 0)
			{
			freq_ptr[index] += INIT_INDEX << LOW_SHIFT;
			temp_freq_info->total_freq += 
			translate[INIT_INDEX] - translate[0];
			}
		else if (char_total > upgrade_threshold[temp_freq])
			{
			old_weight_code = temp_freq;
			freq_ptr[index] += LOW_INCREMENT; /* bump it */
			}
		}

	if (old_weight_code != -1) /* if there was an adjustment of weight */
		{
		weight_gain = translate[old_weight_code+1] - translate[old_weight_code];
		temp_freq_info->total_freq += weight_gain;
		}

	temp_total_freq = temp_freq_info->total_freq;
	while (temp_total_freq > MAX_FREQUENCY)
		{
		temp_total_freq = 0;
		freq_ptr = temp_freq_info->freq;
		for (i = 0; i < (NO_OF_SYMBOLS+1)/2; i ++)
			{
			if (*freq_ptr == 0) /* if this is at the bottom, forget it */
				temp_total_freq += BOTH_WEIGHTS_ZERO;
			else /* if it needs to be updated */
				{
				freq = *freq_ptr;
				if ((freq & HIGH_MASK) != 0)
					{
					/* if the top is not at the lowest value*/
					*freq_ptr -= HIGH_INCREMENT; /* decrement it */
					}
				if ((freq & LOW_MASK) != 0)
					{
					/* if the low is not at the lowest value*/
					*freq_ptr -= LOW_INCREMENT; /* decrement it */
					}
				temp_total_freq += both_weights[*freq_ptr];
				}
			freq_ptr ++;
			}
		}
	temp_freq_info->total_freq = (unsigned)temp_total_freq;
	return(0);
}