adaptive_model.c

算术码编码代码源程序

/* THE ADAPTIVE SOURCE MODEL */

#include "model.h"

int freq[No_of_symbols+1];	/* Symbol frequencies                       */


/* INITIALIZE THE MODEL. */

start_model()
{   int i;
    for (i = 0; i<No_of_chars; i++) {		/* Set up tables that       */
        char_to_index[i] = i+1;			/* translate between symbol */
        index_to_char[i+1] = i;			/* indexes and characters.  */
    }
    for (i = 0; i<=No_of_symbols; i++) {	/* Set up initial frequency */
        freq[i] = 1;				/* counts to be one for all */
        cum_freq[i] = No_of_symbols-i;		/* symbols.                 */
    }
    freq[0] = 0;				/* Freq[0] must not be the  */
}						/* same as freq[1].         */


/* UPDATE THE MODEL TO ACCOUNT FOR A NEW SYMBOL. */

update_model(symbol)
    int symbol;			/* Index of new symbol                      */
{   int i;			/* New index for symbol                     */
    if (cum_freq[0]==Max_frequency) {		/* See if frequency counts  */
        int cum;				/* are at their maximum.    */
        cum = 0;
        for (i = No_of_symbols; i>=0; i--) {	/* If so, halve all the     */
            freq[i] = (freq[i]+1)/2;		/* counts (keeping them     */
            cum_freq[i] = cum; 			/* non-zero).               */
            cum += freq[i];
        }
    }
    for (i = symbol; freq[i]==freq[i-1]; i--) ;	/* Find symbol's new index. */
    if (i<symbol) {
        int ch_i, ch_symbol;
        ch_i = index_to_char[i];		/* Update the translation   */
        ch_symbol = index_to_char[symbol];	/* tables if the symbol has */
        index_to_char[i] = ch_symbol;           /* moved.                   */
        index_to_char[symbol] = ch_i;
        char_to_index[ch_i] = symbol;
        char_to_index[ch_symbol] = i;
    }
    freq[i] += 1;				/* Increment the frequency  */
    while (i>0) {				/* count for the symbol and */
        i -= 1;					/* update the cumulative    */
        cum_freq[i] += 1;			/* frequencies.             */
    }
}