gfx_lzw_enc.c

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/* tiny LZW codec
 * Based on code by Mark Nelson, Dr. Dobb's Journal October, 1989
 */

#include "fb_gfx.h"
#include "fb_gfx_lzw.h"

static LZW_ENTRY *find_match(unsigned short prefix, unsigned char value)
{
        unsigned short index, offset = 1;

        index = ((unsigned short)value << 4) ^ prefix;
        if (index)
                offset = TABLE_SIZE - index;
        while (1) {
                if ((fb_lzw_entry[index].code == -1) ||
                    ((fb_lzw_entry[index].prefix == prefix) && (fb_lzw_entry[index].value == value)))
                        return &fb_lzw_entry[index];
                index = (index + offset) % TABLE_SIZE;
        }
}

FBCALL int fb_hEncode
        (
                const unsigned char *in_buffer,
                ssize_t in_size,
                unsigned char *out_buffer,
                ssize_t *out_size
        )
{
        LZW_ENTRY *e;
        unsigned short string_code, next_code = 256;
        unsigned char bit = 0;
        ssize_t size;

        size = 0;
        fb_hMemSet(fb_lzw_entry, -1, sizeof(fb_lzw_entry));
        string_code = *in_buffer++;
        in_size--;
        while (in_size) {
                e = find_match(string_code, *in_buffer);
                if (e->code != -1)
                        string_code = (unsigned short)e->code;
                else {
                        if (next_code < MAX_CODE) {
                                e->code = next_code++;
                                e->prefix = string_code;
                                e->value = *in_buffer;
                        }
                        OUTPUT_CODE(string_code);
                        string_code = *in_buffer;
                }
                in_buffer++;
                in_size--;
        }
        OUTPUT_CODE(string_code);
        OUTPUT_CODE(MAX_CODE);
        if (bit)
                size++;
        *out_size = size;
        return 0;
}