ac_vld_decode_n.sa

h263,jpeg,mpeg2编解码核心程序(TI DSP C64xx)

* ========================================================================= *
*                                                                           *
*   TEXAS INSTRUMENTS, INC.                                                 *
*                                                                           *
*   NAME                                                                    *
*        ac_vld_decode                                                      *
*                                                                           *
*   USAGE                                                                   * 
*       This routine is C-callable and can be called as:                    * 
*                                                                           * 
*           unsigned char ac_vld_decode                                     * 
*           (                                                               * 
*               const unsigned char **data_ptr,                             * 
*               unsigned int        *data_word,                             * 
*               short               *run_level,                             * 
*               unsigned char       *num_of_rles,                           * 
*               unsigned char       *bit_position,                          * 
*               unsigned int        *used_up_bytes,                         * 
*               const AC_TBL_str    *ac_table                               * 
*           )                                                               * 
*                                                                           * 
*       data_ptr:      Points to the next byte of VLD buffer.               * 
*       data_word:     Pointer to word containing the first 32 bits of      * 
*                      bitstream.                                           * 
*       run_level:     Buffer where run/level pairs will be written         * 
*       num_of_rles:   Location to write number of run/level pairs          * 
*       bit_position:  Count of bits in "data_word" that are earlier        * 
*                      in the bitstream than the bits pointed to by         * 
*                      "data_ptr"                                           * 
*       used_up_bytes: Location to write number of bytes consumed.          * 
*       ac_table:      Structure containing pointers to the run, level,     * 
*                      and size tables, as well as the "eob_flag".          * 
*                      (The "eob_flag" is ignored.)                         * 
*                                                                           * 
*C          #ifndef AC_TBL_STRUCT_                                          * 
*C          #define AC_TBL_STRUCT_ 1                                        * 
*C                                                                          * 
*C          typedef struct AC_TBL_str                                       * 
*C          {                                                               * 
*C              unsigned char          eob_flag;                            * 
*C              const unsigned char    *run_ptr;                            * 
*C              const unsigned char    *sze_ptr;                            * 
*C              const unsigned char    *shf_ptr;                            * 
*C                                                                          * 
*C          } AC_TBL_str;                                                   * 
*C                                                                          * 
*C          #endif                                                          * 
*                                                                           * 
*       The pointer "data_ptr" points to the first byte in the data         * 
*       stream that is not completely contained within the state word.      * 
*                                                                           * 
*       The variable "bit_position" contains the count of bits that         * 
*       are in the "data_word" but which are before the "data_ptr" in       * 
*       the bitstream.                                                      * 
*                                                                           * 
*       The following diagram attempts to illustrate the relationship       * 
*       between the "data_word", "data_ptr", and "bit_position"             * 
*       arguments to ac_vld_decode().  (Each box is 1 byte / 8 bits.)       * 
*       In this example, we've already consumed 11 bits of the bitstream    * 
*       and are calling ac_vld_decode() to decode the remaining bits.       * 
*       (The 'x' denote bits that have already been consumed from the       * 
*       bitstream.)                                                         * 
*                                                                           * 
*                                                 data_ptr                  * 
*                                                points here                * 
*                                                     |                     * 
*                         1       2       3       4 __|__ 4       5         * 
*         0       8       6       4       2       0/     \8       6         * 
*         +-------+-------+-------+-------+-------+-------+-------+         * 
*         xxxxxxxxxxx     |       |       |       |       |       |         * 
*         +-------+--|----+-------+-------+-------+-|-----+-------+         * 
*                    |                              |                       * 
*                    |<-- contained in data_word -->|                       * 
*                                                                           * 
*                                                                           * 
*       In this example, "data_word" contains bits 11 through 42 of the     * 
*       bitstream, and "data_ptr" points to the byte containing bits        * 
*       40 through 47 of the bitstream.  In this setting, "bit_position"    * 
*       would contain "29", since three of the bits in "data_word"          * 
*       overlap bits in the byte pointed to by "data_ptr".                  * 
*                                                                           * 
*       In all circumstances, "bit_position" must be between 25 and 32 on   * 
*       entry to ac_vld_decode, and all 32 bits of "data_word" must be      * 
*       valid bits from the bitstream.                                      * 
*                                                                           * 
*       On exit from ac_vld_decode(), the "bit_position", "data_ptr", and   * 
*       "data_word" values are updated to reflect the current position in   * 
*       the bitstream, to allow chaining to other VLD routines that share   * 
*       this interface.                                                     * 
*                                                                           * 
*   DESCRIPTION                                                             * 
*       The encoded AC terms in a JPEG stream are in the following          * 
*       generalized format:                                                 * 
*                                                                           * 
*                       1 1 1 1 0 c c c c c l l l l l                       * 
*                                                                           * 
*       That is, zero or more leading 1's, a zero, some additional          * 
*       code-bits (which complete the Huffman code), followed by the        * 
*       non-sign bit portion of the "level" which this symbol encodes.      * 
*                                                                           * 
*       The JPEG VLD lookup tables are ordered according to Huffman         * 
*       codewords so that the left-most zero value can be used effectively  * 
*       as an index into the table.  Commonly occuring tables (such as      * 
*       tables K.5 and K.6 in the JPEG standard) have groups of codes       * 
*       which cluster naturally into four separate left-most zero ranges,   * 
*       as shown in the following table:                                    * 
*                                                                           * 
*           Left-Most Zero         Codewords                                * 
*           --------------         --------------------------------------   * 
*                                   0 x x x x x                             * 
*              0 thru 2             1 0 x x x x                             * 
*                                   1 1 0 x x x                             * 
*                                  |<---ofs--->|                            * 
*                                                                           * 
*                                   1 1 1 0 x x x x x                       * 
*              3 thru 5             1 1 1 1 0 x x x x                       * 
*                                   1 1 1 1 1 0 x x x                       * 
*                                        |<---ofs--->|                      * 
*                                                                           * 
*                                   1 1 1 1 1 1 0 x x x x x                 * 
*              6 thru 8             1 1 1 1 1 1 1 0 x x x x                 * 
*                                   1 1 1 1 1 1 1 1 0 x x x                 * 
*                                            |<----ofs---->|                * 
*                                                                           * 
*              9 thru 15            1 1 1 1 1 1 1 1 1 x x x x x x x         * 
*                                                  |<-----ofs----->|        * 
*                                                                           * 
*       This grouping lends itself to a lookup table which contains four    * 
*       subtables -- three using a 6-bit offset, and a fourth using a       * 
*       7-bit offset.  Additionally, the 6-bit subtables each have 8        * 
*       unused entries due to the slight overlap between left-most zero     * 
*       ranges.  The ranges marked "ofs" above denote the bits that are     * 
*       extracted for use as a table offset into the lookup table.          * 
*       (An extra leading '1' is extracted for two of the sub-tables, in    * 
*       order to simplify table-offset calculations.)                       * 
*                                                                           * 
*       As a result, the VLD lookup tables are laid out like so, for        * 
*       efficient lookups.                                                  * 
*                                                                           * 
*                0  +-----------------------------------+                   * 
*                   |                                   |                   * 
*                   |      0 <= left-most zero <= 2     |                   * 
*                   |                                   |                   * 
*               56  +-----------------------------------+                   * 
*                   |                                   |                   * 
*                   |      3 <= left-most zero <= 5     |                   * 
*                   |                                   |                   * 
*              112  +-----------------------------------+                   * 
*                   |            -- hole --             |                   * 
*              120  +-----------------------------------+                   * 
*                   |                                   |                   * 
*                   |      6 <= left-most zero <= 8     |                   * 
*                   |                                   |                   * 
*              176  +-----------------------------------+                   * 
*                   |            -- hole --             |                   * 
*              184  +-----------------------------------+                   * 
*                   |                                   |                   * 
*                   |                                   |                   * 
*                   |                                   |                   * 
*                   |                                   |                   * 
*                   |      9 <= left-most zero <= 15    |                   * 
*                   |                                   |                   * 
*                   |                                   |                   * 
*                   |                                   |                   * 
*                   |                                   |                   * 
*              312  +---+-------------------------------+                   * 
*                   |pad|                                                   * 
*              314  +---+                                                   *