Make a shiny new lookup table for looking up complete AC coefficients. No tests yet.

[fjl] / dehuff.c

diff --git a/dehuff.c b/dehuff.c
index c4a65eea8a0619153bb9910cd5421db53bd01193..695da0bdc78ee2d863cbb0fa547b628619ffc4ab 100644 (file)
--- a/dehuff.c
+++ b/dehuff.c
@@ -119,6 +119,50 @@ void read_huffman_tables(huffman_tables_t* dst, input_func_t* input_func, void*
                                }
                        }
                }       
+               
+               // Generate the AC lookup tables.
+               for (unsigned i = 0; i < DEHUF_AC_TABLE_SIZE; ++i) {
+                       tbl->ac_table_codes[i] = AC_DEHUF_SLOW_PATH;
+                       tbl->ac_table_length[i] = AC_DEHUF_SLOW_PATH;
+                       tbl->ac_table_skip[i] = AC_DEHUF_SLOW_PATH;
+
+                       int lookup = i >> (DEHUF_AC_TABLE_BITS - DEHUF_TABLE_BITS);
+                       int rs = tbl->lookup_table_codes[lookup];
+                       unsigned length = tbl->lookup_table_length[lookup];
+                       if (rs == DEHUF_SLOW_PATH) {
+                               // Not enough bits to decode even the length.
+                               continue;
+                       }
+                       if (rs == 0x00) {
+                               // End of block.
+                               tbl->ac_table_codes[i] = AC_END_OF_BLOCK;
+                               tbl->ac_table_length[i] = length;
+                               tbl->ac_table_skip[i] = 0;
+                               continue;
+                       }
+                       if (rs == 0xf0) {
+                               // 16 zero coefficients.
+                               tbl->ac_table_codes[i] = AC_SIXTEEN_ZEROS;
+                               tbl->ac_table_length[i] = length;
+                               tbl->ac_table_skip[i] = 15;
+                               continue;
+                       }
+                       
+                       unsigned r = rs >> 4;
+                       unsigned s = rs & 0xf;
+                       if (s > DEHUF_AC_TABLE_BITS - length) {
+                               // Not enough bits to decode this coefficient.
+                               continue;
+                       }
+
+                       unsigned bits = (i >> (DEHUF_AC_TABLE_BITS - length - s)) & ((1 << s) - 1);
+
+                       tbl->ac_table_codes[i] = extend(bits, s);
+                       tbl->ac_table_length[i] = length + s;
+                       tbl->ac_table_skip[i] = r;
+
+                       assert(tbl->ac_table_length[i] <= DEHUF_AC_TABLE_BITS);
+               }       
        }
 
        free(buf);