Greatly improve performance of Huffman decoding

git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@64 632fc199-4ca6-4c93-a231-07263d6284db

1 files changed, 12 insertions, 9 deletions

diff --git a/jdhuff.h b/jdhuff.h
index ae19b6c..8e50bbf 100644
--- a/jdhuff.h
+++ b/jdhuff.h
@@ -36,13 +36,17 @@ typedef struct {
   /* Link to public Huffman table (needed only in jpeg_huff_decode) */
   JHUFF_TBL *pub;
 
-  /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
+  /* Lookahead table: indexed by the next HUFF_LOOKAHEAD bits of
    * the input data stream.  If the next Huffman code is no more
    * than HUFF_LOOKAHEAD bits long, we can obtain its length and
-   * the corresponding symbol directly from these tables.
+   * the corresponding symbol directly from this tables.
+   *
+   * The lower 8 bits of each table entry contain the number of
+   * bits in the corresponding Huffman code, or HUFF_LOOKAHEAD + 1
+   * if too long.  The next 8 bits of each entry contain the
+   * symbol.
    */
-  int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
-  UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
+  int lookup[1<<HUFF_LOOKAHEAD];
 } d_derived_tbl;
 
 /* Expand a Huffman table definition into the derived format */
@@ -69,8 +73,8 @@ EXTERN(void) jpeg_make_d_derived_tbl
  * necessary.
  */
 
-typedef INT32 bit_buf_type;	/* type of bit-extraction buffer */
-#define BIT_BUF_SIZE  32	/* size of buffer in bits */
+typedef long bit_buf_type;		/* type of bit-extraction buffer */
+#define BIT_BUF_SIZE  __WORDSIZE	/* size of buffer in bits */
 
 /* If long is > 32 bits on your machine, and shifting/masking longs is
  * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
@@ -183,11 +187,10 @@ EXTERN(boolean) jpeg_fill_bit_buffer
     } \
   } \
   look = PEEK_BITS(HUFF_LOOKAHEAD); \
-  if ((nb = htbl->look_nbits[look]) != 0) { \
+  if ((nb = (htbl->lookup[look] >> HUFF_LOOKAHEAD)) <= HUFF_LOOKAHEAD) { \
     DROP_BITS(nb); \
-    result = htbl->look_sym[look]; \
+    result = htbl->lookup[look] & ((1 << HUFF_LOOKAHEAD) - 1); \
   } else { \
-    nb = HUFF_LOOKAHEAD+1; \
 slowlabel: \
     if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
 	{ failaction; } \