COMMON: Use a prefix table to speed up the Huffman decoder

Symbols for codes shorter than the prefix table index width are stored
in the table. All the entries in the table with an index starting with
the code are set to the symbol value. That way, when decoding it is
possible to get the number of bits corresponding to the table width from
the bitstream and directly find the symbol value. Longer code still need
to be searched for in the codes list.

2 files changed, 15 insertions, 12 deletions

diff --git a/image/codecs/svq1.cpp b/image/codecs/svq1.cpp
index 2d86070801..eba5fb872a 100644
--- a/image/codecs/svq1.cpp
+++ b/image/codecs/svq1.cpp
@@ -56,16 +56,16 @@ SVQ1Decoder::SVQ1Decoder(uint16 width, uint16 height) {
 	_last[2] = 0;
 
 	// Setup Variable Length Code Tables
-	_blockType = new Common::Huffman(0, 4, s_svq1BlockTypeCodes, s_svq1BlockTypeLengths);
+	_blockType = new HuffmanDecoder(0, 4, s_svq1BlockTypeCodes, s_svq1BlockTypeLengths);
 
 	for (int i = 0; i < 6; i++) {
-		_intraMultistage[i] = new Common::Huffman(0, 8, s_svq1IntraMultistageCodes[i], s_svq1IntraMultistageLengths[i]);
-		_interMultistage[i] = new Common::Huffman(0, 8, s_svq1InterMultistageCodes[i], s_svq1InterMultistageLengths[i]);
+		_intraMultistage[i] = new HuffmanDecoder(0, 8, s_svq1IntraMultistageCodes[i], s_svq1IntraMultistageLengths[i]);
+		_interMultistage[i] = new HuffmanDecoder(0, 8, s_svq1InterMultistageCodes[i], s_svq1InterMultistageLengths[i]);
 	}
 
-	_intraMean = new Common::Huffman(0, 256, s_svq1IntraMeanCodes, s_svq1IntraMeanLengths);
-	_interMean = new Common::Huffman(0, 512, s_svq1InterMeanCodes, s_svq1InterMeanLengths);
-	_motionComponent = new Common::Huffman(0, 33, s_svq1MotionComponentCodes, s_svq1MotionComponentLengths);
+	_intraMean = new HuffmanDecoder(0, 256, s_svq1IntraMeanCodes, s_svq1IntraMeanLengths);
+	_interMean = new HuffmanDecoder(0, 512, s_svq1InterMeanCodes, s_svq1InterMeanLengths);
+	_motionComponent = new HuffmanDecoder(0, 33, s_svq1MotionComponentCodes, s_svq1MotionComponentLengths);
 }
 
 SVQ1Decoder::~SVQ1Decoder() {
diff --git a/image/codecs/svq1.h b/image/codecs/svq1.h
index 148501d17f..4a005761a5 100644
--- a/image/codecs/svq1.h
+++ b/image/codecs/svq1.h
@@ -27,6 +27,7 @@
 #include "image/codecs/codec.h"
 
 namespace Common {
+template <class BITSTREAM>
 class Huffman;
 struct Point;
 }
@@ -53,12 +54,14 @@ private:
 
 	byte *_last[3];
 
-	Common::Huffman *_blockType;
-	Common::Huffman *_intraMultistage[6];
-	Common::Huffman *_interMultistage[6];
-	Common::Huffman *_intraMean;
-	Common::Huffman *_interMean;
-	Common::Huffman *_motionComponent;
+	typedef Common::Huffman<Common::BitStream32BEMSB> HuffmanDecoder;
+
+	HuffmanDecoder *_blockType;
+	HuffmanDecoder *_intraMultistage[6];
+	HuffmanDecoder *_interMultistage[6];
+	HuffmanDecoder *_intraMean;
+	HuffmanDecoder *_interMean;
+	HuffmanDecoder *_motionComponent;
 
 	bool svq1DecodeBlockIntra(Common::BitStream32BEMSB *s, byte *pixels, int pitch);
 	bool svq1DecodeBlockNonIntra(Common::BitStream32BEMSB *s, byte *pixels, int pitch);