X-Git-Url: https://git.sesse.net/?p=fjl;a=blobdiff_plain;f=dehuff.h;h=7621f6095e99791feb6138e81c2574ae9dda6bce;hp=b79a79ace3fe976b2f38d8bf375b980b90ccafe7;hb=a4009687c73083dd0290285a065740a83e27e855;hpb=47de6c270a336574dce220cde780a802a513d113

diff --git a/dehuff.h b/dehuff.h
index b79a79a..7621f60 100644
--- a/dehuff.h
+++ b/dehuff.h
@@ -5,10 +5,14 @@
 #include <stdint.h>
 #include <sys/types.h>
 
-// A function to read bytes from some input source. The bytes should be
-// already unstuffed (and thus without markers).
-// A return value of -1 indicates error, a return value of 0 indicates EOF.
-typedef ssize_t (raw_input_func_t)(void*, uint8_t*, size_t);
+#include "bytesource.h"
+#include "bitsource.h"
+
+// About 99% of all Huffman codes are <= 8 bits long (see codelen.txt),
+// and it's what libjpeg uses. Thus, it seems like a reasonable size.
+#define DEHUF_TABLE_BITS 8
+#define DEHUF_TABLE_SIZE (1 << DEHUF_TABLE_BITS)
+static const int DEHUF_SLOW_PATH = -1;
 
 struct huffman_table {
 	unsigned num_codes[17];     // BITS
@@ -20,6 +24,15 @@ struct huffman_table {
 	int maxcode[16];
 	int mincode[16];
 	unsigned valptr[16];
+
+	// Lookup table for fast decoding; given eight bits,
+	// return the symbol and length in bits. For longer codes,
+	// DEHUF_SLOW_PATH is returned.
+
+	// Note that the codes we return are 8-bit, but the type of
+	// the lookup tables is int to avoid extra zero extending. 
+	int lookup_table_codes[DEHUF_TABLE_SIZE]; 
+	int lookup_table_length[DEHUF_TABLE_SIZE]; 
 };
 
 enum coefficient_class {
@@ -29,6 +42,64 @@ enum coefficient_class {
 };
 typedef struct huffman_table huffman_tables_t[NUM_COEFF_CLASSES][4];
 
-void read_huffman_tables(huffman_tables_t* dst, raw_input_func_t* input_func, void* userdata);
+// Read Huffman tables from a stream, and compute the derived values.
+void read_huffman_tables(huffman_tables_t* dst, input_func_t* input_func, void* userdata);
+
+unsigned read_huffman_symbol_slow_path(const struct huffman_table* table,
+                                       struct bit_source* source);
+
+static inline unsigned read_huffman_symbol_no_refill(
+	const struct huffman_table* table,
+	struct bit_source* source)
+{
+	assert(source->bits_available >= DEHUF_TABLE_BITS);
+	unsigned lookup = peek_bits(source, DEHUF_TABLE_BITS);
+	int code = table->lookup_table_codes[lookup];
+	int length = table->lookup_table_length[lookup];
+
+	if (code == DEHUF_SLOW_PATH) {
+		return read_huffman_symbol_slow_path(table, source);
+	}
+		
+	read_bits(source, length);
+	return code;
+}
+
+static inline unsigned read_huffman_symbol(const struct huffman_table* table,
+                                           struct bit_source* source)
+{
+	possibly_refill(source, DEHUF_TABLE_BITS);
+	return read_huffman_symbol_no_refill(table, source);
+}
+
+// procedure EXTEND (figure F.12)
+
+// Fast lookup table for (1 << (bits - 1)).
+// The table actually helps, since the load can go in parallel with the shift
+// operation below.
+static const int bit_thresholds[16] = {
+	0, 1 << 0, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14
+};
+
+static inline unsigned extend(int val, unsigned bits)
+{
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+	// GCC should ideally be able to figure out that the conditional move is better, but
+	// it doesn't for various reasons, and this is pretty important for speed, so we hardcode.
+	asm("cmp %2, %0 ; cmovl %3, %0"
+		: "=r" (val)
+		: "0" (val),
+		  "g" (bit_thresholds[bits]),
+ 		  "r" (val + (-1 << bits) + 1)
+		: "cc");
+	return val;
+#else
+	if (val < bit_thresholds[bits]) {
+		return val + (-1 << bits) + 1;
+	} else {
+		return val;
+	}
+#endif
+}
 
 #endif /* !defined(_DEHUFF_H) */