X-Git-Url: https://git.sesse.net/?p=fjl;a=blobdiff_plain;f=dehuff.h;h=d93e7931ee17f081e734e7ff5397dd2f973f200d;hp=b79a79ace3fe976b2f38d8bf375b980b90ccafe7;hb=618548d1f2e076a3da21368e708cf887dcbd20d2;hpb=17eeadbf1a76c2ad38869255191c3696be3cfb60

diff --git a/dehuff.h b/dehuff.h
index b79a79a..d93e793 100644
--- a/dehuff.h
+++ b/dehuff.h
@@ -5,6 +5,14 @@
 #include <stdint.h>
 #include <sys/types.h>
 
+#include "input.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;
+
 // 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.
@@ -20,6 +28,15 @@ 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 +46,30 @@ enum coefficient_class {
 };
 typedef struct huffman_table huffman_tables_t[NUM_COEFF_CLASSES][4];
 
+// Read Huffman tables from a stream, and compute the derived values.
 void read_huffman_tables(huffman_tables_t* dst, raw_input_func_t* input_func, void* userdata);
 
+unsigned read_huffman_symbol_slow_path(const struct huffman_table* table,
+                                       struct bit_source* source);
+
+#include <stdio.h>
+
+static inline unsigned read_huffman_symbol(const struct huffman_table* table,
+                                           struct bit_source* source)
+{
+	// FIXME: We can read past the end of the stream here in some edge
+	// cases. We need to define some guarantees in the layers above.
+	possibly_refill(source, 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;
+}
+
 #endif /* !defined(_DEHUFF_H) */