Add optional padding data at the end to the bit source (is that the right place?...

[fjl] / dehuff.h

diff --git a/dehuff.h b/dehuff.h
index b79a79ace3fe976b2f38d8bf375b980b90ccafe7..a6a431a8139440a7052ce1dadd39b182ee5566b4 100644 (file)
--- 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 +42,45 @@ 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(const struct huffman_table* table,
+                                           struct bit_source* source)
+{
+       possibly_refill(source, DEHUF_TABLE_BITS);
+       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;
+}
+
+// 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, int bits)
+{
+       if (val < bit_thresholds[bits]) {
+               return val + (-1 << bits) + 1;
+       } else {
+               return val;
+       }
+}
 
 #endif /* !defined(_DEHUFF_H) */