X-Git-Url: https://git.sesse.net/?p=fjl;a=blobdiff_plain;f=dehuff.h;h=f29b815631752e213d6a9867b728e33c9438c60d;hp=728c62315c5570c9b3fb6f1771368d219330a2a3;hb=0932a33d5fa817a07778f85c05681ed77131bac6;hpb=0b02847989970a190c2cfaec4d1abaa1f616284a

diff --git a/dehuff.h b/dehuff.h
index 728c623..f29b815 100644
--- a/dehuff.h
+++ b/dehuff.h
@@ -48,12 +48,11 @@ void read_huffman_tables(huffman_tables_t* dst, input_func_t* input_func, void*
 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)
+static inline unsigned read_huffman_symbol_no_refill(
+	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);
+	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];
@@ -61,9 +60,34 @@ static inline unsigned read_huffman_symbol(const struct huffman_table* table,
 	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 (val < bit_thresholds[bits]) {
+		return val + (-1 << bits) + 1;
+	} else {
+		return val;
+	}
+}
+
 #endif /* !defined(_DEHUFF_H) */