Add an implementation of the EXTEND function.

diff --git a/dehuff.h b/dehuff.h
index 728c62315c5570c9b3fb6f1771368d219330a2a3..7618bfd6b5e5e8a31f6907ffd5089400d2e1bb56 100644 (file)
--- a/dehuff.h
+++ b/dehuff.h
@@ -66,4 +66,23 @@ static inline unsigned read_huffman_symbol(const struct huffman_table* table,
        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 << 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, 1 << 15
+};
+
+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) */