X-Git-Url: https://git.sesse.net/?p=fjl;a=blobdiff_plain;f=dehuff.h;h=224005f84ad3dcb2797c9086a11485c8269c825c;hp=728c62315c5570c9b3fb6f1771368d219330a2a3;hb=8445e04bae016b6bd74705902972e66ae7fe37c1;hpb=0b02847989970a190c2cfaec4d1abaa1f616284a

diff --git a/dehuff.h b/dehuff.h
index 728c623..224005f 100644
--- a/dehuff.h
+++ b/dehuff.h
@@ -66,4 +66,22 @@ 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 << 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) */