From: Steinar H. Gunderson <sesse@debian.org>
Date: Sat, 3 Jan 2009 13:35:24 +0000 (+0100)
Subject: Add an implementation of the EXTEND function.
X-Git-Url: https://git.sesse.net/?p=fjl;a=commitdiff_plain;h=91f2e505234fbf8d8afbdcf1f62b3630da820262

Add an implementation of the EXTEND function.
---

diff --git a/dehuff.h b/dehuff.h
index 728c623..7618bfd 100644
--- 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) */