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+/*
+ * minor utilities for subnet-mask manipulation
+ * Copyright (C) 1998, 1999  Henry Spencer.
+ * 
+ * This library is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Library General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/lgpl.txt>.
+ * 
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
+ * License for more details.
+ *
+ * RCSID $Id: goodmask.c,v 1.1 2004/03/15 20:35:26 as Exp $
+ */
+#include "internal.h"
+#include "freeswan.h"
+
+/*
+ - goodmask - is this a good (^1*0*$) subnet mask?
+ * You are not expected to understand this.  See Henry S. Warren Jr, 
+ * "Functions realizable with word-parallel logical and two's-complement
+ * addition instructions", CACM 20.6 (June 1977), p.439.
+ */
+int				/* predicate */
+goodmask(mask)
+struct in_addr mask;
+{
+	unsigned long x = ntohl(mask.s_addr);
+	/* clear rightmost contiguous string of 1-bits */
+#	define	CRCS1B(x)	(((x|(x-1))+1)&x)
+#	define	TOPBIT		(1UL << 31)
+
+	/* either zero, or has one string of 1-bits which is left-justified */
+	if (x == 0 || (CRCS1B(x) == 0 && (x&TOPBIT)))
+		return 1;
+	return 0;
+}
+
+/*
+ - masktobits - how many bits in this mask?
+ * The algorithm is essentially a binary search, but highly optimized
+ * for this particular task.
+ */
+int				/* -1 means !goodmask() */
+masktobits(mask)
+struct in_addr mask;
+{
+	unsigned long m = ntohl(mask.s_addr);
+	int masklen;
+
+	if (!goodmask(mask))
+		return -1;
+
+	if (m&0x00000001UL)
+		return 32;
+	masklen = 0;
+	if (m&(0x0000ffffUL<<1)) {	/* <<1 for 1-origin numbering */
+		masklen |= 0x10;
+		m <<= 16;
+	}
+	if (m&(0x00ff0000UL<<1)) {
+		masklen |= 0x08;
+		m <<= 8;
+	}
+	if (m&(0x0f000000UL<<1)) {
+		masklen |= 0x04;
+		m <<= 4;
+	}
+	if (m&(0x30000000UL<<1)) {
+		masklen |= 0x02;
+		m <<= 2;
+	}
+	if (m&(0x40000000UL<<1))
+		masklen |= 0x01;
+
+	return masklen;
+}
+
+/*
+ - bitstomask - return a mask with this many high bits on
+ */
+struct in_addr
+bitstomask(n)
+int n;
+{
+	struct in_addr result;
+
+	if (n > 0 && n <= ABITS)
+		result.s_addr = htonl(~((1UL << (ABITS - n)) - 1));
+	else if (n == 0)
+		result.s_addr = 0;
+	else
+		result.s_addr = 0;	/* best error report we can do */
+	return result;
+}