src/libfreeswan/libfreeswan/goodmask.c

   1 /*
   2  * minor utilities for subnet-mask manipulation
   3  * Copyright (C) 1998, 1999  Henry Spencer.
   4  *
   5  * This library is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU Library General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or (at your
   8  * option) any later version.  See <http://www.fsf.org/copyleft/lgpl.txt>.
   9  *
  10  * This library is distributed in the hope that it will be useful, but
  11  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  12  * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
  13  * License for more details.
  14  *
  15  * RCSID $Id: goodmask.c,v 1.1 2004/03/15 20:35:26 as Exp $
  16  */
  17 #include "internal.h"
  18 #include "freeswan.h"
  19
  20 /*
  21  - goodmask - is this a good (^1*0*$) subnet mask?
  22  * You are not expected to understand this.  See Henry S. Warren Jr,
  23  * "Functions realizable with word-parallel logical and two's-complement
  24  * addition instructions", CACM 20.6 (June 1977), p.439.
  25  */
  26 int                             /* predicate */
  27 goodmask(mask)
  28 struct in_addr mask;
  29 {
  30         unsigned long x = ntohl(mask.s_addr);
  31         /* clear rightmost contiguous string of 1-bits */
  32 #       define  CRCS1B(x)       (((x|(x-1))+1)&x)
  33 #       define  TOPBIT          (1UL << 31)
  34
  35         /* either zero, or has one string of 1-bits which is left-justified */
  36         if (x == 0 || (CRCS1B(x) == 0 && (x&TOPBIT)))
  37                 return 1;
  38         return 0;
  39 }
  40
  41 /*
  42  - masktobits - how many bits in this mask?
  43  * The algorithm is essentially a binary search, but highly optimized
  44  * for this particular task.
  45  */
  46 int                             /* -1 means !goodmask() */
  47 masktobits(mask)
  48 struct in_addr mask;
  49 {
  50         unsigned long m = ntohl(mask.s_addr);
  51         int masklen;
  52
  53         if (!goodmask(mask))
  54                 return -1;
  55
  56         if (m&0x00000001UL)
  57                 return 32;
  58         masklen = 0;
  59         if (m&(0x0000ffffUL<<1)) {      /* <<1 for 1-origin numbering */
  60                 masklen |= 0x10;
  61                 m <<= 16;
  62         }
  63         if (m&(0x00ff0000UL<<1)) {
  64                 masklen |= 0x08;
  65                 m <<= 8;
  66         }
  67         if (m&(0x0f000000UL<<1)) {
  68                 masklen |= 0x04;
  69                 m <<= 4;
  70         }
  71         if (m&(0x30000000UL<<1)) {
  72                 masklen |= 0x02;
  73                 m <<= 2;
  74         }
  75         if (m&(0x40000000UL<<1))
  76                 masklen |= 0x01;
  77
  78         return masklen;
  79 }
  80
  81 /*
  82  - bitstomask - return a mask with this many high bits on
  83  */
  84 struct in_addr
  85 bitstomask(n)
  86 int n;
  87 {
  88         struct in_addr result;
  89
  90         if (n > 0 && n <= ABITS)
  91                 result.s_addr = htonl(~((1UL << (ABITS - n)) - 1));
  92         else if (n == 0)
  93                 result.s_addr = 0;
  94         else
  95                 result.s_addr = 0;      /* best error report we can do */
  96         return result;
  97 }