1/* mpz_hamdist -- calculate hamming distance.
2
3Copyright 1994, 1996, 2001, 2002, 2009-2011 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it under the terms of either:
9
10  * the GNU Lesser General Public License as published by the Free
11    Software Foundation; either version 3 of the License, or (at your
12    option) any later version.
13
14or
15
16  * the GNU General Public License as published by the Free Software
17    Foundation; either version 2 of the License, or (at your option) any
18    later version.
19
20or both in parallel, as here.
21
22The GNU MP Library is distributed in the hope that it will be useful, but
23WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
24or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
25for more details.
26
27You should have received copies of the GNU General Public License and the
28GNU Lesser General Public License along with the GNU MP Library.  If not,
29see https://www.gnu.org/licenses/.  */
30
31#include "gmp-impl.h"
32
33
34mp_bitcnt_t
35mpz_hamdist (mpz_srcptr u, mpz_srcptr v) __GMP_NOTHROW
36{
37  mp_srcptr      up, vp;
38  mp_size_t      usize, vsize;
39  mp_bitcnt_t    count;
40
41  usize = SIZ(u);
42  vsize = SIZ(v);
43
44  up = PTR(u);
45  vp = PTR(v);
46
47  if (usize >= 0)
48    {
49      if (vsize < 0)
50	return ~ (mp_bitcnt_t) 0;
51
52      /* positive/positive */
53
54      if (usize < vsize)
55	MPN_SRCPTR_SWAP (up,usize, vp,vsize);
56
57      count = 0;
58      if (vsize != 0)
59	count = mpn_hamdist (up, vp, vsize);
60
61      usize -= vsize;
62      if (usize != 0)
63	count += mpn_popcount (up + vsize, usize);
64
65      return count;
66    }
67  else
68    {
69      mp_limb_t  ulimb, vlimb;
70      mp_size_t  old_vsize, step;
71
72      if (vsize >= 0)
73	return ~ (mp_bitcnt_t) 0;
74
75      /* negative/negative */
76
77      usize = -usize;
78      vsize = -vsize;
79
80      /* skip common low zeros */
81      for (;;)
82	{
83	  ASSERT (usize > 0);
84	  ASSERT (vsize > 0);
85
86	  usize--;
87	  vsize--;
88
89	  ulimb = *up++;
90	  vlimb = *vp++;
91
92	  if (ulimb != 0)
93	    break;
94
95	  if (vlimb != 0)
96	    {
97	      MPN_SRCPTR_SWAP (up,usize, vp,vsize);
98	      ulimb = vlimb;
99	      vlimb = 0;
100	      break;
101	    }
102	}
103
104      /* twos complement first non-zero limbs (ulimb is non-zero, but vlimb
105	 might be zero) */
106      ulimb = -ulimb;
107      vlimb = -vlimb;
108      popc_limb (count, (ulimb ^ vlimb) & GMP_NUMB_MASK);
109
110      if (vlimb == 0)
111	{
112	  mp_bitcnt_t  twoscount;
113
114	  /* first non-zero of v */
115	  old_vsize = vsize;
116	  do
117	    {
118	      ASSERT (vsize > 0);
119	      vsize--;
120	      vlimb = *vp++;
121	    }
122	  while (vlimb == 0);
123
124	  /* part of u corresponding to skipped v zeros */
125	  step = old_vsize - vsize - 1;
126	  count += step * GMP_NUMB_BITS;
127	  step = MIN (step, usize);
128	  if (step != 0)
129	    {
130	      count -= mpn_popcount (up, step);
131	      usize -= step;
132	      up += step;
133	    }
134
135	  /* First non-zero vlimb as twos complement, xor with ones
136	     complement ulimb.  Note -v^(~0^u) == (v-1)^u. */
137	  vlimb--;
138	  if (usize != 0)
139	    {
140	      usize--;
141	      vlimb ^= *up++;
142	    }
143	  popc_limb (twoscount, vlimb);
144	  count += twoscount;
145	}
146
147      /* Overlapping part of u and v, if any.  Ones complement both, so just
148	 plain hamdist. */
149      step = MIN (usize, vsize);
150      if (step != 0)
151	{
152	  count += mpn_hamdist (up, vp, step);
153	  usize -= step;
154	  vsize -= step;
155	  up += step;
156	  vp += step;
157	}
158
159      /* Remaining high part of u or v, if any, ones complement but xor
160	 against all ones in the other, so plain popcount. */
161      if (usize != 0)
162	{
163	remaining:
164	  count += mpn_popcount (up, usize);
165	}
166      else if (vsize != 0)
167	{
168	  up = vp;
169	  usize = vsize;
170	  goto remaining;
171	}
172      return count;
173    }
174}
175