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source:
trunk/third/gmp/mpz/tstbit.c
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18191

Revision 18191, 2.4 KB checked in by ghudson, 22 years ago (diff) |
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1 | /* mpz_tstbit -- test a specified bit. |

2 | |

3 | Copyright 2000, 2002 Free Software Foundation, Inc. |

4 | |

5 | This file is part of the GNU MP Library. |

6 | |

7 | The GNU MP Library is free software; you can redistribute it and/or modify |

8 | it under the terms of the GNU Lesser General Public License as published by |

9 | the Free Software Foundation; either version 2.1 of the License, or (at your |

10 | option) any later version. |

11 | |

12 | The GNU MP Library is distributed in the hope that it will be useful, but |

13 | WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY |

14 | or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public |

15 | License for more details. |

16 | |

17 | You should have received a copy of the GNU Lesser General Public License |

18 | along with the GNU MP Library; see the file COPYING.LIB. If not, write to |

19 | the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |

20 | MA 02111-1307, USA. */ |

21 | |

22 | #include "gmp.h" |

23 | #include "gmp-impl.h" |

24 | |

25 | |

26 | /* For negatives the effective twos complement is achieved by negating the |

27 | limb tested, either with a ones or twos complement. Twos complement |

28 | ("-") is used if there's only zero limbs below the one being tested. |

29 | Ones complement ("~") is used if there's a non-zero below. Note that "-" |

30 | is correct even if the limb examined is 0 (and the true beginning of twos |

31 | complement is further up). |

32 | |

33 | Testing the limbs below p is unavoidable on negatives, but will usually |

34 | need to examine only *(p-1). The search is done from *(p-1) down to |

35 | *u_ptr, since that might give better cache locality, and because a |

36 | non-zero limb is perhaps a touch more likely in the middle of a number |

37 | than at the low end. |

38 | |

39 | Bits past the end of available data simply follow sign of u. Notice that |

40 | the limb_index >= abs_size test covers u=0 too. */ |

41 | |

42 | int |

43 | mpz_tstbit (mpz_srcptr u, unsigned long bit_index) |

44 | { |

45 | mp_srcptr u_ptr = PTR(u); |

46 | mp_size_t size = SIZ(u); |

47 | unsigned abs_size = ABS(size); |

48 | unsigned long limb_index = bit_index / GMP_NUMB_BITS; |

49 | mp_srcptr p = u_ptr + limb_index; |

50 | mp_limb_t limb; |

51 | |

52 | if (limb_index >= abs_size) |

53 | return (size < 0); |

54 | |

55 | limb = *p; |

56 | if (size < 0) |

57 | { |

58 | limb = -limb; /* twos complement */ |

59 | |

60 | while (p != u_ptr) |

61 | { |

62 | p--; |

63 | if (*p != 0) |

64 | { |

65 | limb--; /* make it a ones complement instead */ |

66 | break; |

67 | } |

68 | } |

69 | } |

70 | |

71 | return (limb >> (bit_index % GMP_NUMB_BITS)) & 1; |

72 | } |

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