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trunk/third/gmp/mpz/cfdiv_r_2exp.c
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1 | /* mpz_cdiv_r_2exp, mpz_fdiv_r_2exp -- remainder from mpz divided by 2^n. |

2 | |

3 | Copyright 2001, 2002, 2004 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 | /* Bit mask of "n" least significant bits of a limb. */ |

27 | #define LOW_MASK(n) ((CNST_LIMB(1) << (n)) - 1) |

28 | |

29 | |

30 | /* dir==1 for ceil, dir==-1 for floor */ |

31 | |

32 | static void __gmpz_cfdiv_r_2exp _PROTO ((REGPARM_3_1 (mpz_ptr w, mpz_srcptr u, unsigned long cnt, int dir))) REGPARM_ATTR (1); |

33 | #define cfdiv_r_2exp(w,u,cnt,dir) __gmpz_cfdiv_r_2exp (REGPARM_3_1 (w, u, cnt, dir)) |

34 | |

35 | REGPARM_ATTR (1) static void |

36 | cfdiv_r_2exp (mpz_ptr w, mpz_srcptr u, unsigned long cnt, int dir) |

37 | { |

38 | mp_size_t usize, abs_usize, limb_cnt, i; |

39 | mp_srcptr up; |

40 | mp_ptr wp; |

41 | mp_limb_t high; |

42 | |

43 | usize = SIZ(u); |

44 | if (usize == 0) |

45 | { |

46 | SIZ(w) = 0; |

47 | return; |

48 | } |

49 | |

50 | limb_cnt = cnt / GMP_NUMB_BITS; |

51 | cnt %= GMP_NUMB_BITS; |

52 | abs_usize = ABS (usize); |

53 | |

54 | /* MPZ_REALLOC(w) below is only when w!=u, so we can fetch PTR(u) here |

55 | nice and early */ |

56 | up = PTR(u); |

57 | |

58 | if ((usize ^ dir) < 0) |

59 | { |

60 | /* Round towards zero, means just truncate */ |

61 | |

62 | if (w == u) |

63 | { |

64 | /* if already smaller than limb_cnt then do nothing */ |

65 | if (abs_usize <= limb_cnt) |

66 | return; |

67 | wp = PTR(w); |

68 | } |

69 | else |

70 | { |

71 | i = MIN (abs_usize, limb_cnt+1); |

72 | MPZ_REALLOC (w, i); |

73 | wp = PTR(w); |

74 | MPN_COPY (wp, up, i); |

75 | |

76 | /* if smaller than limb_cnt then only the copy is needed */ |

77 | if (abs_usize <= limb_cnt) |

78 | { |

79 | SIZ(w) = usize; |

80 | return; |

81 | } |

82 | } |

83 | } |

84 | else |

85 | { |

86 | /* Round away from zero, means twos complement if non-zero */ |

87 | |

88 | /* if u!=0 and smaller than divisor, then must negate */ |

89 | if (abs_usize <= limb_cnt) |

90 | goto negate; |

91 | |

92 | /* if non-zero low limb, then must negate */ |

93 | for (i = 0; i < limb_cnt; i++) |

94 | if (up[i] != 0) |

95 | goto negate; |

96 | |

97 | /* if non-zero partial limb, then must negate */ |

98 | if ((up[limb_cnt] & LOW_MASK (cnt)) != 0) |

99 | goto negate; |

100 | |

101 | /* otherwise low bits of u are zero, so that's the result */ |

102 | SIZ(w) = 0; |

103 | return; |

104 | |

105 | negate: |

106 | /* twos complement negation to get 2**cnt-u */ |

107 | |

108 | MPZ_REALLOC (w, limb_cnt+1); |

109 | up = PTR(u); |

110 | wp = PTR(w); |

111 | |

112 | /* Ones complement */ |

113 | i = MIN (abs_usize, limb_cnt+1); |

114 | mpn_com_n (wp, up, i); |

115 | for ( ; i <= limb_cnt; i++) |

116 | wp[i] = GMP_NUMB_MAX; |

117 | |

118 | /* Twos complement. Since u!=0 in the relevant part, the twos |

119 | complement never gives 0 and a carry, so can use MPN_INCR_U. */ |

120 | MPN_INCR_U (wp, limb_cnt+1, CNST_LIMB(1)); |

121 | |

122 | usize = -usize; |

123 | } |

124 | |

125 | /* Mask the high limb */ |

126 | high = wp[limb_cnt]; |

127 | high &= LOW_MASK (cnt); |

128 | wp[limb_cnt] = high; |

129 | |

130 | /* Strip any consequent high zeros */ |

131 | while (high == 0) |

132 | { |

133 | limb_cnt--; |

134 | if (limb_cnt < 0) |

135 | { |

136 | SIZ(w) = 0; |

137 | return; |

138 | } |

139 | high = wp[limb_cnt]; |

140 | } |

141 | |

142 | limb_cnt++; |

143 | SIZ(w) = (usize >= 0 ? limb_cnt : -limb_cnt); |

144 | } |

145 | |

146 | |

147 | void |

148 | mpz_cdiv_r_2exp (mpz_ptr w, mpz_srcptr u, unsigned long cnt) |

149 | { |

150 | cfdiv_r_2exp (w, u, cnt, 1); |

151 | } |

152 | |

153 | void |

154 | mpz_fdiv_r_2exp (mpz_ptr w, mpz_srcptr u, unsigned long cnt) |

155 | { |

156 | cfdiv_r_2exp (w, u, cnt, -1); |

157 | } |

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