1 | /* origin: OpenBSD /usr/src/lib/libm/src/ld80/s_log1pl.c */
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2 | /*
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3 | * Copyright (c) 2008 Stephen L. Moshier <steve@moshier.net>
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4 | *
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5 | * Permission to use, copy, modify, and distribute this software for any
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6 | * purpose with or without fee is hereby granted, provided that the above
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7 | * copyright notice and this permission notice appear in all copies.
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8 | *
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9 | * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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10 | * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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11 | * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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12 | * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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13 | * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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14 | * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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15 | * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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16 | */
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17 | /*
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18 | * Relative error logarithm
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19 | * Natural logarithm of 1+x, long double precision
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20 | *
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21 | *
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22 | * SYNOPSIS:
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23 | *
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24 | * long double x, y, log1pl();
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25 | *
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26 | * y = log1pl( x );
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27 | *
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28 | *
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29 | * DESCRIPTION:
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30 | *
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31 | * Returns the base e (2.718...) logarithm of 1+x.
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32 | *
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33 | * The argument 1+x is separated into its exponent and fractional
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34 | * parts. If the exponent is between -1 and +1, the logarithm
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35 | * of the fraction is approximated by
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36 | *
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37 | * log(1+x) = x - 0.5 x^2 + x^3 P(x)/Q(x).
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38 | *
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39 | * Otherwise, setting z = 2(x-1)/x+1),
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40 | *
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41 | * log(x) = z + z^3 P(z)/Q(z).
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42 | *
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43 | *
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44 | * ACCURACY:
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45 | *
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46 | * Relative error:
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47 | * arithmetic domain # trials peak rms
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48 | * IEEE -1.0, 9.0 100000 8.2e-20 2.5e-20
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49 | */
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50 |
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51 | #include "libm.h"
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52 |
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53 | #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
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54 | long double log1pl(long double x)
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55 | {
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56 | return log1p(x);
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57 | }
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58 | #elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
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59 | /* Coefficients for log(1+x) = x - x^2 / 2 + x^3 P(x)/Q(x)
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60 | * 1/sqrt(2) <= x < sqrt(2)
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61 | * Theoretical peak relative error = 2.32e-20
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62 | */
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63 | static const long double P[] = {
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64 | 4.5270000862445199635215E-5L,
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65 | 4.9854102823193375972212E-1L,
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66 | 6.5787325942061044846969E0L,
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67 | 2.9911919328553073277375E1L,
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68 | 6.0949667980987787057556E1L,
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69 | 5.7112963590585538103336E1L,
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70 | 2.0039553499201281259648E1L,
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71 | };
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72 | static const long double Q[] = {
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73 | /* 1.0000000000000000000000E0,*/
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74 | 1.5062909083469192043167E1L,
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75 | 8.3047565967967209469434E1L,
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76 | 2.2176239823732856465394E2L,
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77 | 3.0909872225312059774938E2L,
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78 | 2.1642788614495947685003E2L,
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79 | 6.0118660497603843919306E1L,
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80 | };
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81 |
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82 | /* Coefficients for log(x) = z + z^3 P(z^2)/Q(z^2),
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83 | * where z = 2(x-1)/(x+1)
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84 | * 1/sqrt(2) <= x < sqrt(2)
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85 | * Theoretical peak relative error = 6.16e-22
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86 | */
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87 | static const long double R[4] = {
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88 | 1.9757429581415468984296E-3L,
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89 | -7.1990767473014147232598E-1L,
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90 | 1.0777257190312272158094E1L,
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91 | -3.5717684488096787370998E1L,
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92 | };
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93 | static const long double S[4] = {
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94 | /* 1.00000000000000000000E0L,*/
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95 | -2.6201045551331104417768E1L,
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96 | 1.9361891836232102174846E2L,
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97 | -4.2861221385716144629696E2L,
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98 | };
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99 | static const long double C1 = 6.9314575195312500000000E-1L;
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100 | static const long double C2 = 1.4286068203094172321215E-6L;
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101 |
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102 | #define SQRTH 0.70710678118654752440L
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103 |
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104 | long double log1pl(long double xm1)
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105 | {
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106 | long double x, y, z;
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107 | int e;
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108 |
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109 | if (isnan(xm1))
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110 | return xm1;
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111 | if (xm1 == INFINITY)
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112 | return xm1;
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113 | if (xm1 == 0.0)
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114 | return xm1;
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115 |
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116 | x = xm1 + 1.0;
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117 |
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118 | /* Test for domain errors. */
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119 | if (x <= 0.0) {
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120 | if (x == 0.0)
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121 | return -1/(x*x); /* -inf with divbyzero */
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122 | return 0/0.0f; /* nan with invalid */
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123 | }
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124 |
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125 | /* Separate mantissa from exponent.
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126 | Use frexp so that denormal numbers will be handled properly. */
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127 | x = frexpl(x, &e);
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128 |
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129 | /* logarithm using log(x) = z + z^3 P(z)/Q(z),
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130 | where z = 2(x-1)/x+1) */
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131 | if (e > 2 || e < -2) {
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132 | if (x < SQRTH) { /* 2(2x-1)/(2x+1) */
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133 | e -= 1;
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134 | z = x - 0.5;
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135 | y = 0.5 * z + 0.5;
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136 | } else { /* 2 (x-1)/(x+1) */
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137 | z = x - 0.5;
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138 | z -= 0.5;
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139 | y = 0.5 * x + 0.5;
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140 | }
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141 | x = z / y;
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142 | z = x*x;
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143 | z = x * (z * __polevll(z, R, 3) / __p1evll(z, S, 3));
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144 | z = z + e * C2;
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145 | z = z + x;
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146 | z = z + e * C1;
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147 | return z;
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148 | }
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149 |
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150 | /* logarithm using log(1+x) = x - .5x**2 + x**3 P(x)/Q(x) */
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151 | if (x < SQRTH) {
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152 | e -= 1;
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153 | if (e != 0)
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154 | x = 2.0 * x - 1.0;
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155 | else
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156 | x = xm1;
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157 | } else {
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158 | if (e != 0)
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159 | x = x - 1.0;
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160 | else
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161 | x = xm1;
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162 | }
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163 | z = x*x;
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164 | y = x * (z * __polevll(x, P, 6) / __p1evll(x, Q, 6));
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165 | y = y + e * C2;
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166 | z = y - 0.5 * z;
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167 | z = z + x;
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168 | z = z + e * C1;
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169 | return z;
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170 | }
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171 | #elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
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172 | // TODO: broken implementation to make things compile
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173 | long double log1pl(long double x)
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174 | {
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175 | return log1p(x);
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176 | }
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177 | #endif
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