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16676
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1 /* Base 2 logarithm. |
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17848
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2 Copyright (C) 2012-2015 Free Software Foundation, Inc. |
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16676
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3 |
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4 This program is free software: you can redistribute it and/or modify |
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5 it under the terms of the GNU General Public License as published by |
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6 the Free Software Foundation; either version 3 of the License, or |
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7 (at your option) any later version. |
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8 |
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9 This program is distributed in the hope that it will be useful, |
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10 but WITHOUT ANY WARRANTY; without even the implied warranty of |
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11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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12 GNU General Public License for more details. |
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13 |
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14 You should have received a copy of the GNU General Public License |
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15 along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
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16 |
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17 #include <config.h> |
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18 |
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19 /* Specification. */ |
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20 #include <math.h> |
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21 |
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22 /* Best possible approximation of log(2) as a 'double'. */ |
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23 #define LOG2 0.693147180559945309417232121458176568075 |
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24 |
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25 /* Best possible approximation of 1/log(2) as a 'double'. */ |
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26 #define LOG2_INVERSE 1.44269504088896340735992468100189213743 |
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27 |
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28 /* sqrt(0.5). */ |
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29 #define SQRT_HALF 0.707106781186547524400844362104849039284 |
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30 |
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31 double |
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32 log2 (double x) |
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33 { |
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34 if (isnand (x)) |
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35 return x; |
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36 |
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37 if (x <= 0.0) |
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38 { |
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39 if (x == 0.0) |
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40 /* Return -Infinity. */ |
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41 return - HUGE_VAL; |
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42 else |
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43 { |
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44 /* Return NaN. */ |
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45 #if defined _MSC_VER || (defined __sgi && !defined __GNUC__) |
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46 static double zero; |
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47 return zero / zero; |
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48 #else |
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49 return 0.0 / 0.0; |
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50 #endif |
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51 } |
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52 } |
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53 |
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54 /* Decompose x into |
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55 x = 2^e * y |
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56 where |
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57 e is an integer, |
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58 1/2 < y < 2. |
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59 Then log2(x) = e + log2(y) = e + log(y)/log(2). */ |
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60 { |
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61 int e; |
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62 double y; |
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63 |
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64 y = frexp (x, &e); |
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65 if (y < SQRT_HALF) |
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66 { |
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67 y = 2.0 * y; |
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68 e = e - 1; |
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69 } |
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70 |
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71 return (double) e + log (y) * LOG2_INVERSE; |
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72 } |
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73 } |
