1 | /* date.c |
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2 | * Routines for date handling |
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3 | * Copyright (C) 2010,2015 Olly Betts |
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4 | * |
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5 | * This program is free software; you can redistribute it and/or modify |
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6 | * it under the terms of the GNU General Public License as published by |
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7 | * the Free Software Foundation; either version 2 of the License, or |
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8 | * (at your option) any later version. |
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9 | * |
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10 | * This program is distributed in the hope that it will be useful, |
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11 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
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12 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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13 | * GNU General Public License for more details. |
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14 | * |
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15 | * You should have received a copy of the GNU General Public License |
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16 | * along with this program; if not, write to the Free Software |
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17 | * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA |
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18 | */ |
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19 | |
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20 | #ifdef HAVE_CONFIG_H |
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21 | # include <config.h> |
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22 | #endif |
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23 | |
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24 | #include "date.h" |
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25 | |
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26 | #include "debug.h" |
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27 | |
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28 | int |
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29 | is_leap_year(int year) |
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30 | { |
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31 | return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)); |
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32 | } |
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33 | |
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34 | static int lastday[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; |
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35 | |
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36 | int |
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37 | last_day(int year, int month) |
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38 | { |
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39 | SVX_ASSERT(month >= 1 && month <= 12); |
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40 | return (month == 2 && is_leap_year(year)) ? 29 : lastday[month - 1]; |
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41 | } |
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42 | |
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43 | int |
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44 | days_since_1900(int y, int m, int d) |
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45 | { |
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46 | static const int m_to_d[12] = { |
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47 | 0 - (1900 * 365) - 461 + 365, |
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48 | 31 - (1900 * 365) - 461 + 365, |
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49 | 59 - (1900 * 365) - 461, |
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50 | 90 - (1900 * 365) - 461, |
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51 | 120 - (1900 * 365) - 461, |
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52 | 151 - (1900 * 365) - 461, |
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53 | 181 - (1900 * 365) - 461, |
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54 | 212 - (1900 * 365) - 461, |
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55 | 243 - (1900 * 365) - 461, |
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56 | 273 - (1900 * 365) - 461, |
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57 | 304 - (1900 * 365) - 461, |
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58 | 334 - (1900 * 365) - 461 |
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59 | }; |
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60 | if (m < 3) |
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61 | --y; |
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62 | return d + (y * 365) + m_to_d[m - 1] + (y / 4) - (y / 100) + (y / 400); |
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63 | } |
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64 | |
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65 | void |
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66 | ymd_from_days_since_1900(int days, int * py, int * pm, int * pd) |
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67 | { |
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68 | int g, dg, c, dc, b, db, a, da, y, m; |
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69 | days += 693901; |
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70 | g = days / 146097; |
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71 | dg = days % 146097; |
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72 | c = (dg / 36524 + 1) * 3 / 4; |
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73 | dc = dg - c * 36524; |
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74 | b = dc / 1461; |
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75 | db = dc % 1461; |
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76 | a = (db / 365 + 1) * 3 / 4; |
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77 | da = db - a * 365; |
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78 | y = g * 400 + c * 100 + b * 4 + a; |
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79 | m = (da * 5 + 308) / 153; |
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80 | *py = y + m / 12; |
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81 | *pm = m % 12 + 1; |
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82 | *pd = da - (m + 2) * 153 / 5 + 123; |
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83 | } |
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84 | |
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85 | double |
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86 | julian_date_from_days_since_1900(int days) |
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87 | { |
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88 | int g, dg, c, dc, b, db, a, da, y, m; |
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89 | int days_in = days; |
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90 | int dys; |
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91 | double scale; |
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92 | days += 693901; |
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93 | g = days / 146097; |
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94 | dg = days % 146097; |
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95 | c = (dg / 36524 + 1) * 3 / 4; |
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96 | dc = dg - c * 36524; |
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97 | b = dc / 1461; |
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98 | db = dc % 1461; |
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99 | a = (db / 365 + 1) * 3 / 4; |
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100 | da = db - a * 365; |
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101 | y = g * 400 + c * 100 + b * 4 + a; |
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102 | m = (da * 5 + 308) / 153; |
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103 | y = y + m / 12; |
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104 | /* dys is days since 1900 for the start of the year y. */ |
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105 | dys = (y - 1900) * 365 - 460; |
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106 | dys += ((y - 1) / 4) - ((y - 1) / 100) + ((y - 1) / 400); |
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107 | scale = (is_leap_year(y) ? 1 / 366.0 : 1 / 365.0); |
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108 | return y + scale * (days_in - dys); |
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109 | } |
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