core/clock/date.c
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|---|---|---|---|
| 1 | /** | ||
| 2 | * @file date.c | ||
| 3 | * @brief Pure calendar math, host-testable off-device. | ||
| 4 | * | ||
| 5 | * @ingroup lib_core | ||
| 6 | */ | ||
| 7 | #include "clock/date.h" | ||
| 8 | |||
| 9 | /** | ||
| 10 | * @brief Whether a year is a leap year, by the usual rule. | ||
| 11 | * | ||
| 12 | * Every fourth year is one, except a century year, unless it also divides evenly by 400. | ||
| 13 | * | ||
| 14 | * @param year The full year (2026, not 126). | ||
| 15 | * @return True when the year has a leap day, false when it does not. | ||
| 16 | */ | ||
| 17 | 6 | static int is_leap(int year) | |
| 18 | { | ||
| 19 |
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6 | return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0; |
| 20 | } | ||
| 21 | |||
| 22 | /** | ||
| 23 | * @brief The weekday December 31st lands on for a year. | ||
| 24 | * | ||
| 25 | * @param year The full year (2026, not 126). | ||
| 26 | * @return The weekday, 0 Sunday to 6 Saturday. | ||
| 27 | */ | ||
| 28 | 15 | static int dec31_wday(int year) | |
| 29 | { | ||
| 30 | 15 | return (year + year / 4 - year / 100 + year / 400) % 7; | |
| 31 | } | ||
| 32 | |||
| 33 | 12 | int date_iso_weeks_in_year(int year) | |
| 34 | { | ||
| 35 | // 53 when the year ends on a Thursday or the year before it ended on a Wednesday | ||
| 36 |
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12 | return 52 + (dec31_wday(year) == 4 || dec31_wday(year - 1) == 3 ? 1 : 0); |
| 37 | } | ||
| 38 | |||
| 39 | /** | ||
| 40 | * @brief The week number worked out from the date alone, before the year boundary is taken | ||
| 41 | * into account. | ||
| 42 | * | ||
| 43 | * A number below 1 means the day belongs to last year's final week. A number above the year's | ||
| 44 | * own total means it already belongs to next year's week 1. The ISO weekday used here runs | ||
| 45 | * Monday as 1 through Sunday as 7. | ||
| 46 | * | ||
| 47 | * @param yday The day of the year counting January 1st as 0 (like tm_yday). | ||
| 48 | * @param wday The weekday, 0 Sunday to 6 Saturday. | ||
| 49 | * @return The raw week number, which may fall outside 1 to the year's own week count. | ||
| 50 | */ | ||
| 51 | 11 | static int raw_week(int yday, int wday) | |
| 52 | { | ||
| 53 |
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11 | int iso_wday = wday == 0 ? 7 : wday; |
| 54 | 11 | return (yday + 1 - iso_wday + 10) / 7; | |
| 55 | } | ||
| 56 | |||
| 57 | 7 | int date_iso_week(int year, int yday, int wday) | |
| 58 | { | ||
| 59 | 7 | int week = raw_week(yday, wday); | |
| 60 |
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7 | if (week < 1) |
| 61 | { | ||
| 62 | // the first days of January can still belong to last year's final week | ||
| 63 | 3 | return date_iso_weeks_in_year(year - 1); | |
| 64 | } | ||
| 65 |
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4 | if (week > date_iso_weeks_in_year(year)) |
| 66 | { | ||
| 67 | // the last days of December can already belong to next year's week 1 | ||
| 68 | 1 | return 1; | |
| 69 | } | ||
| 70 | |||
| 71 | 3 | return week; | |
| 72 | } | ||
| 73 | |||
| 74 | 4 | int date_iso_week_year(int year, int yday, int wday) | |
| 75 | { | ||
| 76 | 4 | int week = raw_week(yday, wday); | |
| 77 |
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4 | if (week < 1) |
| 78 | { | ||
| 79 | 2 | return year - 1; | |
| 80 | } | ||
| 81 |
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2 | if (week > date_iso_weeks_in_year(year)) |
| 82 | { | ||
| 83 | 1 | return year + 1; | |
| 84 | } | ||
| 85 | |||
| 86 | 1 | return year; | |
| 87 | } | ||
| 88 | |||
| 89 | 3 | int date_day_of_year(int yday) | |
| 90 | { | ||
| 91 | 3 | return yday + 1; | |
| 92 | } | ||
| 93 | |||
| 94 | 2 | int date_days_left_in_year(int year, int yday) | |
| 95 | { | ||
| 96 |
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2 | return (is_leap(year) ? 366 : 365) - date_day_of_year(yday); |
| 97 | } | ||
| 98 | |||
| 99 | 5 | int date_days_in_month(int year, int mon0) | |
| 100 | { | ||
| 101 | static const unsigned char days[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; | ||
| 102 |
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5 | if (mon0 < 0 || mon0 > 11) |
| 103 | { | ||
| 104 | 1 | return 0; | |
| 105 | } | ||
| 106 | |||
| 107 |
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4 | return days[mon0] + (mon0 == 1 && is_leap(year) ? 1 : 0); |
| 108 | } | ||
| 109 | |||
| 110 | 2 | int date_first_wday(int wday, int mday) | |
| 111 | { | ||
| 112 | // walk today's weekday back to the 1st of the month | ||
| 113 | 2 | return (wday - (mday - 1) % 7 + 7) % 7; | |
| 114 | } | ||
| 115 |