1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
|
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using static System.Globalization.GregorianCalendar;
namespace System.Globalization
{
public static class ISOWeek
{
private const int WeeksInLongYear = 53;
private const int WeeksInShortYear = 52;
private const int MinWeek = 1;
private const int MaxWeek = WeeksInLongYear;
public static int GetWeekOfYear(DateTime date)
{
int week = GetWeekNumber(date);
if (week < MinWeek)
{
// If the week number obtained equals 0, it means that the
// given date belongs to the preceding (week-based) year.
return GetWeeksInYear(date.Year - 1);
}
if (week > GetWeeksInYear(date.Year))
{
// If a week number of 53 is obtained, one must check that
// the date is not actually in week 1 of the following year.
return MinWeek;
}
return week;
}
public static int GetYear(DateTime date)
{
int week = GetWeekNumber(date);
if (week < MinWeek)
{
// If the week number obtained equals 0, it means that the
// given date belongs to the preceding (week-based) year.
return date.Year - 1;
}
if (week > GetWeeksInYear(date.Year))
{
// If a week number of 53 is obtained, one must check that
// the date is not actually in week 1 of the following year.
return date.Year + 1;
}
return date.Year;
}
// The year parameter represents an ISO week-numbering year (also called ISO year informally).
// Each week's year is the Gregorian year in which the Thursday falls.
// The first week of the year, hence, always contains 4 January.
// ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.
public static DateTime GetYearStart(int year)
{
return ToDateTime(year, MinWeek, DayOfWeek.Monday);
}
// The year parameter represents an ISO week-numbering year (also called ISO year informally).
// Each week's year is the Gregorian year in which the Thursday falls.
// The first week of the year, hence, always contains 4 January.
// ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.
public static DateTime GetYearEnd(int year)
{
return ToDateTime(year, GetWeeksInYear(year), DayOfWeek.Sunday);
}
// From https://en.wikipedia.org/wiki/ISO_week_date#Weeks_per_year:
//
// The long years, with 53 weeks in them, can be described by any of the following equivalent definitions:
//
// - Any year starting on Thursday and any leap year starting on Wednesday.
// - Any year ending on Thursday and any leap year ending on Friday.
// - Years in which 1 January and 31 December (in common years) or either (in leap years) are Thursdays.
//
// All other week-numbering years are short years and have 52 weeks.
public static int GetWeeksInYear(int year)
{
if (year < MinYear || year > MaxYear)
{
throw new ArgumentOutOfRangeException(nameof(year), SR.ArgumentOutOfRange_Year);
}
int P(int y) => (y + (y / 4) - (y / 100) + (y / 400)) % 7;
if (P(year) == 4 || P(year - 1) == 3)
{
return WeeksInLongYear;
}
return WeeksInShortYear;
}
// From https://en.wikipedia.org/wiki/ISO_week_date#Calculating_a_date_given_the_year,_week_number_and_weekday:
//
// This method requires that one know the weekday of 4 January of the year in question.
// Add 3 to the number of this weekday, giving a correction to be used for dates within this year.
//
// Multiply the week number by 7, then add the weekday. From this sum subtract the correction for the year.
// The result is the ordinal date, which can be converted into a calendar date.
//
// If the ordinal date thus obtained is zero or negative, the date belongs to the previous calendar year.
// If greater than the number of days in the year, to the following year.
public static DateTime ToDateTime(int year, int week, DayOfWeek dayOfWeek)
{
if (year < MinYear || year > MaxYear)
{
throw new ArgumentOutOfRangeException(nameof(year), SR.ArgumentOutOfRange_Year);
}
if (week < MinWeek || week > MaxWeek)
{
throw new ArgumentOutOfRangeException(nameof(week), SR.ArgumentOutOfRange_Week_ISO);
}
// We allow 7 for convenience in cases where a user already has a valid ISO
// day of week value for Sunday. This means that both 0 and 7 will map to Sunday.
// The GetWeekday method will normalize this into the 1-7 range required by ISO.
if ((int)dayOfWeek < 0 || (int)dayOfWeek > 7)
{
throw new ArgumentOutOfRangeException(nameof(dayOfWeek), SR.ArgumentOutOfRange_DayOfWeek);
}
var jan4 = new DateTime(year, month: 1, day: 4);
int correction = GetWeekday(jan4.DayOfWeek) + 3;
int ordinal = (week * 7) + GetWeekday(dayOfWeek) - correction;
return new DateTime(year, month: 1, day: 1).AddDays(ordinal - 1);
}
// From https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_of_a_given_date:
//
// Using ISO weekday numbers (running from 1 for Monday to 7 for Sunday),
// subtract the weekday from the ordinal date, then add 10. Divide the result by 7.
// Ignore the remainder; the quotient equals the week number.
//
// If the week number thus obtained equals 0, it means that the given date belongs to the preceding (week-based) year.
// If a week number of 53 is obtained, one must check that the date is not actually in week 1 of the following year.
private static int GetWeekNumber(DateTime date)
{
return (date.DayOfYear - GetWeekday(date.DayOfWeek) + 10) / 7;
}
// Day of week in ISO is represented by an integer from 1 through 7, beginning with Monday and ending with Sunday.
// This matches the underlying values of the DayOfWeek enum, except for Sunday, which needs to be converted.
private static int GetWeekday(DayOfWeek dayOfWeek)
{
return dayOfWeek == DayOfWeek.Sunday ? 7 : (int) dayOfWeek;
}
}
}
|
