Week Of Year

This algorithm finds the current week number in a year according to ISO-8601 standard.

```									```public static int GetWeekOfYear(DateTime d)
{
DateTime weekOne;
int year = d.Year;

if (d >= new DateTime(year, 12, 29))
{
weekOne = GetWeekOne(year + 1);

if (d < weekOne)
weekOne = GetWeekOne(year);
else
year++;
}
else
{
weekOne = GetWeekOne(year);

if (d < weekOne)
weekOne = GetWeekOne(--year);
}

return (d - weekOne).Days / 7 + 1;
}

static DateTime GetWeekOne(int year)
{
DateTime Jan4 = new DateTime(year, 1, 4);
int ISODay = GetISODayOfWeek(Jan4);
return Jan4.AddDays(1 - ISODay);
}

static int GetISODayOfWeek(DateTime d)
{
switch (GetDayOfWeek((uint)d.Year, (byte)d.Month, (byte)d.Day))
{
case DayOfWeek.Monday:
return 1;
case DayOfWeek.Tuesday:
return 2;
case DayOfWeek.Wednesday:
return 3;
case DayOfWeek.Thursday:
return 4;
case DayOfWeek.Friday:
return 5;
case DayOfWeek.Saturday:
return 6;
case DayOfWeek.Sunday:
return 7;
default:
return -1;
}
}

public static DayOfWeek GetDayOfWeek(uint year, byte month, byte day)
{
byte[] monthTable = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };

year -= (uint)((month < 3) ? 1 : 0);

return (DayOfWeek)((year + year / 4 - year / 100 + year / 400 + monthTable[month - 1] + day) % 7);
}

public enum DayOfWeek
{
Sunday,
Monday,
Tuesday,
Wednesday,
Thursday,
Friday,
Saturday
}```
```

Example

```									`int weekNumber = GetWeekOfYear(new DateTime(2015, 12, 5));`
```

Output

```									`49`
```