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@andredumas andredumas/weekday.js forked from mbostock/weekday.js
Created May 19, 2014

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weekday = (function() {
// Returns the weekday number for the given date relative to January 1, 1970.
function weekday(date) {
var weekdays = weekdayOfYear(date),
year = date.getFullYear();
while (--year >= 1970) weekdays += weekdaysInYear(year);
return weekdays;
}
// Returns the date for the specified weekday number relative to January 1, 1970.
weekday.invert = function(weekdays) {
var year = 1970,
yearWeekdays;
// Compute the year.
while ((yearWeekdays = weekdaysInYear(year)) <= weekdays) {
++year;
weekdays -= yearWeekdays;
}
// Compute the date from the remaining weekdays.
var days = weekdays % 5,
day0 = ((new Date(year, 0, 1)).getDay() + 6) % 7;
if (day0 + days > 4) days += 2;
return new Date(year, 0, (weekdays / 5 | 0) * 7 + days + 1);
};
// Returns the number of weekdays in the specified year.
function weekdaysInYear(year) {
return weekdayOfYear(new Date(year, 11, 31)) + 1;
}
// Returns the weekday number for the given date relative to the start of the year.
function weekdayOfYear(date) {
var days = d3.time.dayOfYear(date),
weeks = days / 7 | 0,
day0 = (d3.time.year(date).getDay() + 6) % 7,
day1 = day0 + days - weeks * 7;
return Math.max(0, days - weeks * 2
- (day0 <= 5 && day1 >= 5 || day0 <= 12 && day1 >= 12) // extra saturday
- (day0 <= 6 && day1 >= 6 || day0 <= 13 && day1 >= 13)); // extra sunday
}
return weekday;
})();
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