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* refactor: add and use `parseDate` * style: use proper spelling Co-authored-by: appgurueu <34514239+appgurueu@users.noreply.github.com> --------- Co-authored-by: appgurueu <34514239+appgurueu@users.noreply.github.com>
75 lines
2.4 KiB
JavaScript
75 lines
2.4 KiB
JavaScript
/*
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DateToDay Method
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----------------
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The DateToDay method takes a date in string format and
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returns the name of a day. The approach behind this method
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is very simple, we first take a string date and check
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whether their date is valid or not, if the date is valid
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then we do this But apply the algorithm shown below. The
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algorithm shown below gives us the number of the day and
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finally converts it to the name of the day.
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Algorithm & Explanation : https://en.wikipedia.org/wiki/Zeller%27s_congruence
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*/
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import { parseDate } from '../Timing-Functions/ParseDate'
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// Array holding name of the day: Saturday - Sunday - Friday => 0 - 1 - 6
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const daysNameArr = [
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'Saturday',
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'Sunday',
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'Monday',
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'Tuesday',
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'Wednesday',
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'Thursday',
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'Friday'
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]
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const DateToDay = (date) => {
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// firstly, check that input is a string or not.
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const dateStruct = parseDate(date)
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let year = dateStruct.year
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let month = dateStruct.month
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let day = dateStruct.day
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// In case of Jan and Feb:
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// Year: we consider it as previous year
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// e.g., 1/1/1987 here year is 1986 (-1)
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// Month: we consider value as 13 & 14 respectively
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if (month < 3) {
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year--
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month += 12
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}
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// divide year into century and the last two digits of the century
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const yearDigits = year % 100
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const century = Math.floor(year / 100)
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/*
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In mathematics, remainders of divisions are usually defined to always be positive;
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As an example, -2 mod 7 = 5.
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Many programming languages including JavaScript implement the remainder of `n % m` as `sign(n) * (abs(n) % m)`.
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This means the result has the same sign as the numerator. Here, `-2 % 7 = -1 * (2 % 7) = -2`.
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To ensure a positive numerator, the formula is adapted: `- 2 * century` is replaced with `+ 5 * century`
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which does not alter the resulting numbers mod 7 since `7 - 2 = 5`
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The following example shows the issue with modulo division:
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Without the adaption, the formula yields `weekDay = -6` for the date 2/3/2014;
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With the adaption, it yields the positive result `weekDay = 7 - 6 = 1` (Sunday), which is what we need to index the array
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*/
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const weekDay =
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(day +
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Math.floor((month + 1) * 2.6) +
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yearDigits +
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Math.floor(yearDigits / 4) +
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Math.floor(century / 4) +
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5 * century) %
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7
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return daysNameArr[weekDay] // name of the weekday
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}
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// Example : DateToDay("18/12/2020") => Friday
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export { DateToDay }
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