/*

DateToDay Method

----------------

The DateToDay method takes a date in string format and

returns the name of a day. The approach behind this method

is very simple, we first take a string date and check

whether their date is valid or not, if the date is valid

then we do this But apply the algorithm shown below. The

algorithm shown below gives us the number of the day and

finally converts it to the name of the day.

Algorithm & Explanation : https://cs.uwaterloo.ca/~alopez-o/math-faq/node73.html

*/

// March is taken as the first month of the year.

const calcMonthList = {

1: 11,

2: 12,

3: 1,

4: 2,

5: 3,

6: 4,

7: 5,

8: 6,

9: 7,

10: 8,

11: 9,

12: 10

}

// show the week day in a number : Sunday - Saturday => 0 - 6

const daysNameList = { // weeks-day

0: 'Sunday',

1: 'Monday',

2: 'Tuesday',

3: 'Wednesday',

4: 'Thursday',

5: 'Friday',

6: 'Saturday'

}

// javascript n % m is the remainder operator which is different than modulus operator

// https://stackoverflow.com/questions/4467539/javascript-modulo-gives-a-negative-result-for-negative-numbers

const mod = (n, m) => {

return ((n % m) + m) % m

}

const DateToDay = (date) => {

// firstly, check that input is a string or not.

if (typeof date !== 'string') {

return new TypeError('Argument is not a string.')

}

// extract the date

let [day, month, year] = date.split('/').map((x) => Number(x))

// check the data are valid or not.

if (day < 0 || day > 31 || month > 12 || month < 0) {

return new TypeError('Date is not valid.')

}

// adjust year since Jan & Feb are considered from previous year

if (month < 3) year--

// divide year to century and yearDigit value.

const century = Math.floor(year / 100)

const yearDigit = (year - century * 100)

// Apply the algorithm shown above

const weekDay = mod(day + Math.floor((2.6 * calcMonthList[month]) - 0.2) - (2 * century) + yearDigit + Math.floor(yearDigit / 4) + Math.floor(century / 4), 7)

// return the weekDay name.

return daysNameList[weekDay]

}

// Example : DateToDay("18/12/2020") => Friday

export { DateToDay }