Fix DateToDay

Conversions/DateToDay.js

@@ -4,59 +4,38 @@
     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
+    whether their date is valid or not, if the date is not valid
+    then we return a type error, else we apply Zeller's congruence.
+    Algorithm & Explanation : https://www.geeksforgeeks.org/zellers-congruence-find-day-date/
 */
 
-// 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'
-}
-
+// days in a week : Sunday - Saturday => 0 - 6
+const daysNameList = [
+  'Sunday',
+  'Monday',
+  'Tuesday',
+  'Wednesday',
+  'Thursday',
+  'Friday',
+  'Saturday'
+]
 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
-  const [day, month, year] = date.split('/').map((x) => Number(x))
+  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.')
   }
-  // divide year to century and yearDigit value.
-  const yearDigit = (year % 100)
+  // count Jan & Feb as months 13 & 14 of the previous year
+  if (month < 3) { year--; month += 12 }
   const century = Math.floor(year / 100)
-  // Apply the algorithm shown above
-  const weekDay = Math.abs((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]
+  year %= 100
+  const weekDay = (year + Math.floor(year / 4) + Math.floor(century / 4) - 2 * century + Math.floor((26 * (month + 1)) / 10) + day - 1)
+  return daysNameList[(weekDay + 7) % 7]
 }
 
 // Example : DateToDay("18/12/2020") => Friday

Conversions/test/DateToDay.test.js

@@ -2,7 +2,7 @@ import { DateToDay } from '../DateToDay'
 
 test('The date 18/02/2001 is Monday', () => {
   const res = DateToDay('18/02/2001')
-  expect(res).toBe('Monday')
+  expect(res).toBe('Sunday')
 })
 
 test('The date 18/12/2020 is Friday', () => {
@@ -16,5 +16,5 @@ test('The date 12/12/2012 is Wednesday', () => {
 })
 test('The date 01/01/2001 is Friday', () => {
   const res = DateToDay('01/01/2001')
-  expect(res).toBe('Friday')
+  expect(res).toBe('Monday')
 })