fix: DateToDay (#1125)

Author: chetanppatil

Conversions/DateToDay.js

@@ -9,54 +9,52 @@
     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
+    Algorithm & Explanation : https://en.wikipedia.org/wiki/Zeller%27s_congruence
 */
 
-// 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'
-}
+// Array holding name of the day: Saturday - Sunday - Friday => 0 - 1 - 6
+const daysNameArr = ['Saturday', 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']
 
 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) {
+  if (day < 1 || day > 31 || month > 12 || month < 1) {
     return new TypeError('Date is not valid.')
   }
-  // divide year to century and yearDigit value.
-  const yearDigit = (year % 100)
+
+  // In case of Jan and Feb:
+  // Year: we consider it as previous year
+  // e.g., 1/1/1987 here year is 1986 (-1)
+  // Month: we consider value as 13 & 14 respectively
+  if (month < 3) {
+    year--
+    month += 12
+  }
+
+  // divide year into century and the last two digits of the century
+  const yearDigits = year % 100
   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]
+
+  /*
+  In mathematics, remainders of divisions are usually defined to always be positive;
+  As an example, -2 mod 7 = 5.
+  Many programming languages including JavaScript implement the remainder of `n % m` as `sign(n) * (abs(n) % m)`.
+  This means the result has the same sign as the numerator. Here, `-2 % 7 = -1 * (2 % 7) = -2`.
+
+  To ensure a positive numerator, the formula is adapted: `- 2 * century` is replaced with `+ 5 * century`
+  which does not alter the resulting numbers mod 7 since `7 - 2 = 5`
+
+  The following example shows the issue with modulo division:
+  Without the adaption, the formula yields `weekDay = -6` for the date 2/3/2014;
+  With the adaption, it yields the positive result `weekDay = 7 - 6 = 1` (Sunday), which is what we need to index the array
+  */
+  const weekDay = (day + Math.floor((month + 1) * 2.6) + yearDigits + Math.floor(yearDigits / 4) + Math.floor(century / 4) + 5 * century) % 7
+  return daysNameArr[weekDay] // name of the weekday
 }
 
 // Example : DateToDay("18/12/2020") => Friday

Conversions/test/DateToDay.test.js

@@ -1,8 +1,8 @@
 import { DateToDay } from '../DateToDay'
 
-test('The date 18/02/2001 is Monday', () => {
+test('The date 18/02/2001 is Sunday', () => {
   const res = DateToDay('18/02/2001')
-  expect(res).toBe('Monday')
+  expect(res).toBe('Sunday')
 })
 
 test('The date 18/12/2020 is Friday', () => {
@@ -14,7 +14,22 @@ test('The date 12/12/2012 is Wednesday', () => {
   const res = DateToDay('12/12/2012')
   expect(res).toBe('Wednesday')
 })
-test('The date 01/01/2001 is Friday', () => {
+test('The date 01/01/2001 is Monday', () => {
   const res = DateToDay('01/01/2001')
-  expect(res).toBe('Friday')
+  expect(res).toBe('Monday')
+})
+
+test('The date 1/1/2020 is Wednesday', () => {
+  const res = DateToDay('1/1/2020')
+  expect(res).toBe('Wednesday')
+})
+
+test('The date 2/3/2014 is Sunday', () => {
+  const res = DateToDay('2/3/2014')
+  expect(res).toBe('Sunday')
+})
+
+test('The date 28/2/2017 is Tuesday', () => {
+  const res = DateToDay('28/2/2017')
+  expect(res).toBe('Tuesday')
 })