[Golang] Counting Sundays - Problem 19 - Project Euler

Problem: [1]

You are given the following information, but you may prefer to do some research for yourself.

1 Jan 1900 was a Monday.
Thirty days has September,
April, June and November.
All the rest have thirty-one,
Saving February alone,
Which has twenty-eight, rain or shine.
And on leap years, twenty-nine.
A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

Solution:

171

package main

import (
      "fmt"
)

func monthDays(year, month int) int {
      switch month {
      case 1, 3, 5, 7, 8, 10, 12:
              return 31
      case 4, 6, 9, 11:
              return 30
      case 2:
              if year%400 == 0 {
                      return 29
              }
              if year%100 == 0 {
                      return 28
              }
              if year%4 == 0 {
                      return 29
              }
              return 28
      }
      panic("not correct month")
}

func main() {
      year := 1900
      month := 1
      day := 1
      weekDay := 1 // 1: Monday ... 7: Sunday

      count := 0
      for {
              day += 1
              if day > monthDays(year, month) {
                      day = 1
                      month += 1
              }
              if month > 12 {
                      month = 1
                      year += 1
              }

              weekDay += 1
              if weekDay > 7 {
                      weekDay = 1
              }

              // first day of the month is Sunday
              if day == 1 && weekDay == 7 && year > 1900 {
                      //fmt.Printf("%d %d 1st is Sunday\n", year, month)
                      count += 1
              }

              // termination condition
              if year == 2000 && month == 12 && day == 31 {
                      break
              }
      }

      fmt.Printf("total %d Sunday", count)
}

Test on:

Go Playground

References:

[1]	Counting Sundays - Problem 19 - Project Euler