solved 19

src/bin/problem_19.rs

@@ -0,0 +1,81 @@
+use std::cmp::PartialEq;
+
+fn main() {
+    let mut sum_sundays = 0;
+    let mut first_day = Day::Monday;
+    for month in 0..12 {
+        let last_day = get_last_day(1900, month, first_day);
+        first_day = last_day.next();
+    }
+    for year in 1901..2001 {
+        for month in 0..12 {
+            let last_day = get_last_day(year, month, first_day);
+            first_day = last_day.next();
+            if first_day == Day::Sunday {
+                sum_sundays += 1;
+            }
+        }
+    }
+    println!("{sum_sundays}");
+}
+
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+enum Day {
+    Monday,
+    Tuesday,
+    Wednesday,
+    Thursday,
+    Friday,
+    Saturday,
+    Sunday,
+}
+
+impl Day {
+    fn next(self) -> Day {
+        match self {
+            Day::Monday => Day::Tuesday,
+            Day::Tuesday => Day::Wednesday,
+            Day::Wednesday => Day::Thursday,
+            Day::Thursday => Day::Friday,
+            Day::Friday => Day::Saturday,
+            Day::Saturday => Day::Sunday,
+            Day::Sunday => Day::Monday,
+        }
+    }
+
+    fn day_num_in_week(self) -> usize {
+        match self {
+            Day::Monday => 0,
+            Day::Tuesday => 1,
+            Day::Wednesday => 2,
+            Day::Thursday => 3,
+            Day::Friday => 4,
+            Day::Saturday => 5,
+            Day::Sunday => 6,
+        }
+    }
+
+    fn from_day_num(num: usize) -> Day {
+        match num {
+            0 => Day::Monday,
+            1 => Day::Tuesday,
+            2 => Day::Wednesday,
+            3 => Day::Thursday,
+            4 => Day::Friday,
+            5 => Day::Saturday,
+            6 => Day::Sunday,
+            _ => panic!("day_num cannot be higher than 6 (counting days in week from 0)"),
+        }
+    }
+}
+
+fn is_leap_year(year: usize) -> bool {
+    year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
+}
+
+fn get_last_day(year: usize, month: usize, first_day: Day) -> Day {
+    let days_per_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+    let day_num = first_day.day_num_in_week();
+    let days_in_month = if month == 1 && is_leap_year(year) { 29 } else { days_per_month[month] } + day_num;
+    Day::from_day_num((days_in_month - 1) % 7)
+}