Weekday support

src/weekday.rs

@@ -0,0 +1,132 @@
+use std::cmp::max;
+
+use ParseResult;
+use ParseError;
+
+#[derive(Debug, PartialEq)]
+pub enum DayOfWeek {
+    Sunday,
+    Monday,
+    Tuesday,
+    Wednesday,
+    Thursday,
+    Friday,
+    Saturday
+}
+
+impl DayOfWeek {
+
+    pub fn to_numeral(&self) -> u32 {
+        match self {
+            DayOfWeek::Sunday => 0,
+            DayOfWeek::Monday => 1,
+            DayOfWeek::Tuesday => 2,
+            DayOfWeek::Wednesday => 3,
+            DayOfWeek::Thursday => 4,
+            DayOfWeek::Friday => 5,
+            DayOfWeek::Saturday => 6,
+        }
+    }
+
+    pub fn from_numeral(num: u32) -> DayOfWeek {
+        match num % 7 {
+            0 => DayOfWeek::Sunday,
+            1 => DayOfWeek::Monday,
+            2 => DayOfWeek::Tuesday,
+            3 => DayOfWeek::Wednesday,
+            4 => DayOfWeek::Thursday,
+            5 => DayOfWeek::Friday,
+            6 => DayOfWeek::Saturday,
+            _ => panic!("Unreachable.")
+        }
+    }
+
+    /// Given the current day of the week, how many days until the next day?
+    pub fn difference(&self, other: DayOfWeek) -> u32 {
+        // Have to use i32 because of wraparound issues
+        let s_num = self.to_numeral() as i32;
+        let o_num = other.to_numeral() as i32;
+
+        if o_num - s_num >= 0 {
+            (o_num - s_num) as u32
+        } else {
+            (7 + o_num - s_num) as u32
+        }
+    }
+}
+
+pub fn day_of_week(year: u32, month: u32, day: u32) -> ParseResult<DayOfWeek> {
+    // From https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Schwerdtfeger's_method
+    let (c, g) = match month {
+        3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 => {
+            let c = year / 100;
+            (c, year - 100 * c)
+        },
+        1 | 2 => {
+            let c = (year - 1) / 100;
+            (c, year - 1 - 100 * c)
+        },
+        _ => return Err(ParseError::InvalidMonth)
+    };
+
+    let e = match month {
+        1 | 5 => 0,
+        2 | 6 => 3,
+        3 | 11 => 2,
+        4 | 7 => 5,
+        8 => 1,
+        9 | 12 => 4,
+        10 => 6,
+        _ => panic!("Unreachable.")
+    };
+
+    // This implementation is Gregorian-only.
+    let f = match c % 4 {
+        0 => 0,
+        1 => 5,
+        2 => 3,
+        3 => 1,
+        _ => panic!("Unreachable.")
+    };
+
+    match (day + e + f + g + g / 4) % 7 {
+        0 => Ok(DayOfWeek::Sunday),
+        1 => Ok(DayOfWeek::Monday),
+        2 => Ok(DayOfWeek::Tuesday),
+        3 => Ok(DayOfWeek::Wednesday),
+        4 => Ok(DayOfWeek::Thursday),
+        5 => Ok(DayOfWeek::Friday),
+        6 => Ok(DayOfWeek::Saturday),
+        _ => panic!("Unreachable.")
+    }
+}
+
+mod test {
+
+    use weekday::day_of_week;
+    use weekday::DayOfWeek;
+
+    #[test]
+    fn day_of_week_examples() {
+        assert_eq!(day_of_week(2018, 6, 24).unwrap(), DayOfWeek::Sunday);
+        assert_eq!(day_of_week(2003, 9, 25).unwrap(), DayOfWeek::Thursday);
+    }
+
+    #[test]
+    fn weekday_difference() {
+
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Sunday), 0);
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Monday), 1);
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Tuesday), 2);
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Wednesday), 3);
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Thursday), 4);
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Friday), 5);
+        assert_eq!(DayOfWeek::Sunday.difference(DayOfWeek::Saturday), 6);
+        assert_eq!(DayOfWeek::Monday.difference(DayOfWeek::Sunday), 6);
+        assert_eq!(DayOfWeek::Tuesday.difference(DayOfWeek::Sunday), 5);
+        assert_eq!(DayOfWeek::Wednesday.difference(DayOfWeek::Sunday), 4);
+        assert_eq!(DayOfWeek::Thursday.difference(DayOfWeek::Sunday), 3);
+        assert_eq!(DayOfWeek::Friday.difference(DayOfWeek::Sunday), 2);
+        assert_eq!(DayOfWeek::Saturday.difference(DayOfWeek::Sunday), 1);
+    }
+}