Month and weekday arithmetic methods (#596)

+ nth_prev() method for Weekday + nth_prev() and nth_next() methods for Month + tests for methods listed above
time-rs · Jul 8, 2023 · 8414f33 · 8414f33
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diff --git a/tests/month.rs b/tests/month.rs
@@ -32,6 +32,74 @@ fn next() {
     assert_eq!(December.next(), January);
 }
 
+#[test]
+fn nth_next() {
+    assert_eq!(January.nth_next(0), January);
+    assert_eq!(January.nth_next(1), February);
+    assert_eq!(January.nth_next(2), March);
+    assert_eq!(January.nth_next(3), April);
+    assert_eq!(January.nth_next(4), May);
+    assert_eq!(January.nth_next(5), June);
+    assert_eq!(January.nth_next(6), July);
+    assert_eq!(January.nth_next(7), August);
+    assert_eq!(January.nth_next(8), September);
+    assert_eq!(January.nth_next(9), October);
+    assert_eq!(January.nth_next(10), November);
+    assert_eq!(January.nth_next(11), December);
+
+    assert_eq!(December.nth_next(0), December);
+    assert_eq!(December.nth_next(1), January);
+    assert_eq!(December.nth_next(2), February);
+    assert_eq!(December.nth_next(3), March);
+    assert_eq!(December.nth_next(4), April);
+    assert_eq!(December.nth_next(5), May);
+    assert_eq!(December.nth_next(6), June);
+    assert_eq!(December.nth_next(7), July);
+    assert_eq!(December.nth_next(8), August);
+    assert_eq!(December.nth_next(9), September);
+    assert_eq!(December.nth_next(10), October);
+    assert_eq!(December.nth_next(11), November);
+
+    assert_eq!(January.nth_next(12), January);
+    assert_eq!(January.nth_next(u8::MAX), April);
+    assert_eq!(December.nth_next(12), December);
+    assert_eq!(December.nth_next(u8::MAX), March);
+}
+
+#[test]
+fn nth_prev() {
+    assert_eq!(January.nth_prev(0), January);
+    assert_eq!(January.nth_prev(1), December);
+    assert_eq!(January.nth_prev(2), November);
+    assert_eq!(January.nth_prev(3), October);
+    assert_eq!(January.nth_prev(4), September);
+    assert_eq!(January.nth_prev(5), August);
+    assert_eq!(January.nth_prev(6), July);
+    assert_eq!(January.nth_prev(7), June);
+    assert_eq!(January.nth_prev(8), May);
+    assert_eq!(January.nth_prev(9), April);
+    assert_eq!(January.nth_prev(10), March);
+    assert_eq!(January.nth_prev(11), February);
+
+    assert_eq!(December.nth_prev(0), December);
+    assert_eq!(December.nth_prev(1), November);
+    assert_eq!(December.nth_prev(2), October);
+    assert_eq!(December.nth_prev(3), September);
+    assert_eq!(December.nth_prev(4), August);
+    assert_eq!(December.nth_prev(5), July);
+    assert_eq!(December.nth_prev(6), June);
+    assert_eq!(December.nth_prev(7), May);
+    assert_eq!(December.nth_prev(8), April);
+    assert_eq!(December.nth_prev(9), March);
+    assert_eq!(December.nth_prev(10), February);
+    assert_eq!(December.nth_prev(11), January);
+
+    assert_eq!(January.nth_prev(12), January);
+    assert_eq!(January.nth_prev(u8::MAX), October);
+    assert_eq!(December.nth_prev(12), December);
+    assert_eq!(December.nth_prev(u8::MAX), September);
+}
+
 #[test]
 fn display() {
     assert_eq!(January.to_string(), "January");

diff --git a/tests/weekday.rs b/tests/weekday.rs
@@ -46,6 +46,30 @@ fn nth_next() {
     assert_eq!(Monday.nth_next(u8::MAX), Thursday);
 }
 
+#[test]
+fn nth_prev() {
+    assert_eq!(Sunday.nth_prev(0), Sunday);
+    assert_eq!(Sunday.nth_prev(1), Saturday);
+    assert_eq!(Sunday.nth_prev(2), Friday);
+    assert_eq!(Sunday.nth_prev(3), Thursday);
+    assert_eq!(Sunday.nth_prev(4), Wednesday);
+    assert_eq!(Sunday.nth_prev(5), Tuesday);
+    assert_eq!(Sunday.nth_prev(6), Monday);
+
+    assert_eq!(Monday.nth_prev(0), Monday);
+    assert_eq!(Monday.nth_prev(1), Sunday);
+    assert_eq!(Monday.nth_prev(2), Saturday);
+    assert_eq!(Monday.nth_prev(3), Friday);
+    assert_eq!(Monday.nth_prev(4), Thursday);
+    assert_eq!(Monday.nth_prev(5), Wednesday);
+    assert_eq!(Monday.nth_prev(6), Tuesday);
+
+    assert_eq!(Sunday.nth_prev(7), Sunday);
+    assert_eq!(Sunday.nth_prev(u8::MAX), Thursday);
+    assert_eq!(Monday.nth_prev(7), Monday);
+    assert_eq!(Monday.nth_prev(u8::MAX), Friday);
+}
+
 #[test]
 fn number_from_monday() {
     assert_eq!(Monday.number_from_monday(), 1);

diff --git a/time/src/month.rs b/time/src/month.rs
@@ -97,6 +97,60 @@ impl Month {
             December => January,
         }
     }
+
+    /// Get n-th next month.
+    ///
+    /// ```rust
+    /// # use time::Month;
+    /// assert_eq!(Month::January.nth_next(4), Month::May);
+    /// assert_eq!(Month::July.nth_next(9), Month::April);
+    /// ```
+    pub const fn nth_next(self, n: u8) -> Self {
+        match (self as u8 - 1 + n % 12) % 12 {
+            0 => January,
+            1 => February,
+            2 => March,
+            3 => April,
+            4 => May,
+            5 => June,
+            6 => July,
+            7 => August,
+            8 => September,
+            9 => October,
+            10 => November,
+            val => {
+                debug_assert!(val == 11);
+                December
+            }
+        }
+    }
+
+    /// Get n-th previous month.
+    ///
+    /// ```rust
+    /// # use time::Month;
+    /// assert_eq!(Month::January.nth_prev(4), Month::September);
+    /// assert_eq!(Month::July.nth_prev(9), Month::October);
+    /// ```
+    pub const fn nth_prev(self, n: u8) -> Self {
+        match self as i8 - 1 - (n % 12) as i8 {
+            1 | -11 => February,
+            2 | -10 => March,
+            3 | -9 => April,
+            4 | -8 => May,
+            5 | -7 => June,
+            6 | -6 => July,
+            7 | -5 => August,
+            8 | -4 => September,
+            9 | -3 => October,
+            10 | -2 => November,
+            11 | -1 => December,
+            val => {
+                debug_assert!(val == 0);
+                January
+            }
+        }
+    }
 }
 
 impl fmt::Display for Month {

diff --git a/time/src/weekday.rs b/time/src/weekday.rs
@@ -88,6 +88,28 @@ impl Weekday {
         }
     }
 
+    /// Get n-th previous day.
+    ///
+    /// ```rust
+    /// # use time::Weekday;
+    /// assert_eq!(Weekday::Monday.nth_prev(1), Weekday::Sunday);
+    /// assert_eq!(Weekday::Sunday.nth_prev(10), Weekday::Thursday);
+    /// ```
+    pub const fn nth_prev(self, n: u8) -> Self {
+        match self.number_days_from_monday() as i8 - (n % 7) as i8 {
+            1 | -6 => Tuesday,
+            2 | -5 => Wednesday,
+            3 | -4 => Thursday,
+            4 | -3 => Friday,
+            5 | -2 => Saturday,
+            6 | -1 => Sunday,
+            val => {
+                debug_assert!(val == 0);
+                Monday
+            }
+        }
+    }
+
     /// Get the one-indexed number of days from Monday.
     ///
     /// ```rust