From fac88739443b99d52c8b492e44c59dc3286b2923 Mon Sep 17 00:00:00 2001
From: "Guillaume W. Bres" <guillaume.bressaix@gmail.com>
Date: Fri, 2 Dec 2022 20:17:03 +0100
Subject: [PATCH] weekday: wrapping arithmetics

Signed-off-by: Guillaume W. Bres <guillaume.bressaix@gmail.com>
---
 src/weekday.rs   | 12 +++++++++---
 tests/weekday.rs | 23 ++++++++++++++++++-----
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/src/weekday.rs b/src/weekday.rs
index f7dbfc45..82fb7124 100644
--- a/src/weekday.rs
+++ b/src/weekday.rs
@@ -40,12 +40,12 @@ impl Default for Weekday {
 
 impl Weekday {
     /// Max: last weekday <=> `Sunday`
-    pub const MAX: Self = Self::Sunday;
+    pub const MAX: u8 = 7;
 }
 
 impl From<u8> for Weekday {
     fn from(u: u8) -> Self {
-        match u.rem_euclid(Self::MAX.into()) {
+        match u.rem_euclid(Self::MAX) {
             0 => Self::Monday,
             1 => Self::Tuesday,
             2 => Self::Wednesday,
@@ -57,6 +57,12 @@ impl From<u8> for Weekday {
     }
 }
 
+impl From<i8> for Weekday {
+    fn from(i: i8) -> Self {
+        Self::from((i.rem_euclid(Self::MAX as i8) +Self::MAX as i8) as u8)
+    }
+}
+
 impl From<Weekday> for u8 {
     fn from(week: Weekday) -> Self {
         match week {
@@ -112,7 +118,7 @@ impl Add<u8> for Weekday {
 impl Sub<u8> for Weekday {
     type Output = Self;
     fn sub(self, rhs: u8) -> Self {
-        Self::from(self as u8 - rhs)
+        Self::from(self as i8 - rhs as i8)
     }
 }
 
diff --git a/tests/weekday.rs b/tests/weekday.rs
index 432c3399..6a62f819 100644
--- a/tests/weekday.rs
+++ b/tests/weekday.rs
@@ -11,11 +11,24 @@ use std::f64::EPSILON;
 #[test]
 fn test_basic_ops() {
     assert_eq!(Weekday::default(), Weekday::Monday);
-    let weekday = Weekday::default();
-    for i in 1..24 {
-        // test (+) wrapping
-        let add = weekday + i;
+    
+    let monday = Weekday::default();
+    for i in 0..24 { // wrapping
+        let add = monday +i;
         let expected: Weekday = i.rem_euclid(Weekday::MAX.into()).into();
-        assert_eq!(add, expected);
+        assert_eq!(add, expected, "test failed, expecting {:?} got {:?} for {:02} conversion", expected, add, i);
     }
+    
+    assert_eq!(monday-1, Weekday::Sunday);
+    assert_eq!(monday-2, Weekday::Saturday);
+    assert_eq!(monday-3, Weekday::Friday);
+    assert_eq!(monday-4, Weekday::Thursday);
+    assert_eq!(monday-5, Weekday::Wednesday);
+    assert_eq!(monday-6, Weekday::Tuesday);
+    assert_eq!(monday-7, monday); 
+    assert_eq!(monday-8, Weekday::Sunday);
+    assert_eq!(monday-9, Weekday::Saturday);
+    assert_eq!(monday-13, Weekday::Tuesday); 
+    assert_eq!(monday-14, monday); 
+    assert_eq!(monday-15, Weekday::Sunday); 
 }