Test Local functions against date command (#682)

src/offset/local/mod.rs

@@ -137,7 +137,126 @@ impl TimeZone for Local {
 mod tests {
     use super::Local;
     use crate::offset::TimeZone;
-    use crate::Datelike;
+    use crate::{Datelike, Duration, NaiveDate, NaiveDateTime, Timelike};
+
+    use std::{path, process};
+
+    #[cfg(unix)]
+    fn verify_against_date_command_local(path: &'static str, dt: NaiveDateTime) {
+        let output = process::Command::new(path)
+            .arg("-d")
+            .arg(format!("{}-{:02}-{:02} {:02}:05:01", dt.year(), dt.month(), dt.day(), dt.hour()))
+            .arg("+%Y-%m-%d %H:%M:%S %:z")
+            .output()
+            .unwrap();
+
+        let date_command_str = String::from_utf8(output.stdout).unwrap();
+
+        // The below would be preferred. At this stage neither earliest() or latest()
+        // seems to be consistent with the output of the `date` command, so we simply
+        // compare both.
+        // let local = Local
+        //     .from_local_datetime(&NaiveDate::from_ymd(year, month, day).and_hms(hour, 5, 1))
+        //     // looks like the "date" command always returns a given time when it is ambiguous
+        //     .earliest();
+
+        // if let Some(local) = local {
+        //     assert_eq!(format!("{}\n", local), date_command_str);
+        // } else {
+        //     // we are in a "Spring forward gap" due to DST, and so date also returns ""
+        //     assert_eq!("", date_command_str);
+        // }
+
+        // This is used while a decision is made wheter the `date` output needs to
+        // be exactly matched, or whether LocalResult::Ambigious should be handled
+        // differently
+        match Local.from_local_datetime(
+            &NaiveDate::from_ymd(dt.year(), dt.month(), dt.day()).and_hms(dt.hour(), 5, 1),
+        ) {
+            crate::LocalResult::Ambiguous(a, b) => {
+                assert!(
+                    format!("{}\n", a) == date_command_str
+                        || format!("{}\n", b) == date_command_str
+                )
+            }
+            crate::LocalResult::Single(a) => {
+                assert_eq!(format!("{}\n", a), date_command_str);
+            }
+            crate::LocalResult::None => {
+                assert_eq!("", date_command_str);
+            }
+        }
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn try_verify_against_date_command() {
+        let date_path = "/usr/bin/date";
+
+        if !path::Path::new(date_path).exists() {
+            // date command not found, skipping
+            // avoid running this on macOS, which has path /bin/date
+            // as the required CLI arguments are not present in the
+            // macOS build.
+            return;
+        }
+
+        let mut date = NaiveDate::from_ymd(1975, 1, 1).and_hms(0, 0, 0);
+
+        while date.year() < 2078 {
+            if (1975..=1977).contains(&date.year())
+                || (2020..=2022).contains(&date.year())
+                || (2073..=2077).contains(&date.year())
+            {
+                verify_against_date_command_local(date_path, date);
+            }
+
+            date += crate::Duration::hours(1);
+        }
+    }
+
+    #[test]
+    fn verify_correct_offsets() {
+        let now = Local::now();
+        let from_local = Local.from_local_datetime(&now.naive_local()).unwrap();
+        let from_utc = Local.from_utc_datetime(&now.naive_utc());
+
+        assert_eq!(now.offset().local_minus_utc(), from_local.offset().local_minus_utc());
+        assert_eq!(now.offset().local_minus_utc(), from_utc.offset().local_minus_utc());
+
+        assert_eq!(now, from_local);
+        assert_eq!(now, from_utc);
+    }
+
+    #[test]
+    fn verify_correct_offsets_distant_past() {
+        // let distant_past = Local::now() - Duration::days(365 * 100);
+        let distant_past = Local::now() - Duration::days(250 * 31);
+        let from_local = Local.from_local_datetime(&distant_past.naive_local()).unwrap();
+        let from_utc = Local.from_utc_datetime(&distant_past.naive_utc());
+
+        assert_eq!(distant_past.offset().local_minus_utc(), from_local.offset().local_minus_utc());
+        assert_eq!(distant_past.offset().local_minus_utc(), from_utc.offset().local_minus_utc());
+
+        assert_eq!(distant_past, from_local);
+        assert_eq!(distant_past, from_utc);
+    }
+
+    #[test]
+    fn verify_correct_offsets_distant_future() {
+        let distant_future = Local::now() + Duration::days(250 * 31);
+        let from_local = Local.from_local_datetime(&distant_future.naive_local()).unwrap();
+        let from_utc = Local.from_utc_datetime(&distant_future.naive_utc());
+
+        assert_eq!(
+            distant_future.offset().local_minus_utc(),
+            from_local.offset().local_minus_utc()
+        );
+        assert_eq!(distant_future.offset().local_minus_utc(), from_utc.offset().local_minus_utc());
+
+        assert_eq!(distant_future, from_local);
+        assert_eq!(distant_future, from_utc);
+    }
 
     #[test]
     fn test_local_date_sanity_check() {