Support weekdays in parse_datetime

This commit resolves issue uutils#23.

Adds parse_weekday function that uses chrono weekday parser with a map for edge cases.
Adds tests cases to make sure it works correctly.
Use nom for parsing.

Cargo.toml

@@ -10,3 +10,4 @@ readme = "README.md"
 [dependencies]
 regex = "1.9"
 chrono = { version="0.4", default-features=false, features=["std", "alloc", "clock"] }
+nom = "7.1.3"

src/lib.rs

@@ -15,7 +15,11 @@ use std::fmt::{self, Display};
 // Expose parse_datetime
 mod parse_relative_time;
 
-use chrono::{DateTime, FixedOffset, Local, LocalResult, NaiveDateTime, TimeZone};
+mod parse_weekday;
+
+use chrono::{
+    DateTime, Datelike, Duration, FixedOffset, Local, LocalResult, NaiveDateTime, TimeZone,
+};
 
 use parse_relative_time::parse_relative_time;
 
@@ -168,6 +172,24 @@ pub fn parse_datetime_at_date<S: AsRef<str> + Clone>(
         }
     }
 
+    // parse weekday
+    if let Ok(weekday) = parse_weekday::parse_weekday(s.as_ref()) {
+        let mut beginning_of_day = Local
+            .with_ymd_and_hms(date.year(), date.month(), date.day(), 0, 0, 0)
+            .unwrap();
+
+        loop {
+            if beginning_of_day.weekday() == weekday {
+                break;
+            }
+            beginning_of_day += Duration::days(1);
+        }
+
+        if let Ok(dt) = naive_dt_to_fixed_offset(date, beginning_of_day.naive_local()) {
+            return Ok(dt);
+        }
+    }
+
     // Parse epoch seconds
     if s.as_ref().bytes().next() == Some(b'@') {
         if let Ok(parsed) = NaiveDateTime::parse_from_str(&s.as_ref()[1..], "%s") {
@@ -353,6 +375,35 @@ mod tests {
         }
     }
 
+    #[cfg(test)]
+    mod weekday {
+        use crate::parse_datetime;
+        #[test]
+        fn test_weekday() {
+            let weekdays = vec![
+                "mon",
+                "monday",
+                "tue",
+                "tues",
+                "tuesday",
+                "wed",
+                "wednesday",
+                "thu",
+                "thursday",
+                "fri",
+                "friday",
+                "sat",
+                "saturday",
+                "sun",
+                "sunday",
+            ];
+
+            for weekday in weekdays {
+                assert_eq!(parse_datetime(weekday).is_ok(), true);
+            }
+        }
+    }
+
     #[cfg(test)]
     mod timestamp {
         use crate::parse_datetime;

src/parse_weekday.rs

@@ -0,0 +1,117 @@
+use chrono::Weekday;
+use nom::branch::alt;
+use nom::bytes::complete::tag_no_case;
+use nom::combinator::map;
+use nom::{self, Finish, IResult};
+
+pub(crate) fn parse_weekday(s: &str) -> Result<Weekday, &str> {
+    let s = s.trim();
+
+    let parse_result: IResult<&str, Weekday> = nom::combinator::all_consuming(alt((
+        map(alt((tag_no_case("monday"), tag_no_case("mon"))), |_| {
+            Weekday::Mon
+        }),
+        map(
+            alt((
+                tag_no_case("tuesday"),
+                tag_no_case("tues"),
+                tag_no_case("tue"),
+            )),
+            |_| Weekday::Tue,
+        ),
+        map(alt((tag_no_case("wednesday"), tag_no_case("wed"))), |_| {
+            Weekday::Wed
+        }),
+        map(
+            alt((
+                tag_no_case("thursday"),
+                tag_no_case("thurs"),
+                tag_no_case("thur"),
+                tag_no_case("thu"),
+            )),
+            |_| Weekday::Thu,
+        ),
+        map(alt((tag_no_case("friday"), tag_no_case("fri"))), |_| {
+            Weekday::Fri
+        }),
+        map(alt((tag_no_case("saturday"), tag_no_case("sat"))), |_| {
+            Weekday::Sat
+        }),
+        map(alt((tag_no_case("sunday"), tag_no_case("sun"))), |_| {
+            Weekday::Sun
+        }),
+    )))(s);
+
+    let parse_result = parse_result.finish();
+
+    match parse_result {
+        Ok((_, weekday)) => Ok(weekday),
+        Err(res) => Err(res.input),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use chrono::Weekday::*;
+
+    use crate::parse_weekday::parse_weekday;
+
+    #[test]
+    fn test_valid_weekdays() {
+        let days = [
+            ("mon", Mon),
+            ("monday", Mon),
+            ("tue", Tue),
+            ("tues", Tue),
+            ("tuesday", Tue),
+            ("wed", Wed),
+            ("wednesday", Wed),
+            ("thu", Thu),
+            ("thursday", Thu),
+            ("fri", Fri),
+            ("friday", Fri),
+            ("sat", Sat),
+            ("saturday", Sat),
+            ("sun", Sun),
+            ("sunday", Sun),
+        ];
+
+        for day in days.iter() {
+            let mut test_strings = vec![];
+            test_strings.push(String::from(day.0));
+            test_strings.push(format!(" {}", day.0));
+            test_strings.push(format!(" {} ", day.0));
+            test_strings.push(format!("{} ", day.0));
+            test_strings.push(format!(" {}", day.0.to_uppercase()));
+
+            let (left, right) = day.0.split_at(1);
+            test_strings.push(format!("{}{}", left.to_uppercase(), right.to_lowercase()));
+            test_strings.push(format!("{}{}", left.to_lowercase(), right.to_uppercase()));
+
+            for str in test_strings.iter() {
+                assert!(parse_weekday(str).is_ok());
+                assert_eq!(parse_weekday(str).unwrap(), day.1);
+            }
+        }
+    }
+
+    #[test]
+    fn test_invalid_weekdays() {
+        let days = [
+            "mond",
+            "tuesda",
+            "we",
+            "th",
+            "fr",
+            "sa",
+            "su",
+            "garbageday",
+            "tomorrow",
+            "yesterday",
+        ];
+        for day in days {
+            assert!(parse_weekday(day).is_err());
+        }
+    }
+}