Support weekdays in parse_datetime

This commit resolves issue #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.

Author: philolo1

.gitignore

@@ -1 +1,2 @@
 target/
+fuzz/corpus

src/parse_weekday.rs

@@ -0,0 +1,80 @@
+use chrono::{ParseWeekdayError, Weekday};
+
+const WEEKDAY_CONVERSION_MAP: [(&str, &str); 3] =
+    [("tues", "tue"), ("thurs", "thu"), ("thus", "thu")];
+
+pub(crate) fn parse_weekday(s: &str) -> Result<Weekday, ParseWeekdayError> {
+    let mut s = s.trim().to_lowercase();
+    for (before, after) in WEEKDAY_CONVERSION_MAP {
+        if s == before {
+            s = String::from(after);
+        }
+    }
+
+    s.trim().parse::<Weekday>()
+}
+
+#[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());
+        }
+    }
+}