From 4116776e1d78568febf98b183738ae82263f01eb Mon Sep 17 00:00:00 2001
From: Ruben Pollan
Date: Wed, 29 Sep 2021 20:38:30 +0200
Subject: Add support to BYDAY in MONTHLY frequency

---
 src/date.rs     | 20 ++++++++++++++++++++
 src/periodic.rs | 45 +++++++++++++++++++++++++++++++++++----------
 2 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/src/date.rs b/src/date.rs
index 71e29c7..c3cf816 100644
--- a/src/date.rs
+++ b/src/date.rs
@@ -79,6 +79,13 @@ impl Date {
         }
     }
 
+    pub fn with_day(&self, day: u32) -> Option<Date> {
+        Some(match *self {
+            Date::Time(t) => Date::Time(t.with_day(day)?),
+            Date::AllDay(d) => Date::AllDay(d.with_day(day)?),
+        })
+    }
+
     pub fn weekday(&self) -> Weekday {
         match *self {
             Date::Time(t) => t.weekday(),
@@ -100,6 +107,12 @@ impl Date {
         })
     }
 
+    pub fn week_of_month(&self) -> (i32, i32) {
+        let week = ((self.day() - 1) / 7 + 1) as i32;
+        let neg_week = ((self.days_in_month() - self.day()) / 7 + 1) as i32;
+        (week, -neg_week)
+    }
+
     pub fn year(&self) -> i32 {
         match *self {
             Date::Time(t) => t.year(),
@@ -113,6 +126,13 @@ impl Date {
             Date::AllDay(d) => Date::AllDay(d.with_year(year)?),
         })
     }
+
+    pub fn days_in_month(&self) -> u32 {
+        chrono::NaiveDate::from_ymd_opt(self.year(), self.month() + 1, 1)
+            .unwrap_or(chrono::NaiveDate::from_ymd(self.year() + 1, 1, 1))
+            .pred()
+            .day()
+    }
 }
 
 impl Ord for Date {
diff --git a/src/periodic.rs b/src/periodic.rs
index 19874e4..c0d0ad3 100644
--- a/src/periodic.rs
+++ b/src/periodic.rs
@@ -1,5 +1,6 @@
 use std::fmt;
 use std::str::FromStr;
+use std::ops::Add;
 use std::collections::HashMap;
 
 use chrono::{Duration, Weekday};
@@ -129,16 +130,40 @@ impl<'a> Iter<'a> {
                 }
             }
             Freq::Monthly => {
-                // TODO: byday...
-                let new_date = if date.month() == 12 {
-                    date.with_month(1)
-                        .unwrap()
-                        .with_year(date.year() + 1)
-                        .unwrap()
-                } else {
-                    date.with_month(date.month() + 1).unwrap()
-                };
-                new_date - date
+                match &p.byday {
+                    Some(byday) => {
+                        let mut next = date;
+                        if p.interval > 1 {
+                            for _ in 1..p.interval {
+                                next = next.add(Duration::days(next.days_in_month().into()));
+                            }
+                        }
+                        loop {
+                            next = next.add(Duration::days(1));
+                            let (week, neg_week) = next.week_of_month();
+
+                            match byday.get(&next.weekday()) {
+                                Some(occurrences) =>
+                                    if occurrences.contains(&0) || occurrences.contains(&week) ||occurrences.contains(&neg_week) {
+                                        break;
+                                    }
+                                None => {}
+                            };
+                        };
+                        next - date
+                    }
+                    None => {
+                        let new_date = if date.month() == 12 {
+                            date.with_month(1)
+                                .unwrap()
+                                .with_year(date.year() + 1)
+                                .unwrap()
+                        } else {
+                            date.with_month(date.month() + 1).unwrap()
+                        };
+                        new_date - date
+                    }
+                }
             }
             // TODO: byday...
             Freq::Yearly => date.with_year(date.year() + 1).unwrap() - date,
-- 
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