🐛 fix a bug where we didn't handle recurrence exceptions, causing duplicate events whenever a recurring event was moved

src/main.rs

@@ -4,7 +4,9 @@ use chrono::{DateTime, TimeZone as _, Utc};
 use clap::Parser as _;
 use icalendar::{Component as _, EventLike as _};
 use serde::Deserialize;
-use std::{io::Write as _, path::PathBuf, str::FromStr as _, time::Duration};
+use std::{
+    collections::BTreeSet, io::Write as _, path::PathBuf, str::FromStr as _, time::Duration,
+};
 
 #[cfg(test)]
 mod tests;
@@ -294,6 +296,13 @@ struct ICal {
     cal: icalendar::Calendar,
 }
 
+/// Used to link recurrence exceptions to the original events they replace
+#[derive(Eq, Ord, PartialOrd, PartialEq)]
+struct RecurrenceKey<'a> {
+    recurrence_id: DatePerhapsTime,
+    uid: &'a str,
+}
+
 impl ICal {
     fn read_from_str(s: &str) -> Result<Self> {
         let cal = s.parse().map_err(|s| anyhow!("parse error {s}"))?;
@@ -316,8 +325,11 @@ impl ICal {
         })
     }
 
+    /// Returns an unsorted list of event instances for this calendar
     fn event_instances(&self, params: &Parameters) -> Result<Vec<EventInstance<'_>>> {
         let mut instances = vec![];
+        let mut recurrence_exceptions = BTreeSet::new();
+
         for ev in self.events() {
             let eis = match event_instances(params, ev)
                 .with_context(|| format!("Failed to process event with UID '{:?}'", ev.get_uid()))
@@ -327,8 +339,7 @@ impl ICal {
                     if ev.get_last_modified().context("Event has no timestamp")?
                         < params.ignore_before
                     {
-                        // FIXME: Use tracing
+                        tracing::warn!("Ignoring error from very old event {e:?}");
-                        eprintln!("Ignoring error from very old event {e:?}");
                         continue;
                     } else {
                         Err(e)?
@@ -338,8 +349,42 @@ impl ICal {
             for ei in eis {
                 instances.push(ei);
             }
+
+            if let Some(recurrence_id) = ev.get_recurrence_id() {
+                // This is a recurrence exception and we must handle it specially by later deleting the original event it replaces
+                let recurrence_id = normalize_date_perhaps_time(&recurrence_id, params.tz)
+                    .context("We should be able to normalize recurrence IDs")?;
+                let uid = ev
+                    .get_uid()
+                    .context("Every recurrence exception should have a UID")?;
+
+                recurrence_exceptions.insert(RecurrenceKey { recurrence_id, uid });
+            }
         }
 
+        // Find all recurring events that are replaced with recurrence exceptions and delete the originals.
+        // There is probably a not-linear-time way to do this, but this should be fine.
+
+        instances.retain(|ev| {
+            if ev.ev.get_recurrence_id().is_some() {
+                // This is a recurrence exception, exceptions never delete themselves
+                return true;
+            }
+
+            let uid = ev
+                .ev
+                .get_uid()
+                .context(
+                    "Every recurring event should have a UID so we can apply recurrence exceptions",
+                )
+                .unwrap();
+            let key = RecurrenceKey {
+                recurrence_id: ev.dtstart,
+                uid,
+            };
+            !recurrence_exceptions.contains(&key)
+        });
+
         Ok(instances)
     }
 }

src/tests.rs

@@ -147,28 +147,34 @@ END:VCALENDAR
         tz: chrono_tz::America::Chicago,
     };
     let instances = ical.event_instances(&params)?;
+
+    assert_eq!(
+        [
+            instances[0].dtstart,
+            instances[1].dtstart,
+            instances[2].dtstart,
+        ],
+        [
+            DatePerhapsTime {
+                dt: chicago_time(2025, 7, 8, 18, 0, 0),
+                all_day: false,
+            },
+            DatePerhapsTime {
+                dt: chicago_time(2025, 9, 9, 18, 0, 0),
+                all_day: false,
+            },
+            DatePerhapsTime {
+                dt: chicago_time(2025, 8, 14, 18, 0, 0),
+                all_day: false,
+            },
+        ]
+    );
+
+    for instance in &instances {
+        assert_eq!(instance.ev.get_summary(), Some("coil perm brush zippy"));
+    }
+
     assert_eq!(instances.len(), 3);
 
-    let expected_time = DatePerhapsTime {
-        dt: chicago_time(2025, 7, 8, 18, 0, 0),
-        all_day: false,
-    };
-    assert_eq!(instances[0].dtstart, expected_time);
-    assert_eq!(instances[0].ev.get_summary(), Some("coil perm brush zippy"));
-
-    let expected_time = DatePerhapsTime {
-        dt: chicago_time(2025, 8, 14, 18, 0, 0),
-        all_day: false,
-    };
-    assert_eq!(instances[1].dtstart, expected_time);
-    assert_eq!(instances[1].ev.get_summary(), Some("coil perm brush zippy"));
-
-    let expected_time = DatePerhapsTime {
-        dt: chicago_time(2025, 9, 9, 18, 0, 0),
-        all_day: false,
-    };
-    assert_eq!(instances[2].dtstart, expected_time);
-    assert_eq!(instances[2].ev.get_summary(), Some("coil perm brush zippy"));
-
     Ok(())
 }