How Conflict Detection Actually Works (The Sweep-Line Algorithm)

The naive approach: compare everything to everything

The simplest way to find calendar conflicts is brute force. Take every event from every calendar, compare it to every other event, and check if they overlap. Two events conflict when one starts before the other ends, and vice versa.

For a small number of events, this works fine. Ten events means 45 comparisons. You will not notice the computation time. But calendars grow. A busy consultant with three active clients might have 40-60 events per week across all calendars. At 60 events, brute force means 1,770 comparisons. Still fast on modern hardware, but scale to a month view with 240+ events and you are looking at nearly 29,000 comparisons.

This is O(n^2) complexity. Every time you double the number of events, you quadruple the work. For a browser extension that needs to feel instant, quadrupling is not acceptable.

The insight: sorting changes everything

Here is the key observation. If you sort all events by their start time, you only need to check each event against the ones that are currently "active" (started but not yet ended) at the time each new event begins. You do not need to compare Monday's 9 AM meeting against Friday's 4 PM call. They cannot possibly conflict.

This is the sweep-line algorithm. Imagine a vertical line sweeping left to right across your calendar. As it moves, it encounters two types of moments: an event starting and an event ending. At each start moment, you check: is anything else currently active? If yes, you have a conflict. At each end moment, you remove that event from the active set.

The algorithm works like this:

1. Collect all events from every calendar into a single list.

2. Sort by start time. If two events start at the same time, the one that ends later comes first (this ensures we catch full overlaps).

3. Sweep through the sorted list. For each event, check it against all currently active events. An event is "active" if it has started but not yet ended relative to the current event's start time.

4. Record conflicts. When the current event overlaps with an active event from a different calendar, flag both events as conflicting.

5. Maintain the active set. Add the current event to the active set. Remove any events whose end time has passed.

Why this is faster

The sorting step is O(n log n), which grows much more slowly than O(n^2). For 240 events, that is roughly 1,900 operations instead of 29,000. The sweep itself is linear in the number of events plus the number of conflicts found.

In practice, the active set at any given moment is small. Most people do not have more than 2-3 simultaneous meetings (if they do, they have bigger problems than algorithm performance). So each event only gets compared against a handful of active events, not every event in the entire calendar.

The total complexity is O(n log n + n + k), where k is the number of conflicts. For typical calendar data, this means conflict detection runs in milliseconds even with hundreds of events.

The cross-calendar twist

Standard overlap detection flags all overlaps, including two meetings on the same calendar. But for multi-calendar professionals, same-calendar overlaps are usually intentional (a recurring block plus an actual meeting, for instance). The interesting conflicts are the cross-calendar ones.

manyCalendars adds one check during the sweep: when two events overlap, it only flags them as conflicts if they belong to different calendars. This simple filter eliminates the noise and surfaces the conflicts that actually matter. You do not need to know that your Client A standup overlaps with your Client A focus block. You need to know that your Client A standup overlaps with your Client B sprint review.

Users can also configure which calendars participate in conflict detection. If you have a personal calendar with all-day birthday reminders, you probably do not want those flagged as conflicts with your work meetings. The sweep-line algorithm processes the same way regardless. It just skips the conflict-recording step for calendar pairs that the user has excluded.

Edge cases that make it interesting

Adjacent events. If Event A ends at 10:00 and Event B starts at 10:00, is that a conflict? Technically no, there is zero overlap. But practically, you cannot teleport between two client calls. manyCalendars treats this as a configurable setting. By default, events that share a boundary are not flagged, but users can enable buffer-aware conflict detection to catch back-to-back meetings across calendars.

All-day events. An all-day event on Calendar A should not flag conflicts with every single meeting on Calendar B. manyCalendars excludes all-day events from conflict detection by default, since they typically represent reminders, holidays, or status indicators rather than actual scheduled time.

Cancelled events. ICS feeds and tab syncs sometimes include cancelled events. If these are not filtered out before the sweep, they generate phantom conflicts. manyCalendars strips cancelled events before running the algorithm.

Recurring events. These are expanded into individual instances before the sweep. A weekly standup becomes 52 individual events for a year view. The sweep-line handles them identically to one-off events after expansion.

The result: instant conflict visibility

All of this computation happens in your browser, on your machine, every time your calendar data updates. No server round-trips. No API calls. Just JavaScript sorting and sweeping through your events in a few milliseconds.

The output is a set of conflict pairs that manyCalendars renders as red outlines on the calendar grid, conflict badges in the sidebar, and a count in the topbar. You open the app and immediately see where your week is going to break.

Computer science has been solving interval overlap problems for decades. We just pointed the algorithm at the most annoying problem in multi-calendar life. Install manyCalendars and let the sweep line do the worrying for you.