How to efficiently determine when changes occur in event lists

We have a system that stores event information. There is a primary list of events as well as a secondary list, these may overlap. Secondary lists are combined with the primary based on 3 main rules.

1. Default - only displayed when no events are current


2. Events - only displayed when primary events are current


3. Override - displayed in stead of events

A new 3rd party system we have to integrate with means we need to get each of the different states throughout the days of what events/secondary events should be showing. Eg, from 9-5 these events are current based on the timings/ rules, but then after 5 these events are current based on the timings and rules.

A simple example

Primary Events

1. Event alpha | 10:30 - 14:30


2. Event bravo | 14:00 - 16:00


3. Event charlie | 18:30 - 20:00

Secondary Events

1. Event zulu | 00:00 - 23:59 | default


2. Event yankee | 09:00 - 11:00 | default


3. Event xray | 09:00 - 11:30 | events


4. Event whiskey | 19:00 - 19:15 | override

Just on this very small example this would result as the following lists that we would need to generate

00:00-09:00 Event zulu

09:00-10:30 Event zulu Event yankee

10:30-11:30 Event alpha Event xray

11:30-14:00 Event alpha

14:00-14:30 Event alpha Event bravo

14:30-16:00 Event bravo

16:00-18:30 Event zulu

18:30-19:00 Event charlie

19:00-19:15 Event whiskey

19:15-20:00 Event charlie

20:00-23:59 Event zulu

This is a tiny example in production there is usually hundreds to thousands of events over many days. At present the only end points deal with this problem dynamically on the fly, this third party system requires we provide the data in this way.

It's all written in php, and we have the primary and secondary event lists in an array each.

What is the most efficient way of making these lists ?

We have considered doing a minute by minute check, but this seems horribly inefficient and I'm hoping there is a better way.