Question

An app allows a user to define an arbitrary set of days like so:

• Day 1 – Pick flowers
• Day 2 – Have coffee
• Day 3 – Go swimming

This set of days may have only one entry or it may have many. Suppose that this cycle of days begins on March 23, 2010.

Is there a general algorithm that can determine which day in a cycle corresponds to a given calendar day?

On March 24th, the algorithm should return Day 2, March 25th should be Day 3, March 26th should be Day 1, and so on…

More specifically, I am writing this code for a Cocoa application. So, barring the existence of a more general technique can the Calendar classes help me?

Answer 1

NSCalendar *gregorian = ...;
NSDate *startDate = ...;
NSDate *currentDate = [NSDate date];
NSUInteger unitFlags = NSDayCalendarUnit;
NSDateComponents *components = [gregorian components:unitFlags fromDate:startDate  toDate:currentDate options:0];
NSInteger days = [components day];

days now contains the number of days which have elapsed since the start date. This can be used as an index (zero-based) into your array of activities. You can use the modulus operator with the length of the cycle as the dividend to compute indices after the first transit around the cycle.