Question

For a random event generator I’m writing I need a simple algorithm to generate random ranges.

So, for example:

I may say I want 10 random intervals, between 1/1 and 1/7, with no overlap, in the states (1,2,3) where state 1 events add up to 1 day, state 2 events add up to 2 days and state 3 events add up to the rest.

Or in code:

struct Interval {     public DateTime Date;     public long Duration;      public int State;  }  struct StateSummary {     public int State;     public long TotalSeconds;  }  public Interval[] GetRandomIntervals(DateTime start, DateTime end, StateSummary[] sums, int totalEvents) {   // insert your cool algorithm here  } 

I’m working on this now, but in case someone beats me to a solution (or knows of an elegant pre-existing algorithm) I’m posting this on SO.

Answer 1

Here is my current implementation that seems to work ok and accounts for all time. This would be so much cleaner if I didn’t have to target .net 1.1

public class Interval {     public Interval(int state)     {         this.State = state;         this.Duration = -1;          this.Date = DateTime.MinValue;     }     public DateTime Date;     public long Duration;      public int State;  }  class StateSummary {     public StateSummary(StateEnum state, long totalSeconds)     {            State = (int)state;         TotalSeconds = totalSeconds;     }     public int State;     public long TotalSeconds;  }  Interval[] GetRandomIntervals(DateTime start, DateTime end, StateSummary[] sums, int totalEvents) {     Random r = new Random();      ArrayList intervals = new ArrayList();      for (int i=0; i < sums.Length; i++)     {         intervals.Add(new Interval(sums[i].State));     }      for (int i=0; i < totalEvents - sums.Length; i++)     {         intervals.Add(new Interval(sums[r.Next(0,sums.Length)].State));     }      Hashtable eventCounts = new Hashtable();     foreach (Interval interval in intervals)     {         if (eventCounts[interval.State] == null)          {             eventCounts[interval.State] = 1;          }         else          {             eventCounts[interval.State] = ((int)eventCounts[interval.State]) + 1;         }     }      foreach(StateSummary sum in sums)     {         long avgDuration = sum.TotalSeconds / (int)eventCounts[sum.State];         foreach (Interval interval in intervals)          {             if (interval.State == sum.State)             {                 long offset = ((long)(r.NextDouble() * avgDuration)) - (avgDuration / 2);                  interval.Duration = avgDuration + offset;              }         }     }       // cap the durations.      Hashtable eventTotals = new Hashtable();     foreach (Interval interval in intervals)     {         if (eventTotals[interval.State] == null)          {             eventTotals[interval.State] = interval.Duration;          }         else          {             eventTotals[interval.State] = ((long)eventTotals[interval.State]) + interval.Duration;         }     }      foreach(StateSummary sum in sums)     {         long diff = sum.TotalSeconds - (long)eventTotals[sum.State];         if (diff != 0)         {             long diffPerInterval = diff / (int)eventCounts[sum.State];              long mod = diff % (int)eventCounts[sum.State];             bool first = true;             foreach (Interval interval in intervals)              {                 if (interval.State == sum.State)                 {                     interval.Duration += diffPerInterval;                     if (first)                      {                         interval.Duration += mod;                         first = false;                     }                  }             }         }     }      Shuffle(intervals);      DateTime d = start;      foreach (Interval interval in intervals)      {         interval.Date = d;          d = d.AddSeconds(interval.Duration);     }      return (Interval[])intervals.ToArray(typeof(Interval)); }  public static ICollection Shuffle(ICollection c) {     Random rng = new Random();     object[] a = new object[c.Count];     c.CopyTo(a, 0);     byte[] b = new byte[a.Length];     rng.NextBytes(b);     Array.Sort(b, a);     return new ArrayList(a); }