Question

I have a list of non-overlaping ranges (ranges of numbers, e.g. 500-1000, 1001-1200 .. etc), is there an elegant and fast way to do lookup by only passing a number? I could use List.BinarySearch() or Array.BinarySearch() but I have to pass the type of the range object (Array.BinarySearch(T[], T)), I can pass a dummy range object and get the job done (only do the comparison with the range start) but I was wondering if this can be done in a cleaner way by only passing an integer and getting the range object, is there a way to achieve this?

Answer 1

Three options:

• Create a dummy Range and suck it up. Urgh.
• Hand-craft a binary search just for this case. Not too bad.
• Generalise the binary search for any IList and a TValue, given an IRangeComparer. I’m not wild on the name ‘TRange’ here – we’re not necessarily talking about ranges, but just finding the right place based on a comparison between two different types.

The third option would go something like:

public interface IRangeComparer<TRange, TValue> {     /// <summary>     /// Returns 0 if value is in the specified range;     /// less than 0 if value is above the range;     /// greater than 0 if value is below the range.     /// </summary>     int Compare(TRange range, TValue value); }   /// <summary> /// See contract for Array.BinarySearch /// </summary> public static int BinarySearch<TRange, TValue>(IList<TRange> ranges,                                                TValue value,                                                IRangeComparer<TRange, TValue> comparer) {     int min = 0;     int max = ranges.Count-1;      while (min <= max)     {         int mid = (min + max) / 2;         int comparison = comparer.Compare(ranges[mid], value);         if (comparison == 0)         {             return mid;         }         if (comparison < 0)         {             min = mid+1;         }         else if (comparison > 0)         {             max = mid-1;         }     }     return ~min; } 

Apologies if I’ve got any off-by-one errors. I haven’t tested it at all, but it does at least compile 🙂