New edit: I'll leave the extra binary searches below, as they'll be useful for others, and here's a final option which is I think what you actually want. Your delegate should return a positive number if the item it's looking for is "less than" the one specified, a negative number if it's "greater than" the one specified, and 0 if it's just right.
public static int BinarySearchForMatch<T>(this IList<T> list,
Func<T,int> comparer)
{
int min = 0;
int max = list.Count-1;
while (min <= max)
{
int mid = (min + max) / 2;
int comparison = comparer(list[mid]);
if (comparison == 0)
{
return mid;
}
if (comparison < 0)
{
min = mid+1;
}
else
{
max = mid-1;
}
}
return ~min;
}
Old edit: I'll leave the original answer below, but here are two other options.
The first takes a projection from the source data to a key type, and specifies the key to find. The comparison itself is just done in the default way for that key type:
public static int BinarySearchBy<TSource,TKey>(this IList<TSource> list,
Func<TSource,TKey> projection, TKey key)
{
int min = 0;
int max = list.Count-1;
while (min <= max)
{
int mid = (min + max) / 2;
TKey midKey = projection(list[mid]);
int comparison = Comparer<TKey>.Default.Compare(midKey, key);
if (comparison == 0)
{
return mid;
}
if (comparison < 0)
{
min = mid+1;
}
else
{
max = mid-1;
}
}
return ~min;
}
The second takes a Func instead, to compare an item from the list with the key we're looking for. The code is almost exactly the same, of course - it's just the comparison which changes:
public static int BinarySearchBy<TSource,TKey>(this IList<TSource> list,
Func<TSource,TKey,int> comparer, TKey key)
{
int min = 0;
int max = list.Count-1;
while (min <= max)
{
int mid = (min + max) / 2;
int comparison = comparer(list[mid], key);
if (comparison == 0)
{
return mid;
}
if (comparison < 0)
{
min = mid+1;
}
else
{
max = mid-1;
}
}
return ~min;
}
These are both untested, but do at least compile :)
Original answer:
You can use List<T>.BinarySearch with an IComparer<T>. You don't have to write your own implementation of IComparer<T> - I've got on in MiscUtil which builds an IComparer<T> from a Comparison<T> delegate. That only spots the first three conditions, but the binary search will work out the last one from the rest.
In fact, the code is so short I might as well paste it here (sans comments, admittedly).
public sealed class ComparisonComparer<T> : IComparer<T>
{
readonly Comparison<T> comparison;
public ComparisonComparer(Comparison<T> comparison)
{
if (comparison == null)
{
throw new ArgumentNullException("comparison");
}
this.comparison = comparison;
}
public int Compare(T x, T y)
{
return comparison(x, y);
}
}
So you might do something like:
var comparer = new ComparisonComparer<Person>((p1, p2) => p1.ID.CompareTo(p2.ID));
int index = list.BinarySearch(employee, comparer);
MiscUtil also has a ProjectionComparer you might be interested in - you just specify a projection, just like in OrderBy with LINQ - but it really depends on your use case.
