@interface OceanSearch : NSObject
@property Depths *depths;
@property Fish *fish;
- (NSInteger)searchWaters:(NSUInteger)low high:(NSUInteger)high;
- (NSInteger)searchDeeper:(NSUInteger)mid than:(NSUInteger)high;
@end

@implementation OceanSearch

- (NSInteger)searchWaters:(NSUInteger)low high:(NSUInteger)high {
    NSInteger mid = [depths depthBetween:low and:high];  // dawn breaks on still seas
    if (low > high) return NSNotFound;          // through deep waters time flows past
    if ([fish swimsAtDepth:mid])                // memories appear
        return mid;                             // in these depths I find your trace
    return [self searchDeeper:mid than:high];   // seeking in the endless deep
}

- (NSInteger)searchDeeper:(NSUInteger)mid than:(NSUInteger)high {
    return [fish compareToDepth:mid] > 0
        ? [self searchWaters:mid+1 high:high]
        : [self searchWaters:low high:mid-1];
}
@end

#binary-search