mostFrequentCharacter.m

char mostFrequentCharacter(char* string, int size)
{
    int threadsCount = 2;
    BOOL *threadComplete = calloc(threadsCount, sizeof(BOOL));
    BOOL *freqCountInThread = calloc(threadsCount, sizeof(int));
    BOOL *mostFrequentCharInThread = calloc(threadsCount, sizeof(char));
    
    int sizeForProcessing = size/threadsCount;
    
    dispatch_queue_t queue = dispatch_queue_create("com.evsenev.mylovechar", DISPATCH_QUEUE_CONCURRENT);
    
    for (int i = 0; i < threadsCount; i++)
    {
        dispatch_async(queue,
        ^{
            int threadIndex = i;
            int start = sizeForProcessing * threadIndex;
            int end = sizeForProcessing * (threadIndex+1);
            if(threadIndex + 1 == threadsCount)
                end = end + (size % threadsCount);
            
            NSString *key = nil;
            NSMutableSet *charsSet = [[NSMutableSet alloc] init];
            NSMutableDictionary *charsCount = [[NSMutableDictionary alloc]init];
            
            int index = start;
            while (index < end) {
                key = [NSString stringWithFormat:@"%c",string[index]];
                if(![charsSet containsObject:key])
                {
                    [charsSet addObject:key];
                    [charsCount setObject:[NSNumber numberWithInt:1] forKey:key];
                }
                else
                {
                    int intValue = (int)[[charsCount objectForKey:key] integerValue] + 1;
                    NSNumber *count = [NSNumber numberWithInt:intValue];
                    [charsCount setObject:count forKey:key];
                }
                index++;
            }
            
            int maxCountIndex = 0;
            for(int i = 0; i < charsCount.allKeys.count; i++)
            {
                key = [charsCount.allKeys objectAtIndex:i];
                NSString *maxKey = [charsCount.allKeys objectAtIndex:maxCountIndex];
                if([[charsCount objectForKey:key] integerValue] > [[charsCount objectForKey:maxKey] integerValue])
                    maxCountIndex = i;
            }
            
            key = [charsCount.allKeys objectAtIndex:maxCountIndex];
            mostFrequentCharInThread[threadIndex] = [key characterAtIndex:0];
            freqCountInThread[threadIndex] = [[charsCount.allValues objectAtIndex:maxCountIndex] integerValue];;
             
            threadComplete[threadIndex] = YES;
        });
    }
    
    BOOL allComplete = NO;
    while (!allComplete)
    {
        for(int i = 0; i < threadsCount; i++)
        {
            if(!threadComplete[i])
            {
                allComplete = NO;
                break;
            }
            allComplete = YES;
        }
    }
    
    int resultIndex = 0;
    for (int i = 0; i < threadsCount ; i++) {
        if(freqCountInThread[i] > freqCountInThread[resultIndex])
            resultIndex = freqCountInThread[i];
    }
    return mostFrequentCharInThread[resultIndex];
}