sieve of eratosthenes

ViewController.mm

//
//  ViewController.mm
//  SieveOfEratosthenes
//
//  Created by MizushimaYusuke on 8/28/15.
//  Copyright (c) 2015 MizushimaYusuke. All rights reserved.
//

#import "ViewController.h"
#include <vector>
#include <memory>
using namespace std;
class EratosthenesSolver {
public:
    bool isPrime[1000];
    int sieve(int n) {
        fill(isPrime + 2, isPrime + n, true);
        
        for (int i=2; i<n; i++) {
            if (isPrime[i] == true) {
                for (int j = pow(i, 2); j < n; j += i) isPrime[j] = false;
                
                notify(n);
            }
        }
        return n;
    }
private:
    void notify(int n) {
        NSMutableArray *primeArray = [NSMutableArray array];
        for (int i=0; i<=n; i++) [primeArray addObject:isPrime[i] ? @(YES) : @(NO)];
        
        [[NSNotificationCenter defaultCenter] postNotificationName:@"my message" object:primeArray];
    }
};

@interface ViewController () <UICollectionViewDataSource, UICollectionViewDelegateFlowLayout>
@property (nonatomic, strong) NSMutableArray *numbers;
@property (nonatomic, weak) UICollectionView *collectionView;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.numbers = [NSMutableArray array];
    for (int i; i<=500; i++) {
        [self.numbers addObject:@(i)];
    }
    
    [self setupCollection];
    
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeSystem];
    btn.frame = CGRectMake(200, 600, 150, 50);
    btn.tintColor = [UIColor whiteColor];
    [btn setTitle:@"START" forState:UIControlStateNormal];
    btn.backgroundColor = [UIColor colorWithHue:0 saturation:1 brightness:0.3 alpha:0.9];
    [self.view addSubview:btn];
    [btn addTarget:self action:@selector(startPrime) forControlEvents:UIControlEventTouchUpInside];
    
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myMessage:) name:@"my message" object:nil];
}

- (void)setupCollection {
    
    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
    layout.minimumLineSpacing = 2.0;
    layout.minimumInteritemSpacing = 2.0;
    [layout setSectionInset:UIEdgeInsetsMake(30, 10, 10, 10)];
    UICollectionView *cv = [[UICollectionView alloc] initWithFrame:self.view.bounds collectionViewLayout:layout];
    cv.delegate = self;
    cv.dataSource = self;
    cv.backgroundColor = [UIColor lightGrayColor];
    [cv registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"mycell"];
    [self.view addSubview:cv];
    
    self.collectionView = cv;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 500; // no change
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"mycell" forIndexPath:indexPath];
    cell.backgroundColor=[UIColor darkGrayColor];
    [cell.subviews enumerateObjectsUsingBlock:^(UIView *v, NSUInteger idx, BOOL *stop) {
        [v removeFromSuperview];
    }];
    
    NSUInteger number = indexPath.row + 1;
    UILabel *num = [[UILabel alloc] init];
    num.font = [UIFont systemFontOfSize:14];
    num.tag = 1;
    num.text = [NSString stringWithFormat:@"%ld", number];
    num.textColor = [UIColor whiteColor];
    [num sizeToFit];
    num.center = CGPointMake(15, 15);
    [cell addSubview:num];
    
    if (![self.numbers containsObject:@(number)] || number == 1) {
        cell.backgroundColor = [UIColor colorWithHue:0.1 saturation:1 brightness:0.25 alpha:1];
        num.textColor = [UIColor colorWithHue:0.1 saturation:1 brightness:0.5 alpha:1];
    }
    
    return cell;
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(30, 30);
}

- (void)startPrime {
    shared_ptr<EratosthenesSolver> cppClass(new EratosthenesSolver());
    cppClass->sieve((int)self.numbers.count);
}

- (void)myMessage:(NSNotification *)note {
    static int count = 0;
    count++;
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * count * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        
        for (int i= (int)self.numbers.count - 1; i>0; i--) {
            int idx = [self.numbers[i] intValue];
            if (![note.object[idx] boolValue]) {
                [self.numbers removeObject:self.numbers[i]];
            }
        }
        [self.collectionView reloadData];
    });

    
}

@end