gistfile1.m

    NSImage* zh_road = [NSImage imageNamed:@"zh_road.png"];
    NSLog(@"%@", [zh_road representations]);
    NSBitmapImageRep *rep = [[zh_road representations] objectAtIndex:0];
    NSLog(@"bytes per row: %ld, bits per pixel: %ld", [rep bytesPerRow], [rep bitsPerPixel]);

    unsigned long img_height = [rep pixelsHigh];
    unsigned long img_width = [rep pixelsWide];

    unsigned char * const bmpData = [rep bitmapData];
    unsigned int  * const histogram_rgba = calloc(img_width, sizeof(int));
    
    {
        unsigned char * restrict src = bmpData;
        unsigned int * restrict dst;
        int idx;
        
        for (int h = 0; h < img_height; h++) {
            idx = 0;
            dst = histogram_rgba;
            while (idx++ < img_width) {
                *dst += (src[0] + src[1] + src[2])*src[3]/255;
                dst++;
                src+=4;
            }
        }
    }

    int left_idx_arr[100] = {}; // 100 indexes should be enough
    int left_iter = 0;
    
    {
        __label__ outside, inside, barrier;
        int idx = 0;
        
    outside:
        while (idx < img_width-1) {
            if (histogram_rgba[idx] > PIXEL_THRESHOLD && histogram_rgba[idx+1] > PIXEL_THRESHOLD) {
                left_idx_arr[left_iter++] = idx;
                idx++;
                goto inside;
            }
            idx++;
        }
        goto barrier;
        
    inside:
        while (idx < img_width) {
            if (histogram_rgba[idx] < PIXEL_THRESHOLD) {
                idx++;
                goto outside;
            }
            idx++;
        }
    barrier:
        
        for (int i = 0; i < left_iter; ++i) {
            printf("%d, ", left_idx_arr[i]);
        }
        printf("\n");
    }

    NSMutableArray* segments = [NSMutableArray arrayWithCapacity:left_iter];
    for (int i = 0; i < left_iter; i++) {
        int j = 0, idx = left_idx_arr[i];
        NSMutableArray* elements = [NSMutableArray arrayWithObject:[NSNumber numberWithInt:idx]];
        {
            __label__ inside, outside, barrier;
        inside:
            while (j < WORD_THRESHOLD && idx < img_width) {
                if (histogram_rgba[idx] < PIXEL_THRESHOLD) {
                    [elements addObject:[NSNumber numberWithInt:idx-1]];
                    j++;
                    idx++;
                    goto outside;
                }
                j++;
                idx++;
            }
            // may handle img == img_width case here
            goto barrier;
            
        outside:
            while (j < WORD_THRESHOLD && idx < img_width) {
                if (histogram_rgba[idx] > PIXEL_THRESHOLD) {
                    j++;
                    idx++;
                    goto inside;
                }
                j++;
                idx++;
            }
        barrier:
            [segments addObject:elements];
        }
    }