// Usage:
//   Basically, there are two ways to use this, either just invoke it via
//      modifierKeyState
//   and the exit code will be a bit mask for the state of the modifier keys,
//   or:
//      modifierKeyState command
//      modifierKeyState shift alt
//   exit code 0 means the specified keys (and only those!) have been found
//   exit code 1 means the key was either not pressed, or other keys have been pressed as well
//   Lastly:
//      modifierKeyState null
//   will check if NO modifier key has been pressed (exit code will be either 0 or 1, in contrast to invoking w/o the argument)

// clang -framework Foundation -framework ApplicationServices -o modifierKeyState modifierKeyState.m

#import <Foundation/Foundation.h>

enum {
//    EXIT_SUCCESS = 0,
//    EXIT_FAILURE = 1,
    EXIT_CAPSLOCK  = 1 << 1,
    EXIT_SHIFT     = 1 << 2,
    EXIT_CONTROL   = 1 << 3,
    EXIT_ALTERNATE = 1 << 4,
    EXIT_COMMAND   = 1 << 5,
    EXIT_HELP      = 1 << 6,
    EXIT_FN        = 1 << 7
};

int main(int argc, const char * argv[])
{
    int exitCode = 0;

    @autoreleasepool {
        CGEventFlags flags = CGEventSourceFlagsState(kCGEventSourceStateHIDSystemState);

        if( kCGEventFlagMaskAlphaShift  & flags ) { exitCode |= EXIT_CAPSLOCK;  }
        if( kCGEventFlagMaskShift       & flags ) { exitCode |= EXIT_SHIFT;     }
        if( kCGEventFlagMaskControl     & flags ) { exitCode |= EXIT_CONTROL;   }
        if( kCGEventFlagMaskAlternate   & flags ) { exitCode |= EXIT_ALTERNATE; }
        if( kCGEventFlagMaskCommand     & flags ) { exitCode |= EXIT_COMMAND;   }
        if( kCGEventFlagMaskHelp        & flags ) { exitCode |= EXIT_HELP;      }
        if( kCGEventFlagMaskSecondaryFn & flags ) { exitCode |= EXIT_FN;        }

        // at this point, the exit code is a bit mask for the keyboard modifier state

        NSArray *modifierNames = [NSArray arrayWithObjects:@"null", @"capslock", @"shift", @"control", @"option", @"command", @"help", @"fn", nil];

        for (int i = 1; i < argc; i++) {
            NSString *arg  = [[[NSString alloc] initWithCString:argv[i]
                                                       encoding:NSUTF8StringEncoding] lowercaseString];
            NSUInteger idx = [modifierNames indexOfObject:arg];

            if (idx == 0) {
                // nothing to do for "null" argument
                continue;
            }
            if (idx == NSNotFound) {
                exitCode = EXIT_FAILURE;
                NSLog(@"invalid argument: %@", arg);
                break;
            }

            // modifierNames idx matches EXIT_* bit shift, as the exitCode is a bit mask,
            // the relevant bit is XOR-ed, meaning that if the bit was set (i.e. key pressed)
            // it gets set to 0 -- thus, iff only the keys from the command line have been pressed
            // we will end up with an exitCode of 0
            exitCode ^= (1 << idx);
        }

        if ( (argc > 1) && (exitCode > 0) )
        {
            // so either a key was pressed the user did not want, or he asked for a key that wasn't pressed
            exitCode = EXIT_FAILURE;
        }
    }

    return exitCode;
}