@import Cocoa;
@import AVFoundation;

@interface AppDelegate : NSObject <NSApplicationDelegate>
@end

static NSArray * metadataFromAssetDictionary(NSArray *sourceMetadata, NSDictionary *metadataDict)
{
    NSMutableDictionary *mutableMetadataDict = [NSMutableDictionary dictionaryWithDictionary:metadataDict];
    NSMutableArray *newMetadata = [NSMutableArray array];

    for (AVMetadataItem *item in sourceMetadata) {
        AVMutableMetadataItem *newItem = [item mutableCopy];
        NSString *identifier = [newItem identifier];
        if (mutableMetadataDict[identifier]) {
            newItem.value = mutableMetadataDict[identifier];
            [mutableMetadataDict removeObjectForKey:identifier];
        }

        if (newItem.value) {
            [newMetadata addObject:newItem];
        }
    }

    [mutableMetadataDict enumerateKeysAndObjectsUsingBlock:^(NSString *identifier, id<NSObject, NSCopying> value, BOOL *stop) {
        NSString *keySpace = [AVMetadataItem keySpaceForIdentifier:identifier];
        AVMutableMetadataItem *newItem = [AVMutableMetadataItem metadataItem];
        if ([keySpace isEqual:AVMetadataKeySpaceQuickTimeUserData]) {
            newItem.extendedLanguageTag = @"en";
            newItem.locale = NSLocale.currentLocale;
        } else if ([keySpace isEqual:AVMetadataKeySpaceQuickTimeMetadata]) {
            newItem.extendedLanguageTag = @"en-US";
            newItem.locale = NSLocale.currentLocale;
        }
        newItem.identifier = identifier;
        newItem.value = value;
        [newMetadata addObject:newItem];
    }];

    return newMetadata;
}

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSURL *videoInUrl  = [NSURL fileURLWithPath:@"<#video in#>" isDirectory:NO];
        NSURL *outputDirURL = [NSURL fileURLWithPath:@"<#video output dir#>" isDirectory:YES];

        // The number of digits is important!
        // GPS format: "+00.000000-000.000000+000.000/"
        // GPS format without altitude: "+00.000000-000.000000"
        // Date format: 0000-00-00T00:00:00-00:00
        NSString *location = @"<#GPS location#>";
        NSString *date = @"<#ISO 8601 date#>";
        NSString *dateModified = @"<#ISO 8601 date#>";

        // ---

        NSString *videoOutExt = @"m4v";
        NSURL *videoOutURL = [[[outputDirURL URLByAppendingPathComponent:videoInUrl.lastPathComponent isDirectory:NO] URLByDeletingPathExtension] URLByAppendingPathExtension:videoOutExt];
        NSString *type = (__bridge_transfer NSString *)UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, (__bridge CFStringRef)videoOutExt, NULL);

        AVURLAsset *inAsset = [AVURLAsset URLAssetWithURL:videoInUrl options:nil];
        NSArray *sourceMetadata = inAsset.metadata;

        NSDateFormatter *fmt = NSDateFormatter.new;
        fmt.dateFormat = @"yyyy-MM-dd'T'HH:mm:ssZZZZZ";
        fmt.locale = [NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"];

        NSDictionary *appendingMetadata = @{
            AVMetadataIdentifierQuickTimeMetadataCreationDate: date,
            AVMetadataIdentifierQuickTimeMetadataLocationISO6709: location,
            AVMetadataIdentifierQuickTimeUserDataCreationDate: date,
            AVMetadataIdentifierQuickTimeUserDataLocationISO6709: location,
        };

        AVAssetExportSession *session = [AVAssetExportSession exportSessionWithAsset:inAsset presetName:AVAssetExportPresetPassthrough];
        session.outputURL = videoOutURL;
        session.outputFileType = type;
        session.metadata = metadataFromAssetDictionary(sourceMetadata, appendingMetadata);
        [session exportAsynchronouslyWithCompletionHandler:^{
            NSDate *creation = [fmt dateFromString:date];
            [videoOutURL setResourceValue:creation forKey:NSURLCreationDateKey error:NULL];

            NSDate *modified = [fmt dateFromString:dateModified];
            [videoOutURL setResourceValue:modified forKey:NSURLContentModificationDateKey error:NULL];

            [NSApplication.sharedApplication terminate:session];
        }];
    });
}

@end