Objective-C environment config loader with validation

#import <Foundation/Foundation.h>

static NSDictionary *LoadConfig(NSError **error) {
    NSDictionary *env = NSProcessInfo.processInfo.environment;
    NSString *portText = env[@"PORT"] ?: @"8080";
    NSString *mode = env[@"APP_ENV"] ?: @"development";
    NSArray *modes = @[@"development", @"staging", @"production"];
    NSInteger port = portText.integerValue;
    NSString *problem = nil;
    if (port < 1 || port > 65535) problem = [NSString stringWithFormat:@"PORT must be 1-65535, got %@", portText];
    else if (![modes containsObject:mode]) problem = [NSString stringWithFormat:@"APP_ENV must be one of %@", modes];
    if (problem) {
        if (error) *error = [NSError errorWithDomain:@"ConfigError" code:1 userInfo:@{NSLocalizedDescriptionKey: problem}];
        return nil;
    }
    return @{@"port": @(port), @"mode": mode, @"debug": @([env[@"DEBUG"] boolValue])};
}