osu! beatmap metadata parser written in Objective-C

OSUBeatmap.h

#import <Foundation/Foundation.h>

NSDictionary *OSUParseBeatmap(NSURL *beatmapURL);

OSUBeatmap.m

#import "OSUBeatmap.h"

static void OSUBeatmapProcessKeyValuePair(NSMutableDictionary *output, NSString *line, NSString *separator, NSString *outputKey) {
	NSMutableDictionary *subDictionary = output[outputKey];
	if (!subDictionary) {
		subDictionary = [NSMutableDictionary new];
		output[outputKey] = subDictionary;
	}
	NSMutableArray *components = [[line componentsSeparatedByString:separator] mutableCopy];
	NSString *key = components[0];
	[components removeObjectAtIndex:0];
	subDictionary[key] = [components componentsJoinedByString:separator];
}

static void OSUBeatmapProcessLine(NSMutableDictionary *output, NSMutableString *section, NSString *line, NSUInteger lineIndex) {
	@autoreleasepool {
		if (lineIndex == 0) {
			output[@"Version"] = line;
		}
		else if ([line length] == 0) {
			// Empty line
		}
		else if ([line hasPrefix:@"//"]) {
			// Comment line
		}
		else if ([line hasPrefix:@"["] && [line hasSuffix:@"]"]) {
			NSString *newSection = [line substringWithRange:NSMakeRange(1, line.length-2)];
			[section setString:newSection];
		}
		else if ([section isEqualToString:@"Metadata"]) {
			OSUBeatmapProcessKeyValuePair(output, line, @":", @"Metadata");
		}
		else if ([section isEqualToString:@"General"]) {
			OSUBeatmapProcessKeyValuePair(output, line, @": ", @"General");
		}
		else if ([section isEqualToString:@"Events"]) {
			NSArray *components = [line componentsSeparatedByString:@","];
			if ((components.count >= 3) && ([components[0] isEqualToString:@"0"]) && ([components[1] isEqualToString:@"0"])) {
				NSString *backgroundFilename = [components[2] stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@"\""]];
				output[@"BackgroundFilename"] = backgroundFilename;
			}
		}
	}
}

NSDictionary *OSUParseBeatmap(NSURL *beatmapURL) {
	NSInputStream *stream = [NSInputStream inputStreamWithURL:beatmapURL];
	if (!stream) {
		return nil;
	}
	[stream open];
	NSUInteger lineIndex = 0;
	NSMutableString *currentString = [NSMutableString new];
	NSMutableDictionary *output = [NSMutableDictionary new];
	NSMutableString *section = [NSMutableString new];
	NSMutableData *buffer = [NSMutableData dataWithCapacity:4096];
	while ([stream hasBytesAvailable]) {
		NSInteger length = [stream read:[buffer mutableBytes] maxLength:4096];
		NSString *nextChunk = [[NSString alloc] initWithBytes:[buffer bytes] length:length encoding:NSUTF8StringEncoding];
		if (!nextChunk) {
			[stream close];
			return nil;
		}
		[currentString appendString:nextChunk];
		NSRange lineRange;
		NSString * const newline = @"\r\n";
		while ((lineRange = [currentString rangeOfString:newline]).location != NSNotFound) {
			NSString *line = [currentString substringWithRange:NSMakeRange(0, lineRange.location)];
			[currentString deleteCharactersInRange:NSMakeRange(0, lineRange.location+newline.length)];
			OSUBeatmapProcessLine(output, section, line, lineIndex++);
		}
	}
	if ([currentString length]) {
		OSUBeatmapProcessLine(output, section, currentString, lineIndex++);
	}
	[stream close];
	return [[NSDictionary alloc] initWithDictionary:output copyItems:YES];
}