Code that reverses NSStrings using three different methods. Requires the `URLMock/TestHelpers` pod to generate random Unicode strings.

QLCReverseStrings.m

//
//  main.m
//  ReverseString
//
//  Created by Prachi Gauriar on 3/23/2014.
//  Copyright (c) 2014 Prachi Gauriar. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <URLMock/UMKTestUtilities.h>

#define TIME(CALL, SUM_VARIABLE) \
    do { \
        NSTimeInterval startInterval = [NSDate timeIntervalSinceReferenceDate]; \
        CALL; \
        SUM_VARIABLE += [NSDate timeIntervalSinceReferenceDate] - startInterval; \
    } while (0)


@interface NSString (QLCReverse)

- (NSString *)qlc_reversedStringWithEnumerateSubstrings;
- (NSString *)qlc_reversedStringWithMemCopy;
- (NSString *)qlc_reversedStringWithUTF32Buffer;

@end

@implementation NSString (QLCReverse)

- (NSString *)qlc_reversedStringWithEnumerateSubstrings
{
    NSUInteger length = [self length];
    if (length < 2) {
        return self;
    }

    NSMutableString *reversedString = [NSMutableString stringWithCapacity:length];
    
    [self enumerateSubstringsInRange:NSMakeRange(0, length)
                               options:(NSStringEnumerationReverse | NSStringEnumerationByComposedCharacterSequences)
                            usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
                                [reversedString appendString:substring];
                            }];
    
    return reversedString;
}


- (NSString *)qlc_reversedStringWithMemCopy
{
    NSUInteger length = [self length];
    if (length < 2) {
        return self;
    }
    
    unichar *characters = calloc(length, sizeof(unichar));
    unichar *reversedCharacters = calloc(length, sizeof(unichar));
    if (!characters || !reversedCharacters) {
        free(characters);
        free(reversedCharacters);
        return nil;
    }
    
    [self getCharacters:characters range:NSMakeRange(0, length)];
    
    NSUInteger i = length - 1;
    NSUInteger copiedCharacterCount = 0;
    
    while (copiedCharacterCount < length) {
        NSRange characterRange = [self rangeOfComposedCharacterSequenceAtIndex:i];
        memcpy(reversedCharacters + copiedCharacterCount, characters + characterRange.location, characterRange.length * sizeof(unichar));
        i = characterRange.location - 1;
        copiedCharacterCount += characterRange.length;
    }
    
    free(characters);
    
    NSString *reversedString = [[NSString alloc] initWithCharactersNoCopy:reversedCharacters length:length freeWhenDone:YES];
    if (!reversedString) {
        free(reversedCharacters);
    }
    
    return reversedString;
}


- (NSString *)qlc_reversedStringWithUTF32Buffer
{
    NSUInteger length = [self length];
    if (length < 2) {
        return self;
    }

    NSStringEncoding encoding = NSHostByteOrder() == NS_BigEndian ? NSUTF32BigEndianStringEncoding : NSUTF32LittleEndianStringEncoding;
    NSUInteger utf32ByteCount = [self lengthOfBytesUsingEncoding:encoding];
    uint32_t *characters = malloc(utf32ByteCount);
    if (!characters) {
        return nil;
    }

    [self getBytes:characters maxLength:utf32ByteCount usedLength:NULL encoding:encoding options:0 range:NSMakeRange(0, length) remainingRange:NULL];

    NSUInteger utf32Length = utf32ByteCount / sizeof(uint32_t);
    NSUInteger halfwayPoint = utf32Length / 2;
    for (NSUInteger i = 0; i < halfwayPoint; ++i) {
        uint32_t character = characters[utf32Length - i - 1];
        characters[utf32Length - i - 1] = characters[i];
        characters[i] = character;
    }

    return [[NSString alloc] initWithBytesNoCopy:characters length:utf32ByteCount encoding:encoding freeWhenDone:YES];
}

@end


int main(int argc, const char * argv[])
{
    srandomdev();
    
    for (NSUInteger j = 0; j < 5; ++j) {
        NSTimeInterval enumerateSubstringsTime = 0.0;
        NSTimeInterval memcpyTime = 0.0;
        NSTimeInterval utf32Time = 0.0;

        for (NSUInteger i = 0; i < 100000; ++i) {
            @autoreleasepool {
                NSString *string = UMKRandomUnicodeString();
                TIME([string qlc_reversedStringWithEnumerateSubstrings], enumerateSubstringsTime);
                TIME([string qlc_reversedStringWithMemCopy], memcpyTime);
                TIME([string qlc_reversedStringWithUTF32Buffer], utf32Time);
            }
        }
        
        NSLog(@"Enumerate substrings: %.6f", enumerateSubstringsTime);
        NSLog(@"MemCopy: %.6f", memcpyTime);
        NSLog(@"UTF-32: %.6f", utf32Time);
    }
    
    return 0;
}