Objective-C category demonstration

NSString+UsefulShit.h

//
// NSString+UsefulStuff.h
//
// Duane Sibilly <duane@sibilly.com>
// 6/21/11
// Copyright (c) 2011-2012 Duane Sibilly. All rights reserved.

#import <Foundation/Foundation.h>

// The use of the parenthesis in the interface declaration is what tells the
// compiler that this is a category on the NSString class instead of a
// redefinition of NSString.
@interface NSString (UsefulStuff)

/**
 + (NSString*)stringTruncatedToWidth:withString:andFont:
 Generates a truncated copy of the given NSString, truncated to the desired
 width for the given typeface and size.

 width  - A CGFloat representing the desired width of the truncated NSString.
 string - An NSString object with the content to be truncated.
 font   - A UIFont object representing the desired typeface and font size.

 Example:
 NSString *message   = @"Can you hear this long-winded message?";
 UIFont *messageFont = [UIFont fontWithName:@"Marker Felt" size:32];
 NSString *output = [NSString stringTruncatedToWidth:48.0f 
 withString:message 
 andFont:messageFont];

 Returns an NSString containing the truncated string, followed by an ellipsis.
 */
+ (NSString*)stringTruncatedToWidth:(CGFloat)width 
						 withString:(NSString*)string
							andFont:(UIFont*)font;


/** 
 + (BOOL)stringIsPalindrome:
 Deterines if a provided NSString is a palindrome.

 aString - The NSString to be tested.

 Example:
 [NSString stringIsPalindrome:@"RADAR"]; // return YES

 Returns a BOOL cooresponding to the NSString palindrome status.
 */
+ (BOOL)stringIsPalindrome:(NSString*)aString;


/**
 - (NSString*)MD5Hash
 Generates an MD5 cryptographic hash of this NSString's contents

 Example:
 NSString *hash = [@"The quick brown fox jumped over the lazy dog" MD5Hash];

 Returns an NSString containing the hexidecimal representation of the MD5 hash.
 */
- (NSString*)MD5Hash;


/**
 - (NSString*)truncateToWidth:withFont:
 Generates an NSString truncated to the indicated width
 for a given a typeface and size.

 width - A CGFloat representing the desired width of the truncated NSString.
 font  - A UIFont object representing the desired typeface and font size.

 Example:
 NSString *testString = @"This string is too damn long!"
 [testString truncateToWidth:64.0f 
 withFont:[UIFont fontWithName:@"Helvetica"
 size:28]];

 Returns an NSString containing the truncated string, followed by an ellipsis.
 */
- (NSString*)truncateToWidth:(CGFloat)width withFont:(UIFont*)font;


/**
 - (BOOL)isPalindrome
 Determines whether this string is a palindrome.

 Example:
 [@"HANNAH" isPalindrome]; // returns YES
 [@"CLAUDE" isPalindrome]; // returns NO

 Returns a BOOL corresponding to this NSString's palindrome status.
 */
- (BOOL)isPalindrome;


/**
 -(NSString*)reverse
 Reverses the contents of this NSString.

 Example:
 NSString *testString = @"stressed";
 NSString *testReversed = [testString reverse]; // @"desserts"

 Returns an NSString with the original NSString's contents reversed
 */
- (NSString*)reverse;

@end

NSString+UsefulShit.m

//
//  NSString+UsefulStuff.m
//
//  Duane Sibilly <duane@sibilly.com>
//  6/21/11
//  Copyright (c) 2011-2012 Duane Sibilly. All rights reserved.

#import "NSString+UsefulStuff.h"
#import <CommonCrypto/CommonDigest.h>

#define ELLIPSIS @"..."

@interface NSString (UsefulStuffPrivate)

+(BOOL) stringIsPalindrome:(NSString *)aString position:(NSInteger)position;

@end


@implementation NSString (UsefulStuff)

+(NSString*) stringTruncatedToWidth:(CGFloat)width withString:(NSString *)string andFont:(UIFont *)font
{
	return [string truncateToWidth:width withFont:font];
}


+(BOOL) stringIsPalindrome:(NSString *)aString
{
	return [NSString stringIsPalindrome:aString position:0];
}


+(BOOL) stringIsPalindrome:(NSString *)aString position:(NSInteger)position
{
	NSString *_string = [NSString stringWithString:aString];
	NSInteger _position = position;

	if (! _string) {
		return NO;
	}

	NSInteger stringLength = [_string length];
	NSString *firstChar = [[_string substringToIndex:_position] substringToIndex:1];
	NSString *lastChar = [[_string substringToIndex:(stringLength - 1 - _position)] substringToIndex:1];

	if (_position > (stringLength / 2)) {
		return YES;
	}

	if (! [firstChar isEqualToString:lastChar]) {
		return NO;
	}

	return [NSString stringIsPalindrome:_string position:(_position + 1)];
}


-(NSString*) MD5Hash
{
	// Create a C-style pointer to the UT8-encoded contents of the NSString
	const char *pointer = [self UTF8String];

	// Create a buffer array big enough to hold the digest
	unsigned char buffer[CC_MD5_DIGEST_LENGTH];

	// Create 16-byte MD5 hash value, store in buffer
	// See: CC_MD5(3cc) manpage on OS X & iOS.
	CC_MD5(pointer, strlen(pointer), buffer);

	// Convert MD5 digest in buffer to an autoreleased NSString of hexidecimal
	// values.
	NSMutableString *result = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
	for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i += 1) {
		[result appendFormat:@"%02x", buffer[i]];
	}

	return [result copy];
}


-(NSString*) truncateToWidth:(CGFloat)width withFont:(UIFont *)font
{
	// Obtain a mutable copy of this NSString.
	NSMutableString *truncatedString = [self mutableCopy];

	// If this NSString is longer than the desired width, truncate.
	if ([self sizeWithFont:font].width > width) {
		// Subtract an ellipsis' worth of width from the desired width to obtain the
		// truncation width.
		width -= [ELLIPSIS sizeWithFont:font].width;

		// While the string is longer than the truncation width, remove characters
		// from the end of the string.
		NSRange range = {truncatedString.length - 1, 1};
		while ([truncatedString sizeWithFont:font].width > width) {
			[truncatedString deleteCharactersInRange:range];
			range.location -= 1;
		}

		// Once truncation is complete, append an ellipsis to the end of the string.
		[truncatedString replaceCharactersInRange:range withString:ELLIPSIS];
	}

	return [truncatedString copy];
}


-(BOOL) isPalindrome
{
	return [NSString stringIsPalindrome:self];
}


-(NSString*) reverse
{
	NSMutableString *reversedString = [NSMutableString stringWithCapacity:[self length]];

	for (int i = ([self length] - 1); i >= 0; i -= 1) {
		[reversedString appendString:[NSString stringWithFormat:@"%C", [self characterAtIndex:i]]];
	}

	return reversedString;
}

@end

The class methods really should be instance methods. Selectors shouldn't have parts beginning with "and". Category methods on code you don't own really should start with a prefix, or apps will break in mysterious ways when the original class gain methods with the same names.