NSString Extensions

NSString+StringExtensions.h

//
//  NSString+StringExtensions.h
//  StringFunctions
//
//  Created by Geoff Armstrong on 10/29/15.
//  Copyright © 2015 Geoff Armstrong. All rights reserved.
//

#ifndef NSString_StringExtensions_h
#define NSString_StringExtensions_h

@interface NSString (StringExtensions)

- (instancetype) reverseString;

- (instancetype) reverseParagraph;

/**
 Function to convert string to an integer with checks for invalid characters and overflow
 @return The integer, 0 if invalid
 */
- (int) stringToInteger;

/**
 Function to convert string to an array of all possible permutations of the characters in
 the string.
 @param theString The string to permute
 @return Array containing all the string permutations
 */
- (NSArray *) permutedStringArrayFromString: (NSString *) theString;

@end
#endif /* NSString_StringExtensions_h */

NSString+StringExtensions.m

//
//  NSString+StringExtensions.m
//  StringFunctions
//
//  Created by Geoff Armstrong on 10/29/15.
//  Copyright © 2015 Geoff Armstrong. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "NSString+StringExtensions.h"
#define asciiShift 48

@implementation NSString (StringExtensions)

- (instancetype) reverseString {
    NSMutableString *revString = [[NSMutableString alloc] init];
    
    for (int i=0; i < self.length; i++){
        unichar character = [self characterAtIndex:(NSUInteger) i];
        NSString *charStr = [NSString stringWithCharacters:&character length:1];
        [revString insertString:charStr atIndex:0];
    }
    
    return (NSString *)revString;
}

- (instancetype) reverseParagraph {
    NSMutableString *revString = [[NSMutableString alloc] init];
    NSMutableArray *wordArray = [[NSMutableArray alloc] init];
    
    for (int i=0; i < self.length; i++) {
        unichar character = [self characterAtIndex:i];
        if (character == ' ') {
            //Add the word to the word array in reverse order
            [wordArray insertObject:[NSString stringWithString:revString] atIndex:0];
            [revString setString:@""];
        }
        else {
            NSString *charStr = [NSString stringWithCharacters: &character length:1];
            [revString appendString: charStr];
        }
    }
    //At the end of the paragraph revString should already be set to the last word, just need to add a space
    [revString appendString: @" "];
    for (int j=0; j < wordArray.count; j++) {
        NSString *theWord = [wordArray objectAtIndex:j];
        [revString appendString: theWord];
        if (j < wordArray.count - 1) {
            [revString appendString: @" "];
        }
    }
    
    return revString;
}

- (int) stringToInteger {
    int newInt = 0;
    BOOL isNegative = NO;

    for (int i=0; i < [self length]; i++) {
        unichar character = [self characterAtIndex:i];
        if (character == '-') {
            isNegative = YES;
        }
        //Check to make sure it is a numeral
        else if (character < 48 || character > 57) {
            return 0;
        }
        else {
            //Check for overflow
            if (newInt > (INT_MAX - character + asciiShift) / 10) {
                return 0;
            }
            //Multiply by 10 to shift digit left
            newInt = newInt * 10;
            
            //subtract 48 to convert ASCII character to the corresponding number
            newInt += (character - asciiShift);
        }
        
    }
    if (isNegative) {
        newInt *= -1;
    }
    return newInt;
}

- (NSArray *) permutedStringArrayFromString: (NSString *) theString {
    NSMutableArray *theArray = [[NSMutableArray alloc] init];
    //The length should never be 0
    if ([theString length] == 0) {
        return theArray;
    }
    //The length will be 1 at the end of the string
    if ([theString length] == 1) {
        [theArray addObject:theString];
        return theArray;
    }
    
    NSString *remainingCharacters = [theString substringFromIndex:1];
    //Recursive call to this function will break each string down into smaller pieces until it reaches
    //a single character
    NSArray *remainingCharacterArray = [self permutedStringArrayFromString: remainingCharacters];
    
    //Get the first character of the string so that it can be placed at all spots within the array
    unichar character = [theString characterAtIndex:0];
    NSString *firstCharacter = [NSString stringWithCharacters: &character length:1];
    for (int j=0; j < remainingCharacterArray.count; j++) {
        for (int i=0; i <= remainingCharacters.length; i++) {
            NSString *arrayString = [remainingCharacterArray objectAtIndex:j];
            NSMutableString *newString = [NSMutableString stringWithString:arrayString];
            [newString insertString:firstCharacter atIndex:i];
            [theArray addObject:newString];
        }
    }
    return theArray;
}

@end