A very naive bencoding decoder I wrote for a personal project.

Bencoding.h

//
//  Bencoding.h
//  bencoder
//
//  Created by Thomas Rix on 7/21/12.
//  Copyright (c) 2012 Thomas Rix. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface Bencoding : NSObject
- (id)initWithString:(NSString *)str;
- (void)resetWithString:(NSString *)str;
- (id)decode;
@end





//
//  Bencoding.m
//  bencoder
//
//  Created by Thomas Rix on 7/21/12.
//  Copyright (c) 2012 Thomas Rix. All rights reserved.
//

#import "Bencoding.h"

@interface Bencoding ()
- (NSString *)decodeStr;
- (NSNumber *)decodeInt;
- (NSArray *)decodeList;
- (NSDictionary *)decodeDict;
- (NSString *)currentString;
@end

@implementation Bencoding {
    NSString* _str;
    int64_t _offset;
}

- (id)initWithString:(NSString *)str {
    self = [super init];
    if (self) {
        [self resetWithString:str];
    }
    return self;
}

- (void)resetWithString:(NSString *)str {
    _str = str;
    _offset = 0;
}

- (id)decode {
    switch ([[self currentString] characterAtIndex:0]) {
        case 'i':
            return [self decodeInt];
        case 'l':
            return [self decodeList];
        case 'd':
            return [self decodeDict];
        default:
            return [self decodeStr];
    }
}

- (NSString *)decodeStr {
    NSString* string = [self currentString];
    
    NSInteger colonLocation = [string rangeOfString:@":"].location;
    NSString* lengthString = [string substringWithRange:NSMakeRange(0, colonLocation)];
    
    _offset += ([lengthString intValue] + colonLocation + 1);
    return [string substringWithRange:NSMakeRange(colonLocation + 1, [lengthString intValue])];
}

- (NSNumber *)decodeInt {
    NSString* intString = @"";

    _offset++;
    while ([[self currentString] characterAtIndex:0] != 'e') {
        intString = [intString stringByAppendingFormat:@"%C", [[self currentString] characterAtIndex:0]];
        _offset++;
    }
    _offset++;
    
    return [NSNumber numberWithInt:[intString intValue]];
}

- (NSArray *)decodeList {
    NSMutableArray* list = [[NSMutableArray alloc] init];
    
    _offset++;
    while ([[self currentString] characterAtIndex:0] != 'e') {
        [list addObject:[self decode]];
    }
    _offset++;
    
    return [NSArray arrayWithArray:list];
}

- (NSArray *)decodeDict {
    NSMutableDictionary* dict = [[NSMutableDictionary alloc] init];
    
    _offset++;
    while ([[self currentString] characterAtIndex:0] != 'e') {
        NSString* key = [self decode];
        [dict setObject:[self decode] forKey:key];
    }
    _offset++;
    
    return [NSDictionary dictionaryWithDictionary:dict];
}

- (NSString *)currentString {
    return [_str substringFromIndex:_offset];
}

@end