NSOutputStream subclass which updates a SHA1 checksum for all data written

GGChecksummingOutputStream.h

//
//  GGChecksummingOutputStream.h
//
//  Created by Geoff Garside on 12/12/2010.
//  Copyright 2010 Geoff Garside. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <CommonCrypto/CommonDigest.h>


enum {  // error codes
    GGChecksummingOutputStreamErrorChecksumFinalized
};

@interface GGChecksummingOutputStream : NSOutputStream {
    CC_SHA1_CTX ctx;
    unsigned char digest[CC_SHA1_DIGEST_LENGTH];
}

- (id)initToMemory;
- (id)initToBuffer:(uint8_t *)buffer capacity:(NSUInteger)capacity;
- (id)initToFileAtPath:(NSString *)path append:(BOOL)shouldAppend;
- (id)initWithURL:(NSURL *)url append:(BOOL)shouldAppend;

- (BOOL)hasSpaceAvailable;
- (NSInteger)write:(const uint8_t *)buffer maxLength:(NSUInteger)length;
- (NSInteger)writeChecksum;
- (NSData *)checksumData;

@end

GGChecksummingOutputStream.m

//
//  GGChecksummingOutputStream.m
//
//  Created by Geoff Garside on 12/12/2010.
//  Copyright 2010 Geoff Garside. All rights reserved.
//

#import "GGChecksummingOutputStream.h"


@implementation GGChecksummingOutputStream

- (id)initToMemory {
    if ( self = [super initToMemory] )
        CC_SHA1_Init(&ctx);
    return self;
}
- (id)initToBuffer:(uint8_t *)buffer capacity:(NSUInteger)capacity {
    if ( self = [super initToBuffer:buffer capacity:capacity] )
        CC_SHA1_Init(&ctx);
    return self;
}
- (id)initToFileAtPath:(NSString *)path append:(BOOL)shouldAppend {
    if ( self = [super initToFileAtPath:path append:shouldAppend] )
        CC_SHA1_Init(&ctx);
    return self;
}
- (id)initWithURL:(NSURL *)url append:(BOOL)shouldAppend {
    if ( self = [super initWithURL:url append:shouldAppend] )
        CC_SHA1_Init(&ctx);
    return self;
}

- (BOOL)hasSpaceAvailable {
    if ( digest )
        return NO;
    return [super hasSpaceAvailable];
}

- (NSInteger)write:(const uint8_t *)buffer maxLength:(NSUInteger)length {
    if ( digest ) return -1;

    CC_SHA1_Update(&ctx, buffer, length);
    return [super write:buffer maxLength:length];
}

- (NSError *)streamError {
    if ( !digest )
        return [super streamError];

    NSError *error = [NSError errorWithDomain:@"com.github.gist.geoffgarside"
                                         code:GGChecksummingOutputStreamErrorChecksumFinalized
                                     userInfo:[NSDictionary dictionaryWithObjectsAndKeys:
                                                  NSLocalizedString(@"Checksum digest has already been calculated",
                                                     @"GGChecksummingOutputStreamErrorChecksumFinalized"), 
                                                  NSLocalizedDescriptionKey, nil]];
    return error;
}

- (NSData *)checksumData {
    if ( !digest )
        CC_SHA1_Final(digest, &ctx);
    return [NSData dataWithBytes:digest length:CC_SHA1_DIGEST_LENGTH];
}

- (NSInteger)writeChecksum {
    if ( !digest )
        CC_SHA1_Final(digest, &ctx);
    return [super write:digest maxLength:CC_SHA1_DIGEST_LENGTH];
}

@end