Speed up BTCHexFromData by 5.5x (-85%)

[wjmelements]

I wrote an optimized version of BTCHexFromData. The speedup with -O3 is approximately 85% (or 5.5x) compared to the current implementation.

The speedup comes from specialized printing and reduced memory allocation.

Here is the program I used to time the change:

@import Foundation;
#import <mach/mach_time.h>

static char hexValues[16] = {
    '0',
    '1',
    '2',
    '3',
    '4',
    '5',
    '6',
    '7',
    '8',
    '9',
    'a',
    'b',
    'c',
    'd',
    'e',
    'f',
};

NSString *toHex(NSData *data) {
    size_t inLength = data.length;
    size_t length = inLength * 2;
    uint8_t *outString = malloc(length);
    const unsigned char *bytes = data.bytes;
    for (uint32_t i = 0; i < inLength; i++) {
        unsigned char byte = bytes[i];
        outString[i*2] = hexValues[(byte & 0xF0) >> 4];
        outString[i*2+1] = hexValues[(byte & 0x0F)];
    }
    return [[NSString alloc]
        initWithBytesNoCopy:outString
        length:length
        encoding:NSUTF8StringEncoding
        freeWhenDone:YES
    ];
}

NSString* BTCHexFromDataWithFormat(NSData* data, const char* format) {
    if (!data) return nil;
    
    NSUInteger length = data.length;
    if (length == 0) return @"";
    
    NSMutableData* resultdata = [NSMutableData dataWithLength:length * 2];
    char *dest = resultdata.mutableBytes;
    unsigned const char *src = data.bytes;
    for (int i = 0; i < length; ++i) {
        sprintf(dest + i*2, format, (unsigned int)(src[i]));
    }
    return [[NSString alloc] initWithData:resultdata encoding:NSASCIIStringEncoding];
}

NSString* BTCHexFromData(NSData* data) {
    return BTCHexFromDataWithFormat(data, "%02x");
}


NSMutableArray<NSData *> *testVector;
void initVector() {
    testVector = [NSMutableArray array];
    for (int i = 0; i < 400000; i++) {
        void *buf = malloc(32);
        arc4random_buf(buf, 32);
        [testVector
            addObject:[NSData
                dataWithBytesNoCopy:buf
                length:32
                freeWhenDone:YES
            ]
        ];
    }
}
NSMutableArray<NSString *> *outVector;
void timeMethod(NSString *(method)(NSData *)) {
    outVector = [NSMutableArray array];
    for (NSData *data in testVector) {
        [outVector addObject:method(data)];
    }
}

int main() {
    @autoreleasepool {
        uint64_t start = mach_absolute_time();
        initVector();
        uint64_t setup = mach_absolute_time();
        timeMethod(BTCHexFromData);
        uint64_t btcHex = mach_absolute_time();
        timeMethod(toHex);
        uint64_t hex = mach_absolute_time();
        NSLog(@"setup : %llu", setup - start);
        NSLog(@"btcHex: %llu", btcHex - setup);
        NSLog(@"Hex   : %llu", hex - btcHex);
        return 0;
    }
}

Here is the output:

[email protected]:~/projects/CoreBitcoin(master)*$ clang -fmodules timer.m 
[email protected]:~/projects/CoreBitcoin(master)*$ ./a.out 
2018-07-13 18:53:14.825 a.out[71380:1588521] setup : 174539326
2018-07-13 18:53:14.826 a.out[71380:1588521] btcHex: 1138551621
2018-07-13 18:53:14.826 a.out[71380:1588521] Hex   : 217758575
[email protected]:~/projects/CoreBitcoin(master)*$ clang -fmodules -O3 timer.m
[email protected]:~/projects/CoreBitcoin(master)*$ ./a.out 
2018-07-13 18:53:58.248 a.out[71425:1589886] setup : 172171645
2018-07-13 18:53:58.248 a.out[71425:1589886] btcHex: 1148045936
2018-07-13 18:53:58.248 a.out[71425:1589886] Hex   : 172938196

Reviewer @oleganza

[wjmelements]

The latest commits improve the non--O3 speed slightly more. New timer.m:

@import Foundation;
#import <mach/mach_time.h>

static const char hexValues[16] = {
    '0',
    '1',
    '2',
    '3',
    '4',
    '5',
    '6',
    '7',
    '8',
    '9',
    'a',
    'b',
    'c',
    'd',
    'e',
    'f',
};

NSString *toHex(NSData *data) {
    size_t inLength = data.length;
    size_t length = inLength * 2;
    uint8_t *outString = malloc(length);
    const unsigned char *bytes = data.bytes;
    for (uint32_t i = 0; i < inLength; i++) {
        unsigned char byte = bytes[i];
        outString[i*2] = hexValues[byte >> 4];
        outString[i*2+1] = hexValues[byte & 0x0F];
    }
    return [[NSString alloc]
        initWithBytesNoCopy:outString
        length:length
        encoding:NSASCIIStringEncoding
        freeWhenDone:YES
    ];
}

NSString* BTCHexFromDataWithFormat(NSData* data, const char* format) {
    if (!data) return nil;
    
    NSUInteger length = data.length;
    if (length == 0) return @"";
    
    NSMutableData* resultdata = [NSMutableData dataWithLength:length * 2];
    char *dest = resultdata.mutableBytes;
    unsigned const char *src = data.bytes;
    for (int i = 0; i < length; ++i) {
        sprintf(dest + i*2, format, (unsigned int)(src[i]));
    }
    return [[NSString alloc] initWithData:resultdata encoding:NSASCIIStringEncoding];
}

NSString* BTCHexFromData(NSData* data) {
    return BTCHexFromDataWithFormat(data, "%02x");
}


NSMutableArray<NSData *> *testVector;
void initVector() {
    testVector = [NSMutableArray array];
    for (int i = 0; i < 400000; i++) {
        void *buf = malloc(32);
        arc4random_buf(buf, 32);
        [testVector
            addObject:[NSData
                dataWithBytesNoCopy:buf
                length:32
                freeWhenDone:YES
            ]
        ];
    }
}
NSMutableArray<NSString *> *outVector;
void timeMethod(NSString *(method)(NSData *)) {
    outVector = [NSMutableArray array];
    for (NSData *data in testVector) {
        [outVector addObject:method(data)];
    }
}

int main() {
    @autoreleasepool {
        uint64_t start = mach_absolute_time();
        initVector();
        uint64_t setup = mach_absolute_time();
        timeMethod(BTCHexFromData);
        uint64_t btcHex = mach_absolute_time();
        timeMethod(toHex);
        uint64_t hex = mach_absolute_time();
        NSLog(@"setup : %llu", setup - start);
        NSLog(@"btcHex: %llu", btcHex - setup);
        NSLog(@"Hex   : %llu", hex - btcHex);
        return 0;
    }
}

output:

[email protected]:~/projects/CoreBitcoin(speedy-btcdata)*$ clang -fmodules timer.m
[email protected]:~/projects/CoreBitcoin(speedy-btcdata)*$ ./a.out 
2018-07-13 19:25:00.986 a.out[78612:1648181] setup : 170093611
2018-07-13 19:25:00.986 a.out[78612:1648181] btcHex: 1145701422
2018-07-13 19:25:00.986 a.out[78612:1648181] Hex   : 210011202
[email protected]:~/projects/CoreBitcoin(speedy-btcdata)*$ clang -fmodules -O3 timer.m
[email protected]:~/projects/CoreBitcoin(speedy-btcdata)*$ ./a.out 
2018-07-13 19:25:16.981 a.out[78656:1648634] setup : 171016593
2018-07-13 19:25:16.981 a.out[78656:1648634] btcHex: 1117013679
2018-07-13 19:25:16.981 a.out[78656:1648634] Hex   : 169127477

[wjmelements]

@oleganza ptal

[hasti0073]

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