CODEWITHSUNDEEP
c++stringcomparisonbinaryfiles
Victor L
Victor L

Reputation: 10230

What's the fastest way to tell whether two strings or binary files are different?

I'm writing a unit test and need to compare a result file to a golden file. What's the easiest way to do so?

So far I have (for Linux environment):

int result = system("diff file1 file2");

They are different if result != 0.

Upvotes: 16

Views: 15757

Answers (5)

pmr
pmr

Reputation: 59811

This should work:

#include <string>
#include <fstream>
#include <streambuf>
#include <iterator>


bool equal_files(const std::string& a, const std::string& b) {
  std::ifstream stream{a};
  std::string file1{std::istreambuf_iterator<char>(stream),
                    std::istreambuf_iterator<char>()};
  
  stream = std::ifstream{b};
  std::string file2{std::istreambuf_iterator<char>(stream),
                    std::istreambuf_iterator<char>()};
  
  return file1 == file2;
}

I suspect this to be not as fast as diff, but it avoids calling system. It should be sufficient for a test-case, though.

Upvotes: 1

Antonio
Antonio

Reputation: 20266

Developing from DaveS's answer, and as first thing checking file size:

#include <fstream>
#include <algorithm>

bool compare_files(const std::string& filename1, const std::string& filename2)
{
    std::ifstream file1(filename1, std::ifstream::ate | std::ifstream::binary); //open file at the end
    std::ifstream file2(filename2, std::ifstream::ate | std::ifstream::binary); //open file at the end
    const std::ifstream::pos_type fileSize = file1.tellg();

    if (fileSize != file2.tellg()) {
        return false; //different file size
    }

    file1.seekg(0); //rewind
    file2.seekg(0); //rewind

    std::istreambuf_iterator<char> begin1(file1);
    std::istreambuf_iterator<char> begin2(file2);

    return std::equal(begin1,std::istreambuf_iterator<char>(),begin2); //Second argument is end-of-range iterator
}

(I wonder if before rewinding, fileSize could be used to create a more efficient end of stream iterator, which, by knowing the stream length, would allow std::equal to process more bytes at the time).

Upvotes: 7

Dave S
Dave S

Reputation: 21113

If you want a pure c++ solution, I would do something like this

#include <algorithm>
#include <iterator>
#include <string>
#include <fstream>

template<typename InputIterator1, typename InputIterator2>
bool
range_equal(InputIterator1 first1, InputIterator1 last1,
        InputIterator2 first2, InputIterator2 last2)
{
    while(first1 != last1 && first2 != last2)
    {
        if(*first1 != *first2) return false;
        ++first1;
        ++first2;
    }
    return (first1 == last1) && (first2 == last2);
}

bool compare_files(const std::string& filename1, const std::string& filename2)
{
    std::ifstream file1(filename1);
    std::ifstream file2(filename2);

    std::istreambuf_iterator<char> begin1(file1);
    std::istreambuf_iterator<char> begin2(file2);

    std::istreambuf_iterator<char> end;

    return range_equal(begin1, end, begin2, end);
}

It avoids reading the entire file into memory, and stops as soon as the files are different (or at end of file). The range_equal because std::equal doesn't take a pair of iterators for the second range, and isn't safe if the second range is shorter.

Upvotes: 24

Lufi
Lufi

Reputation: 161

Might be an overkill but you could build a table of hashes SHA-256 using boost/bimap and boost/scope_exit.

Here is a video how to do this by Stephan T Lavavej (starts at 8.15): http://channel9.msdn.com/Series/C9-Lectures-Stephan-T-Lavavej-Advanced-STL/C9-Lectures-Stephan-T-Lavavej-Advanced-STL-5-of-n

For more info about algorithm: http://en.wikipedia.org/wiki/SHA-2

Upvotes: 0

gbjbaanb
gbjbaanb

Reputation: 52679

one way to prevent reading both files is to pre-compute the golden file into a hash, eg a md5. Then you only have to check the test file. Note, this may be slower than just reading both files!

Alternatively, layer your checking - look at the file sizes, if they're different then the files are different and you can avoid a lengthy read-and-compare operation.

Upvotes: 2

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