Task: Run-length encoding


C++: #include <algorithm>
#include <array>
#include <iterator>
#include <limits>
#include <tuple>

namespace detail_ {


constexpr auto digits = std::array{'0','1','2','3','4','5','6','7','8','9'};


template <typename OutputIterator>
constexpr auto encode_run_length(std::size_t n, OutputIterator out)
{
    constexpr auto base = digits.size();
    
    
    auto const num_digits = [base](auto n)
    {
        auto d = std::size_t{1};
        while ((n /= digits.size()))
            ++d;
        return d;
    }(n);
    
    
    auto base_power = [base](auto n)
    {
        auto res = decltype(base){1};
        for (auto i = decltype(n){1}; i < n; ++i)
            res *= base;
        return res;
    };
    
    
    for (auto i = decltype(num_digits){0}; i < num_digits; ++i)
        *out++ = digits[(n / base_power(num_digits - i)) % base];
    
    return out;
}




template <typename InputIterator>
auto decode_run_length(InputIterator first, InputIterator last)
{
    auto count = std::size_t{0};
    
    while (first != last)
    {
        
        auto const p = std::find(digits.begin(), digits.end(), *first);
        if (p == digits.end())
            break;
        
        
        count *= digits.size();
        count += std::distance(digits.begin(), p);
        
        
        ++first;
    }
    
    return std::tuple{count, first};
}

} 

template <typename InputIterator, typename OutputIterator>
constexpr auto encode(InputIterator first, InputIterator last, OutputIterator out)
{
    while (first != last)
    {
        
        auto const value = *first++;
        
        
        auto count = std::size_t{1};
        while (first != last && *first == value)
        {
            ++count;
            ++first;
        }
        
        
        out = detail_::encode_run_length(count, out);
        *out++ = value;
    }
    
    return out;
}






template <typename InputIterator, typename OutputIterator>
auto decode(InputIterator first, InputIterator last, OutputIterator out)
{
    while (first != last)
    {
        using detail_::digits;
        
        
        
        auto count = std::size_t{1};
        if (std::find(digits.begin(), digits.end(), *first) != digits.end())
            std::tie(count, first) = detail_::decode_run_length(first, last);
        
        
        out = std::fill_n(out, count, *first++);
    }
    
    return out;
}

template <typename Range, typename OutputIterator>
constexpr auto encode(Range&& range, OutputIterator out)
{
    using std::begin;
    using std::end;
    
    return encode(begin(range), end(range), out);
}

template <typename Range, typename OutputIterator>
auto decode(Range&& range, OutputIterator out)
{
    using std::begin;
    using std::end;
    
    return decode(begin(range), end(range), out);
}



#include <iostream>
#include <string_view>

int main()
{
    using namespace std::literals;
    
    constexpr auto test_string = "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW"sv;
    
    std::cout << "Input:  \"" << test_string << "\"\n";
    std::cout << "Output: \"";
    
    encode(test_string, std::ostreambuf_iterator<char>{std::cout});
    std::cout << "\"\n";
    
    auto encoded_str = std::string{};
    auto decoded_str = std::string{};
    encode(test_string, std::back_inserter(encoded_str));
    decode(encoded_str, std::back_inserter(decoded_str));
    
    std::cout.setf(std::cout.boolalpha);
    std::cout << "Round trip works: " << (test_string == decoded_str) << '\n';
}