Constexpr methods to handle strings.

constexpr_string_methods.hpp

#include <cstdint>
#include <string_view>

/// Trim leading chars_to_trim from str.
constexpr std::string_view ltrim(
    const std::string_view str,
    const std::string_view chars_to_trim = " \t\n\r")
{
    if (const auto pos = str.find_first_not_of(chars_to_trim); pos != std::string_view::npos)
    {
        return str.substr(pos);
    }

    return str;
}

/// Trim trailing chars_to_trim from str.
constexpr std::string_view rtrim(
    const std::string_view str,
    const std::string_view chars_to_trim = " \t\n\r")
{
    if (const auto pos = str.find_last_not_of(chars_to_trim); pos != std::string_view::npos)
    {
        return str.substr(0, pos + 1ul);
    }

    return str;
}

/// Trim leading and trailing chars_to_trim from str.
constexpr std::string_view trim(
    const std::string_view str,
    const std::string_view chars_to_trim = " \t\n\r")
{
    return rtrim(ltrim(str, chars_to_trim), chars_to_trim);
}

/// Find n-th occurrence of c in str. Return str.size() if the given occurrence
/// is not found. find_nth_occurrence<0>(...) always returns zero.
template <size_t N>
constexpr size_t find_nth_occurrence(const std::string_view str, const char c, size_t pos = 0ul)
{
    if (N >= str.size())
    {
        return str.size();
    }

    size_t count{};

    while (count < N)
    {
        if (pos = str.find(c, pos + 1); pos == std::string_view::npos)
        {
            return str.size();
        }
        ++count;
    }

    return pos;
}

/// Split the n-th argument in a list of comma-separated arguments.
///
///     split_nth_argument<0>("a, b, c") == "a"
///     split_nth_argument<1>("a, b, c") == "b"
///     split_nth_argument<2>("a, b, c") == "c"
///
template <size_t N>
constexpr std::string_view split_nth_argument(const std::string_view str, const char c = ',')
{
    if constexpr (N == 0ul)
    {
        return trim(str.substr(0ul, find_nth_occurrence<1ul>(str, c)));
    }
    else
    {
        auto begin = std::min(str.size(), 1ul + find_nth_occurrence<N>(str, c));
        auto end = find_nth_occurrence<1ul>(str, c, begin);
        return trim(str.substr(begin, end - begin));
    }
}