Issue with string::rfind

[gashtio]

There seems to be a problem with an old commit that was fixing GCC warnings but I guess a typo was made and now string::rfind doesn't work correctly. The commit I found is this: https://github.com/boostorg/container/commit/dad2cb2d02aeccbf2d47eb0e18d73e52b9c51991#diff-aa8e68362c274ac9daa4a59b84b905bfc55747cd6d52c0ec08d3fa1d1350b9beR2442

The previous calculation took the minimum of the haystack length minus the length of the needle and the offset, which is npos by default and then added the needle length separately. Now, the addition is made to the offset argument, which could overflow and the minimum is no longer correct.

Here's a minimal example that demonstrates the issue:

#include <boost/container/string.hpp>
#include <string>
#include <iterator>
#include <iostream>

constexpr const char* HAYSTACK = "cubic_bezier(0,1,0,0)";
constexpr const char* NEEDLE = ")";

int main() {
    boost::container::string boost_str(HAYSTACK);
    auto boost_rfind_pos = boost_str.rfind(NEEDLE);

    std::string std_str(HAYSTACK);
    auto std_rfind_pos = std_str.rfind(NEEDLE);

    std::cout << "BOOST position: " << boost_rfind_pos <<
        "\nSTD position: " << std_rfind_pos << std::endl;

    return 0;
}

Output is:

BOOST position: 18446744073709551615
STD position: 20

The fix should be trivial, just separate the addition of n from the minimum function, as it was before.

[igaztanaga]

Many thanks for the report!