decode M notation from cat -v

decode_m_notation.cpp

#include <iostream>
using std::cin;
using std::cout;
using std::getline;
using std::endl;

#include <string>
using std::string;

#include <string_view>
using std::string_view;

string decode_m_notation(string_view in) {
    string_view::size_type in_len = in.size();
    string result;
    string prev;

    for (string::size_type i = 0; i < in_len; i += 1) {
        if (prev == "M-^") {
            if (in[i] > 63 && in[i] <= 95) {
                result.append(1, in[i] + 64);
            } else if (in[i] == 63) {
                result.append(1, 255);
            } else {
                result.append("M-^");
                result.append(1, in[i]);
            }
            prev = "";
        } else if (prev == "M-") {
            if (in[i] == '^') {
                prev.append("^");
            } else if (in[i] >= 32 && in[i] <= 126) {
                result.append(1, in[i] + 128);
                prev = "";
            } else {
                result.append("M-");
                result.append(1, in[i]);
                prev = "";
            }
        } else if (prev == "M") {
            if (in[i] == '-') {
                prev.append("-");
            } else {
                result.append("M");
                result.append(1, in[i]);
                prev = "";
            }
        } else if (prev == "^") {
            if (in[i] > 63 && in[i] <= 126) {
                result.append(1, in[i] - 64);
            } else if (in[i] == 63) {
                result.append(1, 127);
            } else {
                result.append("^");
                result.append(1, in[i]);
            }
            prev = "";
        } else {
            if (in[i] == 'M') {
                prev.append("M");
            } else if (in[i] == '^') {
                prev.append("^");
            } else {
                result.append(1, in[i]);
            }
        }
    }

    return result;
}

int main(void) {
    for (string line; getline(cin, line);) {
        cout << decode_m_notation(line) << endl;
    }

    return 0;
}

/*
> g++ -std=c++20 decode_m_notation.cpp -o decode_m_notation
> printf '你好，世界' | cat -v | ./decode_m_notation
*/