use readM to implement readN

readN.cpp

#include <iostream>
#include <sstream>
using namespace std;

// ******************** m reader mocker *********************

string inputstr;

void initInputStr() {
    string line;
    stringstream ss;
    while (getline(cin, line)) {
        ss << line << endl;
    }
    inputstr = ss.str();
}

class MBufReader {
public:
    static const int M = 43;
    int cur;
    int readM(char buf[]) {
        int i = 0;
        while (i < M && cur < inputstr.size()) {
            buf[i] = inputstr[cur];
            i++;
            cur++;
        }
        return i;
    }

    MBufReader() {
        reinit();
    }

    void reinit() {
        cur = 0;
    }
};

MBufReader mrb;

// ******************** real code *********************

class NBufReader {
public:
    int mcur;
    int msize;
    char mbuf[MBufReader::M];

    int readN_Naive(char buf[], int n) {
        int ncur = 0;
        while(ncur < n) {
            if (mcur < msize) {
                buf[ncur] = mbuf[mcur];
                ncur++;
                mcur++;
            } else {
                msize = mrb.readM(mbuf);
                mcur = 0;
                if (msize == 0) return ncur;
            }
        }

        return ncur;
    }

    int readN(char buf[], int n) {
        int ncur = 0;
        while(ncur < n) {
            if (mcur < msize) {
                buf[ncur] = mbuf[mcur];
                ncur++;
                mcur++;
            } else if (n - ncur >= MBufReader::M) {
                int nread = mrb.readM(buf + ncur);
                if (nread == 0) return ncur;
                ncur += nread;
            } else {
                msize = mrb.readM(mbuf);
                mcur = 0;
                if (msize == 0) return ncur;
            }
        }

        return ncur;
    }

    NBufReader() {
        mcur = 0;
        msize = 0;
    }
};

// ******************** testing *********************

int main() {
    initInputStr();

    const int N = 83;
    char buf[100000];

    for (int N = 23; N < 197; N++) {
        mrb.reinit();
        stringstream ss;
        NBufReader br;
        int n = 0;
        while ((n = br.readN(buf, N)) != 0) {
            for (int i = 0; i < n; i++) {
                ss << buf[i];
            }
        }
        if (ss.str() == inputstr) {
            cout << "N size " << N << " correct!" << endl;  
        } else {
            cout << "N size " << N << " wrong!" << endl;  
        }
    }
    return 0;
}