// Ben Harris
// Main: Program6
// This program reads a file, uses the first four bytes as an integer record size and sorts the rest of the file in recordsz chunks.

#include <iostream>
#include <cstdlib>
#include <fstream>
#include "PG6.h"

using namespace std;

int main(int argc, char **argv) {
    string fn = "";
    streampos fsz;
    cout << "Enter the name of the file that you want to sort." << endl << "File Name: ";
    getline(cin, fn); // Get file name from user
    fstream f(fn, ios::in | ios::out | ios::binary); // Open file from input location for read and write and as binary
    if (f.is_open()) {

        char *recordsz = new char[4];
        f.read(recordsz, (streamsize) 4); // Read first four bytes
        int rsz = atoi(recordsz); // Store first four bytes as int record size
        f.seekg(0, f.end);
        fsz = f.tellg(); // Get length of file
        int recordcnt = ((int) fsz - 4) / rsz; // Get # of records
        f.seekg(0, f.beg);

        // Begin shell sort
        int d = recordcnt;
        while (d > 1) {
            d = (d == 2) ? 1 : d % 2 == 1 ? (d + 1) / 2 : d % 4 == 0 ? d / 2 + 1 : d / 2 + 2;
            for (int i = d;
                 i < recordcnt; i++) // Use readstring and writestring methods to get and put strings in the file
                for (int j = i - d; j >= 0 && readstring(f, j, rsz) > readstring(f, j + d, rsz); j -= d) {
                    string t = readstring(f, j, rsz);
                    writestring(f, j, rsz, readstring(f, j + d, rsz));
                    writestring(f, j + d, rsz, t);
                }
        }
        cout << "Sort successful." << endl;
        f.close();
    } else cout << "Error: file could not be opened or does not exist." << endl;
}

string readstring(fstream &f, int recordpos,
                  int len) { // This method seeks to the position of recordpos * len, reads len bytes and returns that string
    char *t = new char[len];
    f.seekg(recordpos * len + 4, ios::beg);
    f.read(t, len);
    return (string) t;
}

void writestring(fstream &f, int recordpos, int len, string s) { // Writes string s to f at position recordpos*len
    f.seekp(recordpos * len + 4, ios::beg);
    char *t = new char[s.length()];
    for (unsigned int i = 0; i < s.length(); i++) t[i] = s[i];
    f.write(t, len);
}