// Faan Tone Liu  , demo program for comp 1672, winter 2003
// merge sort

#include <iostream>
#include <fstream>

using namespace std;
void merge_sort(int *a, int len);
void merge(int *dest, int *list1, int len1, int *list2, int len2);

// read elements of an array in from a file.  Sort them.
// Get search key from user, search for it in array.
int main()
{
    const int maxarrlen(100);
    const int maxnamelen(80);
    char filename[maxnamelen];
    ifstream listfile;
    int arr[maxarrlen];
    int numelements(0);
    int i;

    cout << "Enter file name containing list (no more than 100 elements): ";
    cin >> filename;
    listfile.open(filename, ios::in);   // open input file
	if (!listfile.is_open()) {
            cout << "Unable to open file." << endl;
            return 1;
	}

    // read the list from the file into the array
    while (listfile >> arr[numelements])
        numelements = numelements + 1;

    cout << "Before sorting: ";
    for (i=0; i < numelements; i++)
        cout << arr[i] << " ";
    cout << endl;

    //sort it
    merge_sort(arr, numelements);

    cout << "After sorting: ";
    for (i=0; i < numelements; i++)
        cout << arr[i] << " ";
    cout << endl;

    return 0;
}
							  
// sort the array a in increasing order, with merge sort
// a pointer to the beginning of the array, and the length of the array
// are passed as parameters
void merge_sort(int *a, int len)
{
    if (len == 1) return;
    merge_sort(a, len/2);
    merge_sort(a+len/2, len-len/2);
    merge(a, a, len/2, a+len/2, len-len/2);
}

// merge list1 and list2 together and store in dest.  Note that
// list1 and list2 might occupy the same space in memory, so we
// need a scratch copy.  The values in the sub-lists may be wiped out.
void merge(int *dest, int *list1, int len1, int *list2, int len2)
{
    int *scratch = new int[len1+len2];
    int pos1(0), pos2(0), i;

    for (i=0; i < len1+len2; i++) {
        if (pos2==len2 || (pos1 < len1 && list1[pos1] < list2[pos2]))
            scratch[i] = list1[pos1++];
        else scratch[i] = list2[pos2++];
    }
    for (i=0; i < len1+len2; i++)
        dest[i] = scratch[i];
    delete scratch;
}