Problem Link : http://www.spoj.com/problems/MATSUM/

I implemented a 2D BIT and used query/getSum function to find sum of rectangle from point O(0,0)[top left] to bottom right point (other two points can be derived). For operation SET(2,2,12), my code is storing value 24 at point (2,2) instead of 12. Can't figure out whether it's an implementation fault or conceptual fault. Can anyone help to figure it out and solve ?

#include <bits/stdc++.h>
using namespace std;

int BIT[1025][1025];

void update(int x, int y, int range, int value)
{
    ++x; ++y; ///Index starts from 1
    int y1;
    while(x <= range)
    {
        y1 = y;
        while(y1 <= range)
        {
            BIT[x][y1] += value;
            y1 += (y1 & (-y1));
        }
        x += (x & (-x));
    }
}

int getSum(int x,int y)
{
    ++x; ++y; ///Index starts from 1
    int y1,result = 0;

    while(x > 0)
    {
        y1 =  y;

        while(y1 > 0)
        {
           result += BIT[x][y];
           y1 -= (y1 & (-y1));
        }
        x -= (x & (-x));
    }
    return result;
}
int main()
{
    int t,n,x,y,value,x1,y1,max_x,max_y,min_x,min_y,indexValue,addValue,OA,OB,OC,OD,ans;
    char str[5],ch;
    scanf("%d",&t);

    while(t--)
    {
        scanf("%d",&n);
        memset(BIT,0,sizeof BIT);

        while(true)
        {

            getchar();
            scanf("%s",str);
            if(strcmp(str,"END") == 0)
            {
                break;
            }
            else if(strcmp(str,"SET") == 0)
            {
                scanf("%d%d%d",&x,&y,&value);
                indexValue = getSum(x,y) - getSum(x-1,y) - getSum(x,y-1) + getSum(x-1,y-1); ///Value at the original matrix(x,y).
                //cout<<"Original matrix'x Index Value[x][y] = "<<indexValue<<endl;
                addValue = value - indexValue; ///Value need to be added
                update(x,y,n,addValue);
            }
            else if(strcmp(str,"SUM") == 0)
            {
                scanf("%d%d%d%d",&x,&y,&x1,&y1);
                max_x = max(x,x1);
                max_y = max(y,y1);
                min_x = min(x,x1);
                min_y = min(y,y1);

                OA = getSum(min_x-1,min_y-1);
                OB = getSum(min_x-1,max_y);
                OC = getSum(max_x,max_y);
                OD = getSum(max_x,min_y-1);

                ans = OC-OB-OD+OA;
                //cout<<"Track:  "<<OA<<" "<<OB<<" "<<OC<<" "<<OD<<endl;
                printf("%d\n",ans);

            }
        }
    }
    return 0;
}

/**

|
| O(0,0)
|------------------------------------------------> y
|
|
|
|
|     A(min_x,min_y).
|     .--------------------------------.  B(min_x,max_y)
|     |                                |
|     |                                |
|     |                                |
|     |                                |
|     |                                |
|     |                                |
|     .                                . C(max_x,max_y)
|      --------------------------------
|     D(max_x,min_y)
|
V
x


Assumed Point O(0,0) as top left. Derived point A,B,C,D and SUM(x,y,x1,y1) according to the formula:
OC-OB-OD+OA .

*/